diff --git a/.github/workflows/behat-tests.yml b/.github/workflows/behat-tests.yml
index 4d201c2a..86bd0426 100644
--- a/.github/workflows/behat-tests.yml
+++ b/.github/workflows/behat-tests.yml
@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php: ['8.3']
+        php: ['8.4']
         coverage-driver: [pcov]
     name: PHP ${{ matrix.php }}
     steps:
diff --git a/.github/workflows/phpunit-tests.yml b/.github/workflows/phpunit-tests.yml
index 44af1076..ebdfe652 100644
--- a/.github/workflows/phpunit-tests.yml
+++ b/.github/workflows/phpunit-tests.yml
@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php: ['8.3']
+        php: ['8.4']
         coverage-driver: [pcov]
     name: PHP ${{ matrix.php }}
     steps:
diff --git a/.github/workflows/psalm-analysis.yml b/.github/workflows/psalm-analysis.yml
index e0b766da..88d0341c 100644
--- a/.github/workflows/psalm-analysis.yml
+++ b/.github/workflows/psalm-analysis.yml
@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php: [ '8.3' ]
+        php: [ '8.4' ]
         coverage-driver: [ pcov ]
     name: PHP ${{ matrix.php }}
     steps:
@@ -41,4 +41,4 @@ jobs:
         run: composer install -n
 
       - name: Run Psalm
-        run: vendor/bin/psalm
\ No newline at end of file
+        run: vendor/bin/psalm
diff --git a/composer.json b/composer.json
index befd66e2..093e86b6 100644
--- a/composer.json
+++ b/composer.json
@@ -65,7 +65,8 @@
         "opis/closure": "3.x-dev as 3.6.x-dev",
         "cache/array-adapter": "*",
         "matthiasnoback/behat-expect-exception": "^v0.3.0",
-        "behat/gherkin": "~v4.11.0"
+        "behat/gherkin": "~v4.11.0",
+        "rector/rector": "dev-main"
     },
     "suggest": {
         "ext-intl": "intl extension is required for formula support",
diff --git a/rector.php b/rector.php
new file mode 100644
index 00000000..7468ed74
--- /dev/null
+++ b/rector.php
@@ -0,0 +1,80 @@
+<?php
+
+declare(strict_types=1);
+
+use Rector\Caching\ValueObject\Storage\FileCacheStorage;
+use Rector\CodeQuality\Rector\BooleanAnd\SimplifyEmptyArrayCheckRector;
+use Rector\CodeQuality\Rector\Concat\JoinStringConcatRector;
+use Rector\CodeQuality\Rector\Empty_\SimplifyEmptyCheckOnEmptyArrayRector;
+use Rector\CodeQuality\Rector\FuncCall\SimplifyRegexPatternRector;
+use Rector\CodeQuality\Rector\FunctionLike\SimplifyUselessVariableRector;
+use Rector\CodeQuality\Rector\Ternary\SwitchNegatedTernaryRector;
+use Rector\Config\RectorConfig;
+use Rector\DeadCode\Rector\ClassConst\RemoveUnusedPrivateClassConstantRector;
+use Rector\DeadCode\Rector\ClassMethod\RemoveUnusedPrivateMethodRector;
+use Rector\DeadCode\Rector\ConstFetch\RemovePhpVersionIdCheckRector;
+use Rector\EarlyReturn\Rector\Foreach_\ChangeNestedForeachIfsToEarlyContinueRector;
+use Rector\EarlyReturn\Rector\If_\ChangeIfElseValueAssignToEarlyReturnRector;
+use Rector\EarlyReturn\Rector\If_\ChangeNestedIfsToEarlyReturnRector;
+use Rector\EarlyReturn\Rector\If_\RemoveAlwaysElseRector;
+use Rector\Php71\Rector\FuncCall\RemoveExtraParametersRector;
+use Rector\Php73\Rector\FuncCall\JsonThrowOnErrorRector;
+use Rector\Strict\Rector\Empty_\DisallowedEmptyRuleFixerRector;
+
+/**
+ * How to run:
+ *
+ * ```bash
+ * composer require rector/rector --dev
+ * ./vendor/bin/rector process
+ * ```
+ */
+return RectorConfig::configure()
+    ->withAutoloadPaths([
+        file_exists(__DIR__ . '/../../autoload.php') ? __DIR__ . '/../../autoload.php' : null,
+        file_exists(__DIR__ . '/vendor/autoload.php') ? __DIR__ . '/vendor/autoload.php' : null,
+    ])
+    ->withCache(__DIR__ . '/../../../runtime/cache/rector', FileCacheStorage::class)
+    ->withPreparedSets(deadCode: true, codeQuality: true)
+    ->withPhpSets(php83: true, php84: true)
+    ->withPhpVersion(\Rector\ValueObject\PhpVersion::PHP_84)
+    ->withoutParallel()
+    ->withPaths([
+        __DIR__ . '/src',
+    ])
+    ->withRules([
+        RemoveAlwaysElseRector::class,
+        ChangeNestedIfsToEarlyReturnRector::class,
+        ChangeIfElseValueAssignToEarlyReturnRector::class,
+        ChangeNestedForeachIfsToEarlyContinueRector::class,
+    ])
+    ->withSkip([
+        // Too many places where json_decode was used intentionally on wrong data =\
+        JsonThrowOnErrorRector::class,
+
+        // Frequently used for debug. Maybe, such places should be annotated with @noRector?
+        SimplifyUselessVariableRector::class,
+
+        // Legacy modules are full of `_doSomeShit()` methods that are likely called as `$this->_do{$operation}`
+        RemoveUnusedPrivateMethodRector::class,
+
+        // Does not make sense for us
+        SimplifyRegexPatternRector::class,
+
+        // Requires whole project autoloaded to work well
+        RemoveUnusedPrivateClassConstantRector::class,
+
+        // don't refactor with `empty` check function
+        DisallowedEmptyRuleFixerRector::class,
+        SimplifyEmptyArrayCheckRector::class,
+        SimplifyEmptyCheckOnEmptyArrayRector::class,
+
+        // do not refactor `query` attribute like " . '$select' . " to 'SELECT $select ...'
+        JoinStringConcatRector::class,
+
+        // do not refactor php version id checkers
+        RemovePhpVersionIdCheckRector::class,
+        RemoveExtraParametersRector::class,
+
+        SwitchNegatedTernaryRector::class,
+    ]);
diff --git a/src/EntityInterface.php b/src/EntityInterface.php
index 497cd470..1e70fee6 100644
--- a/src/EntityInterface.php
+++ b/src/EntityInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing;
 
 /**
diff --git a/src/Exception/CannotReassignException.php b/src/Exception/CannotReassignException.php
index 2c4491ae..9219567e 100644
--- a/src/Exception/CannotReassignException.php
+++ b/src/Exception/CannotReassignException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\Exception;
 
 use Exception;
diff --git a/src/Exception/EntityNotFoundException.php b/src/Exception/EntityNotFoundException.php
index fe160170..fbcb920b 100644
--- a/src/Exception/EntityNotFoundException.php
+++ b/src/Exception/EntityNotFoundException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\Exception;
 
 use hiqdev\php\billing\ExceptionInterface;
diff --git a/src/Exception/NotSupportedException.php b/src/Exception/NotSupportedException.php
index 9c1a7ba6..819491a3 100644
--- a/src/Exception/NotSupportedException.php
+++ b/src/Exception/NotSupportedException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\Exception;
 
 use Exception;
diff --git a/src/Exception/UnknownEntityException.php b/src/Exception/UnknownEntityException.php
index 22100b10..5cecae4d 100644
--- a/src/Exception/UnknownEntityException.php
+++ b/src/Exception/UnknownEntityException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\Exception;
 
 use hiqdev\php\billing\ExceptionInterface;
diff --git a/src/ExceptionInterface.php b/src/ExceptionInterface.php
index f3d1e26c..a140ca57 100644
--- a/src/ExceptionInterface.php
+++ b/src/ExceptionInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing;
 
 /**
diff --git a/src/Money/MultipliedMoney.php b/src/Money/MultipliedMoney.php
index 3cc10b9c..0d78957e 100644
--- a/src/Money/MultipliedMoney.php
+++ b/src/Money/MultipliedMoney.php
@@ -35,7 +35,6 @@ private function __construct(
 
     /**
      * @param numeric-string $amount
-     * @param string $currencyCode
      */
     public static function create(string $amount, string $currencyCode): MultipliedMoney
     {
@@ -87,7 +86,7 @@ private static function calculateMultiplierToInteger(string $amount): int
         }
 
         [, $fraction] = explode('.', $amount, 2);
-        return (int)('1' . implode(array_fill(0, strlen($fraction), 0)));
+        return (int)('1' . implode('', array_fill(0, strlen($fraction), 0)));
     }
 
     /**
diff --git a/src/action/AbstractAction.php b/src/action/AbstractAction.php
index a28cddc8..ff8ec252 100755
--- a/src/action/AbstractAction.php
+++ b/src/action/AbstractAction.php
@@ -28,9 +28,6 @@
  */
 abstract class AbstractAction implements \JsonSerializable, ActionInterface
 {
-    /** @var int */
-    protected $id;
-
     /** @var TypeInterface */
     protected $type;
 
@@ -55,25 +52,21 @@ abstract class AbstractAction implements \JsonSerializable, ActionInterface
     /** @var ActionInterface */
     protected $parent;
 
-    protected float $fractionOfMonth = 0.0;
-
     /**
-     * @param SaleInterface $sale
-     * @param ActionInterface $parent
+     * @param int $id
      */
     public function __construct(
-        $id,
+        protected $id,
         TypeInterface $type,
         TargetInterface $target,
         QuantityInterface $quantity,
         CustomerInterface $customer,
         DateTimeImmutable $time,
-        SaleInterface $sale = null,
-        ActionState $state = null,
-        ActionInterface $parent = null,
-        float $fractionOfMonth = 0.0
+        ?SaleInterface $sale = null,
+        ?ActionState $state = null,
+        ?ActionInterface $parent = null,
+        protected float $fractionOfMonth = 0.0
     ) {
-        $this->id       = $id;
         $this->type     = $type;
         $this->target   = $target;
         $this->quantity = $quantity;
@@ -82,7 +75,6 @@ public function __construct(
         $this->sale     = $sale;
         $this->state    = $state;
         $this->parent   = $parent;
-        $this->fractionOfMonth = $fractionOfMonth;
     }
 
     /**
diff --git a/src/action/Action.php b/src/action/Action.php
index af25f5e9..af92c4f9 100644
--- a/src/action/Action.php
+++ b/src/action/Action.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\action;
 
 use hiqdev\php\billing\price\PriceInterface;
diff --git a/src/action/ActionCreationDto.php b/src/action/ActionCreationDto.php
index 5a94fdb1..378cefb0 100644
--- a/src/action/ActionCreationDto.php
+++ b/src/action/ActionCreationDto.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\action;
 
 /**
diff --git a/src/action/ActionFactoryInterface.php b/src/action/ActionFactoryInterface.php
index 93650e4c..7576ad72 100644
--- a/src/action/ActionFactoryInterface.php
+++ b/src/action/ActionFactoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\action;
 
 /**
diff --git a/src/action/ActionInterface.php b/src/action/ActionInterface.php
index 456a120a..874e97de 100644
--- a/src/action/ActionInterface.php
+++ b/src/action/ActionInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\action;
 
 use DateTimeImmutable;
diff --git a/src/action/ActionState.php b/src/action/ActionState.php
index 5dfbbb49..56209569 100644
--- a/src/action/ActionState.php
+++ b/src/action/ActionState.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\action;
 
 /**
diff --git a/src/action/TemporaryAction.php b/src/action/TemporaryAction.php
index ccf7a923..61107103 100644
--- a/src/action/TemporaryAction.php
+++ b/src/action/TemporaryAction.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\action;
 
 use hiqdev\php\billing\customer\CustomerInterface;
diff --git a/src/action/TemporaryActionInterface.php b/src/action/TemporaryActionInterface.php
index 1c97dbcf..ee64ce47 100644
--- a/src/action/TemporaryActionInterface.php
+++ b/src/action/TemporaryActionInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\action;
 
 /**
diff --git a/src/action/UsageInterval.php b/src/action/UsageInterval.php
index 3251b8db..d62c725d 100644
--- a/src/action/UsageInterval.php
+++ b/src/action/UsageInterval.php
@@ -7,23 +7,15 @@
 use InvalidArgumentException;
 use JsonSerializable;
 
-/** @readonly */
-final class UsageInterval implements JsonSerializable
+final readonly class UsageInterval implements JsonSerializable
 {
-    /** @readonly */
-    private DateTimeImmutable $start;
-    /** @readonly */
-    private DateTimeImmutable $end;
-    /** @readonly */
     private DateTimeImmutable $month;
 
     private function __construct(
-        DateTimeImmutable $start,
-        DateTimeImmutable $end
+        private DateTimeImmutable $start,
+        private DateTimeImmutable $end
     ) {
-        $this->start = $start;
-        $this->end = $end;
-        $this->month = self::toMonth($start);
+        $this->month = self::toMonth($this->start);
     }
 
     private static function toMonth(DateTimeImmutable $date): DateTimeImmutable
@@ -48,7 +40,6 @@ public static function wholeMonth(DateTimeImmutable $time): self
      * @param DateTimeImmutable $start the start date of the sale
      * @param DateTimeImmutable|null $end the end date of the sale or null if the sale is active
      * @throws InvalidArgumentException if the start date is greater than the end date
-     * @return static
      */
     public static function withinMonth(
         DateTimeImmutable $month,
@@ -58,7 +49,7 @@ public static function withinMonth(
         $month = self::toMonth($month);
         $nextMonth = $month->modify('+1 month');
 
-        if ($end !== null && $start > $end) {
+        if ($end instanceof \DateTimeImmutable && $start > $end) {
             throw new InvalidArgumentException('Start date must be less than end date');
         }
 
@@ -67,7 +58,7 @@ public static function withinMonth(
             $end = $month;
         }
 
-        if ($end !== null && $end < $month) {
+        if ($end instanceof \DateTimeImmutable && $end < $month) {
             $start = $month;
             $end = $month;
         }
@@ -90,7 +81,6 @@ public static function withinMonth(
      * @param DateTimeImmutable $month the month to calculate the usage interval for
      * @param DateTimeImmutable $start the start date of the sale
      * @param float $fractionOfMonth the fraction of manth
-     * @return static
      */
     public static function withMonthAndFraction(
         DateTimeImmutable $month,
@@ -173,9 +163,6 @@ public function ratioOfMonth(): float
 
     /**
      * Extends the usage interval to include both current and other intervals.
-     *
-     * @param UsageInterval $other
-     * @return self
      */
     public function extend(self $other): self
     {
diff --git a/src/action/mux/ActionMuxInterface.php b/src/action/mux/ActionMuxInterface.php
index 76c6e0b9..906813fb 100644
--- a/src/action/mux/ActionMuxInterface.php
+++ b/src/action/mux/ActionMuxInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\action\mux;
 
 use hiqdev\php\billing\action\ActionInterface;
diff --git a/src/action/mux/MuxException.php b/src/action/mux/MuxException.php
index 1db551b6..49416313 100644
--- a/src/action/mux/MuxException.php
+++ b/src/action/mux/MuxException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\action\mux;
 
 use Exception;
diff --git a/src/bill/Bill.php b/src/bill/Bill.php
index 81782680..6ecf2ea1 100755
--- a/src/bill/Bill.php
+++ b/src/bill/Bill.php
@@ -28,9 +28,6 @@
  */
 class Bill implements BillInterface
 {
-    /** @var int|string */
-    protected $id;
-
     /** @var TypeInterface */
     protected $type;
 
@@ -67,19 +64,21 @@ class Bill implements BillInterface
     /** @var UsageInterval */
     protected $usageInterval;
 
+    /**
+     * @param int|string $id
+     */
     public function __construct(
-                            $id,
+        protected $id,
         TypeInterface $type,
         DateTimeImmutable $time,
         Money $sum,
         QuantityInterface $quantity,
         CustomerInterface $customer,
-        TargetInterface $target = null,
-        PlanInterface $plan = null,
+        ?TargetInterface $target = null,
+        ?PlanInterface $plan = null,
         array $charges = [],
-        BillState $state = null
+        ?BillState $state = null
     ) {
-        $this->id           = $id;
         $this->type         = $type;
         $this->time         = $time;
         $this->sum          = $sum;
diff --git a/src/bill/BillCreationDto.php b/src/bill/BillCreationDto.php
index 897ac8f2..ffb64f55 100644
--- a/src/bill/BillCreationDto.php
+++ b/src/bill/BillCreationDto.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\bill;
 
