Add a style guide

CONTRIBUTING.md

@@ -32,7 +32,9 @@
 
 ## **Do you want to add an exercise?**
 
-- **Ensure that someone else isn't already adding it** by searching the [forum](https://forum.exercism.org/c/programming/moonscript) and the repository's [issues](https://github.com/exercism/moonscript/issues) and [pull requests](https://github.com/exercism/moonscript/pulls).
+- **Ensure that someone else isn't already adding it** 
+    - start with the [Practice exercises to implement](https://github.com/exercism/moonscript/issues/102) issue.
+    - also search the [forum](https://forum.exercism.org/c/programming/moonscript) and the repository's [issues](https://github.com/exercism/moonscript/issues) and [pull requests](https://github.com/exercism/moonscript/pulls).
 
 - If nobody is yet adding the exercise, [open a conversation](https://forum.exercism.org/c/programming/moonscript) and indicate you'd like to add the exercise.
 
@@ -89,6 +91,8 @@
 
 1. Create the example solution: `.meta/example.moon`
 
+   * follow the [style guide](./STYLE.md).
+
 1. Test it with
 
     ```sh

STYLE.md

@@ -0,0 +1,165 @@
+# MoonScript Style Guide
+
+There isn't "one true" style guide.
+But as a language that inherits both from Lua and CoffeeScript, let's take inspiration from both.
+
+Note that this guide is not comprehensive, it's just a starting point.
+
+## Lua Style
+
+Start with the [Lua Style Guide][lua-style] from the lua-users wiki.
+Read it and note that it takes inspiration from other languages.
+
+## CoffeeScript Style
+
+Like Lua, there is no one single style guide.
+I've been looking at [polarmobile/coffeescript-style-guide][coffeescript-style]
+
+## Style Guide
+
+### Significant Whitespace
+
+* use spaces not tabs
+* 2-space indentation
+* no trailing whitespace on any line.
+
+### Naming
+
+* variable and function names should use either `snake_case` or `camelCase` _but not both_.
+* variables intended to be constants can use `SCREAMING_SNAKE_CASE`.
+* class names should use `PascalCase`.
+* use underscore as a variable names for "throw-away" values, like:
+
+    ```moonscript
+    values = [v for _, v in pairs myDict]
+    ```
+
+### Return Value of Modules
+
+This is particularly key to Exercism, as all MoonScript exercises are implemented with modules.
+We don't have a convention in the implemented exercises.
+Pick a style that you think is particularly readable for the module you're creating.
+
+* a module that implements a class might look like this: it returns the class without using the `return` keyword.
+
+  ```moonscript
+  class MyClassName
+    -- .. class definition here
+
+  MyClassName
+  ```
+
+* a module that defines some functions can return the functions defined inside a table (note the colon after the function name):
+
+  ```moonscript
+  {
+    func1: (args) -> -- ...
+    func2: (args) -> -- ...
+    -- etc
+  }
+  ```
+
+* alternately, the functions can be defined locally in the module, and the return table uses the `:` prefix operator:
+
+  ```moonscript
+  func1 = (args) -> -- ...
+  func2 = (args) -> --...
+  -- etc
+
+  { :func1, :func2 }
+  ```
+
+### Loops
+
+* use the `*` operator, not `ipairs`
+
+  ```moonscript
+  # Yes
+  for elem in *array
+  # No
+  for _, elem in ipairs array
+  ```
+
+* use list comprehensions where possible, unless a multi-line for loop is much more readable.
+
+  ```moonscript
+  # Yes
+  result = [v * 2 for v in *my_list when v % 2 == 0]
+
+  # No
+  results = []
+  for v in *my_list
+    if v % 2 == 0
+      table.insert results, v * 2
+
+  # Yes
+  result = [fn i, j for i = 1, 100 for j = 1, 100]
+
+  # No
+  results = []
+  for i = 1, 100
+    for j = 1, 100
+      table.insert results, fn i, j
+  ```
+
+### `is_empty`
+
+Given an arbitrary table, `#tbl == 0` is insufficient to determine if the table is empty.
+The `#` length operator only "works" if the table is a _sequence_, with numeric indices starting at 1 with no gaps.
+
+```moonscript
+import p from require 'moon'
+
+t = {1, 2, 3, 4}
+p {len: #t, next: next(t)}       -- {len: 4, next: 1}
+
+t = {[2]: 2, [3]: 3, [4]: 4}
+p {len: #t, next: next(t)}       -- {len: 0, next: 3}
+                                 -- non-sequence tables are unordered
+
+t = {foo: 'bar', baz: 'qux'}
+p {len: #t, next: next(t)}       -- {len: 0, next: "baz"}
+```
+
+The best way to check for emptiness is with the [`next`][lua-next] builtin function:
+
+```moonscript
+is_empty = (t) -> not next t
+
+t = {}
+p is_empty t    -- true
+
+t = {1, 2, 3}
+p is_empty t    -- false
+
+t = {foo: 'bar', baz: 'qux'}
+p is_empty t    -- false
+
+```
+
+
+### Other MoonScript Syntactic Sugar
+
+* for no-argument function definitions, omit the arglist
+
+  ```moonscript
+  # Yes
+  f = -> ...
+  # No
+  f = () -> ...
+  ```
+
+* for calling no-argument functions, prefer `func!` over `func()`
+* for class methods, use `@` not `self`
+
+  ```moonscript
+  class MyClass
+    aMethod: =>
+      @instVar = 42
+      @helperMethod!
+  ```
+
+
+[lua-style]: http://lua-users.org/wiki/LuaStyleGuide
+[coffeescript-style]: https://github.com/polarmobile/coffeescript-style-guide
+[lua-next]: https://www.lua.org/manual/5.4/manual.html#pdf-next