Add support for standalone yield

This breaks compatibility with
->
  yield for i in [1..3]
    i * 2
and
->
  yield
    i * 2

yield's behaviour now mirrors that of return in that it can be used stand alone as well as with expressions. Thus, it currently also inherits the above limitations.

lib/coffee-script/grammar.js

@@ -47,7 +47,18 @@
         return new Literal($1);
       })
     ],
-    Expression: [o('Value'), o('Invocation'), o('Code'), o('Operation'), o('Assign'), o('If'), o('Try'), o('While'), o('For'), o('Switch'), o('Class'), o('Throw')],
+    Expression: [o('Value'), o('Invocation'), o('Code'), o('Operation'), o('Assign'), o('If'), o('Try'), o('While'), o('For'), o('Switch'), o('Class'), o('Throw'), o('Yield')],
+    Yield: [
+      o('YIELD', function() {
+        return new Op($1, new Value(new Literal('')));
+      }), o('YIELD Statement', function() {
+        return new Op($1, $2);
+      }), o('YIELD Expression', function() {
+        return new Op($1, $2);
+      }), o('YIELD FROM Expression', function() {
+        return new Op($1.concat($2), $3);
+      })
+    ],
     Block: [
       o('INDENT OUTDENT', function() {
         return new Block;
@@ -580,12 +591,6 @@
         return new Op('+', $2);
       }), {
         prec: 'UNARY_MATH'
-      }), o('YIELD Statement', function() {
-        return new Op($1, $2);
-      }), o('YIELD Expression', function() {
-        return new Op($1, $2);
-      }), o('YIELD FROM Expression', function() {
-        return new Op($1.concat($2), $3);
       }), o('-- SimpleAssignable', function() {
         return new Op('--', $2);
       }), o('++ SimpleAssignable', function() {

lib/coffee-script/lexer.js

@@ -794,7 +794,7 @@
 
     Lexer.prototype.unfinished = function() {
       var ref2;
-      return LINE_CONTINUER.test(this.chunk) || ((ref2 = this.tag()) === '\\' || ref2 === '.' || ref2 === '?.' || ref2 === '?::' || ref2 === 'UNARY' || ref2 === 'MATH' || ref2 === 'UNARY_MATH' || ref2 === '+' || ref2 === '-' || ref2 === 'YIELD' || ref2 === '**' || ref2 === 'SHIFT' || ref2 === 'RELATION' || ref2 === 'COMPARE' || ref2 === 'LOGIC' || ref2 === 'THROW' || ref2 === 'EXTENDS');
+      return LINE_CONTINUER.test(this.chunk) || ((ref2 = this.tag()) === '\\' || ref2 === '.' || ref2 === '?.' || ref2 === '?::' || ref2 === 'UNARY' || ref2 === 'MATH' || ref2 === 'UNARY_MATH' || ref2 === '+' || ref2 === '-' || ref2 === '**' || ref2 === 'SHIFT' || ref2 === 'RELATION' || ref2 === 'COMPARE' || ref2 === 'LOGIC' || ref2 === 'THROW' || ref2 === 'EXTENDS');
     };
 
     Lexer.prototype.formatString = function(str) {

src/grammar.coffee

@@ -113,6 +113,14 @@ grammar =
     o 'Switch'
     o 'Class'
     o 'Throw'
+    o 'Yield'
+  ]
+
+  Yield: [
+    o 'YIELD',                                  -> new Op $1, new Value new Literal ''
+    o 'YIELD Statement',                        -> new Op $1, $2
+    o 'YIELD Expression',                       -> new Op $1, $2
+    o 'YIELD FROM Expression',                  -> new Op $1.concat($2), $3
   ]
 
   # An indented block of expressions. Note that the [Rewriter](rewriter.html)
@@ -558,9 +566,6 @@ grammar =
     o 'UNARY_MATH Expression',                  -> new Op $1 , $2
     o '-     Expression',                      (-> new Op '-', $2), prec: 'UNARY_MATH'
     o '+     Expression',                      (-> new Op '+', $2), prec: 'UNARY_MATH'
-    o 'YIELD Statement',                        -> new Op $1 , $2
-    o 'YIELD Expression',                       -> new Op $1 , $2
-    o 'YIELD FROM Expression',                  -> new Op $1.concat($2) , $3
 
     o '-- SimpleAssignable',                    -> new Op '--', $2
     o '++ SimpleAssignable',                    -> new Op '++', $2

src/lexer.coffee

@@ -682,7 +682,7 @@ exports.Lexer = class Lexer
   # Are we in the midst of an unfinished expression?
   unfinished: ->
     LINE_CONTINUER.test(@chunk) or
-    @tag() in ['\\', '.', '?.', '?::', 'UNARY', 'MATH', 'UNARY_MATH', '+', '-', 'YIELD',
+    @tag() in ['\\', '.', '?.', '?::', 'UNARY', 'MATH', 'UNARY_MATH', '+', '-',
                '**', 'SHIFT', 'RELATION', 'COMPARE', 'LOGIC', 'THROW', 'EXTENDS']
 
   formatString: (str) ->

test/generators.coffee

@@ -4,7 +4,7 @@
 # * Generator Definition
 
 test "most basic generator support", ->
-  ok -> yield 0
+  ok -> yield
 
 test "empty generator", ->
   x = do -> yield return
@@ -15,19 +15,21 @@ test "empty generator", ->
 test "generator iteration", ->
   x = do ->
     yield 0
-    yield 1
+    yield
     yield 2
+    3
+
   y = x.next()
   ok y.value is 0 and y.done is false
 
   y = x.next()
-  ok y.value is 1 and y.done is false
+  ok y.value is undefined and y.done is false
 
   y = x.next()
   ok y.value is 2 and y.done is false
 
   y = x.next()
-  ok y.value is undefined and y.done is true
+  ok y.value is 3 and y.done is true
 
 test "last line yields are returned", ->
   x = do ->
@@ -71,9 +73,6 @@ test "bound generator", ->
 test "error if `yield` occurs outside of a function", ->
   throws -> CoffeeScript.compile 'yield 1'
 
-test "`yield` by itself not at the end of a function errors", ->
-  throws -> CoffeeScript.compile 'x = -> yield; return'
-
 test "`yield from` support", ->
   x = do ->
     yield from do ->
@@ -131,18 +130,6 @@ test "yield in for loop expressions", ->
   arrayEq z.value, [10, 20, 30]
   ok z.done is true
 
-test "yielding for loop expressions", ->
-  x = do ->
-    yield for i in [1..3]
-      i * 2
-
-  y = x.next()
-  arrayEq y.value, [2, 4, 6]
-  ok y.done is false
-
-  y = x.next 42
-  ok y.value is 42 and y.done is true
-
 test "yield in switch expressions", ->
   x = do ->
     y = switch yield 1
@@ -223,8 +210,9 @@ test "yield handles 'this' correctly", ->
   x = ->
     yield switch
       when true then yield => this
-    yield for item in [1]
+    array = for item in [1]
       yield => this
+    yield array
     yield if true then yield => this
     yield try throw yield => this
     throw yield => this