 /**
diff --git a/src/bill/BillFactoryInterface.php b/src/bill/BillFactoryInterface.php
index f5598ebb..08ce757b 100644
--- a/src/bill/BillFactoryInterface.php
+++ b/src/bill/BillFactoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\bill;
 
 /**
diff --git a/src/bill/BillInterface.php b/src/bill/BillInterface.php
index 13bb58d5..2c06d54e 100644
--- a/src/bill/BillInterface.php
+++ b/src/bill/BillInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\bill;
 
 use DateTimeImmutable;
diff --git a/src/bill/BillRepositoryInterface.php b/src/bill/BillRepositoryInterface.php
index dadc97d3..85e95a55 100644
--- a/src/bill/BillRepositoryInterface.php
+++ b/src/bill/BillRepositoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\bill;
 
 /**
diff --git a/src/bill/BillRequisite.php b/src/bill/BillRequisite.php
index 0d6db484..fe51843a 100644
--- a/src/bill/BillRequisite.php
+++ b/src/bill/BillRequisite.php
@@ -1,18 +1,18 @@
 <?php
 
+declare(strict_types=1);
+
 namespace hiqdev\php\billing\bill;
 
 class BillRequisite
 {
-    /** @var int|string|null */
-    protected $id;
-
-    protected ?string $name = null;
-
-    public function __construct($id = null, string $name = null)
-    {
-        $this->id = $id;
-        $this->name = $name;
+    /**
+     * @param int|string|null $id
+     */
+    public function __construct(
+        protected $id = null,
+        protected ?string $name = null
+    ) {
     }
 
     public function getId()
diff --git a/src/bill/BillState.php b/src/bill/BillState.php
index 72601b62..6937d516 100644
--- a/src/bill/BillState.php
+++ b/src/bill/BillState.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\bill;
 
 /**
diff --git a/src/charge/AddonsContainerInterface.php b/src/charge/AddonsContainerInterface.php
index e8d2bf62..57fa9bc8 100644
--- a/src/charge/AddonsContainerInterface.php
+++ b/src/charge/AddonsContainerInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge;
 
 use hiqdev\php\billing\charge\modifiers\AddonInterface;
@@ -22,8 +24,6 @@ interface AddonsContainerInterface
     /**
      * Adds the $addon into the container
      *
-     * @param string $name
-     * @param AddonInterface $addon
      * @return ChargeModifier|AddonsContainerInterface
      * @throw Exception when the addon $name already exists in the container
      */
@@ -38,7 +38,6 @@ public function addAddon(string $name, AddonInterface $addon);
     public function getAddon(string $name): ?AddonInterface;
 
     /**
-     * @param string $name
      * @return bool whether addon is in the container
      */
     public function hasAddon(string $name): bool;
diff --git a/src/charge/Charge.php b/src/charge/Charge.php
index 34e43f10..74e207ee 100755
--- a/src/charge/Charge.php
+++ b/src/charge/Charge.php
@@ -31,9 +31,6 @@ class Charge implements ChargeInterface
 {
     use EventAwareTrait;
 
-    /** @var int */
-    protected $id;
-
     /** @var TypeInterface */
     protected $type;
 
@@ -64,17 +61,19 @@ class Charge implements ChargeInterface
     /** @var ChargeInterface|null */
     protected $parent;
 
+    /**
+     * @param int $id
+     */
     public function __construct(
-                            $id,
+        protected $id,
         TypeInterface $type,
         TargetInterface $target,
         ActionInterface $action,
         ?PriceInterface $price,
         QuantityInterface $usage,
         Money $sum,
-        BillInterface $bill = null
+        ?BillInterface $bill = null
     ) {
-        $this->id       = $id;
         $this->type     = $type;
         $this->target   = $target;
         $this->action   = $action;
diff --git a/src/charge/ChargeCreationDto.php b/src/charge/ChargeCreationDto.php
index 028ce6a1..5d21459f 100644
--- a/src/charge/ChargeCreationDto.php
+++ b/src/charge/ChargeCreationDto.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge;
 
 /**
diff --git a/src/charge/ChargeFactoryInterface.php b/src/charge/ChargeFactoryInterface.php
index c9aef9e0..9d06aa8e 100644
--- a/src/charge/ChargeFactoryInterface.php
+++ b/src/charge/ChargeFactoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge;
 
 /**
diff --git a/src/charge/ChargeInterface.php b/src/charge/ChargeInterface.php
index d2767842..74e3b884 100644
--- a/src/charge/ChargeInterface.php
+++ b/src/charge/ChargeInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge;
 
 use hiqdev\php\billing\action\ActionInterface;
diff --git a/src/charge/ChargeModifier.php b/src/charge/ChargeModifier.php
index 16be4963..200373de 100644
--- a/src/charge/ChargeModifier.php
+++ b/src/charge/ChargeModifier.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge;
 
 use hiqdev\php\billing\action\ActionInterface;
diff --git a/src/charge/ChargeState.php b/src/charge/ChargeState.php
index a5c63324..8dbbbe4b 100644
--- a/src/charge/ChargeState.php
+++ b/src/charge/ChargeState.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge;
 
 /**
diff --git a/src/charge/GeneralizerInterface.php b/src/charge/GeneralizerInterface.php
index b8b64e11..ce44075f 100644
--- a/src/charge/GeneralizerInterface.php
+++ b/src/charge/GeneralizerInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge;
 
 use hiqdev\php\billing\bill\BillInterface;
diff --git a/src/charge/SettableChargeModifierTrait.php b/src/charge/SettableChargeModifierTrait.php
index 19f9fe0b..a3305960 100644
--- a/src/charge/SettableChargeModifierTrait.php
+++ b/src/charge/SettableChargeModifierTrait.php
@@ -31,7 +31,7 @@ public function modifyCharge(?ChargeInterface $charge, ActionInterface $action):
             return $this->modifier->modifyCharge($charge, $action);
         }
 
-        return $charge ? [$charge] : [];
+        return $charge instanceof ChargeInterface ? [$charge] : [];
     }
 
     /** {@inheritdoc} */
diff --git a/src/charge/TimeLimitedModifierInterface.php b/src/charge/TimeLimitedModifierInterface.php
index 68293a65..87eea9f0 100644
--- a/src/charge/TimeLimitedModifierInterface.php
+++ b/src/charge/TimeLimitedModifierInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge;
 
 use DateTimeImmutable;
@@ -18,7 +20,6 @@
 interface TimeLimitedModifierInterface
 {
     /**
-     * @param DateTimeImmutable $time
      * @return bool whether the modifier is applicable at the passed $time
      * @throws FormulaRuntimeError when modifier is misconfigured
      */
diff --git a/src/charge/derivative/ChargeDerivativeInterface.php b/src/charge/derivative/ChargeDerivativeInterface.php
index f8ede3da..c3b0e9b3 100644
--- a/src/charge/derivative/ChargeDerivativeInterface.php
+++ b/src/charge/derivative/ChargeDerivativeInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\derivative;
 
 use hiqdev\php\billing\charge\ChargeInterface;
diff --git a/src/charge/derivative/ChargeDerivativeQueryInterface.php b/src/charge/derivative/ChargeDerivativeQueryInterface.php
index a643eb74..bb6527f1 100644
--- a/src/charge/derivative/ChargeDerivativeQueryInterface.php
+++ b/src/charge/derivative/ChargeDerivativeQueryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\derivative;
 
 use hiqdev\php\billing\action\ActionInterface;
diff --git a/src/charge/modifiers/AddonInterface.php b/src/charge/modifiers/AddonInterface.php
index 5d898dba..366a489e 100644
--- a/src/charge/modifiers/AddonInterface.php
+++ b/src/charge/modifiers/AddonInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers;
 
 /**
diff --git a/src/charge/modifiers/FixedDiscount.php b/src/charge/modifiers/FixedDiscount.php
index 203d5283..44a24764 100644
--- a/src/charge/modifiers/FixedDiscount.php
+++ b/src/charge/modifiers/FixedDiscount.php
@@ -13,7 +13,9 @@
 use hiqdev\php\billing\action\ActionInterface;
 use hiqdev\php\billing\charge\Charge;
 use hiqdev\php\billing\charge\ChargeInterface;
+use hiqdev\php\billing\charge\modifiers\addons\ChargeType;
 use hiqdev\php\billing\charge\modifiers\addons\Discount;
+use hiqdev\php\billing\charge\modifiers\addons\Reason;
 use hiqdev\php\billing\type\Type;
 use hiqdev\php\units\Quantity;
 use Money\Money;
@@ -33,12 +35,13 @@ public function __construct($value, array $addons = [])
         $this->addAddon(self::VALUE, new Discount($value));
     }
 
+    #[\Override]
     public function getNext()
     {
         return $this;
     }
 
-    public function getValue(ChargeInterface $charge = null): Discount
+    public function getValue(?ChargeInterface $charge = null): Discount
     {
         return $this->getAddon(self::VALUE);
     }
@@ -53,7 +56,7 @@ public function isRelative()
         return !$this->isAbsolute();
     }
 
-    public function calculateSum(ChargeInterface $charge = null): Money
+    public function calculateSum(?ChargeInterface $charge = null): Money
     {
         return $this->getValue($charge)->calculateSum($charge);
     }
@@ -62,14 +65,14 @@ private function getType()
     {
         $chargeTypeAddon = $this->getChargeType();
 
-        $type = $chargeTypeAddon ? $chargeTypeAddon->getValue() : 'discount,discount';
+        $type = $chargeTypeAddon instanceof ChargeType ? $chargeTypeAddon->getValue() : 'discount,discount';
 
         return new Type(Type::ANY, $type);
     }
 
     public function modifyCharge(?ChargeInterface $charge, ActionInterface $action): array
     {
-        if ($charge === null) {
+        if (!$charge instanceof ChargeInterface) {
             return [];
         }
 
@@ -88,7 +91,7 @@ public function modifyCharge(?ChargeInterface $charge, ActionInterface $action):
         $discount->setParent($charge);
 
         $reason = $this->getReason();
-        if ($reason) {
+        if ($reason instanceof Reason) {
             $discount->setComment($reason->getValue());
         }
 
diff --git a/src/charge/modifiers/FullCombination.php b/src/charge/modifiers/FullCombination.php
index 5b5ba510..1fb9400b 100644
--- a/src/charge/modifiers/FullCombination.php
+++ b/src/charge/modifiers/FullCombination.php
@@ -57,9 +57,7 @@ public function modifyCharge(?ChargeInterface $charge, ActionInterface $action):
                 return []; // If there was at least one charge, but it disappeared – modifier does not want this charge to happen. Stop.
             }
 
-            $originalChargeExists = array_reduce($leftCharges, static function ($result, Charge $item) use ($charge) {
-                return $result || $charge === $item;
-            }, false);
+            $originalChargeExists = array_reduce($leftCharges, static fn($result, Charge $item) => $result || $charge === $item, false);
             if ($charge && !$originalChargeExists) {
                 return $leftCharges;
             }
@@ -155,7 +153,7 @@ private function unique(array $charges): array
      */
     private function sumCharges(?ChargeInterface $originalCharge, array $producedCharges): ?ChargeInterface
     {
-        if ($originalCharge === null) {
+        if (!$originalCharge instanceof ChargeInterface) {
             return null;
         }
 
@@ -175,9 +173,9 @@ private function sumCharges(?ChargeInterface $originalCharge, array $producedCha
             return $originalCharge;
         }
 
-        $query = (new ChargeDerivativeQuery())->changeSum($sum);
+        $query = new ChargeDerivativeQuery()->changeSum($sum);
 
-        return (new ChargeDerivative())->__invoke($originalCharge, $query);
+        return new ChargeDerivative()->__invoke($originalCharge, $query);
     }
 
     /**
diff --git a/src/charge/modifiers/GrowingDiscount.php b/src/charge/modifiers/GrowingDiscount.php
index 47506f44..99f6cd4c 100644
--- a/src/charge/modifiers/GrowingDiscount.php
+++ b/src/charge/modifiers/GrowingDiscount.php
@@ -16,6 +16,7 @@
 use hiqdev\php\billing\charge\modifiers\addons\Maximum;
 use hiqdev\php\billing\charge\modifiers\addons\Minimum;
 use hiqdev\php\billing\charge\modifiers\addons\Period;
+use hiqdev\php\billing\charge\modifiers\addons\Since;
 use hiqdev\php\billing\charge\modifiers\addons\Step;
 use Money\Money;
 
@@ -40,6 +41,7 @@ public function __construct($step, $min = null, array $addons = [])
         }
     }
 
+    #[\Override]
     public function isAbsolute()
     {
         return $this->getStep()->isAbsolute();
@@ -87,6 +89,7 @@ public function every($string = 1)
         return $this->addAddon(self::PERIOD, Period::fromString($string));
     }
 
+    #[\Override]
     public function calculateSum(?ChargeInterface $charge = null): Money
     {
         $sum = parent::calculateSum($charge);
@@ -101,9 +104,10 @@ public function calculateSum(?ChargeInterface $charge = null): Money
         return $sum;
     }
 
+    #[\Override]
     public function getValue(?ChargeInterface $charge = null): Discount
     {
-        $time = $charge ? $charge->getAction()->getTime() : new DateTimeImmutable();
+        $time = $charge instanceof ChargeInterface ? $charge->getAction()->getTime() : new DateTimeImmutable();
         $num = (int) $this->countPeriodsPassed($time);
 
         return $this->getStep()->calculateFor($num, $this->getMin());
@@ -112,7 +116,7 @@ public function getValue(?ChargeInterface $charge = null): Discount
     protected function countPeriodsPassed(DateTimeImmutable $time)
     {
         $since = $this->getSince();
-        if ($since === null) {
+        if (!$since instanceof Since) {
             throw new \Exception('no since given for growing discount');
         }
 
diff --git a/src/charge/modifiers/Increase.php b/src/charge/modifiers/Increase.php
index e1d7f5af..2dc189cb 100644
--- a/src/charge/modifiers/Increase.php
+++ b/src/charge/modifiers/Increase.php
@@ -19,11 +19,6 @@
  */
 class Increase extends Modifier
 {
-    public function __construct(array $addons = [])
-    {
-        parent::__construct($addons);
-    }
-
     public function grows($step, $min = null): GrowingDiscount
     {
         $increase = new GrowingDiscount($step, $min, $this->addons);
diff --git a/src/charge/modifiers/Installment.php b/src/charge/modifiers/Installment.php
index dcfe6a10..b97817a5 100644
--- a/src/charge/modifiers/Installment.php
+++ b/src/charge/modifiers/Installment.php
@@ -17,6 +17,8 @@
 use hiqdev\php\billing\charge\Charge;
 use hiqdev\php\billing\charge\ChargeInterface;
 use hiqdev\php\billing\charge\modifiers\addons\Period;
+use hiqdev\php\billing\charge\modifiers\addons\Reason;
+use hiqdev\php\billing\charge\modifiers\addons\Since;
 use hiqdev\php\billing\charge\modifiers\event\InstallmentWasFinished;
 use hiqdev\php\billing\charge\modifiers\event\InstallmentWasStarted;
 use hiqdev\php\billing\formula\FormulaSemanticsError;
@@ -56,21 +58,22 @@ public function getTarget()
         return new Target(Target::ANY, Target::ANY);
     }
 
-    public function till($time)
+    #[\Override]
+    public function till($time): never
     {
         throw new FormulaSemanticsError('till can not be defined for installment');
     }
 
     public function modifyCharge(?ChargeInterface $charge, ActionInterface $action): array
     {
-        if ($charge === null) {
+        if (!$charge instanceof ChargeInterface) {
             throw new \Exception('unexpected null charge in Installment, to be implemented');
         }
 
         $this->ensureIsValid();
 
         $reason = $this->getReason();
-        if ($reason) {
+        if ($reason instanceof Reason) {
             $charge->setComment($reason->getValue());
         }
 
@@ -89,12 +92,12 @@ public function modifyCharge(?ChargeInterface $charge, ActionInterface $action):
     protected function ensureIsValid(): void
     {
         $since = $this->getSince();
-        if ($since === null) {
+        if (!$since instanceof Since) {
             throw new FormulaSemanticsError('no since given for installment');
         }
 
         $term = $this->getTerm();
-        if ($term === null) {
+        if (!$term instanceof Period) {
             throw new FormulaSemanticsError('no term given for installment');
         }
     }
@@ -105,12 +108,7 @@ private function isFirstMonthInInstallmentPassed(DateTimeImmutable $time): bool
         if ($since && $since->getValue() > $time) {
             return false;
         }
-
-        if ($since->getValue()->diff($time)->format('%a') === '0') {
-            return true;
-        }
-
-        return false;
+        return $since->getValue()->diff($time)->format('%a') === '0';
     }
 
     private function isFirstMonthAfterInstallmentPassed(DateTimeImmutable $time): bool
@@ -121,18 +119,12 @@ private function isFirstMonthAfterInstallmentPassed(DateTimeImmutable $time): bo
         }
 
