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

[Edward Kmett]

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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