tc39 / proposal-iterator-unique

a proposal to add a method to iterators to produce an iterator of only its unique values

Iterator Unique

A TC39 proposal to produce an iterator of unique values from any iterator.

Stage: 1

See the January 2024 presentation to committee.

motivation

Removing duplicates from any kind of collection is a common operation. It's not very easy to do for iterators.

For some iterables, you can do something like the following:

let uniques = new Set(iter).values();

This has a few downsides, though:

1. Consumes the whole iterator before producing any results.
2. Doesn't work for infinite iterators.
3. Yields 0 when the underlying iterator yields -0.
4. Can't yield both 0 and -0.
5. Doesn't work for non-iterable iterators.

A better solution is much harder to write and doesn't work well with chaining, as it requires a bunch of side variables for state.

let objSeen = new WeakSet,
  primSeen = new Set,
  seenNegZero = false;

let uniques = iter.filter(e => {
  if (e === 0 && 1/e < 0) {
    if (seenNegZero) return false;
    seenNegZero = true;
    return true;
  }
  let seen = Object(e) === e ? objSeen : primSeen;
  if (seen.has(e)) return false;
  seen.add(e);
  return true;
});

Worse, when you want to unique by some applied transform, you need to surround the filter with tupling and untupling maps.

let uniques = iter
  .map(e => [e, f(e)])
  .filter([, criteria] => { ... })
  .map(([e]) => e);

chosen solution

Iterator.prototype.uniqBy which takes an optional mapper.

let uniques = iter.uniqBy();
let uniques = iter.uniqBy(obj => obj.field);

design space

• still no good solution for composite keys, but that's an unsolved problem generally
• mapper? comparator? both? neither?
  • separate methods or combined with optional params?
  • would mapper be passed an index?
• naming: distinct is also common

prior art

other languages

language	library	simple API	with comparator	with mapping
Clojure	core	distinct	--	--
Elm	List.Extra	unique	--	uniqueBy
Haskell	Data.List	nub	nubBy	--
Java	Stream	distinct	--	--
Kotlin	Sequence	distinct	--	distinctBy
.NET	System.Linq	Distinct	Distinct, DistinctBy	DistinctBy
PHP	array	array_unique	--	--
Python	more-itertools	unique_everseen	--	unique_everseen
Ruby	Enumerable	uniq	--	uniq
Rust	Iterator	--	--	--
Scala	Seq	distinct	--	distinctBy
Shell	GNU coreutils	uniq	--	--
Swift	Sequence	--	--	--

JS libraries

library	simple API	with comparator	with mapping
extra-iterable	unique	unique	unique
iter-ops	distinct	--	distinct
iter-tools	distinct	--	distinct
itertools-ts	distinct	--	distinct
Lodash / Underscore	uniq	uniqWith	uniqBy
Ramda	uniq	uniqWith	uniqBy
sequency	distinct	--	distinctBy
wu	unique	--	--