indexM or (!?)-style accessor for arrays?

[Zemyla]

Also, even if the base implementation of indexM were Monad-only, you could do

indexA :: (Applicative m, Array a e, Ix i) => a i e -> i -> m e
indexA a e = runCont (indexM a e) pure

to pass it to an Applicative.

On Tue, Aug 13, 2019 at 12:47 PM Edward Kmett <ekmett at gmail.com> wrote:
>
> I'd for some reason assumed that vector's indexM actually combined values for the Unboxed case, but it looks like _it_ should be generalized to Applicative, too.
>
> -Edwrd
>
> On Tue, Aug 13, 2019 at 10:33 AM Zemyla <zemyla at gmail.com> wrote:
>>
>> Well, the reason I say Applicative is because the only method from
>> Monad which is used is return, which is the same as pure in
>> Applicative. I'm preparing for when the "Monad of no return" proposal
>> gets implemented.
>>
>> On Tue, Aug 13, 2019 at 11:29 AM Edward Kmett <ekmett at gmail.com> wrote:
>> >
>> > I'm pretty strongly +1 on this, assuming someone is willing to chase down all the implementation issues, as the current API doesn't really allow for efficient usage.
>> >
>> > I'm not sure that Applicative is strong enough to give the desired behavior, though.
>> >
>> > e.g. to write a strict fmap (<$!>) you need Monad.
>> >
>> > -Edward
>> >
>> > On Tue, Aug 13, 2019 at 9:24 AM Zemyla <zemyla at gmail.com> wrote:
>> >>
>> >> This is an issue I originally proposed on GHC Trac, but I'm posting it
>> >> here because (I think) the Data.Array package is under the purview of
>> >> the Libraries committee.
>> >>
>> >> The vector package has indexM and its cousins so that a user can be
>> >> strict in the array without necessarily being strict in the value
>> >> retrieved from that array. Arrays don't have that sort of thing,
>> >> meaning that anything you do that takes an array will necessarily
>> >> leave references to the array unless you force the whole thing, and
>> >> that's not only inefficient, it's untenable for general libraries.
>> >>
>> >> What I'm thinking is that the IArray class should have a function like
>> >>
>> >> unsafeAtM :: (Array a e, Ix i, Applicative m) => a i e -> Int -> m e
>> >>
>> >> For compatibility with older code that wouldn't necessarily define
>> >> this but would define unsafeAt, we'd have the default implementation
>> >>
>> >> unsafeAtM a !n = pure (unsafeAt a n)
>> >>
>> >> Also, you could have unsafeAt defined in terms of unsafeAtM, so the
>> >> minimal implementation could require only one of them:
>> >>
>> >> unsafeAt = (coerce :: (a i e -> Int -> Identity e) -> a i e -> Int ->
>> >> e) unsafeAtM
>> >>
>> >> Also, (!?) would be a "safe" indexing tool for arrays, which would
>> >> incidentally also force the array without forcing the value inside.
>> >> You would have
>> >>
>> >> (!?) :: (Array a e, Ix i) => a i e -> i -> Maybe e
>> >> (!?) a e = case bounds a of
>> >>   p@(l, u) -> case inRange p e of
>> >>     True -> unsafeAtM arr $ unsafeIndex p e
>> >>     False -> Nothing
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