Map constructors taking Iterables

[floitschG]

Add:

Map<K, V>.fromIterable(Iterable iterable, { K key(element), V value(element) });
key and value default to the identity function.  (Still a small discussion going for the name of the optional arguments).

and
Map<K, V>.fromIterables(Iterable<K> keys, Iterable<V> values);

Examples:
new Map<int, String>.fromIterable([1, 2], value: (x) => "$x");
new Map<int, String>.fromIterables([1, 2], ["eins", "zwei"]);

[DartBot]

This comment was originally written by @seaneagan

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So it looks like there will be no Pair/tuple concept in dart (see issue #7088)?

Corresponding examples for that approach would be:

new Map<int, String>.fromPairs([1, 2].map((i) => new Pair(x, "$x")));
new Map<int, String>.fromPairs([1, 2].zip(["eins", "zwei"])));

where Iterable has:

Iterable<Pair<K, dynamic>> zip(Iterable other);

If so, do you plan on instead adding Iterable-like functional methods directly to Map:

Iterable map(f(K k, V v));
Map<K, V> where(test(K k, V v));
fold(initialValue, transform(K k, V v));
// etc.

[lrhn]

The operations that work on key and value can usually be written in terms of just the key (and access to the map). For example
   Iterable map(f(K key))
already exists on Map.keys.

For ease of use, you can write a function from map and binary function to unary function:
  Function withMapValue(Map map, func(key, value)) => (key) => func(key, map[key])

Then you can do
  map.keys.map(withMapValue(map, (k, v) => ...))
instead of
  map.map((k, v) => ...)
Not as short, but not bad either.

[DartBot]

This comment was originally written by odw...@google.com

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Having Map<K, V>.fromIterable(Iterable iterable, { K key(element), V value(element) });
is great, and I really like the idea of passing two functions on elements for the keys and the values.
What about having, in addition, a method on Iterable to build a map out of it (in the spirit of toList and toSet)?

Iterable<K>.toMap({ K key(element), V value(element) }

That way we keep the fluentness of Iterable:

Map<K,V> map = Iterable.generate(....)
    .map(....)
    .where(....)
    .toMap(key: toKey)

instead of:
Iterable<K> keys = Iterable.generate(....)
    .map(....)
    .where(....)
Map<K,V> map = new Map.fromIterable(keys, key: toKey)

[floitschG]

We thought of adding a toMap on Iterables, but don't think it is needed that often. Now, with the Iterable constructors it becomes even less useful.

[szakarias]

Set owner to @szakarias.
Added Started label.

[szakarias]

Fixed in r=24531.

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Added Fixed label.