         $till = $this->getTill();
-        if ($till && $till->getValue() <= $time) {
-            if ($till->getValue()->diff($time)->format('%a') === '0') {
-                return true;
-            }
-        }
-
-        $term = $this->getTerm();
-        if ($term && $term->addTo($since->getValue())->diff($time)->format('%a') === '0') {
+        if ($till && $till->getValue() <= $time && $till->getValue()->diff($time)->format('%a') === '0') {
             return true;
         }
 
-        return false;
+        $term = $this->getTerm();
+        return $term && $term->addTo($since->getValue())->diff($time)->format('%a') === '0';
     }
 
     private function createInstallmentFinishingCharge(ChargeInterface $charge, DateTimeImmutable $month): ChargeInterface
@@ -189,11 +181,11 @@ public function getRemainingPeriods(DateTimeImmutable $currentDate): ?Period
         $since = $this->getSince();
         $term = $this->getTerm();
 
-        if ($since === null || $term === null) {
+        if (!$since instanceof Since || !$term instanceof Period) {
             return null;
         }
 
-        $className = get_class($term);
+        $className = $term::class;
         $passedRatio = $term->countPeriodsPassed($since->getValue(), $currentDate);
 
         return new $className(
diff --git a/src/charge/modifiers/Modifier.php b/src/charge/modifiers/Modifier.php
index ee9965b8..58771562 100644
--- a/src/charge/modifiers/Modifier.php
+++ b/src/charge/modifiers/Modifier.php
@@ -16,6 +16,8 @@
 use hiqdev\php\billing\charge\AddonsContainerInterface;
 use hiqdev\php\billing\charge\ChargeInterface;
 use hiqdev\php\billing\charge\ChargeModifier;
+use hiqdev\php\billing\charge\modifiers\addons\Period;
+use hiqdev\php\billing\charge\modifiers\addons\Since;
 use hiqdev\php\billing\charge\TimeLimitedModifierInterface;
 use hiqdev\php\billing\formula\FormulaRuntimeError;
 
@@ -52,11 +54,7 @@ public function isSuitable(?ChargeInterface $charge, ActionInterface $action): b
         $month = $action->getTime()->modify('first day of this month midnight');
 
         $since = $this->getSince();
-        if ($since && $since->getValue() > $month) {
-            return false;
-        }
-
-        return true;
+        return !($since && $since->getValue() > $month);
     }
 
     public function discount()
@@ -125,8 +123,8 @@ public function checkPeriod(DateTimeImmutable $time): bool
         }
 
         $term = $this->getTerm();
-        if ($term) {
-            if (!$since) {
+        if ($term instanceof Period) {
+            if (!$since instanceof Since) {
                 throw new FormulaRuntimeError('since must be set to use term');
             }
             if ($term->countPeriodsPassed($since->getValue(), $time) >= 1) {
diff --git a/src/charge/modifiers/MonthlyCap.php b/src/charge/modifiers/MonthlyCap.php
index 4126c52a..f4dc9f4d 100644
--- a/src/charge/modifiers/MonthlyCap.php
+++ b/src/charge/modifiers/MonthlyCap.php
@@ -20,6 +20,7 @@
 use hiqdev\php\billing\charge\modifiers\addons\Boolean;
 use hiqdev\php\billing\charge\modifiers\addons\DayPeriod;
 use hiqdev\php\billing\charge\modifiers\addons\Period;
+use hiqdev\php\billing\charge\modifiers\addons\Reason;
 use hiqdev\php\billing\Exception\NotSupportedException;
 use hiqdev\php\units\Quantity;
 use hiqdev\php\units\QuantityInterface;
@@ -34,7 +35,7 @@
  */
 class MonthlyCap extends Modifier
 {
-    private const PERIOD = 'period';
+    private const string PERIOD = 'period';
 
     protected ChargeDerivative $chargeDerivative;
 
@@ -56,6 +57,7 @@ public function forNonProportionalizedQuantity(): Modifier
         return $this->addAddon('non-proportionalized', new Boolean(true));
     }
 
+    #[\Override]
     public function getNext()
     {
         return $this;
@@ -63,7 +65,7 @@ public function getNext()
 
     public function modifyCharge(?ChargeInterface $charge, ActionInterface $action): array
     {
-        if ($charge === null) {
+        if (!$charge instanceof ChargeInterface) {
             return [];
         }
 
@@ -81,8 +83,6 @@ public function modifyCharge(?ChargeInterface $charge, ActionInterface $action):
     }
 
     /**
-     * @param ChargeInterface $charge
-     * @param ActionInterface $action
      * @return ChargeInterface[]
      */
     private function propotionalizeCharge(ChargeInterface $charge, ActionInterface $action): array
@@ -133,7 +133,7 @@ private function splitCapFromCharge(ChargeInterface $charge, ActionInterface $ac
         $zeroChargeQuery->changeUsage($usageHours->subtract($cappedHours));
         $zeroChargeQuery->changeParent($newCharge);
         $reason = $this->getReason();
-        if ($reason) {
+        if ($reason instanceof Reason) {
             $zeroChargeQuery->changeComment($reason->getValue());
         }
         $newZeroCharge = $this->chargeDerivative->__invoke($charge, $zeroChargeQuery);
diff --git a/src/charge/modifiers/Once.php b/src/charge/modifiers/Once.php
index 1679e9ee..222d8b58 100644
--- a/src/charge/modifiers/Once.php
+++ b/src/charge/modifiers/Once.php
@@ -9,10 +9,12 @@
 use hiqdev\php\billing\charge\derivative\ChargeDerivativeQuery;
 use hiqdev\php\billing\charge\modifiers\addons\MonthPeriod;
 use hiqdev\php\billing\charge\modifiers\addons\Period;
+use hiqdev\php\billing\charge\modifiers\addons\Reason;
 use hiqdev\php\billing\charge\modifiers\addons\Since;
 use hiqdev\php\billing\charge\modifiers\addons\WithSince;
 use hiqdev\php\billing\charge\modifiers\addons\YearPeriod;
 use hiqdev\php\billing\formula\FormulaEngineException;
+use hiqdev\php\billing\sale\SaleInterface;
 use Money\Money;
 
 /**
@@ -26,7 +28,7 @@
  */
 class Once extends Modifier
 {
-    private const MONTHS_IN_YEAR_ON_EARTH = 12;
+    private const int MONTHS_IN_YEAR_ON_EARTH = 12;
 
     protected ChargeDerivative $chargeDerivative;
 
@@ -85,7 +87,7 @@ private function createNewZeroCharge(ChargeInterface $charge): ChargeInterface
         $zeroChargeQuery = new ChargeDerivativeQuery();
         $zeroChargeQuery->changeSum(new Money(0, $charge->getSum()->getCurrency()));
         $reason = $this->getReason();
-        if ($reason) {
+        if ($reason instanceof Reason) {
             $zeroChargeQuery->changeComment($reason->getValue());
         } else {
             $zeroChargeQuery->changeComment('Billed once per ' . $this->getPer()->toString());
@@ -96,14 +98,14 @@ private function createNewZeroCharge(ChargeInterface $charge): ChargeInterface
 
     private function assertPeriod(?Period $period)
     {
-        if ($period === null) {
+        if (!$period instanceof Period) {
             throw new FormulaEngineException('Period cannot be null in Once');
         }
     }
 
     private function assertCharge(?ChargeInterface $charge)
     {
-        if ($charge === null) {
+        if (!$charge instanceof ChargeInterface) {
             throw new FormulaEngineException('Charge cannot be null in Once');
         }
     }
@@ -128,7 +130,7 @@ private function getSinceDate(ActionInterface $action): DateTimeImmutable
         }
 
         $sale = $action->getSale();
-        if ($sale !== null) {
+        if ($sale instanceof SaleInterface) {
             return $sale->getTime();
         }
 
@@ -145,7 +147,8 @@ private function getIntervalMonthsFromPeriod(Period $period): int
     {
         if ($period instanceof YearPeriod) {
             return self::MONTHS_IN_YEAR_ON_EARTH;
-        } elseif ($period instanceof MonthPeriod) {
+        }
+        if ($period instanceof MonthPeriod) {
             return $period->getValue();
         }
 
diff --git a/src/charge/modifiers/PercentPoint.php b/src/charge/modifiers/PercentPoint.php
index 5c8205fc..4b7adc2d 100644
--- a/src/charge/modifiers/PercentPoint.php
+++ b/src/charge/modifiers/PercentPoint.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers;
 
 /**
@@ -17,17 +19,11 @@
  */
 class PercentPoint
 {
-    /**
-     * @var int|float|string
-     */
-    private $number;
-
     /**
      * @param int|float|string $number
      */
-    public function __construct($number)
+    public function __construct(private $number)
     {
-        $this->number = $number;
     }
 
     /**
diff --git a/src/charge/modifiers/addons/Boolean.php b/src/charge/modifiers/addons/Boolean.php
index 6a63be7f..e0029f5f 100644
--- a/src/charge/modifiers/addons/Boolean.php
+++ b/src/charge/modifiers/addons/Boolean.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 use hiqdev\php\billing\charge\modifiers\AddonInterface;
diff --git a/src/charge/modifiers/addons/ChargeType.php b/src/charge/modifiers/addons/ChargeType.php
index 78b224b0..fd1bf73f 100644
--- a/src/charge/modifiers/addons/ChargeType.php
+++ b/src/charge/modifiers/addons/ChargeType.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 use hiqdev\php\billing\charge\modifiers\AddonInterface;
@@ -36,7 +38,7 @@ public function getValue()
 
     private function ensureValidValue($value): string
     {
-        if (!preg_match('/^[\w_,]+$/', $value)) {
+        if (!preg_match('/^[\w_,]+$/', (string) $value)) {
             throw new \InvalidArgumentException('Charge type is not valid');
         }
 
diff --git a/src/charge/modifiers/addons/Date.php b/src/charge/modifiers/addons/Date.php
index 78fb114d..95404a33 100644
--- a/src/charge/modifiers/addons/Date.php
+++ b/src/charge/modifiers/addons/Date.php
@@ -42,19 +42,19 @@ public static function ensureValidValue($value)
             return $value;
         }
 
-        if (preg_match('/^(\d{4})-(\d{2})$/', $value, $ms)) {
+        if (preg_match('/^(\d{4})-(\d{2})$/', (string) $value, $ms)) {
             return new DateTimeImmutable("$ms[1]-$ms[2]-01");
         }
 
-        if (preg_match('/^(\d{4})-(\d{1,2})-(\d{1,2})$/', $value, $ms)) {
+        if (preg_match('/^(\d{4})-(\d{1,2})-(\d{1,2})$/', (string) $value, $ms)) {
             return new DateTimeImmutable("$ms[1]-$ms[2]-$ms[3]");
         }
 
-        if (preg_match('/^(\d{2})\.(\d{4})$/', $value, $ms)) {
+        if (preg_match('/^(\d{2})\.(\d{4})$/', (string) $value, $ms)) {
             return new DateTimeImmutable("$ms[2]-$ms[1]-01");
         }
 
-        if (preg_match('/^(\d{1,2})\.(\d{1,2})\.(\d{4})$/', $value, $ms)) {
+        if (preg_match('/^(\d{1,2})\.(\d{1,2})\.(\d{4})$/', (string) $value, $ms)) {
             return new DateTimeImmutable("$ms[3]-$ms[2]-$ms[1]");
         }
 
diff --git a/src/charge/modifiers/addons/DayPeriod.php b/src/charge/modifiers/addons/DayPeriod.php
index a54d44c6..e74681ad 100644
--- a/src/charge/modifiers/addons/DayPeriod.php
+++ b/src/charge/modifiers/addons/DayPeriod.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 use DateInterval;
diff --git a/src/charge/modifiers/addons/Discount.php b/src/charge/modifiers/addons/Discount.php
index 008f6719..013f2c1a 100644
--- a/src/charge/modifiers/addons/Discount.php
+++ b/src/charge/modifiers/addons/Discount.php
@@ -94,7 +94,7 @@ public function ensureValidValue($value)
             if (preg_match('/^(-?\d{1,5}(\.\d+)?) ([a-z]{2,})$/', $value, $ms)) {
                 try {
                     return Quantity::create($ms[3], $ms[1]);
-                } catch (\Exception $e) {
+                } catch (\Exception) {
                     throw new FormulaSemanticsError("invalid $name value: $value");
                 }
             }
@@ -119,11 +119,9 @@ public function add($addend)
         }
         $this->ensureSameType($addend, 'addend');
 
-        if ($this->isAbsolute()) {
-            $sum = $this->getValue()->add($addend->getValue());
-        } else {
-            $sum = $this->getValue() + $addend->getValue();
-        }
+        $sum = $this->isAbsolute()
+            ? $this->getValue()->add($addend->getValue())
+            : $this->getValue() + $addend->getValue();
 
         return new static($sum);
     }
@@ -137,9 +135,9 @@ public function compare($other)
 
         if ($this->isAbsolute()) {
             return $this->value->compare($other->getValue());
-        } else {
-            return $this->value - $other->getValue();
         }
+
+        return $this->value - $other->getValue();
     }
 
     public function ensureSameType(self $other, $name)
diff --git a/src/charge/modifiers/addons/Extremum.php b/src/charge/modifiers/addons/Extremum.php
index 0043f845..77ff0f22 100644
--- a/src/charge/modifiers/addons/Extremum.php
+++ b/src/charge/modifiers/addons/Extremum.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 /**
diff --git a/src/charge/modifiers/addons/Maximum.php b/src/charge/modifiers/addons/Maximum.php
index 577eb322..2ce66c95 100644
--- a/src/charge/modifiers/addons/Maximum.php
+++ b/src/charge/modifiers/addons/Maximum.php
@@ -22,6 +22,7 @@ class Maximum extends Extremum
 {
     protected static $name = 'maximum';
 
+    #[\Override]
     public function calculateSum(ChargeInterface $charge): Money
     {
         return $this->value instanceof Money
diff --git a/src/charge/modifiers/addons/Minimum.php b/src/charge/modifiers/addons/Minimum.php
index 09bcf09b..0e79c1ae 100644
--- a/src/charge/modifiers/addons/Minimum.php
+++ b/src/charge/modifiers/addons/Minimum.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 /**
diff --git a/src/charge/modifiers/addons/MonthPeriod.php b/src/charge/modifiers/addons/MonthPeriod.php
index 93534550..69a85510 100644
--- a/src/charge/modifiers/addons/MonthPeriod.php
+++ b/src/charge/modifiers/addons/MonthPeriod.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 use DateTimeImmutable;
diff --git a/src/charge/modifiers/addons/Period.php b/src/charge/modifiers/addons/Period.php
index 9f9ce889..54263209 100644
--- a/src/charge/modifiers/addons/Period.php
+++ b/src/charge/modifiers/addons/Period.php
@@ -19,7 +19,7 @@
  *
  * @author Andrii Vasyliev <sol@hiqdev.com>
  */
-abstract class Period implements AddonInterface
+abstract class Period implements AddonInterface, \Stringable
 {
     /**
      * @var int
@@ -49,12 +49,9 @@ public function getValue()
 
     public static function fromString($string)
     {
-        if (preg_match('/^((\d+) +)?(\w+)$/', trim($string), $ms)) {
-            if (isset(static::$periods[$ms[3]])) {
-                $class = static::$periods[$ms[3]];
-
-                return new $class($ms[1] ?: 1);
-            }
+        if (preg_match('/^((\d+) +)?(\w+)$/', trim((string) $string), $ms) && isset(static::$periods[$ms[3]])) {
+            $class = static::$periods[$ms[3]];
+            return new $class($ms[1] ?: 1);
         }
 
         throw new FormulaSemanticsError("invalid period given: $string");
diff --git a/src/charge/modifiers/addons/Reason.php b/src/charge/modifiers/addons/Reason.php
index 8b66e55e..dffd2c9f 100644
--- a/src/charge/modifiers/addons/Reason.php
+++ b/src/charge/modifiers/addons/Reason.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 use hiqdev\php\billing\charge\modifiers\AddonInterface;
diff --git a/src/charge/modifiers/addons/Since.php b/src/charge/modifiers/addons/Since.php
index 719c5e30..8d35c82f 100644
--- a/src/charge/modifiers/addons/Since.php
+++ b/src/charge/modifiers/addons/Since.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 use hiqdev\php\billing\charge\modifiers\AddonInterface;
diff --git a/src/charge/modifiers/addons/Step.php b/src/charge/modifiers/addons/Step.php
index 7dfccd8e..5b799bd0 100644
--- a/src/charge/modifiers/addons/Step.php
+++ b/src/charge/modifiers/addons/Step.php
@@ -21,7 +21,7 @@ class Step extends Discount
 
     public function calculateFor(int $num, ?Discount $min): Discount
     {
-        $start = $min ? $min : $this;
+        $start = $min instanceof Discount ? $min : $this;
 
         if ($this->isAbsolute() || $this->isPercentPoint()) {
             return $this->multiply($num)->add($start);
@@ -30,7 +30,7 @@ public function calculateFor(int $num, ?Discount $min): Discount
         $start = $start->getValue()/100.0;
         $factor = $this->getValue()/100.0;
 
