Implement type checking of map literals (issue 221).

runtime/lib/literal_map_factory.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.
+// Immutable map class for compiler generated map literals.
+
+class _LiteralMapFactory {
+  // [elements] contains n key-value pairs. The keys are at position
+  // 2*n, the values at position 2*n+1.
+  factory Map<K, V>.fromLiteral(int location,
+                                String value_type,
+                                List elements) {
+    var map = new LinkedHashMap<K, V>();

[hausner, 2011/11/22 18:37:35]

Why don't you declare the map as <String, V> ?

[regis, 2011/11/22 20:15:20]

Done.

+    var len = elements.length;
+    for (int i = 1; i < len; i += 2) {
+      // The type of the key has been checked in the parser already.
+      if (elements[i] is !V) {

[hausner, 2011/11/22 18:37:35]

Why do you have to explicitly implement the type check? Would the assignment in
line 22 below not throw a type error if elements[i] isn't of type V? (because
map is declared to be of type <K, V>)

[regis, 2011/11/22 20:15:20]

Yes, we would get a type error here, but the user would not know how to deal
with it. So I intercept the error and report it with a more useful error
message.

[hausner, 2011/11/23 15:40:10]

At the cost of duplicated type checks?  Why not declare map without type
arguments then to avoid the second (redundant) check? The user cannot add
anything to that map later, since it is immutable, so I'm not sure how the the
type arguments would ever be helpful.
 
On 2011/11/22 20:15:20, regis wrote:

[regis, 2011/11/23 15:51:32]

Actually, this code has changed now. See the other pending cl fixing issue 220.
This was wrong, as the test was done in non checked mode as well.

+        TypeError._throwNew(location,
+                            elements[i],
+                            value_type,
+                            "map literal value at index ${i ~/ 2}");
+      }
+      map[elements[i-1]] = elements[i];
+    }
+    return map;
+  }
+}