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

From discussion https://groups.google.com/a/dartlang.org/forum/?fromgroups#!topic/misc/Y0TfPuQD4uU

Original request:

Sometimes when I have a map, I'd like to iterate over it which I can do with the forEach() method, but if I want to stop the iteration at some point, returning false won't work and neither can I break it.

I can break with a for-in loop, but it does not support multiple values:

for (String key, List<int> value in someMap) {
}

I know that I could do something like:

for (var key in someMap.keys) {
  if (someMap[key] ... )
    break;
}

However, it makes me think why can't we have a for-in with multiple values or why can't we return false in a callback for forEach()? There's also no StopIteration to throw.

Is there a reason or is it something that "just isn't there"?

This comment was originally written by @seaneagan

The "pairs" getter also makes sense for List, see:

https://code.google.com/p/dart/issues/detail?id=1322#c8

In JavaScript, the higher order Array method callbacks receive both the index and value, and this could provide similar functionality to Dart. Another example:

void printEvens(List list) {
  list.pairs.where((pair) {
    if(pair.key.isEven) print(pair.value);
  });
}

printEvens(['a', 'b', 'c', 'd']);
// a
// c

This comment was originally written by @seaneagan

Now that we have List.asMap, we can probably settle for:

list.asMap().pairs

instead of:

list.pairs

Issue #12766 has been merged into this issue.

This comment was originally written by @seaneagan

Tuples could be a good way to represent Map pairs. I guess the best bug for tuples is issue #10310. As seen here:

google/quiver-dart#112

tuples could be Iterables which also provide strongly typed fields for each index.

So pairs could initially just return Iterables of length 2:

Iterable<Iterable> get pairs;
Map.fromPairs(Iterable<Iterable> pairs);

and then if we manage to get tuples which extend Iterable in the future, then these APIs can be updated to use those backwards compatibly.

This comment was originally written by @Andersmholmgren

I agree building on some tuple solution would be the nicest

This comment was originally written by @Andersmholmgren

Actually maybe the cleanest solution is if map directly implements Iterable<Tup2<A,B>>

Assuming a language level syntax for tuples then you could do things like

final m = {'foo':'1', 'bar':'2'};
Final m2 = m.map((k,v) => (k, int.parse(v))
        . where((k,v) => v >= 1);

Note (at least as a first step) you could do this today without first adding support for tuples except map couldn't actually declare that it implements Iterable. That would already provide most of the value.

This comment was originally written by @seaneagan

Any comment from the dart team on this one? Most other mainstream languages support this:

JavaScript (ES 6): http://people.mozilla.org/~jorendorff/es6-draft.html#sec-map.prototype.entries
Java: http://stackoverflow.com/questions/46898/how-do-i-iterate-over-each-entry-in-a-map
Python: https://docs.python.org/3.3/library/stdtypes.html#dict.items
Ruby: http://www.ruby-doc.org/core-1.9.3/Hash.html#method-i-each_pair

There is obviously an overhead to creating the pair objects when iterating, but it is negligible for many use cases, and the use cases where it does matter can simply not use this API.

I understand the hesitance to have Map implement Iterable<Pair> directly, but a "composition over inheritance" approach (e.g. a "pairs" or "entries" getter) seems pretty reasonable.

Issue #16117 has been merged into this issue.

Issue #21491 has been merged into this issue.

Issue #16117 has been merged into this issue.

This comment was originally written by jirkada...@gmail.com

Angular.dart implemented KeyValue class dart-archive/angular.dart#394

Map.forEach() does not really work if I need to await something. In this example, I want to run asynchronous function doWork() for each k-v pair and I don't want to have more than one doWork executing.

import 'dart:async';

Future doWork(v) {
  var completer = new Completer();
  new Timer(new Duration(seconds: 1), () => completer.complete(v*v));
  return completer.future;
}

main() async {

  // With List, all works fine

  var l = [1,2,3,4,5,6];

  // I can either use "smart" for loop,
  
  for (var v in l) {
    var r = await doWork(v);
    print(r);
  }
  
  // or Future.forEach()

  Future.forEach(l, (v) async {
    var r = await doWork(v);
    print(r);
  });
  
  // With Map it sucks

  var m = {"I":1, "you":2, "he/she/it":3, "we": 4};
  
  // This obviously runs all iterations concurrently

  m.forEach((k,v) async {
    var r = await doWork(v);
    print("$k: $v");
  });

  // and there is no Future.forEach for Maps
}

This comment was originally written by greg...@gmail.com

Would it be possible to just iterate on they key and look up the value?

For example:

Future doWork(key, value) => new Future.delayed(new Duration(seconds: 1));

main() {
  Future.forEach(m.keys, (key) => doWork(key, m[key]));
}

This comment was originally written by jirkad...@gmail.com

@greg Sure. I came up with something similar

Future forEach(Map map, Function f) {
  return Future.forEach(map.keys, (k) async => await f(k,map[k]));
}

main() {
  forEach(m, doWork);
}

Dart is Turing complete. Every problem can be "solved".

This comment was originally written by greg.low...@gmail.com

My point is that Future.forEach() works fine for maps.

  Future.forEach(m.keys, (key) => doWork(key, m[key]));

  Future.forEach(m.pairs, (pair) => doWork(pair.key, pair.value));

  Future.forEachMap(m, doWork);

These options all work, and are all pretty short. But if you'd like to advocate for adding a Future.forEachMap() function, then consider opening a bug for this rather than commenting against the bug for Map.pairs.

Also worth considering, if using async/await, then Future.forEach() is not actually required at all:
  
   for (var key in m) { await doWork(key, m[key]); }

In the example from #­16, the await statement in the closure passed to Future.forEach() can be omitted, as Future.forEach() already waits if the closure returns a Future.

This comment was originally written by @seaneagan

Milestone-2.0 ?

This comment was originally written by @paulevans

Is the reason that these closure implementations are being discussed over pairs / tuples because the allocations are actually harder on the vm then the setup for the forEach closures? Or is it a previous language bias?

I'll put my hat in the ring and say I do miss the ability to iterate over pairs like I did in C#.

Though looking over the implementation of one of the maps, say https://github.com/dart-lang/bleeding_edge/blob/3c02a5f133ca8d687fe21fd4beec0081745a376d/dart/runtime/lib/linked_hash_map.dart ... you may as well just iterate over the keys and ask for the values because that is exactly what this implementation is doing anyway.

To involve Futures and the extra work + overhead they do compared to just moving over a collection seems like convenience triumphing over sanity.

Maps now have an entries getter that returns an Iterable<MapEntry<K, V>>.
It was added in development version 2.0.0-dev.22.0.