-        $value = 1 - (1 - $start)*pow(1 - $factor, $num);
+        $value = 1 - (1 - $start)*(1 - $factor) ** $num;
 
         return new Discount($value*100);
     }
diff --git a/src/charge/modifiers/addons/Till.php b/src/charge/modifiers/addons/Till.php
index b49fea03..579474d6 100644
--- a/src/charge/modifiers/addons/Till.php
+++ b/src/charge/modifiers/addons/Till.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 use hiqdev\php\billing\charge\modifiers\AddonInterface;
diff --git a/src/charge/modifiers/addons/YearPeriod.php b/src/charge/modifiers/addons/YearPeriod.php
index 15620287..6a4b80cd 100644
--- a/src/charge/modifiers/addons/YearPeriod.php
+++ b/src/charge/modifiers/addons/YearPeriod.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\charge\modifiers\addons;
 
 use DateTimeImmutable;
diff --git a/src/charge/modifiers/event/InstallmentWasFinished.php b/src/charge/modifiers/event/InstallmentWasFinished.php
index a037a325..a6babb19 100755
--- a/src/charge/modifiers/event/InstallmentWasFinished.php
+++ b/src/charge/modifiers/event/InstallmentWasFinished.php
@@ -15,19 +15,10 @@
 
 class InstallmentWasFinished extends AbstractEvent implements \JsonSerializable
 {
-    /**
-     * @var ChargeInterface
-     */
-    private $charge;
-    /**
-     * @var \DateTimeImmutable
-     */
-    private $time;
-
-    private function __construct(ChargeInterface $charge, \DateTimeImmutable $time)
-    {
-        $this->charge = $charge;
-        $this->time = $time;
+    private function __construct(
+        private readonly ChargeInterface $charge,
+        private readonly \DateTimeImmutable $time
+    ) {
     }
 
     public static function onCharge(ChargeInterface $charge, \DateTimeImmutable $time): self
diff --git a/src/charge/modifiers/event/InstallmentWasStarted.php b/src/charge/modifiers/event/InstallmentWasStarted.php
index 4731b6a2..01a5f36b 100644
--- a/src/charge/modifiers/event/InstallmentWasStarted.php
+++ b/src/charge/modifiers/event/InstallmentWasStarted.php
@@ -17,19 +17,10 @@
 
 class InstallmentWasStarted extends AbstractEvent implements \JsonSerializable
 {
-    /**
-     * @var ChargeInterface
-     */
-    private $charge;
-    /**
-     * @var \DateTimeImmutable
-     */
-    private $time;
-
-    private function __construct(ChargeInterface $charge, DateTimeImmutable $time)
-    {
-        $this->charge = $charge;
-        $this->time = $time;
+    private function __construct(
+        private readonly ChargeInterface $charge,
+        private readonly DateTimeImmutable $time
+    ) {
     }
 
     public static function onCharge(ChargeInterface $charge, DateTimeImmutable $time): self
diff --git a/src/customer/Customer.php b/src/customer/Customer.php
index 51658920..6e18c38a 100755
--- a/src/customer/Customer.php
+++ b/src/customer/Customer.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\customer;
 
 /**
@@ -17,16 +19,6 @@
  */
 class Customer implements CustomerInterface
 {
-    /**
-     * @var integer
-     */
-    protected $id;
-
-    /**
-     * @var string
-     */
-    protected $login;
-
     /**
      * @var CustomerInterface
      */
@@ -37,10 +29,15 @@ class Customer implements CustomerInterface
      */
     protected $sellers = [];
 
-    public function __construct($id, $login, ?CustomerInterface $seller = null)
-    {
-        $this->id = $id;
-        $this->login = $login;
+    /**
+     * @param int $id
+     * @param string $login
+     */
+    public function __construct(
+        protected $id,
+        protected $login,
+        ?CustomerInterface $seller = null
+    ) {
         $this->seller = $seller;
     }
 
@@ -75,11 +72,9 @@ public function getSeller()
 
     public static function fromArray(array $info)
     {
-        if (!empty($info['seller_id']) && !empty($info['seller'])) {
-            $seller = new static($info['seller_id'], $info['seller']);
-        } else {
-            $seller = null;
-        }
+        $seller = !empty($info['seller_id']) && !empty($info['seller'])
+            ? new static($info['seller_id'], $info['seller'])
+            : null;
 
         return new static($info['id'], $info['login'], $seller);
     }
diff --git a/src/customer/CustomerCreationDto.php b/src/customer/CustomerCreationDto.php
index 21ff612c..7e19f7ba 100644
--- a/src/customer/CustomerCreationDto.php
+++ b/src/customer/CustomerCreationDto.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\customer;
 
 /**
diff --git a/src/customer/CustomerFactoryInterface.php b/src/customer/CustomerFactoryInterface.php
index da92a7c2..7e4ed2e0 100644
--- a/src/customer/CustomerFactoryInterface.php
+++ b/src/customer/CustomerFactoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\customer;
 
 /**
diff --git a/src/customer/CustomerInterface.php b/src/customer/CustomerInterface.php
index d121d8c4..369fd563 100644
--- a/src/customer/CustomerInterface.php
+++ b/src/customer/CustomerInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\customer;
 
 use hiqdev\php\billing\EntityInterface;
diff --git a/src/customer/CustomerRepositoryInterface.php b/src/customer/CustomerRepositoryInterface.php
index 866bcba5..75e9be2f 100644
--- a/src/customer/CustomerRepositoryInterface.php
+++ b/src/customer/CustomerRepositoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\customer;
 
 /**
diff --git a/src/formula/Asserter.php b/src/formula/Asserter.php
index 6379b2d0..8a997483 100644
--- a/src/formula/Asserter.php
+++ b/src/formula/Asserter.php
@@ -21,12 +21,12 @@
 class Asserter extends \Hoa\Ruler\Visitor\Asserter
 {
     /**
-     * @param Context $context context
+     * @param Context|null $context context
      */
-    public function __construct(Context $context = null)
+    public function __construct(?Context $context = null)
     {
         parent::__construct($context);
-        $this->setOperator('and', [$this, 'makeAnd']);
+        $this->setOperator('and', $this->makeAnd(...));
     }
 
     public function makeAnd($lhs, $rhs)
@@ -34,6 +34,7 @@ public function makeAnd($lhs, $rhs)
         return new FullCombination($lhs, $rhs);
     }
 
+    #[\Override]
     public function visitModel(Model $element, &$handle = null, $eldnah = null)
     {
         return $this->getExpression($element)->accept($this, $handle, $eldnah);
diff --git a/src/formula/FormulaEngine.php b/src/formula/FormulaEngine.php
index a1718b21..dea854c6 100755
--- a/src/formula/FormulaEngine.php
+++ b/src/formula/FormulaEngine.php
@@ -17,7 +17,9 @@
 use hiqdev\php\billing\charge\modifiers\Increase;
 use hiqdev\php\billing\charge\modifiers\Installment;
 use hiqdev\php\billing\charge\modifiers\Once;
+use Hoa\Compiler\Exception\Exception as CompilerException;
 use Hoa\Ruler\Context;
+use Hoa\Ruler\Exception\Interpreter;
 use Hoa\Ruler\Model\Model;
 use Hoa\Ruler\Ruler;
 use Hoa\Visitor\Visit;
@@ -47,7 +49,7 @@ class FormulaEngine implements FormulaEngineInterface
     protected ?Once $once = null;
 
     public function __construct(
-        private CacheInterface $cache
+        private readonly CacheInterface $cache
     ) {
         if (!class_exists(Context::class)) {
             throw new Exception('to use formula engine install `hoa/ruler`');
@@ -96,9 +98,7 @@ public function interpret(string $formula): Model
             }
 
             return $model;
-        } catch (\Hoa\Compiler\Exception\Exception $exception) {
-            throw FormulaSyntaxError::fromException($exception, $formula);
-        } catch (\Hoa\Ruler\Exception\Interpreter $exception) {
+        } catch (CompilerException|Interpreter $exception) {
             throw FormulaSyntaxError::fromException($exception, $formula);
         } catch (\Throwable $exception) {
             throw FormulaSyntaxError::create($formula, 'Failed to interpret formula: ' . $exception->getMessage());
@@ -139,7 +139,7 @@ public function validate(string $formula): ?string
 
     public function getRuler(): Ruler
     {
-        if ($this->ruler === null) {
+        if (!$this->ruler instanceof Ruler) {
             $this->ruler = new Ruler();
             $this->ruler->setAsserter($this->getAsserter());
         }
@@ -150,7 +150,7 @@ public function getRuler(): Ruler
     public function setAsserter(Visit $asserter): self
     {
         $this->asserter = $asserter;
-        if ($this->ruler !== null) {
+        if ($this->ruler instanceof Ruler) {
             $this->ruler->setAsserter($asserter);
         }
 
@@ -159,7 +159,7 @@ public function setAsserter(Visit $asserter): self
 
     public function getAsserter(): Visit
     {
-        if ($this->asserter === null) {
+        if (!$this->asserter instanceof Visit) {
             $this->asserter = new Asserter();
         }
 
@@ -168,7 +168,7 @@ public function getAsserter(): Visit
 
     public function getContext(): Context
     {
-        if ($this->context === null) {
+        if (!$this->context instanceof Context) {
             $this->context = $this->buildContext();
         }
 
@@ -189,7 +189,7 @@ protected function buildContext(): Context
 
     public function getDiscount(): ChargeModifier
     {
-        if ($this->discount === null) {
+        if (!$this->discount instanceof ChargeModifier) {
             $this->discount = new Discount();
         }
 
@@ -198,7 +198,7 @@ public function getDiscount(): ChargeModifier
 
     public function getInstallment(): ChargeModifier
     {
-        if ($this->installment === null) {
+        if (!$this->installment instanceof ChargeModifier) {
             $this->installment = new Installment();
         }
 
@@ -207,7 +207,7 @@ public function getInstallment(): ChargeModifier
 
     public function getIncrease(): ChargeModifier
     {
-        if ($this->increase === null) {
+        if (!$this->increase instanceof ChargeModifier) {
             $this->increase = new Increase();
         }
 
@@ -216,7 +216,7 @@ public function getIncrease(): ChargeModifier
 
     private function getCap(): ChargeModifier
     {
-        if ($this->cap === null) {
+        if (!$this->cap instanceof Cap) {
             $this->cap = new Cap();
         }
 
@@ -225,7 +225,7 @@ private function getCap(): ChargeModifier
 
     private function getOnce(): ChargeModifier
     {
-        if ($this->once === null) {
+        if (!$this->once instanceof Once) {
             $this->once = new Once();
         }
 
@@ -234,13 +234,13 @@ private function getOnce(): ChargeModifier
 
     public function __clone()
     {
-        if ($this->context !== null) {
+        if ($this->context instanceof Context) {
             $this->context = clone $this->context;
         }
-        if ($this->ruler !== null) {
+        if ($this->ruler instanceof Ruler) {
             $this->ruler = clone $this->ruler;
         }
-        if ($this->asserter !== null) {
+        if ($this->asserter instanceof Visit) {
             $this->asserter = clone $this->asserter;
         }
     }
diff --git a/src/formula/FormulaEngineException.php b/src/formula/FormulaEngineException.php
index f68ff2f3..1b376888 100644
--- a/src/formula/FormulaEngineException.php
+++ b/src/formula/FormulaEngineException.php
@@ -25,7 +25,7 @@ class FormulaEngineException extends Exception
      */
     private $formula;
 
-    public static function fromException(Throwable $previous, string $formula, string $message = null): FormulaEngineException
+    public static function fromException(Throwable $previous, string $formula, ?string $message = null): FormulaEngineException
     {
         if ($message !== null) {
             $message .= ': ';
diff --git a/src/formula/FormulaEngineInterface.php b/src/formula/FormulaEngineInterface.php
index 889a7627..94f1d25d 100644
--- a/src/formula/FormulaEngineInterface.php
+++ b/src/formula/FormulaEngineInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\formula;
 
 use hiqdev\php\billing\charge\ChargeModifier;
diff --git a/src/formula/FormulaInterface.php b/src/formula/FormulaInterface.php
index cd994bbd..d91173a0 100644
--- a/src/formula/FormulaInterface.php
+++ b/src/formula/FormulaInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\formula;
 
 /**
diff --git a/src/formula/FormulaRuntimeError.php b/src/formula/FormulaRuntimeError.php
index 318fe2ba..dba83895 100644
--- a/src/formula/FormulaRuntimeError.php
+++ b/src/formula/FormulaRuntimeError.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\formula;
 
 /**
diff --git a/src/formula/FormulaSemanticsError.php b/src/formula/FormulaSemanticsError.php
index 46ec0e54..2c1c0f48 100644
--- a/src/formula/FormulaSemanticsError.php
+++ b/src/formula/FormulaSemanticsError.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\formula;
 
 /**
diff --git a/src/formula/FormulaSyntaxError.php b/src/formula/FormulaSyntaxError.php
index cf5420de..993a02ed 100644
--- a/src/formula/FormulaSyntaxError.php
+++ b/src/formula/FormulaSyntaxError.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\formula;
 
 /**
diff --git a/src/helpers/PriceChargesEstimator.php b/src/helpers/PriceChargesEstimator.php
index b9cb7867..e923c7ad 100644
--- a/src/helpers/PriceChargesEstimator.php
+++ b/src/helpers/PriceChargesEstimator.php
@@ -29,11 +29,6 @@ class PriceChargesEstimator
      */
     protected $calculations = [];
 
-    /**
-     * @var string[] array of strings compatible with `strtotime()`, e.g. `first day of next month`
-     */
-    private $periods = [];
-
     public function __construct(array $calculations)
     {
         $this->calculations = $calculations;
@@ -46,12 +41,9 @@ public function __invoke($periods)
 
     /**
      * @var string[] array of strings compatible with `strtotime()`, e.g. `first day of next month`
-     * @return array
      */
     public function calculateForPeriods($periods): array
     {
-        $this->periods = $periods;
-
         return $this->groupCalculationsByTarget();
     }
 
@@ -86,16 +78,14 @@ private function groupCalculationsByTarget()
             }
             unset($action);
 
-            if (!empty($chargesByTargetAndAction['targets'])) {
-                foreach ($chargesByTargetAndAction['targets'] as &$actions) {
-                    foreach ($actions as &$action) {
-                        $this->decorateAction($action);
-                    }
+            foreach ($chargesByTargetAndAction['targets'] as &$actions) {
+                foreach ($actions as &$action) {
+                    $this->decorateAction($action);
                 }
             }
             unset($action, $actions);
 
-            $result[date("Y-m-d", strtotime($period))] = $chargesByTargetAndAction;
+            $result[date("Y-m-d", strtotime((string) $period))] = $chargesByTargetAndAction;
         }
 
         return $result;
diff --git a/src/order/Billing.php b/src/order/Billing.php
index 97edaf47..7cb588c1 100644
--- a/src/order/Billing.php
+++ b/src/order/Billing.php
@@ -63,7 +63,7 @@ public function __construct(
         $this->collector = $collector ?? new Collector();
     }
 
-    public function calculate($source, DateTimeImmutable $time = null): array
+    public function calculate($source, ?DateTimeImmutable $time = null): array
     {
         $charges = $this->calculateCharges($source, $time);
         $bills = $this->aggregator->aggregateCharges($charges);
@@ -71,7 +71,7 @@ public function calculate($source, DateTimeImmutable $time = null): array
         return $this->merger->mergeBills($bills);
     }
 
-    public function perform($source, DateTimeImmutable $time = null): array
+    public function perform($source, ?DateTimeImmutable $time = null): array
     {
         $charges = $this->calculateCharges($source, $time);
         $bills = $this->getRepoAggregator()->aggregateCharges($charges);
@@ -79,7 +79,7 @@ public function perform($source, DateTimeImmutable $time = null): array
         return $this->saveBills($bills);
     }
 
