using 'yield' automatically turns functions into generators

package.json

@@ -17,7 +17,8 @@
     "cake":       "./bin/cake"
   },
   "scripts": {
-    "test": "node ./bin/cake test"
+    "test":         "node ./bin/cake test",
+    "test-harmony": "node --harmony ./bin/cake test"
   },
   "homepage":     "http://coffeescript.org",
   "bugs":         "https://github.com/jashkenas/coffee-script/issues",

src/lexer.coffee

@@ -710,7 +710,7 @@ exports.Lexer = class Lexer
 JS_KEYWORDS = [
   'true', 'false', 'null', 'this'
   'new', 'delete', 'typeof', 'in', 'instanceof'
-  'return', 'throw', 'break', 'continue', 'debugger'
+  'return', 'throw', 'break', 'continue', 'debugger', 'yield'
   'if', 'else', 'switch', 'for', 'while', 'do', 'try', 'catch', 'finally'
   'class', 'extends', 'super'
 ]
@@ -739,7 +739,7 @@ RESERVED = [
   'case', 'default', 'function', 'var', 'void', 'with', 'const', 'let', 'enum'
   'export', 'import', 'native', '__hasProp', '__extends', '__slice', '__bind'
   '__indexOf', 'implements', 'interface', 'package', 'private', 'protected'
-  'public', 'static', 'yield'
+  'public', 'static'
 ]
 
 STRICT_PROSCRIBED = ['arguments', 'eval']
@@ -831,7 +831,7 @@ COMPOUND_ASSIGN = [
 ]
 
 # Unary tokens.
-UNARY   = ['!', '~', 'NEW', 'TYPEOF', 'DELETE', 'DO']
+UNARY   = ['!', '~', 'NEW', 'TYPEOF', 'DELETE', 'DO', 'YIELD']
 
 # Logical tokens.
 LOGIC   = ['&&', '||', '&', '|', '^']

src/nodes.coffee

@@ -456,7 +456,6 @@ exports.Return = class Return extends Base
     answer.push @makeCode ";"
     return answer
 
-
 #### Value
 
 # A value, variable or literal or parenthesized, indexed or dotted into,
@@ -1302,6 +1301,9 @@ exports.Code = class Code extends Base
     @params  = params or []
     @body    = body or new Block
     @bound   = tag is 'boundfunc'
+    @isGenerator = false
+    @body.traverseChildren false, (child) =>
+      @isGenerator = true if child.operator is 'yield'
 
   children: ['params', 'body']
 
@@ -1369,6 +1371,7 @@ exports.Code = class Code extends Base
       uniqs.push name
     @body.makeReturn() unless wasEmpty or @noReturn
     code  = 'function'
+    code += '*' if @isGenerator
     code  += ' ' + @name if @ctor
     code  += '('
     answer = [@makeCode(code)]
@@ -1699,7 +1702,7 @@ exports.Op = class Op extends Base
     if o.level >= LEVEL_ACCESS
       return (new Parens this).compileToFragments o
     plusMinus = op in ['+', '-']
-    parts.push [@makeCode(' ')] if op in ['new', 'typeof', 'delete'] or
+    parts.push [@makeCode(' ')] if op in ['new', 'typeof', 'delete', 'yield'] or
                       plusMinus and @first instanceof Op and @first.operator is op
     if (plusMinus and @first instanceof Op) or (op is 'new' and @first.isStatement o)
       @first = new Parens @first

test/generators.coffee

@@ -0,0 +1,35 @@
+# Generators
+# -----------------
+
+# * Generator Definition
+
+# ensure that these tests are only run if generators are available
+generatorsAreAvailable = ->
+  for execArg in process.execArgv
+    if execArg.match 'harmony'
+      return 1
+  return 0
+
+if generatorsAreAvailable()
+
+  # Using the keyword yield should not cause a syntax error.
+  -> yield 0
+
+  test "Generator Definition", ->
+    x = ->
+      yield 0
+      yield 1
+      yield 2
+    y = x()
+    z = y.next()
+    eq z.value, 0
+    eq z.done, false
+    z = y.next()
+    eq z.value, 1
+    eq z.done, false
+    z = y.next()
+    eq z.value, 2
+    eq z.done, false
+    z = y.next()
+    eq z.value, undefined
+    eq z.done, true

test/strict.coffee

@@ -140,7 +140,7 @@ test "`Future Reserved Word`s, `eval` and `arguments` restrictions", ->
   tryCatch = (keyword, check = strict) ->
     check "try new Error catch #{keyword}"
 
-  future = 'implements interface let package private protected public static yield'.split ' '
+  future = 'implements interface let package private protected public static'.split ' '
   for keyword in future
     access   keyword
     assign   keyword