using 'yield' automatically turns functions into generators

lib/coffee-script/nodes.js

@@ -1798,6 +1798,14 @@
       this.params = params || [];
       this.body = body || new Block;
       this.bound = tag === 'boundfunc';
+      this.isGenerator = false;
+      this.body.traverseChildren(false, (function(_this) {
+        return function(child) {
+          if (child.operator === 'yield') {
+            return _this.isGenerator = true;
+          }
+        };
+      })(this));
     }
 
     Code.prototype.children = ['params', 'body'];
@@ -1910,6 +1918,9 @@
         this.body.makeReturn();
       }
       code = 'function';
+      if (this.isGenerator) {
+        code += '*';
+      }
       if (this.ctor) {
         code += ' ' + this.name;
       }

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)]