feat(corelib): Iterator::peekable

[hudem1]

I suggest the addition of Iterator::peekable to be able to use peek() on iterators.

Example

let xs = array![1, 2, 3];

let mut iter = xs.into_iter().peekable();

// peek() lets us see one step into the future
assert_eq!(iter.peek(), Option::Some(1));
assert_eq!(iter.next(), Option::Some(1));
assert_eq!(iter.next(), Option::Some(2));

// The iterator does not advance even if we `peek` multiple times
assert_eq!(iter.peek(), Option::Some(3));
assert_eq!(iter.peek(), Option::Some(3));
assert_eq!(iter.next(), Option::Some(3));

// After the iterator is finished, so is `peek()`
assert_eq!(iter.peek(), Option::None);
assert_eq!(iter.next(), Option::None);

I set my PR as a draft because i have 1 issue and 1 question, possibly related:

1. When I run my test, it says Error: Compilation failed without any diagnostics.. It does not display the reason of the compilation error and the test file doesn't show any particular error from the LS. I do get an LS error in the iterator.cairo file saying Trait has no implementation in context: core::iter::traits::iterator::Iterator::<T>., which does not make much sense to me as T obviously implements Iterator as we are in the Iterator<T> trait.

2. I am unsure about how to use the custom trait for the peek method. I exported it in the adapters.cairo and iter.cairo files to be able to import it in my test file, but I am not sure it is the correct way to use it.

[reviewable-StarkWare]

This change is 

[orizi]

it is required to be added to the prelude in that case, but lets postpone that.

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @hudem1)

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a discussion (no related file):
@gilbens-starkware for 2nd eye.

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corelib/src/test/iter_test.cairo line 44 at r1 (raw file):

    let xs = array![1, 2, 3];

    let mut iter = xs.into_iter().peekable();

Suggestion:

fn test_iter_adapter_peekable() {
    let mut iter = (1..4).into_iter().peekable();

[gilbens-starkware]

Reviewed 2 of 5 files at r1, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @hudem1)

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corelib/src/iter/traits/iterator.cairo line 305 at r1 (raw file):

    #[must_use]
    fn peekable(self: T) -> Peekable<T, Self::Item> {
        peekable_iterator(self)

We should find the impl we in, but meanwhile, maybe this will work:

Suggestion:

peekable_iterator::<T, Self>(self)

[hudem1]

Ok.

Reviewable status: 2 of 5 files reviewed, 2 unresolved discussions (waiting on @gilbens-starkware and @orizi)

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corelib/src/iter/traits/iterator.cairo line 305 at r1 (raw file):

Previously, gilbens-starkware (Gil Ben-Shachar) wrote…

We should find the impl we in, but meanwhile, maybe this will work:

Thank you for the suggestion. I tried but it doesn't work, it says Unknown impl over the Self, and running the test still gives the same error Error: Compilation failed without any diagnostics..

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corelib/src/test/iter_test.cairo line 44 at r1 (raw file):

    let xs = array![1, 2, 3];

    let mut iter = xs.into_iter().peekable();

Done.

[orizi]

Reviewed all commit messages.
Reviewable status: 2 of 5 files reviewed, 3 unresolved discussions (waiting on @gilbens-starkware and @hudem1)

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corelib/src/iter/adapters/peekable.cairo line 17 at r2 (raw file):

pub fn peekable_iterator<I, impl IterI: Iterator<I>>(iter: I) -> Peekable<I, IterI::Item> {
    Peekable { iter, peeked: Option::None }

remove this - it would remove the requirement in the trait - that you don't actually need.

Suggestion:

pub fn peekable_iterator<I>(iter: I) -> Peekable<I, IterI::Item> {
    Peekable { iter, peeked: Option::None }

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corelib/src/iter/adapters/peekable.cairo line 21 at r2 (raw file):

impl PeekableIterator<
    I, impl IterI: Iterator<I>, +Drop<I>, +Drop<IterI::Item>,

Suggestion:

    I, impl IterI: Iterator<I>, +Destruct<I>, +Destruct<IterI::Item>,

[hudem1]

Reviewable status: 2 of 5 files reviewed, 3 unresolved discussions (waiting on @gilbens-starkware and @orizi)

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corelib/src/iter/adapters/peekable.cairo line 17 at r2 (raw file):

Previously, orizi wrote…

remove this - it would remove the requirement in the trait - that you don't actually need.

Oh yeah as we are sure that if we reach this function, the parameter iter will be an iterator, and we don't need to use actual iterator methods in this function, good idea! As expected, it now works! :)

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corelib/src/iter/adapters/peekable.cairo line 21 at r2 (raw file):

impl PeekableIterator<
    I, impl IterI: Iterator<I>, +Drop<I>, +Drop<IterI::Item>,

I tried, but then i get the error that the variable from self.peeked.take() is not dropped.
Please find attached a screenshot for more details.

[orizi]

Reviewed 2 of 3 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @gilbens-starkware and @hudem1)

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corelib/src/iter/adapters/peekable.cairo line 21 at r2 (raw file):

Previously, hudem1 wrote…

I tried, but then i get the error that the variable from self.peeked.take() is not dropped.
Please find attached a screenshot for more details.

applied changes to the code itself to make it work.
(the causing issue is the fact that we don't lookup negative impls, but just the lack of positive impls, should be solved soon enough, but this will suffice for the time being)

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corelib/src/iter/adapters/peekable.cairo line 32 at r3 (raw file):

        }
    }
}

Suggestion:

impl PeekableIterator<
    I, impl IterI: Iterator<I>, +Destruct<I>, +Destruct<IterI::Item>,
> of Iterator<Peekable<I, IterI::Item>> {
    type Item = IterI::Item;

    fn next(ref self: Peekable<I, IterI::Item>) -> Option<Self::Item> {
        let next_value = match self.peeked {
            Option::Some(v) => v,
            Option::None => self.iter.next(),
        };
        self.peeked = Option::None;
        next_value
    }
}

[hudem1]

Reviewable status: 3 of 5 files reviewed, 2 unresolved discussions (waiting on @gilbens-starkware and @orizi)

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corelib/src/iter/adapters/peekable.cairo line 21 at r2 (raw file):

Previously, orizi wrote…

applied changes to the code itself to make it work.
(the causing issue is the fact that we don't lookup negative impls, but just the lack of positive impls, should be solved soon enough, but this will suffice for the time being)

Done. And for my understanding, why do we want to use +Destruct instead of +Drop here ?

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corelib/src/iter/adapters/peekable.cairo line 32 at r3 (raw file):

        }
    }
}

Done.

[orizi]

Reviewed 1 of 2 files at r4.
Reviewable status: 4 of 5 files reviewed, 1 unresolved discussion (waiting on @gilbens-starkware)

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corelib/src/iter/adapters/peekable.cairo line 21 at r2 (raw file):

Previously, hudem1 wrote…

Done. And for my understanding, why do we want to use +Destruct instead of +Drop here ?

It just supports more cases.