If two Maps are compared with an equals() matcher and they have the same length,

pkg/unittest/lib/src/core_matchers.dart

 // Copyright (c) 2012, 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.
 
 part of matcher;
 
 /**
  * Returns a matcher that matches empty strings, maps or iterables
  * (including collections).
  */
@@ skipping 123 matching lines @@
       reason = new StringDescription('recursion depth limit exceeded');
     } else {
       if (expected is Iterable && canRecurse) {
         String r = _compareIterables(expected, actual,
           _recursiveMatch, depth+1);
         if (r != null) reason = new StringDescription(r);
       } else if (expected is Map && canRecurse) {
         if (actual is !Map) {
           reason = new StringDescription('expected a map');
         } else if (expected.length != actual.length) {
           reason = new StringDescription('different map lengths');

[Jennifer Messerly, 2013/02/09 23:53:06]

The bug was about this line. Unless I'm really confused :), if map length is
different, we only print "different map lengths". The "missing map key" message
only prints if the lengths are the same.

[Jennifer Messerly, 2013/02/10 00:01:40]

Here's a possible way to refactor this code. Not sure if I'm using the
StringDescription class correctly but this is the the general idea. Basically
the problem I'm trying to solve is that "different map length" isn't a useful
message on its own, it helps to know why it's different -- was their an extra
key, or too few keys? Basically I need to see the full "diff" :)  to quickly
diagnose why the test failed.



if (actual is !Map) {
 ...
} else {
  var reason = new StringDescription();
  if (expected.length != actual.length) {
    reason.add('different map length, ');
  }
  for (var key in expected.keys) {
    if (!actual.containsKey(key)) {
      reason.add('missing map key');
      reason.addDescriptionOf(key);
      break;
    }
    var valueReason = _recursiveMatch(expected[key], actual[key], ...));
    if (valueReason != null) {
      reason.add(valueReason);
      break;
    }
  }
  // NOTE: down here it also needs to handle the case where "actual" has a key
but "expected" does not.
}

[gram, 2013/02/11 20:43:35]

 I have changed it so that it will print the mismatched length message plus one
key message (missing/extra)

         } else {
           for (var key in expected.keys) {
             if (!actual.containsKey(key)) {
-              reason = new StringDescription('missing map key ');
+              reason = new StringDescription('missing map key $key ');
               reason.addDescriptionOf(key);
               break;
             }
             reason = _recursiveMatch(expected[key], actual[key],
                 'with key <${key}> ${location}', depth+1);
             if (reason != null) {
               break;
             }
           }
         }
@@ skipping 538 matching lines @@
       description.add(_featureDescription).add(' ').addDescriptionOf(_matcher);
 
   Description describeMismatch(item, Description mismatchDescription,
                                MatchState matchState, bool verbose) {
     mismatchDescription.add(_featureName).add(' ');
     _matcher.describeMismatch(matchState.state['feature'], mismatchDescription,
         matchState.state['innerState'], verbose);
     return mismatchDescription;
   }
 }