-    public function calculateCharges($source, DateTimeImmutable $time = null): array
+    public function calculateCharges($source, ?DateTimeImmutable $time = null): array
     {
         $order = $this->collector->collect($source, $time);
 
@@ -109,9 +109,6 @@ private function saveBills(array $bills): array
         return $res;
     }
 
-    /**
-     * @return CalculatorInterface
-     */
     public function getCalculator(): CalculatorInterface
     {
         return $this->calculator;
diff --git a/src/order/BillingInterface.php b/src/order/BillingInterface.php
index 57fe201c..e71fb0d7 100644
--- a/src/order/BillingInterface.php
+++ b/src/order/BillingInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\order;
 
 use DateTimeImmutable;
@@ -24,27 +26,21 @@ interface BillingInterface
 {
     /**
      * @param OrderInterface|ActionInterface|mixed $source
-     * @param DateTimeImmutable|null $time
      * @return BillInterface[]
      */
-    public function calculate($source, DateTimeImmutable $time = null): array;
+    public function calculate($source, ?DateTimeImmutable $time = null): array;
 
     /**
      * @param OrderInterface|ActionInterface|ActionInterface[]|mixed $source
-     * @param DateTimeImmutable|null $time
      * @return BillInterface[] array of charges
      */
-    public function perform($source, DateTimeImmutable $time = null): array;
+    public function perform($source, ?DateTimeImmutable $time = null): array;
 
     /**
      * @param OrderInterface|ActionInterface|mixed $source
-     * @param DateTimeImmutable|null $time
      * @return ChargeInterface[]
      */
-    public function calculateCharges($source, DateTimeImmutable $time = null): array;
+    public function calculateCharges($source, ?DateTimeImmutable $time = null): array;
 
-    /**
-     * @return CalculatorInterface
-     */
     public function getCalculator(): CalculatorInterface;
 }
diff --git a/src/order/Calculator.php b/src/order/Calculator.php
index 15d2ee57..75c341c4 100644
--- a/src/order/Calculator.php
+++ b/src/order/Calculator.php
@@ -26,6 +26,8 @@
 use hiqdev\php\billing\sale\SaleRepositoryInterface;
 use hiqdev\php\billing\tools\ActualDateTimeProvider;
 use hiqdev\php\billing\tools\CurrentDateTimeProviderInterface;
+use hiqdev\php\units\QuantityInterface;
+use Money\Money;
 use Throwable;
 
 /**
@@ -35,21 +37,14 @@
  */
 class Calculator implements CalculatorInterface
 {
-    protected GeneralizerInterface $generalizer;
-    private SaleRepositoryInterface $saleRepository;
-    private PlanRepositoryInterface $planRepository;
-
-    private CurrentDateTimeProviderInterface $dateTimeProvider;
+    private readonly CurrentDateTimeProviderInterface $dateTimeProvider;
 
     public function __construct(
-        GeneralizerInterface $generalizer,
-        SaleRepositoryInterface $saleRepository,
-        PlanRepositoryInterface $planRepository,
-        CurrentDateTimeProviderInterface $dateTimeProvider = null
+        protected GeneralizerInterface $generalizer,
+        private readonly SaleRepositoryInterface $saleRepository,
+        private readonly PlanRepositoryInterface $planRepository,
+        ?CurrentDateTimeProviderInterface $dateTimeProvider = null
     ) {
-        $this->generalizer    = $generalizer;
-        $this->saleRepository = $saleRepository;
-        $this->planRepository = $planRepository;
         $this->dateTimeProvider = $dateTimeProvider ?? new ActualDateTimeProvider();
     }
 
@@ -92,7 +87,7 @@ public function calculatePlan(PlanInterface $plan, ActionInterface $action): arr
     public function calculatePrice(PriceInterface $price, ActionInterface $action): array
     {
         $charge = $this->calculateCharge($price, $action);
-        if ($charge === null) {
+        if (!$charge instanceof ChargeInterface) {
             return [];
         }
 
@@ -114,8 +109,6 @@ public function calculatePrice(PriceInterface $price, ActionInterface $action):
     /**
      * Calculates charge for given action and price.
      * Returns `null`, if $price is not applicable to $action.
-     *
-     * @return ChargeInterface|Charge|null
      */
     public function calculateCharge(PriceInterface $price, ActionInterface $action): ?ChargeInterface
     {
@@ -123,17 +116,17 @@ public function calculateCharge(PriceInterface $price, ActionInterface $action):
             return null;
         }
 
-        if ($action->getSale() !== null && $action->getSale()->getTime() > $this->dateTimeProvider->dateTimeImmutable()) {
+        if ($action->getSale() instanceof SaleInterface && $action->getSale()->getTime() > $this->dateTimeProvider->dateTimeImmutable()) {
             return null;
         }
 
         $usage = $price->calculateUsage($action->getQuantity());
-        if ($usage === null) {
+        if (!$usage instanceof QuantityInterface) {
             return null;
         }
 
         $sum = $price->calculateSum($action->getQuantity());
-        if ($sum === null) {
+        if (!$sum instanceof Money) {
             return null;
         }
 
@@ -183,7 +176,7 @@ private function findPlans(OrderInterface $order): array
             }
         }
 
-        if ($lookPlanIds) {
+        if ($lookPlanIds !== []) {
             $foundPlans = $this->planRepository->findByIds($lookPlanIds);
             foreach ($foundPlans as $plan) {
                 $foundPlans[$plan->getId()] = $plan;
@@ -200,7 +193,6 @@ private function findPlans(OrderInterface $order): array
     }
 
     /**
-     * @param OrderInterface $order
      * @return SaleInterface[]
      */
     private function findSales(OrderInterface $order): array
@@ -216,7 +208,7 @@ private function findSales(OrderInterface $order): array
             }
         }
 
-        if ($lookActions) {
+        if ($lookActions !== []) {
             $lookOrder = new Order(null, $order->getCustomer(), $lookActions);
             $foundSales = $this->saleRepository->findByOrder($lookOrder);
             foreach ($foundSales as $actionKey => $sale) {
diff --git a/src/order/CalculatorInterface.php b/src/order/CalculatorInterface.php
index 51b2a780..0ef4e197 100644
--- a/src/order/CalculatorInterface.php
+++ b/src/order/CalculatorInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\order;
 
 use hiqdev\php\billing\action\ActionInterface;
@@ -25,7 +27,6 @@ interface CalculatorInterface
     /**
      * Calculates Charges for the given $order
      *
-     * @param OrderInterface $order
      * @return ChargeInterface[]
      */
     public function calculateOrder(OrderInterface $order): array;
@@ -34,8 +35,6 @@ public function calculateOrder(OrderInterface $order): array;
      * Calculates all Charges for the given $price and $action by running {@see calculateCharge}
      * and applying all possible modifiers that produce more charges.
      *
-     * @param PriceInterface $price
-     * @param ActionInterface $action
      * @return ChargeInterface[]
      */
     public function calculatePrice(PriceInterface $price, ActionInterface $action): array;
@@ -44,18 +43,12 @@ public function calculatePrice(PriceInterface $price, ActionInterface $action):
      * Calculates a single charge out of $price and $action.
      * In difference to {@see calculatePrice}, this method will only create a primary charge,
      * without trying to produce more charges using modifiers.
-     *
-     * @param PriceInterface $price
-     * @param ActionInterface $action
-     * @return ChargeInterface|null
      */
     public function calculateCharge(PriceInterface $price, ActionInterface $action): ?ChargeInterface;
 
     /**
      * Calculates all possible Charges for the given $plan and $action
      *
-     * @param PlanInterface $plan
-     * @param ActionInterface $action
      * @return ChargeInterface[]
      */
     public function calculatePlan(PlanInterface $plan, ActionInterface $action): array;
diff --git a/src/order/Collector.php b/src/order/Collector.php
index 4bc62c39..1e54091d 100644
--- a/src/order/Collector.php
+++ b/src/order/Collector.php
@@ -22,7 +22,7 @@
  */
 class Collector implements CollectorInterface
 {
-    public function collect($source, DateTimeImmutable $time = null): OrderInterface
+    public function collect($source, ?DateTimeImmutable $time = null): OrderInterface
     {
         if ($source instanceof OrderInterface) {
             return $source;
diff --git a/src/order/CollectorInterface.php b/src/order/CollectorInterface.php
index 24775a09..d602e58c 100644
--- a/src/order/CollectorInterface.php
+++ b/src/order/CollectorInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\order;
 
 use DateTimeImmutable;
@@ -31,8 +33,6 @@ interface CollectorInterface
 {
     /**
      * @param OrderInterface|ActionInterface|mixed $source
-     * @param DateTimeImmutable|null $time
-     * @return OrderInterface
      */
-    public function collect($source, DateTimeImmutable $time = null): OrderInterface;
+    public function collect($source, ?DateTimeImmutable $time = null): OrderInterface;
 }
diff --git a/src/order/Order.php b/src/order/Order.php
index c8b15132..70311eb5 100755
--- a/src/order/Order.php
+++ b/src/order/Order.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\order;
 
 use hiqdev\php\billing\action\ActionInterface;
@@ -18,11 +20,6 @@
  */
 class Order implements OrderInterface
 {
-    /**
-     * @var int|string|null
-     */
-    protected $id;
-
     /**
      * @var CustomerInterface
      */
@@ -33,9 +30,11 @@ class Order implements OrderInterface
      */
     protected $actions = [];
 
-    public function __construct($id, CustomerInterface $customer, array $actions = [])
+    /**
+     * @param int|string|null $id
+     */
+    public function __construct(protected $id, CustomerInterface $customer, array $actions = [])
     {
-        $this->id = $id;
         $this->customer = $customer;
         $this->actions = $actions;
     }
diff --git a/src/order/OrderInterface.php b/src/order/OrderInterface.php
index a1f6c3c3..854e79b9 100644
--- a/src/order/OrderInterface.php
+++ b/src/order/OrderInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\order;
 
 use hiqdev\php\billing\action\ActionInterface;
diff --git a/src/plan/Plan.php b/src/plan/Plan.php
index 6f79dabf..8b94c4ae 100755
--- a/src/plan/Plan.php
+++ b/src/plan/Plan.php
@@ -22,28 +22,11 @@
  */
 class Plan implements PlanInterface
 {
-    /**
-     * @var int
-     */
-    protected $id;
-
-    /**
-     * @var string
-     */
-    protected $name;
-
-    protected ?int $parent_id = null;
-
     /**
      * @var CustomerInterface
      */
     protected $seller;
 
-    /**
-     * @var PriceInterface[]
-     */
-    protected $prices = [];
-
     /**
      * @var ?TypeInterface
      */
@@ -55,19 +38,15 @@ class Plan implements PlanInterface
      * @param PriceInterface[] $prices
      */
     public function __construct(
-        $id,
-        $name,
-        CustomerInterface $seller = null,
-        $prices = [],
-        TypeInterface $type = null,
-        $parent_id = null
+        protected $id,
+        protected $name,
+        ?CustomerInterface $seller = null,
+        protected $prices = [],
+        ?TypeInterface $type = null,
+        protected ?int $parent_id = null
     ) {
-        $this->id = $id;
-        $this->name = $name;
         $this->seller = $seller;
-        $this->prices = $prices;
         $this->type = $type;
-        $this->parent_id = $parent_id;
     }
 
     public function getUniqueId()
@@ -83,9 +62,6 @@ public function getId()
         return $this->id;
     }
 
-    /**
-     * @return string
-     */
     public function getName(): string
     {
         return $this->name;
diff --git a/src/plan/PlanCreationDto.php b/src/plan/PlanCreationDto.php
index 92633ed6..95083c7b 100644
--- a/src/plan/PlanCreationDto.php
+++ b/src/plan/PlanCreationDto.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\plan;
 
 /**
@@ -29,5 +31,5 @@ class PlanCreationDto
 
     public $type;
     
-    public ?int $parent_id;
+    public ?int $parent_id = null;
 }
diff --git a/src/plan/PlanFactory.php b/src/plan/PlanFactory.php
index 9a99055a..40ba18f7 100644
--- a/src/plan/PlanFactory.php
+++ b/src/plan/PlanFactory.php
@@ -26,9 +26,9 @@ public function create(PlanCreationDto $dto)
         return $this->createAnyPlan($dto);
     }
 
-    protected function createAnyPlan(PlanCreationDto $dto, string $class = null)
+    protected function createAnyPlan(PlanCreationDto $dto, ?string $class = null)
     {
-        $class = $class ?? Plan::class;
+        $class ??= Plan::class;
 
         return new $class(
             $dto->id,
diff --git a/src/plan/PlanFactoryInterface.php b/src/plan/PlanFactoryInterface.php
index 32e26ac1..67e187b4 100644
--- a/src/plan/PlanFactoryInterface.php
+++ b/src/plan/PlanFactoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\plan;
 
 /**
diff --git a/src/plan/PlanInterface.php b/src/plan/PlanInterface.php
index d3c01ecb..e5475b9f 100644
--- a/src/plan/PlanInterface.php
+++ b/src/plan/PlanInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\plan;
 
 use hiqdev\php\billing\customer\CustomerInterface;
diff --git a/src/plan/PlanRepositoryInterface.php b/src/plan/PlanRepositoryInterface.php
index c3e817ca..17fe8c4a 100644
--- a/src/plan/PlanRepositoryInterface.php
+++ b/src/plan/PlanRepositoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\plan;
 
 use hiqdev\DataMapper\Repository\RepositoryInterface;
diff --git a/src/price/AbstractPrice.php b/src/price/AbstractPrice.php
index 938a77d1..bc4a30e9 100755
--- a/src/price/AbstractPrice.php
+++ b/src/price/AbstractPrice.php
@@ -32,11 +32,6 @@ abstract class AbstractPrice implements PriceInterface, ChargeModifier
 {
     use SettableChargeModifierTrait;
 
-    /**
-     * @var integer
-     */
-    protected $id;
-
     /**
      * @var TypeInterface
      */
@@ -52,13 +47,15 @@ abstract class AbstractPrice implements PriceInterface, ChargeModifier
      */
     protected $plan;
 
+    /**
+     * @param int $id
+     */
     public function __construct(
-        $id,
+        protected $id,
         TypeInterface $type,
         TargetInterface $target,
-        PlanInterface $plan = null
+        ?PlanInterface $plan = null
     ) {
-        $this->id = $id;
         $this->type = $type;
         $this->target = $target;
         $this->plan = $plan;
@@ -122,12 +119,12 @@ public function setPlan(PlanInterface $plan)
     public function calculateSum(QuantityInterface $quantity): ?Money
     {
         $usage = $this->calculateUsage($quantity);
-        if ($usage === null) {
+        if (!$usage instanceof QuantityInterface) {
             return null;
         }
 
         $price = $this->calculatePrice($quantity);
-        if ($price === null) {
+        if (!$price instanceof Money) {
             return null;
         }
 
@@ -151,8 +148,6 @@ public function calculateSum(QuantityInterface $quantity): ?Money
      * Where we are using the result of this method?
      * Magic calls can't be determined and I don't know what can be broken if we change the method result.
      * Which structure must have the result, because array can contain anything?
-     *
-     * @return array
      */
     public function jsonSerialize(): array
     {
diff --git a/src/price/EnumPrice.php b/src/price/EnumPrice.php
index a2eabb45..71abc4e8 100644
--- a/src/price/EnumPrice.php
+++ b/src/price/EnumPrice.php
@@ -28,25 +28,16 @@
  */
 class EnumPrice extends AbstractPrice implements PriceWithSumsInterface, PriceWithCurrencyInterface, PriceWithUnitInterface
 {
-    protected UnitInterface $unit;
-
-    protected Currency $currency;
-
-    protected Sums $sums;
-
     public function __construct(
         $id,
         TypeInterface $type,
         TargetInterface $target,
         ?PlanInterface $plan,
-        UnitInterface $unit,
-        Currency $currency,
-        Sums $sums,
+        protected UnitInterface $unit,
+        protected Currency $currency,
+        protected Sums $sums,
     ) {
         parent::__construct($id, $type, $target, $plan);
-        $this->unit = $unit;
-        $this->currency = $currency;
-        $this->sums = $sums;
     }
 
     public function getUnit(): UnitInterface
@@ -67,6 +58,7 @@ public function getSums(): Sums
     /**
      * {@inheritdoc}
      */
+    #[\Override]
     public function calculateSum(QuantityInterface $quantity): ?Money
     {
         $usage = $this->calculateUsage($quantity)->getQuantity();
@@ -86,12 +78,12 @@ public function calculateSum(QuantityInterface $quantity): ?Money
     public function calculatePrice(QuantityInterface $quantity): ?Money
     {
         $sum = $this->calculateSum($quantity);
-        if ($sum === null) {
+        if (!$sum instanceof Money) {
             return null;
         }
 
