iter: document general guidance for writing iterator APIs

[mvdan]

While https://pkg.go.dev/iter does a good job at explaining the basics of iterators, it leaves out a few important bits of information which may be really useful when writing APIs with iterators. The two most important of them being:

1. Whether an exported func or method should itself be an iterator, or return an iterator.

The consensus seems to be to export funcs which return iterators, e.g. func (*Foo) Bars() iter.Seq[Bar] used like for bar := range foo.Bars(), rather than func (*Foo) Bars(yield func(Bar) bool) used like for bar := range foo.Bars. This is a bit more consistent with cases where one needs to supply parameters to obtain an iterator, as then the iterator must be a return parameter.

See #66626 (comment), for example, where @adonovan originally proposed adding methods to go/types which were directly iterators.

2. How errors should be returned to the caller.

If an iteration can fail with an error, it's not obvious whether to return one top-level error, like func Foo() (iter.Seq[Bar], error), or to provide an error at each iteration step, like func Foo() iter.Seq2[Bar, error]. Arguments can be made either way, but I think fundamentally one can implement any reasonable semantics with either signature.

The original proposal at #61897 seemed to clearly favor iter.Seq2[Bar, error] via its func Lines(file string) iter.Seq2[string, error] example, yet none of the value-error examples or text have survived into the final godoc we have today.

As of today I think there is no clear consensus for errors; as recently as last October it was still being discussed as part of a new API proposal.

There may be other API conventions that the iter godoc should mention as well, but these two seem like the most important to me. I would suggest that we document these guidelines sooner than later, so that iterator APIs across the Go ecosystem can be reasonably consistent and predictable.

[thepudds]

yet none of the value-error examples or text have survived into the final godoc we have today.

FWIW, that was a conscious decision. See for example the commit message here:
https://go-review.googlesource.com/c/go/+/591096

(There was a separate comment on the rationale that I couldn’t dig up immediately, but I think in short it was something like it wasn’t 100% obvious if it was the right idiom, and I think also an element of “let’s first see how people use it for real”).

[gabyhelp]

Related Discussions

• discussion: standard iterator interface

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[dsnet]

I understand the sentiment to deliberately not document until we first get experience, but I also agree with @mvdan that we should eventually (hopefully sooner rather than later) guidance on what to do. Doing something like iter.Seq2[Bar, error] is what I've been leaning towards lately.

[jub0bs]

@mvdan

If an iteration can fail with an error, it's not obvious whether to return one top-level error, like func Foo() (iter.Seq[Bar], error) [...]

That error result can communicate that iterator instantiation failed, but it cannot communicate that iteration itself fails. You'd need something like func Foo() (iter.Seq[Bar], func() error) (which was proposed in some other issue), wouldn't you?

[jub0bs]

Another point of clarification that would be welcome: iterator classification. I feel that differentiating single-use iterators from all other iterators was a poor choice. "Stateless" iterators (i.e. iter.Seq[Foo] and iter.Seq2[Bar, Baz] that are pure functions) vs. all others would be more useful.

"Stateless" iterators are easy to reason about, whereas "stateful" iterators come in all shapes and forms: single-use, resumable, etc. An analogy that comes to mind is linear systems vs nonlinear systems in control theory:

the theory of nonlinear systems is like a theory of non-elephants

[gazerro]

@jub0bs

You'd need something like func Foo() (iter.Seq[Bar], func() error) (which was proposed in some other issue), wouldn't you?

func Foo() (iter.Seq[Bar], func() error) was proposed by @jba (70084 comment), and it is indeed a convincing alternative to func Foo() iter.Seq2[Bar, error]:

iter, errf := Foo()
for x := range iter {
    ...
}
if err := errf(); err != nil {
    return err
}

[jub0bs]

@gazerro Yes, that's the issue I was thinking of. Thanks. I'm not sure I'd be in favour of promoting this approach, though.

[mvdan]

Oops, of course, I meant to write something like that - so that an error (or a wrapped list of errors) can be returned once the iterator is done.

[ianlancetaylor]

This discussion is why we don't have general guidance about how to return errors. People don't yet agree.

I think a commonly known method, such as All, should return an iterator rather than be an iterator. That ensure consistency of the common method across different types. This is mentioned (briefly) at https://go.dev/blog/range-functions#standard-push-iterators. But I don't know that we need a convention for methods that are specific to a given type.

[prattmic]

cc @rsc

[jub0bs]

FWIW, I've just published https://jub0bs.com/posts/2025-05-29-pure-vs-impure-iterators-in-go/, which builds upon one of my earlier comments.

TL;DR

• Go has now standardised iterators.
• Iterators are powerful.
• Being functions under the hood, iterators can be closures.
• The classification of iterators suggested by the documentation is ambiguous.
• Dividing iterators into two categories, “pure” and “impure”, seems to me preferrable.
• Whether iterators should be designed as “pure” whenever possible is unclear.