Allowing pattern matching within for..loops

lib/grammar.js

@@ -450,12 +450,20 @@
         return new ForNode($4, $3, $2[0], $2[1]);
       })
     ],
+    // An array of all accepted values for a variable inside the loop. This
+    // enables support for pattern matching.
+    ForValue: [o("Identifier"), o("Array", function() {
+        return new ValueNode($1);
+      }), o("Object", function() {
+        return new ValueNode($1);
+      })
+    ],
     // An array or range comprehension has variables for the current element and
     // (optional) reference to the current index. Or, *key, value*, in the case
     // of object comprehensions.
-    ForVariables: [o("Identifier", function() {
+    ForVariables: [o("ForValue", function() {
         return [$1];
-      }), o("Identifier , Identifier", function() {
+      }), o("ForValue , ForValue", function() {
         return [$1, $3];
       })
     ],

lib/nodes.js

@@ -1461,6 +1461,10 @@
         this.name = _b[0];
         this.index = _b[1];
       }
+      this.pattern = this.name instanceof ValueNode;
+      if (this.index instanceof ObjectNode) {
+        throw new Error('index cannot be a pattern matching expression');
+      }
       this.children = compact([this.body, this.source, this.filter]);
       this.returns = false;
       return this;
@@ -1491,7 +1495,7 @@
       scope = o.scope;
       name = this.name && this.name.compile(o);
       index = this.index && this.index.compile(o);
-      if (name) {
+      if (name && !this.pattern) {
         scope.find(name);
       }
       if (index) {
@@ -1516,9 +1520,14 @@
         svar = scope.free_variable();
         index_var = null;
         source_part = ("" + svar + " = " + (this.source.compile(o)) + ";\n" + this.tab);
-        if (name) {
+        if (name && !this.pattern) {
           var_part = ("" + body_dent + name + " = " + svar + "[" + ivar + "];\n");
         }
+        if (this.pattern) {
+          o.indent = this.idt(1);
+          o.top = true;
+          var_part = new AssignNode(this.name, literal(("" + svar + "[" + ivar + "]"))).compile(o) + "\n";
+        }
         if (!this.object) {
           lvar = scope.free_variable();
           step_part = this.step ? ("" + ivar + " += " + (this.step.compile(o))) : ("" + ivar + "++");

src/grammar.coffee

@@ -433,12 +433,20 @@ grammar: {
     o "FOR ForVariables ForSource Block",       -> new ForNode $4, $3, $2[0], $2[1]
   ]
 
+  # An array of all accepted values for a variable inside the loop. This
+  # enables support for pattern matching.
+  ForValue: [
+    o "Identifier"
+    o "Array",                                  -> new ValueNode $1
+    o "Object",                                 -> new ValueNode $1
+  ]
+
   # An array or range comprehension has variables for the current element and
   # (optional) reference to the current index. Or, *key, value*, in the case
   # of object comprehensions.
   ForVariables: [
-    o "Identifier",                             -> [$1]
-    o "Identifier , Identifier",                -> [$1, $3]
+    o "ForValue",                               -> [$1]
+    o "ForValue , ForValue",                    -> [$1, $3]
   ]
 
   # The source of a comprehension is an array or object with an optional filter

src/nodes.coffee

@@ -1079,8 +1079,10 @@ exports.ForNode: class ForNode extends BaseNode
     @step:    source.step
     @object:  !!source.object
     [@name, @index]: [@index, @name] if @object
+    @pattern:  @name instanceof ValueNode
+    throw new Error('index cannot be a pattern matching expression') if @index instanceof ObjectNode
     @children: compact [@body, @source, @filter]
-    @returns: false
+    @returns:  false
 
   top_sensitive: ->
     true
@@ -1104,7 +1106,7 @@ exports.ForNode: class ForNode extends BaseNode
     scope:          o.scope
     name:           @name and @name.compile o
     index:          @index and @index.compile o
-    scope.find name  if name
+    scope.find name  if name and not @pattern
     scope.find index if index
     body_dent:      @idt 1
     rvar:           scope.free_variable() unless top_level
@@ -1120,7 +1122,11 @@ exports.ForNode: class ForNode extends BaseNode
       svar:         scope.free_variable()
       index_var:    null
       source_part:  "$svar = ${ @source.compile(o) };\n$@tab"
-      var_part:     "$body_dent$name = $svar[$ivar];\n" if name
+      var_part:     "$body_dent$name = $svar[$ivar];\n" if name and not @pattern
+      if @pattern
+        o.indent:   @idt 1
+        o.top:      true
+        var_part:   new AssignNode(@name, literal("$svar[$ivar]")).compile(o) + "\n"
       if not @object
         lvar:       scope.free_variable()
         step_part:  if @step then "$ivar += ${ @step.compile(o) }" else "$ivar++"

test/test_pattern_matching.coffee

@@ -110,3 +110,30 @@ ok name   is "Bob"
 ok age    is 26
 ok first  is "Prince"
 ok second is "Bowie"
+
+# Pattern matching within for..loops
+
+persons: {
+  George: { name: "Bob" },
+  Bob: { name: "Alice" }
+  Christopher: { name: "Stan" }
+}
+
+join1: "$key: $name" for key, { name } of persons
+
+deepEqual join1, ["George: Bob", "Bob: Alice", "Christopher: Stan"]
+
+persons: [
+  { name: "Bob", parent: { name: "George" } },
+  { name: "Alice", parent: { name: "Bob" } },
+  { name: "Stan", parent: { name: "Christopher" } }
+]
+
+join2: "$parent: $name" for { name, parent: { name: parent } } in persons
+
+deepEqual join1, join2
+
+persons: [['Bob', ['George']], ['Alice', ['Bob']], ['Stan', ['Christopher']]]
+join3: "$parent: $name" for [name, [parent]] in persons
+
+deepEqual join2, join3