Add support to Map interface to access entries as an Iterable and optionally to directly filter map entries

[DartBot]

This issue was originally filed by @Andersmholmgren

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i.e. something like

Iterable<KeyValuePair<K, V>> get entries;

It's often useful to be able to iterate of the entry set of a map and process entry by entry. This is common in other languages like Java.

e.g you could do stuff like

final Map<Symbol, MethodMirror> instanceMembers = reflect(o).type.instanceMembers;

final Iterator<KeyValuePair<Symbol, MethodMirror>> getters = instanceMembers.entries.where((kv) =>
    kv.value.isGetter);

final Iterable<KeyValuePair<Symbol, Object>> getterValues = getters.map((kv) =>
        new KeyValuePair(kv.key, invokeGetter(kv.value)));

Sadly in this example I ended up into https://code.google.com/p/dart/issues/detail?id=10975 so it proved futile but this sort of thing is very useful

Also (but less important) it can be useful to filter a map directly.

i.e.

Map<K,V> where(bool test(K key, V value));

This allows you then to create a subset of the first map and work on that.

[lrhn]

We generally try to avoid creating extra "pair" objects when you iterate a map.
There is no requirement that the map is implemented using an object per entry internally, so it would be an extra overhead to create an object for each entry during iteration. Also, a Map is not just a "Set of Pairs", and I personally don't want to encourage using it as such.

If you want to do something for each entry, you can use the forEach method with a binary function argument. That gives you a call per entry.

If you want to return something per entry, I'd use the keys iterable, and possibly lookup the value for the key if I need it.

For filtering, we could consider adding a Map.where (Map.removeWhere/Map.retainWhere for destructively updating).
   

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Removed Type-Defect label.
Added Type-Enhancement, Area-Library, Library-Core, Triaged labels.

[DartBot]

This comment was originally written by @seaneagan

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The "entries as an Iterable" part is a duplicate of issue #7088

This is at least the third bug that's been filed for this.

[lrhn]

Added Duplicate label.
Marked as being merged into #7088.

[DartBot]

This comment was originally written by @mezoni

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We generally try to avoid creating extra "pair" objects when you iterate a map.

What means your "when you iterate a map"?
How we can iterate a map if the map does not implement interface "Iterable"?

============
for (var keyWithValue in map) {
  someMethod(keyWithValue);
}
============

[lrhn]

The "when iterating a map" refers to using Map.forEach. It expects a function with two parameters instead of a function taking a single Pair object.

[DartBot]

This comment was originally written by @mezoni

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The "when iterating a map" refers to using Map.forEach. It expects a function with two parameters instead of a function taking a single Pair object.

Thanks.
How I can obtain an Iterator from this "iteration"?
How I can terminate this "iteration"?

As you know (as a programmer) "iteration" does not means traverse from the begin to the end without the possiblity to terminate this process.

The "iteration" only means a possibility to traverse.

Also, why in the documentation I do not found that this is an "iteration"?
Why I found that this is an "action where to each pair of key and value applied specified function"?

/**
 * Applies [f] to each key-value pair of the map.
 *
 * Calling f must not add or remove keys from the map.
 */
void forEach(void f(K key, V value));