         $usage = $this->calculateUsage($quantity);
-        if ($usage === null) {
+        if (!$usage instanceof QuantityInterface) {
             return null;
         }
 
diff --git a/src/price/FailedCalculatePriceException.php b/src/price/FailedCalculatePriceException.php
index 910a41be..a14308a5 100644
--- a/src/price/FailedCalculatePriceException.php
+++ b/src/price/FailedCalculatePriceException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\price;
 
 use hiqdev\php\billing\ExceptionInterface;
diff --git a/src/price/FailedCreatePriceException.php b/src/price/FailedCreatePriceException.php
index 0e12a41f..df3b4959 100644
--- a/src/price/FailedCreatePriceException.php
+++ b/src/price/FailedCreatePriceException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\price;
 
 use hiqdev\php\billing\ExceptionInterface;
diff --git a/src/price/PriceCreationDto.php b/src/price/PriceCreationDto.php
index 69c3ce78..83f27d43 100644
--- a/src/price/PriceCreationDto.php
+++ b/src/price/PriceCreationDto.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\price;
 
 use hiqdev\php\billing\plan\PlanInterface;
diff --git a/src/price/PriceFactory.php b/src/price/PriceFactory.php
index 02e794d8..37572f6e 100644
--- a/src/price/PriceFactory.php
+++ b/src/price/PriceFactory.php
@@ -31,14 +31,16 @@ class PriceFactory implements PriceFactoryInterface
     ];
 
     /**
-     * @var string default price class, when given will be used for not found types
+     * @param string $defaultClass
      */
-    protected $defaultClass = null;
-
-    public function __construct(array $types = [], $defaultClass = null)
-    {
+    public function __construct(
+        array $types = [],
+        /**
+         * @var string default price class, when given will be used for not found types
+         */
+        protected $defaultClass = null
+    ) {
         $this->types = $types;
-        $this->defaultClass = $defaultClass;
     }
 
 
@@ -48,7 +50,7 @@ public function __construct(array $types = [], $defaultClass = null)
     public function create(PriceCreationDto $dto): PriceInterface
     {
         $class = $this->findClassForTypes([
-            get_class($dto),
+            $dto::class,
             $dto->type->getName(),
         ]);
         $method = $this->findMethodForClass($class);
diff --git a/src/price/PriceFactoryInterface.php b/src/price/PriceFactoryInterface.php
index 2be827e7..88fc28bf 100644
--- a/src/price/PriceFactoryInterface.php
+++ b/src/price/PriceFactoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\price;
 
 /**
diff --git a/src/price/PriceInterface.php b/src/price/PriceInterface.php
index 47c248f2..187965ab 100644
--- a/src/price/PriceInterface.php
+++ b/src/price/PriceInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\price;
 
 use hiqdev\php\billing\action\ActionInterface;
diff --git a/src/price/ProgressivePrice.php b/src/price/ProgressivePrice.php
index 99de67a7..492d25cf 100644
--- a/src/price/ProgressivePrice.php
+++ b/src/price/ProgressivePrice.php
@@ -21,20 +21,17 @@ class ProgressivePrice extends AbstractPrice implements PriceWithThresholdsInter
     use HasMoney;
     use HasQuantity;
 
-    protected ProgressivePriceThresholdList $thresholds;
-
     public function __construct(
         $id,
         TypeInterface $type,
         TargetInterface $target,
         QuantityInterface $prepaid,
         Money $price,
-        ProgressivePriceThresholdList $thresholds,
+        protected ProgressivePriceThresholdList $thresholds,
         ?PlanInterface $plan = null
     )
     {
         parent::__construct($id, $type, $target, $plan);
-        $this->thresholds = $thresholds;
         $this->price = $price;
         $this->prepaid = $prepaid;
     }
@@ -81,6 +78,7 @@ public function getCalculationTraces(): array
         return $this->calculationTraces;
     }
 
+    #[\Override]
     public function calculateSum(QuantityInterface $quantity): ?Money
     {
         $this->calculationTraces = [];
@@ -105,8 +103,8 @@ public function calculateSum(QuantityInterface $quantity): ?Money
             $price = $threshold->price();
 
             $chargedAmount = $price->money()
-                                   ->multiply((string)(sprintf('%.14F', $billedUsage->getQuantity())))
-                                   ->divide((string)(sprintf('%.14F', $price->multiplier())));
+                                   ->multiply(sprintf('%.14F', $billedUsage->getQuantity()))
+                                   ->divide(sprintf('%.14F', $price->multiplier()));
 
             $this->calculationTraces[] = new ProgressivePriceCalculationTrace(
                 $threshold, $billedUsage, $chargedAmount
diff --git a/src/price/ProgressivePriceThreshold.php b/src/price/ProgressivePriceThreshold.php
index 120e4e99..f4697d5c 100644
--- a/src/price/ProgressivePriceThreshold.php
+++ b/src/price/ProgressivePriceThreshold.php
@@ -14,27 +14,24 @@
 
 class ProgressivePriceThreshold implements JsonSerializable
 {
-    /**
-     * @var numeric-string $price The price of the progressive price threshold in currency (not cents)
-     */
-    private string $price;
-
-    private string $currency;
-
-    private string $quantity;
-
-    private string $unit;
-
-    private function __construct(string $price, string $currency, string $quantity, string $unit)
-    {
+    private readonly string $currency;
+
+    private readonly string $quantity;
+
+    private function __construct(
+        /**
+         * @var numeric-string $price The price of the progressive price threshold in currency (not cents)
+         */
+        private readonly string $price,
+        string $currency,
+        string $quantity,
+        private readonly string $unit
+    ) {
         if ($quantity < 0) {
             throw new InvalidArgumentException('Quantity of the progressive price threshold must be positive');
         }
-
-        $this->price = $price;
         $this->currency = strtoupper($currency);
         $this->quantity = $quantity;
-        $this->unit = $unit;
     }
 
     public static function createFromScalar(string $price, string $currency, string $quantity, string $unit): self
diff --git a/src/price/ProgressivePriceThresholdList.php b/src/price/ProgressivePriceThresholdList.php
index fa3b3a09..b8244ee6 100644
--- a/src/price/ProgressivePriceThresholdList.php
+++ b/src/price/ProgressivePriceThresholdList.php
@@ -39,14 +39,12 @@ public function __construct(array $thresholds)
      */
     public static function fromScalarsArray(array $thresholds): self
     {
-        return new self(array_map(function ($threshold) {
-            return ProgressivePriceThreshold::createFromScalar(
-                $threshold['price'],
-                $threshold['currency'],
-                $threshold['quantity'],
-                $threshold['unit']
-            );
-        }, $thresholds));
+        return new self(array_map(fn($threshold) => ProgressivePriceThreshold::createFromScalar(
+            $threshold['price'],
+            $threshold['currency'],
+            $threshold['quantity'],
+            $threshold['unit']
+        ), $thresholds));
     }
 
     private function checkCanBeAdded(ProgressivePriceThreshold $threshold): void
diff --git a/src/price/RatePrice.php b/src/price/RatePrice.php
index 71fff5f2..360df752 100644
--- a/src/price/RatePrice.php
+++ b/src/price/RatePrice.php
@@ -26,17 +26,14 @@
  */
 class RatePrice extends AbstractPrice implements PriceWithRateInterface
 {
-    protected float $rate;
-
     public function __construct(
         $id,
         TypeInterface $type,
         TargetInterface $target,
         ?PlanInterface $plan,
-        float $rate
+        protected float $rate
     ) {
         parent::__construct($id, $type, $target, $plan);
-        $this->rate = $rate;
     }
 
     public function getRate(): float
@@ -44,6 +41,7 @@ public function getRate(): float
         return $this->rate;
     }
 
+    #[\Override]
     public function calculateSum(QuantityInterface $quantity): ?Money
     {
         $sum = $quantity->multiply((string) -$this->rate);
@@ -55,12 +53,12 @@ public function calculateSum(QuantityInterface $quantity): ?Money
     public function calculatePrice(QuantityInterface $quantity): ?Money
     {
         $sum = $this->calculateSum($quantity);
-        if ($sum === null) {
+        if (!$sum instanceof Money) {
             return null;
         }
 
         $usage = $this->calculateUsage($quantity);
-        if ($usage === null) {
+        if (!$usage instanceof QuantityInterface) {
             return null;
         }
 
diff --git a/src/price/SinglePrice.php b/src/price/SinglePrice.php
index 99c1ba33..0b667aac 100644
--- a/src/price/SinglePrice.php
+++ b/src/price/SinglePrice.php
@@ -36,9 +36,9 @@ public function __construct(
         $id,
         TypeInterface $type,
         TargetInterface $target,
-        PlanInterface $plan = null,
-        QuantityInterface $prepaid,
-        Money $price
+        ?PlanInterface $plan = null,
+        ?QuantityInterface $prepaid = null,
+        ?Money $price = null
     ) {
         parent::__construct($id, $type, $target, $plan);
         $this->prepaid  = $prepaid;
diff --git a/src/price/SumsHydrator.php b/src/price/SumsHydrator.php
index 6ce98fe9..cad2e5d5 100644
--- a/src/price/SumsHydrator.php
+++ b/src/price/SumsHydrator.php
@@ -6,6 +6,7 @@
 
 class SumsHydrator extends GeneratedHydrator
 {
+    #[\Override]
     public function hydrate(array $data, $object): object
     {
         if (!isset($data['sums'])) {
diff --git a/src/product/Application/BillingRegistryService.php b/src/product/Application/BillingRegistryService.php
index 8f9e53af..33be4a15 100644
--- a/src/product/Application/BillingRegistryService.php
+++ b/src/product/Application/BillingRegistryService.php
@@ -17,9 +17,9 @@
 use hiqdev\php\billing\type\Type;
 use hiqdev\php\billing\type\TypeInterface;
 
-final class BillingRegistryService implements BillingRegistryServiceInterface
+final readonly class BillingRegistryService implements BillingRegistryServiceInterface
 {
-    public function __construct(private readonly BillingRegistryInterface $registry)
+    public function __construct(private BillingRegistryInterface $registry)
     {
     }
 
@@ -88,7 +88,7 @@ public function getBehavior(string $type, string $behaviorClassWrapper): Behavio
             if ($priceTypeDefinition->hasType($billingType)) {
                 $behavior = $this->findBehaviorInPriceType($priceTypeDefinition, $behaviorClassWrapper);
 
-                if ($behavior) {
+                if ($behavior instanceof BehaviorInterface) {
                     return $behavior;
                 }
             }
diff --git a/src/product/Application/BillingRegistryServiceInterface.php b/src/product/Application/BillingRegistryServiceInterface.php
index 7d38ddb3..3707cf74 100644
--- a/src/product/Application/BillingRegistryServiceInterface.php
+++ b/src/product/Application/BillingRegistryServiceInterface.php
@@ -16,36 +16,25 @@
 interface BillingRegistryServiceInterface
 {
     /**
-     * @param string $representationClass
      * @return RepresentationInterface[]
      */
     public function getRepresentationsByType(string $representationClass): array;
 
-    /**
-     * @deprecated - please use getPriceTypeDefinitionByPriceTypeName() method instead
-     * @param string $type
-     * @return AggregateInterface
-     */
+    #[\Deprecated(message: '- please use getPriceTypeDefinitionByPriceTypeName() method instead')]
     public function getAggregate(string $type): AggregateInterface;
 
     /**
-     * @param string $typeName
-     * @return PriceTypeDefinitionInterface
      * @throws PriceTypeDefinitionNotFoundException
      */
     public function getPriceTypeDefinitionByPriceTypeName(string $typeName): PriceTypeDefinitionInterface;
 
     /**
-     * @param string $tariffName
-     * @return TariffTypeDefinitionInterface
      * @throws TariffTypeDefinitionNotFoundException
      */
     public function getTariffTypeDefinitionByTariffName(string $tariffName): TariffTypeDefinitionInterface;
 
     /**
      * @param string $type - full type like 'overuse,lb_capacity_unit'
-     * @param string $behaviorClassWrapper
-     * @return BehaviorInterface
      * @throws BehaviorNotFoundException
      * @throws InvalidBehaviorException
      */
@@ -55,7 +44,6 @@ public function getBehavior(string $type, string $behaviorClassWrapper): Behavio
     /**
      * Find all behaviors attached to any TariffType or PriceType by specified Behavior class.
      *
-     * @param string $behaviorClassWrapper
      * @return Generator<BehaviorInterface>
      */
     public function getBehaviors(string $behaviorClassWrapper): Generator;
@@ -63,7 +51,6 @@ public function getBehaviors(string $behaviorClassWrapper): Generator;
     /**
      * Find all PriceTypeDefinition in registry by specified Behavior class.
      *
-     * @param string $behaviorClassWrapper
      * @return Generator<PriceTypeDefinitionInterface>
      */
     public function findPriceTypeDefinitionsByBehavior(string $behaviorClassWrapper): Generator;
diff --git a/src/product/Domain/Model/Price/Exception/InvalidPriceTypeCollectionException.php b/src/product/Domain/Model/Price/Exception/InvalidPriceTypeCollectionException.php
index c3bc435e..17e7e6ad 100644
--- a/src/product/Domain/Model/Price/Exception/InvalidPriceTypeCollectionException.php
+++ b/src/product/Domain/Model/Price/Exception/InvalidPriceTypeCollectionException.php
@@ -10,7 +10,7 @@ class InvalidPriceTypeCollectionException extends InvalidArgumentException
 {
     public static function becauseContainsNonPriceType(mixed $value): self
     {
-        $given = is_object($value) ? get_class($value) : gettype($value);
+        $given = get_debug_type($value);
 
         return new self(sprintf(
             'PriceTypeCollection can only contain instances of PriceTypeInterface. Got: %s',
diff --git a/src/product/Domain/Model/Price/PriceTypeCollection.php b/src/product/Domain/Model/Price/PriceTypeCollection.php
index 9d374958..5b2b0904 100644
--- a/src/product/Domain/Model/Price/PriceTypeCollection.php
+++ b/src/product/Domain/Model/Price/PriceTypeCollection.php
@@ -17,7 +17,7 @@ class PriceTypeCollection implements IteratorAggregate, Countable
     /**
      * @var string[] - flipped type names for fast search
      */
-    private array $flippedTypeNames;
+    private readonly array $flippedTypeNames;
 
     public function __construct(private readonly array $types = [])
     {
diff --git a/src/product/InvoiceDescriptionsBuilder.php b/src/product/InvoiceDescriptionsBuilder.php
index 8607ba0a..3e24f51a 100644
--- a/src/product/InvoiceDescriptionsBuilder.php
+++ b/src/product/InvoiceDescriptionsBuilder.php
@@ -4,11 +4,9 @@
 
 class InvoiceDescriptionsBuilder
 {
-    private BillingRegistry $registry;
-
-    public function __construct(BillingRegistry $registry)
-    {
-        $this->registry = $registry;
+    public function __construct(
+        private readonly BillingRegistry $registry
+    ) {
     }
 
     public function build(): array
diff --git a/src/product/TariffTypeDefinition.php b/src/product/TariffTypeDefinition.php
index 730517eb..50a197f7 100644
--- a/src/product/TariffTypeDefinition.php
+++ b/src/product/TariffTypeDefinition.php
@@ -60,7 +60,7 @@ public function getProduct(): ProductInterface
 
     private function ensureProductExists(): void
     {
-        if ($this->product === null) {
+        if (!$this->product instanceof ProductInterface) {
             throw new ProductNotDefinedException('Product is not set. Call the ofProduct() method first.');
         }
     }
diff --git a/src/product/TariffTypeDefinitionInterface.php b/src/product/TariffTypeDefinitionInterface.php
index f6e7a847..5c31a490 100644
--- a/src/product/TariffTypeDefinitionInterface.php
+++ b/src/product/TariffTypeDefinitionInterface.php
@@ -19,9 +19,6 @@ public function tariffType(): TariffTypeInterface;
 
