Joint Iteration

A TC39 proposal to synchronise the advancement of multiple iterators.

Stage: 2.7

Demo: https://tc39.es/proposal-joint-iteration/demo/

Specification: https://tc39.es/proposal-joint-iteration/

motivation

Often you have 2 or more iterators that are positionally aligned (the first value yielded by the first iterator corresponds to the first value yielded by the other iterators, and so on), and you would like to operate on the corresponding values together. A common solution to this is zip, which produces an iterator of the combined values. zipWith allows combination of values in some way other than tupling. Some languages express zipWith as a variadic map.

presentations to committee

• September 2023
• November 2023
• January 2024
• June 2024

considered design space

1. do we support just 2 iterators or something else? 2+? 1+? 0+?
   1. if 0 is allowed, is that considered never-ending or already completed?
   2. should the iterators be passed positionally (combining to arrays) or named (combining to objects)?
   3. do we take the iterators as varargs or as an iterable/object?
      1. varargs eliminates design space for potentially passing an options bag or a combining function
2. do we support iterators and iterables like Iterator.from and flatMap?
   1. if so, which string handling do we match? Iterator.from iterates strings; flatMap rejects strings
3. if an iterator completes, do we still advance the other iterators?
   1. do we return them?
4. if an iterator fails to advance, do we still advance the other iterators?
   1. if so, do we return an AggregateError? Only if 2+ failures?
5. do we want -With variants for combining the values in other ways than tupling?
   1. what about always requiring the combiner?
6. do we want a zipLongest/zipFilled/zipAll?
   1. if so, do we want a filler element or to call a function to provide the filler?
   2. what about a variant that matches the length of a privileged iterator (this)?
7. do we want a zipEqual/zipStrict that throws if they do not complete after the same number of yields?

prior art

other languages

language	shortest	longest	privileged	strict	-With	3+ sources	1 source	0 sources
C++	std::ranges::views::zip				::zip_transform	yes	yes
Clojure	variadic map				yes	yes	yes
Elm					List.map2	yes	yes
Haskell	zip				zipWith	yes
OCaml	zip			combine	map2		yes
Python	zip	itertools.zip_longest		zip(..., strict=True)		yes	yes	yes, empty
Ruby			Enumerable#zip		zip	yes	yes
Rust	Iterator::zip
Scala	zip	it.zipAll(jt, x, y)
Swift	zip

JS libraries

library	shortest	longest	privileged	strict	-With	3+ sources	1 source	0 sources
@iterable-iterator/zip	zip	zipLongest				yes	yes
@softwareventures/iterator	zipOnce
extra-iterable	zip	zip			zip	yes	yes	yes, empty
immutable.js	Seq::zip				zipWith	yes	yes
iter-ops	zip					yes	yes	yes, empty
iter-tools	zip	zipAll				yes	yes	yes, empty
iterablefu	zip	zipAll				yes	yes	yes, empty
iterare	zip
itertools-ts	zip	zipFilled, zipLongest		zipEqual		yes	yes	yes, empty
ixjs	zip					yes	yes	yes, empty
lodash		zip			zipWith	yes	yes	yes, empty
ramda	zip				zipWith
sequency	zip
wu	zip	zipLongest			zipWith	yes	yes
zipiterators		zipiterators