Replace PlaceholderExpression with it

Author: rattrayalex

src/index.js

@@ -950,16 +950,22 @@ export default function (babel) {
     return test;
   }
 
+  function containsAliasReference(path, alias) {
+    let containsAlias = false;
+    path.traverse({
+      ReferencedIdentifier(refPath) {
+        if (refPath.node.name === alias.name) {
+          containsAlias = true;
+          refPath.stop();
+        }
+      }
+    });
+    return containsAlias;
+  }
+
   function transformMatchCases(argRef, cases) {
     return cases.reduce((rootIf, path) => {
 
-      // fill in placeholders
-      path.get("test").traverse({
-        PlaceholderExpression(placeholderPath) {
-          placeholderPath.replaceWith(argRef);
-        }
-      });
-
       // add in ===, etc
       transformMatchCaseTest(path.get("test"), argRef);
 
@@ -1243,9 +1249,6 @@ export default function (babel) {
     // only allowed in MatchCase, so don't alias to Expression
   });
 
-  definePluginType("PlaceholderExpression", {
-    aliases: ["Expression"]
-  });
 
   // traverse as top-level item so as to run before other babel plugins
   // (and avoid traversing any of their output)
@@ -1547,9 +1550,9 @@ export default function (babel) {
       },
 
       MatchExpression(path) {
-        const { discriminant } = path.node;
+        const { discriminant, alias = null } = path.node;
 
-        const argRef = path.scope.generateUidIdentifier("it");
+        const argRef = alias || t.identifier("it");
         const matchBody = transformMatchCases(argRef, path.get("cases"));
 
         const iife = t.callExpression(
@@ -1561,12 +1564,13 @@ export default function (babel) {
 
       MatchStatement(path) {
         const { discriminant } = path.node;
+        const alias = path.node.alias || t.identifier("it");
 
         let argRef;
-        if (t.isIdentifier(discriminant)) {
+        if (t.isIdentifier(discriminant) && !containsAliasReference(path, alias)) {
           argRef = discriminant;
         } else {
-          argRef = path.scope.generateUidIdentifier("it");
+          argRef = alias;
           path.insertBefore(t.variableDeclaration("const", [
             t.variableDeclarator(argRef, discriminant)
           ]));

test/fixtures/match/arr-pattern/actual.js

@@ -1,13 +1,13 @@
 match x:
-  | []:
+  | with []:
     "empty"
-  | [ a, b ]:
+  | with [ a, b ]:
     a - b
-  | [ a, b = 2 ]:
+  | with [ a, b = 2 ]:
     a + b - 2
-  | [ a, ...b ]:
+  | with [ a, ...b ]:
     b.concat(a)
-  | [
+  | with [
       [
         b
         d = 'e'

test/fixtures/match/arr-pattern/exec.js

@@ -1,73 +1,73 @@
 assert.equal(
   "empty",
   match []:
-    | []:
+    | with []:
       "empty"
 )
 assert.equal(
   "empty",
   match []:
-    | []:
+    | with []:
       "empty"
 )
 assert.equal(
   undefined,
   match []:
-    | [a]:
+    | with [a]:
       a + 1
 )
 assert.equal(
   undefined,
   match [1]:
-    | [a, b]:
+    | with [a, b]:
       a + b
 )
 assert.equal(
   5,
   match [1]:
-    | [a, b = 4]:
+    | with [a, b = 4]:
       a + b
 )
 assert.equal(
   3,
   match [1, 2]:
-    | [a, b]:
+    | with [a, b]:
       a + b
 )
 assert.equal(
   4,
   match [1, 2, 3]:
-    | [a,, b]:
+    | with [a,, b]:
       a + b
 )
 assert.equal(
   undefined,
   match [1, 2]:
-    | [a,, b]:
+    | with [a,, b]:
       a + b
 )
 assert.deepEqual(
   [2, 3, 1],
   match [1, 2, 3]:
-    | [a, ...b]:
+    | with [a, ...b]:
       b.concat(a)
 )
 assert.deepEqual(
   [1, 4],
   match [4]:
-    | [a, b = 1, ...c]:
+    | with [a, b = 1, ...c]:
       c.concat([b, a])
 )
 assert.deepEqual(
   [6, 7, 5, 4],
   match [4, 5, 6, 7]:
-    | [a, b = 1, ...c]:
+    | with [a, b = 1, ...c]:
       c.concat([b, a])
 )
 assert.deepEqual(
   [1, 2, 4, 6, 7, 8],
   match [[1], [4, 5, 6], 7, 8]:
-    | [
+    | with [
         [
           b
           d = 2
@@ -80,7 +80,7 @@ assert.deepEqual(
 assert.deepEqual(
   undefined,
   match [[1], [4, 5], 7, 8]:
-    | [
+    | with [
         [
           b
           d = 2

test/fixtures/match/as-call-expression/actual.js

@@ -0,0 +1,7 @@
+z = match foo.bar() as y:
+  | y < 1: "lt one"
+  | y + 1 == 1: "eq zero"
+  | y == 2: "eq two"
+  | y~f(): "f(x) truthy"
+  | y.prop: "has prop"
+  | y.0: "has first child"

test/fixtures/match/as-call-expression/expected.js

@@ -0,0 +1,15 @@
+const z = (y => {
+  if (y < 1) {
+      return "lt one";
+    } else if (y + 1 === 1) {
+      return "eq zero";
+    } else if (y === 2) {
+      return "eq two";
+    } else if (f(y)) {
+      return "f(x) truthy";
+    } else if (y.prop) {
+      return "has prop";
+    } else if (y[0]) {
+      return "has first child";
+    }
+})(foo.bar());

test/fixtures/match/as-call-statement/actual.js

@@ -0,0 +1,7 @@
+match foo() as y:
+  | y < 1: "lt one"
+  | y + 1 == 1: "eq zero"
+  | y == 2: "eq two"
+  | y~f(): "f(y) truthy"
+  | y.prop: "has prop"
+  | y.0: "has first child"

test/fixtures/match/as-call-statement/expected.js

@@ -0,0 +1,15 @@
+const y = foo();
+
+if (y < 1) {
+    "lt one";
+  } else if (y + 1 === 1) {
+    "eq zero";
+  } else if (y === 2) {
+    "eq two";
+  } else if (f(y)) {
+    "f(y) truthy";
+  } else if (y.prop) {
+    "has prop";
+  } else if (y[0]) {
+    "has first child";
+  }

test/fixtures/match/as-identifier-expression/actual.js

@@ -0,0 +1,7 @@
+z = match x as y:
+  | y < 1: "lt one"
+  | y + 1 == 1: "eq zero"
+  | y == 2: "eq two"
+  | y~f(): "f(x) truthy"
+  | y.prop: "has prop"
+  | y.0: "has first child"

test/fixtures/match/as-identifier-expression/expected.js

@@ -0,0 +1,15 @@
+const z = (y => {
+  if (y < 1) {
+      return "lt one";
+    } else if (y + 1 === 1) {
+      return "eq zero";
+    } else if (y === 2) {
+      return "eq two";
+    } else if (f(y)) {
+      return "f(x) truthy";
+    } else if (y.prop) {
+      return "has prop";
+    } else if (y[0]) {
+      return "has first child";
+    }
+})(x);

test/fixtures/match/as-identifier-statement-body-reference/actual.js

@@ -0,0 +1,3 @@
+match x:
+  | 1: "foo"
+  | "1": it