     /**
      * Check if TariffTypeDefinition belongs to specified TariffType
-     *
-     * @param TariffTypeInterface $tariffType
-     * @return bool
      */
     public function belongToTariffType(TariffTypeInterface $tariffType): bool;
 
diff --git a/src/product/behavior/BehaviorInterface.php b/src/product/behavior/BehaviorInterface.php
index cc18ac94..1202489a 100644
--- a/src/product/behavior/BehaviorInterface.php
+++ b/src/product/behavior/BehaviorInterface.php
@@ -18,7 +18,6 @@ public function getTariffType(): TariffTypeInterface;
      * The description can be either static, or use the object values.
      *
      * It can be later used for UI or documentation purposes.
-     * @return string
      */
     public function description(): string;
 }
diff --git a/src/product/behavior/BehaviorPriceTypeDefinitionCollection.php b/src/product/behavior/BehaviorPriceTypeDefinitionCollection.php
index 780b2a6c..b9b64f57 100644
--- a/src/product/behavior/BehaviorPriceTypeDefinitionCollection.php
+++ b/src/product/behavior/BehaviorPriceTypeDefinitionCollection.php
@@ -13,20 +13,16 @@
  */
 class BehaviorPriceTypeDefinitionCollection extends BehaviorCollection
 {
-    /**
-     * @psalm-var TPriceDefinition
-     */
-    private readonly PriceTypeDefinitionInterface $parent;
-
     /**
      * @psalm-param TPriceDefinition $parent
      */
     public function __construct(
-        PriceTypeDefinitionInterface $parent,
+        /**
+         * @psalm-var TPriceDefinition
+         */
+        private readonly PriceTypeDefinitionInterface $parent,
         TariffTypeInterface $tariffType
     ) {
-        $this->parent = $parent;
-
         parent::__construct($tariffType);
     }
 
diff --git a/src/product/behavior/PriceTypeBehaviorRegistry.php b/src/product/behavior/PriceTypeBehaviorRegistry.php
index cc09de2b..f446866f 100644
--- a/src/product/behavior/PriceTypeBehaviorRegistry.php
+++ b/src/product/behavior/PriceTypeBehaviorRegistry.php
@@ -15,7 +15,7 @@ final class PriceTypeBehaviorRegistry extends BehaviorRegistry
     /**
      * @var BehaviorPriceTypeDefinitionCollection<T>
      */
-    private BehaviorPriceTypeDefinitionCollection $behaviorCollection;
+    private readonly BehaviorPriceTypeDefinitionCollection $behaviorCollection;
 
     /**
      * @psalm-param T $priceTypeDefinition
diff --git a/src/product/behavior/TariffTypeBehaviorRegistry.php b/src/product/behavior/TariffTypeBehaviorRegistry.php
index 1644bb16..8629bec3 100644
--- a/src/product/behavior/TariffTypeBehaviorRegistry.php
+++ b/src/product/behavior/TariffTypeBehaviorRegistry.php
@@ -31,7 +31,7 @@ final class TariffTypeBehaviorRegistry extends BehaviorRegistry
     /**
      * @var BehaviorTariffTypeCollection<T>
      */
-    private BehaviorTariffTypeCollection $behaviorCollection;
+    private readonly BehaviorTariffTypeCollection $behaviorCollection;
 
     /**
      * @psalm-param T $tariffTypeDefinition
diff --git a/src/product/invoice/RepresentationCollection.php b/src/product/invoice/RepresentationCollection.php
index ba90e2eb..dcf327bd 100644
--- a/src/product/invoice/RepresentationCollection.php
+++ b/src/product/invoice/RepresentationCollection.php
@@ -21,18 +21,15 @@ class RepresentationCollection implements IteratorAggregate, HasLockInterface
 
     private RepresentationUniquenessGuard $uniquenessGuard;
 
-    /**
-     * @psalm-var T
-     */
-    private readonly PriceTypeDefinition $priceTypeDefinition;
-
     /**
      * @psalm-param T $priceTypeDefinition
      */
     public function __construct(
-        PriceTypeDefinition $priceTypeDefinition,
+        /**
+         * @psalm-var T
+         */
+        private readonly PriceTypeDefinition $priceTypeDefinition,
     ) {
-        $this->priceTypeDefinition = $priceTypeDefinition;
         $this->uniquenessGuard = new RepresentationUniquenessGuard();
     }
 
diff --git a/src/product/price/PriceTypeDefinition.php b/src/product/price/PriceTypeDefinition.php
index 1e3b4aa4..5df4e35e 100644
--- a/src/product/price/PriceTypeDefinition.php
+++ b/src/product/price/PriceTypeDefinition.php
@@ -48,20 +48,17 @@ class PriceTypeDefinition implements PriceTypeDefinitionInterface
 
     private ?AggregateInterface $aggregate = null;
 
-    /** @psalm-var TParentCollection */
-    private readonly PriceTypeDefinitionCollectionInterface $parent;
-
     private readonly PriceTypeBehaviorRegistry $behaviorRegistry;
 
     /**
      * @param TParentCollection $parent
      */
     public function __construct(
-        PriceTypeDefinitionCollectionInterface $parent,
+        /** @psalm-var TParentCollection */
+        private readonly PriceTypeDefinitionCollectionInterface $parent,
         private readonly TypeInterface $type,
         TariffTypeInterface $tariffType,
     ) {
-        $this->parent = $parent;
         $this->representationCollection = new RepresentationCollection($this);
         $this->behaviorRegistry = new PriceTypeBehaviorRegistry($this, $tariffType);
 
@@ -204,12 +201,11 @@ public function aggregation(AggregateInterface $aggregate): static
     }
 
     /**
-     * @return AggregateInterface
      * @throws AggregateNotDefinedException
      */
     public function getAggregate(): AggregateInterface
     {
-        if ($this->aggregate === null) {
+        if (!$this->aggregate instanceof AggregateInterface) {
             throw new AggregateNotDefinedException('Aggregate is not set. Call the aggregation() method first.');
         }
 
diff --git a/src/product/price/PriceTypeDefinitionCollection.php b/src/product/price/PriceTypeDefinitionCollection.php
index b0a45693..dd5d72f7 100644
--- a/src/product/price/PriceTypeDefinitionCollection.php
+++ b/src/product/price/PriceTypeDefinitionCollection.php
@@ -31,7 +31,7 @@ public function __construct(
          */
         private readonly TariffTypeDefinitionInterface $parent,
         private readonly PriceTypeDefinitionFactoryInterface $factory,
-        PriceTypeDefinitionCollectionInterface $collectionInstance = null,
+        ?PriceTypeDefinitionCollectionInterface $collectionInstance = null,
     ) {
         $this->storage = new PriceTypeStorage();
         $this->collectionInstance = $collectionInstance ?? $this;
diff --git a/src/product/price/PriceTypeDefinitionCollectionInterface.php b/src/product/price/PriceTypeDefinitionCollectionInterface.php
index d800a15e..033043d7 100644
--- a/src/product/price/PriceTypeDefinitionCollectionInterface.php
+++ b/src/product/price/PriceTypeDefinitionCollectionInterface.php
@@ -27,7 +27,6 @@ public function getIterator(): Traversable;
     public function priceType(TypeInterface $type): PriceTypeDefinitionInterface;
 
     /**
-     * @return TariffTypeDefinitionInterface
      * @plsam-return M
      */
     public function end(): TariffTypeDefinitionInterface;
@@ -36,7 +35,6 @@ public function end(): TariffTypeDefinitionInterface;
      * For easier understanding and establishing a relationship between PriceTypeDefinitionCollection
      * and TariffTypeDefinition
      *
-     * @return TariffTypeDefinitionInterface
      * @plsam-return M
      */
     public function getTariffTypeDefinition(): TariffTypeDefinitionInterface;
diff --git a/src/product/price/PriceTypeDefinitionInterface.php b/src/product/price/PriceTypeDefinitionInterface.php
index 2098f833..3c972f71 100644
--- a/src/product/price/PriceTypeDefinitionInterface.php
+++ b/src/product/price/PriceTypeDefinitionInterface.php
@@ -55,9 +55,6 @@ public function getUnit(): UnitInterface;
     /**
      * це параметер визначає агрегатну функцію яка застосовується для щоденно записаних ресурсів щоб визнизначти
      * місячне споживання за яке потрібно пробілити клієнта
-     *
-     * @param AggregateInterface $aggregate
-     * @return static
      */
     public function aggregation(AggregateInterface $aggregate): static;
 
@@ -65,8 +62,6 @@ public function getAggregate(): AggregateInterface;
 
     /**
      * For establishing a relationship between PriceTypeDefinition and TariffTypeDefinition
-     *
-     * @return TariffTypeDefinitionInterface
      */
     public function getTariffTypeDefinition(): TariffTypeDefinitionInterface;
 
diff --git a/src/product/quantity/FractionQuantityData.php b/src/product/quantity/FractionQuantityData.php
index 5f6d409e..e94561a5 100644
--- a/src/product/quantity/FractionQuantityData.php
+++ b/src/product/quantity/FractionQuantityData.php
@@ -4,11 +4,11 @@
 
 use hiqdev\php\units\Quantity;
 
-final class FractionQuantityData
+final readonly class FractionQuantityData
 {
     public function __construct(
-        public readonly Quantity $quantity,
-        public readonly string $time,
-        public readonly ?float $fractionOfMonth
+        public Quantity $quantity,
+        public string $time,
+        public ?float $fractionOfMonth
     ) {}
 }
diff --git a/src/product/quantity/QuantityFormatterDefinition.php b/src/product/quantity/QuantityFormatterDefinition.php
index 15da673f..a968c445 100644
--- a/src/product/quantity/QuantityFormatterDefinition.php
+++ b/src/product/quantity/QuantityFormatterDefinition.php
@@ -6,20 +6,14 @@
 
 class QuantityFormatterDefinition
 {
-    /** @var class-string<QuantityFormatterInterface> */
-    private string $formatterClass;
-
-    /** @var FractionUnitInterface|null|string */
-    private $fractionUnit;
-
     /**
      * @param class-string<QuantityFormatterInterface> $formatterClass
      * @param FractionUnitInterface|string|null $fractionUnit
      */
-    public function __construct(string $formatterClass, $fractionUnit = null)
-    {
-        $this->formatterClass = $formatterClass;
-        $this->fractionUnit = $fractionUnit;
+    public function __construct(
+        private readonly string $formatterClass,
+        private $fractionUnit = null
+    ) {
     }
 
     /**
diff --git a/src/product/quantity/QuantityFormatterInterface.php b/src/product/quantity/QuantityFormatterInterface.php
index 80966e83..90414e6e 100644
--- a/src/product/quantity/QuantityFormatterInterface.php
+++ b/src/product/quantity/QuantityFormatterInterface.php
@@ -7,22 +7,16 @@ interface QuantityFormatterInterface
     /**
      * Returns textual user friendly representation of the quantity.
      * E.g. 20 days, 30 GB, 1 year.
-     *
-     * @return string
      */
     public function format(): string;
 
     /**
      * Returns numeric to be saved in DB.
-     *
-     * @return string
      */
     public function getValue(): string;
 
     /**
      * Returns numeric user friendly representation of the quantity.
-     *
-     * @return string
      */
     public function getClientValue(): string;
 }
diff --git a/src/product/trait/HasLock.php b/src/product/trait/HasLock.php
index b27b0124..22a42ce1 100644
--- a/src/product/trait/HasLock.php
+++ b/src/product/trait/HasLock.php
@@ -35,7 +35,6 @@ protected function afterLock(): void
 
     /**
      * @param HasLockInterface[] $items
-     * @return void
      */
     protected function lockItems(array $items): void
     {
diff --git a/src/sale/AbstractSaleRepository.php b/src/sale/AbstractSaleRepository.php
index d5b7c42a..b0a593b1 100644
--- a/src/sale/AbstractSaleRepository.php
+++ b/src/sale/AbstractSaleRepository.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\sale;
 
 /**
diff --git a/src/sale/Sale.php b/src/sale/Sale.php
index b26eadcc..6297b551 100755
--- a/src/sale/Sale.php
+++ b/src/sale/Sale.php
@@ -25,11 +25,6 @@
  */
 class Sale implements SaleInterface
 {
-    /**
-     * @var int
-     */
-    protected $id;
-
     /**
      * @var TargetInterface
      */
@@ -52,22 +47,21 @@ class Sale implements SaleInterface
 
     protected ?DateTimeImmutable $closeTime = null;
 
-    protected ?array $data = null;
-
+    /**
+     * @param int $id
+     */
     public function __construct(
-        $id,
+        protected $id,
         TargetInterface $target,
         CustomerInterface $customer,
         ?PlanInterface $plan = null,
         ?DateTimeImmutable $time = null,
-        ?array $data = null,
+        protected ?array $data = null,
     ) {
-        $this->id = $id;
         $this->target = $target;
         $this->customer = $customer;
         $this->plan = $plan;
         $this->time = $time ?? new DateTimeImmutable();
-        $this->data = $data;
     }
 
     public function getId()
@@ -107,7 +101,7 @@ public function getCloseTime(): ?DateTimeImmutable
 
     public function close(DateTimeImmutable $closeTime): self
     {
-        if ($this->closeTime !== null) {
+        if ($this->closeTime instanceof \DateTimeImmutable) {
             throw new InvariantException('Sale is already closed');
         }
 
diff --git a/src/sale/SaleCreationDto.php b/src/sale/SaleCreationDto.php
index 144b6d50..c66e2519 100644
--- a/src/sale/SaleCreationDto.php
+++ b/src/sale/SaleCreationDto.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\sale;
 
 /**
diff --git a/src/sale/SaleFactoryInterface.php b/src/sale/SaleFactoryInterface.php
index 25c777ed..d1eec8ab 100644
--- a/src/sale/SaleFactoryInterface.php
+++ b/src/sale/SaleFactoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\sale;
 
 /**
diff --git a/src/sale/SaleInterface.php b/src/sale/SaleInterface.php
index 5bce2252..90cbd938 100644
--- a/src/sale/SaleInterface.php
+++ b/src/sale/SaleInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\sale;
 
 use DateTimeImmutable;
@@ -61,13 +63,9 @@ public function getTime();
 
     public function getCloseTime(): ?DateTimeImmutable;
 
-    /**
-     * @return array|null
-     */
     public function getData(): ?array;
 
     /**
-     * @param DateTimeImmutable $closeTime
      * @throws InvariantException
      * @throws ConstraintException
      */
diff --git a/src/sale/SaleRepositoryInterface.php b/src/sale/SaleRepositoryInterface.php
index 7cb03330..4e5dbda7 100644
--- a/src/sale/SaleRepositoryInterface.php
+++ b/src/sale/SaleRepositoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\sale;
 
 use hiqdev\php\billing\order\OrderInterface;
@@ -21,7 +23,6 @@
 interface SaleRepositoryInterface
 {
     /**
-     * @param \hiqdev\php\billing\sale\SaleInterface $sale
      * @return string
      */
     public function findId(SaleInterface $sale);
diff --git a/src/statement/Statement.php b/src/statement/Statement.php
index edf3a66e..eb842518 100755
--- a/src/statement/Statement.php
+++ b/src/statement/Statement.php
@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 /**
  * PHP Billing Library
@@ -25,48 +26,18 @@ class Statement implements \JsonSerializable
     const PERIOD_MONTH = 'month';
     const PERIOD_YEAR = 'year';
 
-    private CustomerInterface $customer;
-
-    private DateTimeImmutable $month;
-
-    private DateTimeImmutable $time;
-
-    private Money $balance;
-
-    private Money $total;
-
-    private Money $payment;
-
-    private Money $amount;
-
-    private array $bills = [];
-
-    private string $period = self::PERIOD_MONTH;
-
-    private array $plans = [];
-
     public function __construct(
-        CustomerInterface $customer,
-        DateTimeImmutable $time,
-        DateTimeImmutable $month,
-        Money $balance,
-        Money $total,
-        Money $payment,
-        Money $amount,
-        array $bills = [],
-        string $period = self::PERIOD_MONTH,
-        array $plans = []
+        private readonly CustomerInterface $customer,
+        private readonly DateTimeImmutable $time,
+        private readonly DateTimeImmutable $month,
+        private readonly Money $balance,
+        private readonly Money $total,
+        private readonly Money $payment,
+        private readonly Money $amount,
+        private array $bills = [],
+        private readonly string $period = self::PERIOD_MONTH,
+        private array $plans = []
     ) {
-        $this->customer = $customer;
-        $this->time = $time;
-        $this->balance = $balance;
-        $this->bills = $bills;
-        $this->period = $period;
-        $this->month = $month;
-        $this->total = $total;
-        $this->payment = $payment;
-        $this->amount = $amount;
-        $this->plans = $plans;
     }
 
     public function getCustomer(): CustomerInterface
diff --git a/src/statement/StatementBill.php b/src/statement/StatementBill.php
index 943f51aa..d93d4ab6 100644
--- a/src/statement/StatementBill.php
+++ b/src/statement/StatementBill.php
@@ -43,9 +43,6 @@ class StatementBill extends Bill implements StatementBillInterface
     /** @var string */
     protected $from;
 
-    /** @var int */
-    protected int $unique_objects_count = 0;
-
     /** @var TypeInterface */
     protected $tariff_type;
 
