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

[Edward Kmett]

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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>
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