frog: better errors/warnigns about type annotations in map literals.

frog/parser.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(jimhug): Error recovery needs major work!
 /**
  * A simple recursive descent parser for the dart language.
  *
  * This parser is designed to be more permissive than the official
  * Dart grammar.  It is expected that many grammar errors would be
@@ skipping 1438 matching lines @@
   finishTypedLiteral(int start, bool isConst) {
     var span = _makeSpan(start);
     var typeToBeNamedLater = new NameTypeReference(false, null, null, span);
     var genericType = addTypeArguments(typeToBeNamedLater, 0);
 
     if (_peekKind(TokenKind.LBRACK) || _peekKind(TokenKind.INDEX)) {
       genericType.baseType = new TypeReference(span, world.listType);
       return finishListLiteral(start, isConst, genericType);
     } else if (_peekKind(TokenKind.LBRACE)) {
       genericType.baseType = new TypeReference(span, world.mapType);
-      if (genericType.typeArguments.length != 1) {
-        _error('a map literal takes one type argument specfying the value type',
-            genericType.typeArguments.length == 0
-              ? genericType.typeArguments.span
-              : genericType.typeArguments[1].span);
-      }
-      genericType.typeArguments = [new TypeReference(span, world.stringType),
-          genericType.typeArguments[0]];
+      if (genericType.typeArguments.length == 1) {
+        genericType.typeArguments = [new TypeReference(span, world.stringType),
+            genericType.typeArguments[0]];
+      } else if (genericType.typeArguments.length == 2) {
+        var keyType = genericType.typeArguments[0];
+        if (keyType is! NameTypeReference || keyType.name.name !== "String") {
+          world.error('the key type of a map literal is implicitly "String"',
+              keyType.span);
+        } else {
+          // making key explicit is just a warning.
+          world.warning(
+              'a map literal takes one type argument specifying the value type',
+              keyType.span);
+        }
+      } // o.w. the type system will detect the mismatch in type arguments.
       return finishMapLiteral(start, isConst, genericType);
     } else {
       _errorExpected('array or map literal');
     }
   }
 
   ///////////////////////////////////////////////////////////////////
   // Some auxilary productions.
   ///////////////////////////////////////////////////////////////////
   _readModifiers() {
@@ skipping 274 matching lines @@
   int _pos = 0;
   next() {
     var token = tokens[_pos];
     ++_pos;
     if (_pos == tokens.length) {
       parser.tokenizer = previousTokenizer;
     }
     return token;
   }
 }