@@ -57,16 +54,16 @@ public function __construct(
         QuantityInterface $quantity,
         CustomerInterface $customer,
         DateTimeImmutable $month,
-        int $unique_objects_count,
-        Money $price = null,
-        Money $overuse = null,
-        QuantityInterface $prepaid = null,
+        protected int $unique_objects_count,
+        ?Money $price = null,
+        ?Money $overuse = null,
+        ?QuantityInterface $prepaid = null,
         array $charges = [],
-        TargetInterface $target = null,
-        PlanInterface $plan = null,
-        BillState $state = null,
+        ?TargetInterface $target = null,
+        ?PlanInterface $plan = null,
+        ?BillState $state = null,
         ?string $from = null,
-        TypeInterface $tariff_type = null
+        ?TypeInterface $tariff_type = null
     ) {
         parent::__construct(
             $id,
@@ -81,7 +78,6 @@ public function __construct(
             $state
         );
         $this->month                = $month;
-        $this->unique_objects_count = $unique_objects_count;
         $this->price                = $price;
         $this->overuse              = $overuse;
         $this->prepaid              = $prepaid;
diff --git a/src/statement/StatementBillInterface.php b/src/statement/StatementBillInterface.php
index 43db5979..2fbe06e7 100644
--- a/src/statement/StatementBillInterface.php
+++ b/src/statement/StatementBillInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2021, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\statement;
 
 use hiqdev\php\billing\bill\BillInterface;
diff --git a/src/target/AbstractTarget.php b/src/target/AbstractTarget.php
index 55fbad19..7cd157fb 100755
--- a/src/target/AbstractTarget.php
+++ b/src/target/AbstractTarget.php
@@ -19,34 +19,21 @@
  */
 abstract class AbstractTarget implements TargetInterface
 {
-    /**
-     * @var int|string
-     */
-    protected $id;
-
-    /**
-     * @var string
-     */
-    protected $type;
-
     /**
      * @var string
      */
     protected $state;
 
-    /**
-     * @var string
-     */
-    protected $name;
-
     /** @var string */
     protected $label;
 
-    public function __construct($id, $type, $name = null)
+    /**
+     * @param int|string $id
+     * @param string $type
+     * @param string $name
+     */
+    public function __construct(protected $id, protected $type, protected $name = null)
     {
-        $this->id = $id;
-        $this->type = $type;
-        $this->name = $name;
     }
 
     /**
@@ -67,7 +54,7 @@ public function hasId()
 
     public function setId($id)
     {
-        if ((string) $this->id === (string) $id) {
+        if ((string)$this->id === (string)$id) {
             return;
         }
         if ($this->hasId()) {
@@ -172,20 +159,22 @@ protected function matchStrings($lhs, $rhs)
             return false;
         }
 
-        return (string) $lhs === (string) $rhs;
+        return (string)$lhs === (string)$rhs;
     }
 
     public function jsonSerialize(): array
     {
-        return array_filter(array_merge(
-            get_object_vars($this),
-            [
-                'id'    => $this->getId(),
-                'type'  => $this->getType(),
-                'state'  => $this->getState(),
-                'name'  => $this->getName(),
-            ],
-        ));
+        return array_filter(
+            array_merge(
+                get_object_vars($this),
+                [
+                    'id' => $this->getId(),
+                    'type' => $this->getType(),
+                    'state' => $this->getState(),
+                    'name' => $this->getName(),
+                ],
+            )
+        );
     }
 
     protected static $anyTarget;
diff --git a/src/target/RenewableTarget.php b/src/target/RenewableTarget.php
index dad98fc9..3e1b908f 100644
--- a/src/target/RenewableTarget.php
+++ b/src/target/RenewableTarget.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\target;
 
 use DateTimeImmutable;
diff --git a/src/target/Target.php b/src/target/Target.php
index 10d21bee..4f115e42 100644
--- a/src/target/Target.php
+++ b/src/target/Target.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\target;
 
 /**
diff --git a/src/target/TargetCreationDto.php b/src/target/TargetCreationDto.php
index 8168a4f3..2bc79fd4 100644
--- a/src/target/TargetCreationDto.php
+++ b/src/target/TargetCreationDto.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\target;
 
 /**
diff --git a/src/target/TargetFactory.php b/src/target/TargetFactory.php
index 652ab7b8..b352e202 100644
--- a/src/target/TargetFactory.php
+++ b/src/target/TargetFactory.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\target;
 
 /**
diff --git a/src/target/TargetFactoryInterface.php b/src/target/TargetFactoryInterface.php
index 5bb16169..ab544120 100644
--- a/src/target/TargetFactoryInterface.php
+++ b/src/target/TargetFactoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\target;
 
 /**
diff --git a/src/target/TargetInterface.php b/src/target/TargetInterface.php
index 07446a6f..318e7a5d 100644
--- a/src/target/TargetInterface.php
+++ b/src/target/TargetInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\target;
 
 use hiqdev\php\billing\EntityInterface;
@@ -51,7 +53,6 @@ public function getType();
 
     /**
      * Get target state.
-     * @return null|string
      */
     public function getState(): ?string;
 
diff --git a/src/target/TargetRepositoryInterface.php b/src/target/TargetRepositoryInterface.php
index 9ed8b11e..28874341 100644
--- a/src/target/TargetRepositoryInterface.php
+++ b/src/target/TargetRepositoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\target;
 
 /**
diff --git a/src/target/TargetWasCreated.php b/src/target/TargetWasCreated.php
index 647bbe35..48fe78f4 100644
--- a/src/target/TargetWasCreated.php
+++ b/src/target/TargetWasCreated.php
@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 /**
  * PHP Billing Library
@@ -20,11 +21,8 @@
  */
 class TargetWasCreated extends AbstractEvent
 {
-    private TargetInterface $target;
-
-    public function __construct(TargetInterface $target)
+    public function __construct(private readonly TargetInterface $target)
     {
-        $this->target = $target;
     }
 
     public function getTarget(): TargetInterface
diff --git a/src/tools/AggregationException.php b/src/tools/AggregationException.php
index 400f05b5..15bdf46c 100644
--- a/src/tools/AggregationException.php
+++ b/src/tools/AggregationException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\tools;
 
 use hiqdev\php\billing\ExceptionInterface;
diff --git a/src/tools/Aggregator.php b/src/tools/Aggregator.php
index befefa29..4ac6d57f 100644
--- a/src/tools/Aggregator.php
+++ b/src/tools/Aggregator.php
@@ -66,11 +66,9 @@ protected function aggregateBills(array $bills, array $others): array
     {
         foreach ($others as $bill) {
             $uid = $bill->getUniqueString();
-            if (empty($bills[$uid])) {
-                $bills[$uid] = $bill;
-            } else {
-                $bills[$uid] = $this->aggregateBill($bills[$uid], $bill);
-            }
+            $bills[$uid] = empty($bills[$uid])
+                ? $bill
+                : $this->aggregateBill($bills[$uid], $bill);
         }
 
         return $bills;
diff --git a/src/tools/AggregatorInterface.php b/src/tools/AggregatorInterface.php
index 83737a28..d738b5e7 100644
--- a/src/tools/AggregatorInterface.php
+++ b/src/tools/AggregatorInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\tools;
 
 use hiqdev\php\billing\bill\Bill;
diff --git a/src/tools/CachedDateTimeProvider.php b/src/tools/CachedDateTimeProvider.php
index 53f12854..3f2e430c 100644
--- a/src/tools/CachedDateTimeProvider.php
+++ b/src/tools/CachedDateTimeProvider.php
@@ -11,13 +11,10 @@
  *
  * @author Dmytro Naumenko <d.naumenko.a@gmail.com>
  */
-final class CachedDateTimeProvider implements CurrentDateTimeProviderInterface
+final readonly class CachedDateTimeProvider implements CurrentDateTimeProviderInterface
 {
-    private DateTimeImmutable $dateTimeImmutable;
-
-    public function __construct(DateTimeImmutable $dateTimeImmutable)
+    public function __construct(private DateTimeImmutable $dateTimeImmutable)
     {
-        $this->dateTimeImmutable = $dateTimeImmutable;
     }
 
     public function dateTimeImmutable(): DateTimeImmutable
diff --git a/src/tools/DbMergingAggregator.php b/src/tools/DbMergingAggregator.php
index cf70c280..d95431b5 100644
--- a/src/tools/DbMergingAggregator.php
+++ b/src/tools/DbMergingAggregator.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -20,18 +23,11 @@
  */
 class DbMergingAggregator implements AggregatorInterface, MergerInterface
 {
-    protected BillRepositoryInterface $billRepository;
-    protected MergerInterface $merger;
-    private AggregatorInterface $localAggregator;
-
     public function __construct(
-        AggregatorInterface $localAggregator,
-        BillRepositoryInterface $billRepository,
-        MergerInterface $merger
+        private readonly AggregatorInterface $localAggregator,
+        protected BillRepositoryInterface $billRepository,
+        protected MergerInterface $merger
     ) {
-        $this->billRepository = $billRepository;
-        $this->merger = $merger;
-        $this->localAggregator = $localAggregator;
     }
 
     public function mergeBills(array $bills): array
diff --git a/src/tools/Factory.php b/src/tools/Factory.php
index 95be85ad..23a4ff72 100644
--- a/src/tools/Factory.php
+++ b/src/tools/Factory.php
@@ -27,11 +27,8 @@ class Factory implements FactoryInterface
 {
     private $entities = [];
 
-    private $factories = [];
-
-    public function __construct(array $factories)
+    public function __construct(private array $factories)
     {
-        $this->factories = $factories;
     }
 
     public function getMoney($data)
@@ -51,7 +48,7 @@ public function getSums($data)
 
     public function parseMoney($str)
     {
-        [$amount, $currency] = explode(' ', $str);
+        [$amount, $currency] = explode(' ', (string) $str);
 
         return [
             'amount' => $amount*100,
@@ -61,7 +58,7 @@ public function parseMoney($str)
 
     public function createMoney($data)
     {
-        return new Money($data['amount'], new Currency(strtoupper($data['currency'])));
+        return new Money($data['amount'], new Currency(strtoupper((string) $data['currency'])));
     }
 
     public function getCurrency($data)
@@ -76,7 +73,7 @@ public function getQuantity($data)
 
     public function parseQuantity($str)
     {
-        [$quantity, $unit] = explode(' ', $str);
+        [$quantity, $unit] = explode(' ', (string) $str);
 
         return [
             'quantity' => $quantity,
@@ -218,12 +215,8 @@ public function parseByUnique(string $entity, $str, $delimiter = ':')
         if (count($keys) === 1) {
             return [reset($keys) => $str];
         }
-        $parts = explode($delimiter, $str, count($keys));
-        if (count($parts) === count($keys)) {
-            $res = array_combine($keys, $parts);
-        } else {
-            $res = [];
-        }
+        $parts = explode($delimiter, (string) $str, count($keys));
+        $res = count($parts) === count($keys) ? array_combine($keys, $parts) : [];
         $res['id'] = $str;
 
         return $res;
diff --git a/src/tools/FactoryNotFoundException.php b/src/tools/FactoryNotFoundException.php
index 095859fb..c26a98cd 100644
--- a/src/tools/FactoryNotFoundException.php
+++ b/src/tools/FactoryNotFoundException.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\tools;
 
 use hiqdev\php\billing\ExceptionInterface;
diff --git a/src/tools/Merger.php b/src/tools/Merger.php
index 693d3c9a..07be7ede 100644
--- a/src/tools/Merger.php
+++ b/src/tools/Merger.php
@@ -29,11 +29,7 @@ public function mergeBills(array $bills): array
         $res = [];
         foreach ($bills as $bill) {
             $uid = $bill->getUniqueString();
-            if (empty($res[$uid])) {
-                $res[$uid] = $bill;
-            } else {
-                $res[$uid] = $this->mergeBill($res[$uid], $bill);
-            }
+            $res[$uid] = empty($res[$uid]) ? $bill : $this->mergeBill($res[$uid], $bill);
         }
 
         return $res;
@@ -71,11 +67,7 @@ protected function mergeCharges(array $charges): array
         $res = [];
         foreach ($charges as $charge) {
             $uid = $charge->getUniqueString();
-            if (empty($res[$uid])) {
-                $res[$uid] = $charge;
-            } else {
-                $res[$uid] = $this->mergeCharge($res[$uid], $charge);
-            }
+            $res[$uid] = empty($res[$uid]) ? $charge : $this->mergeCharge($res[$uid], $charge);
         }
 
         return $res;
diff --git a/src/tools/MergerInterface.php b/src/tools/MergerInterface.php
index c2d97d8d..7c6f248a 100644
--- a/src/tools/MergerInterface.php
+++ b/src/tools/MergerInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\tools;
 
 use hiqdev\php\billing\bill\BillInterface;
diff --git a/src/type/Type.php b/src/type/Type.php
index c171bf50..8add09b5 100755
--- a/src/type/Type.php
+++ b/src/type/Type.php
@@ -18,24 +18,24 @@
 class Type implements TypeInterface
 {
     /**
-     * @var int|string|null|float The unique identifier of the type. Can be an integer or string.
-     *                            Special values:
-     *                            - `Type::ANY` indicates that the type can match any ID.
-     *                            - `Type::NONE` indicates that there is no valid ID.
+     * @param int|string|null|float $id
+     * @param string|null|float $name
      */
-    protected $id;
-
-    /**
-     * @var string|null|float The name of the type. Can be a specific name or one of the special values:
-     *                        - `Type::ANY` indicates that the type can match any name.
-     *                        - `Type::NONE` indicates that there is no valid name.
-     */
-    protected $name;
-
-    public function __construct($id, $name = self::ANY)
-    {
-        $this->id = $id;
-        $this->name = $name;
+    public function __construct(
+        /**
+         * @var int|string|null|float The unique identifier of the type. Can be an integer or string.
+         *                            Special values:
+         *                            - `Type::ANY` indicates that the type can match any ID.
+         *                            - `Type::NONE` indicates that there is no valid ID.
+         */
+        protected $id,
+        /**
+         * @var string|null|float The name of the type. Can be a specific name or one of the special values:
+         *                        - `Type::ANY` indicates that the type can match any name.
+         *                        - `Type::NONE` indicates that there is no valid name.
+         */
+        protected $name = self::ANY
+    ) {
     }
 
     /**
@@ -83,7 +83,7 @@ protected function checkMatches($lhs, $rhs)
             return false;
         }
 
-        return (string) $lhs === (string) $rhs;
+        return (string)$lhs === (string)$rhs;
     }
 
     public function jsonSerialize(): array
diff --git a/src/type/TypeCreationDto.php b/src/type/TypeCreationDto.php
index 86a38240..19a60250 100644
--- a/src/type/TypeCreationDto.php
+++ b/src/type/TypeCreationDto.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\type;
 
 /**
diff --git a/src/type/TypeFactory.php b/src/type/TypeFactory.php
index 5184356e..8d7627d7 100644
--- a/src/type/TypeFactory.php
+++ b/src/type/TypeFactory.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\type;
 
 /**
diff --git a/src/type/TypeFactoryInterface.php b/src/type/TypeFactoryInterface.php
index 55710965..3a621912 100644
--- a/src/type/TypeFactoryInterface.php
+++ b/src/type/TypeFactoryInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\type;
 
 /**
diff --git a/src/type/TypeInterface.php b/src/type/TypeInterface.php
index f12ee582..84eaed23 100644
--- a/src/type/TypeInterface.php
+++ b/src/type/TypeInterface.php
@@ -1,4 +1,7 @@
 <?php
+
+declare(strict_types=1);
+
 /**
  * PHP Billing Library
  *
@@ -7,7 +10,6 @@
  * @license   BSD-3-Clause
  * @copyright Copyright (c) 2017-2020, HiQDev (http://hiqdev.com/)
  */
-
 namespace hiqdev\php\billing\type;
 
 /**
diff --git a/src/usage/Usage.php b/src/usage/Usage.php
index fb6fa589..3e705960 100644
--- a/src/usage/Usage.php
+++ b/src/usage/Usage.php
@@ -1,4 +1,5 @@
 <?php
+
 declare(strict_types=1);
 
 namespace hiqdev\php\billing\usage;
@@ -16,17 +17,12 @@
  */
 class Usage implements UsageInterface
 {
-    private TargetInterface $target;
-    private DateTimeImmutable $time;
-    private TypeInterface $type;
-    private Quantity $amount;
-
-    public function __construct(TargetInterface $target, DateTimeImmutable $time, TypeInterface $type, Quantity $amount)
-    {
-        $this->target = $target;
-        $this->time = $time;
-        $this->type = $type;
-        $this->amount = $amount;
+    public function __construct(
+        private readonly TargetInterface $target,
+        private readonly DateTimeImmutable $time,
+        private readonly TypeInterface $type,
+        private readonly Quantity $amount
+    ) {
     }
 
     public function target(): TargetInterface