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

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 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 wrote:

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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 wrote:

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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 wrote:

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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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries