在Dart中减少/过滤地图的首选方法？

Question

Trying to find the cleanest way to reduce a Map in Dart. Looking for something akin to JavaScript's Array.prototype.filter()

Any suggestions? The following method leaves a lot to be desired:

Map ingredients = {
  'flour': '250g',
  'butter': '50g',
  'egg': 1,
  'water': '0.2l'
};

final flour = new Map.fromIterable(
  ingredients.keys.where((k) => ingredients[k] == '250g'),
  value: (k) => ingredients[k]
);

print(flour); // {flour: 250g}

Answer 1

There is no built in support for Pairs in the core library. However if you find yourself doing this type of thing often, you could write a utility library. For example:

library pairs;

class Pair<K,V> {
  Pair(this.k, this.v)
    final K k;
    final V v;
    String toString() => '($k, $v)';
}

Iterable<Pair> asPairs(Map map) => map.keys.map((k) => new Pair(k, map[k]));

Map fromPairs(Iterable<Pair> pairs) => new Map.fromIterables(
    pairs.map((p) => p.k),
    pairs.map((p) => p.v));

Then you can use it like this:

import 'src/pairs.dart';

Map flour = fromPairs(asPairs(ingredients).where((p) => p.v == '250g'));

print(flour); // {flour: 250g}

Perhaps this is something that could be contributed to the quiver package , a set of non-core utilities.