Add type safe IterableZip

[narumincho]

import 'dart:collection';
import 'package:collection/collection.dart';

void main() {
  final listA = [0, 1, 2];
  final listB = ['aaa', 'bbb', 'ccc'];

  final zipped = IterableZip([listA, listB]);
  for (final [valueAsInt, valueAsString] in zipped) {
    print(valueAsString.substring(valueAsInt)); // type error. (The method 'substring' isn't defined for the type 'Object'.)
  }

  final zipped2 = IterableZip2(listA, listB);
  for (final (valueAsInt, valueAsString) in zipped2) {
    print(valueAsString.substring(valueAsInt)); // no type error. (output aaa, bb, c)
  }
}

Implementation Example

import 'dart:collection';

class IterableZip2<T, U> extends IterableBase<(T, U)> {
  final Iterable<T> _iterablesT;
  final Iterable<U> _iterablesU;

  IterableZip2(Iterable<T> iterablesT, Iterable<U> iterablesU)
      : _iterablesT = iterablesT,
        _iterablesU = iterablesU;

  @override
  Iterator<(T, U)> get iterator {
    return _IteratorZip2<T, U>(_iterablesT.iterator, _iterablesU.iterator);
  }
}

class _IteratorZip2<T, U> implements Iterator<(T, U)> {
  final Iterator<T> _iteratorT;
  final Iterator<U> _iteratorU;
  (T, U)? _current;

  _IteratorZip2(Iterator<T> iteratorsT, Iterator<U> iteratorU)
      : _iteratorT = iteratorsT,
        _iteratorU = iteratorU;

  @override
  bool moveNext() {
    if (!_iteratorT.moveNext()) {
      _current = null;
      return false;
    }
    if (!_iteratorU.moveNext()) {
      _current = null;
      return false;
    }
    _current = (_iteratorT.current, _iteratorU.current);
    return true;
  }

  @override
  (T, U) get current => _current ?? (throw StateError('No element'));
}

Examples of zip functions in other languages

• Kotlin infix fun <T, R> Array<out T>.zip( other: Array<out R>): List<Pair<T, R>> https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/zip.html
• Rust pub fn zip<A, B>(a: A, b: B) -> Zip<<A as IntoIterator>::IntoIter, <B as IntoIterator>::IntoIter> where A: IntoIterator, B: IntoIterator https://doc.rust-lang.org/std/iter/fn.zip.html
• Haskell zip :: [a] -> [b] -> [(a, b)] https://hackage.haskell.org/package/base-4.19.1.0/docs/Prelude.html#v:zip
• PureScript zip :: forall a b. Array a -> Array b -> Array (Tuple a b) https://pursuit.purescript.org/packages/purescript-arrays/7.3.0/docs/Data.Array#v:zip
• Elm zip : List a -> List b -> List ( a, b ) https://package.elm-lang.org/packages/elm-community/list-extra/latest/List-Extra#zip-