Question about iter.Seq & iter.Pull

[Miki Tebeka]

Hi,

I'm writing the following Max function:

func Max[T cmp.Ordered](seq iter.Seq[T]) (T, error) {
pull, stop := iter.Pull(seq)
defer stop()

m, ok := pull()
if !ok {
return m, fmt.Errorf("Max of empty sequence")
}
fmt.Println(">>> m:", m)

for v := range seq {
fmt.Println(">>> v:", v)
if v > m {
m = v
}
}

return m, nil
}

When I run it (see https://go.dev/play/p/9kkQUj3wZiV), I see that "m" and the first "v" are the same. It's like the iterator is restarted.

When I use pull instead of the "range seq", I don't see the first value twice.

    for v, ok := pull(); ok; v, ok = pull() {

Is that the intended behaviour or a bug?

Thanks,

Miki

[Ian Lance Taylor]

On Thu, Mar 14, 2024 at 7:45 AM Miki Tebeka <miki....@gmail.com> wrote:
>
> I'm writing the following Max function:
>
> func Max[T cmp.Ordered](seq iter.Seq[T]) (T, error) {
> pull, stop := iter.Pull(seq)
> defer stop()
>
> m, ok := pull()
> if !ok {
> return m, fmt.Errorf("Max of empty sequence")
> }
> fmt.Println(">>> m:", m)
>
> for v := range seq {
> fmt.Println(">>> v:", v)
> if v > m {
> m = v
> }
> }
>
> return m, nil
> }
>
> When I run it (see https://go.dev/play/p/9kkQUj3wZiV), I see that "m" and the first "v" are the same. It's like the iterator is restarted.

You have one iteration that uses pull (once) and one iteration that
ranges over seq. Those are two independent iterations, so naturally
they both see the first element.

Ian

[Miki Tebeka]

Thanks Ian.

Those are two independent iterations, so naturally they both see the first element.

Coming from Python, this is different - that's what confused me. 

[Ethan Reesor]

`seq` is a function that iterates, yielding values via the callback. `seq` presumably does not have state. Each time you call it, it iterates. Of course you could write an iterator that maintained state, but personally I would expect `func(yield func(V) bool)` not to retain state between invocations.

On the other hand, a push iterator defined as `var next func() (V, bool)` must have state, otherwise it would not be able to iterate.

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