[Haskell-cafe] Data.Foldable UArray
Marcus D. Gabriel
marcus at gabriel.name
Sun Feb 23 22:17:49 UTC 2014
For the record, Michael's suggestion worked for me, and it was quite space and time efficient.
Here is a code fragment in case someone like me is looking for this:
> {-# LANGUAGE FlexibleContexts, TypeFamilies #-}
> import Data.Array.IArray (IArray)
> import Data.Array.Unboxed (UArray, (!), array, elems, bounds)
> import Data.Ix (Ix)
> import qualified Data.List as L
> import qualified Data.Foldable.Mono as F
> instance (Ix i, IArray UArray e) => F.MFoldable (UArray i e) where
> type Elem (UArray i e) = e
> foldl f z = L.foldl f z . elems
> foldl' f z = L.foldl' f z . elems
> foldl1 f = L.foldl1 f . elems
> foldr f z = L.foldr f z . elems
> foldr1 f = L.foldr1 f . elems
To be honest, I was aware of TypeFamilies, but they are new to me
so I need a little study.
Cheers,
- Marcus
On 21/02/2014 07:06, Michael Snoyman wrote:
>
>
> On Thu, Feb 20, 2014 at 11:12 PM, Marcus D. Gabriel <marcus at gabriel.name <mailto:marcus at gabriel.name>> wrote:
>
> Hello,
>
> I wanted to make a simple Data.Foldable UArray, and I naively modelled
> it on
>
> > instance Ix i => Foldable (Array i) where
> > foldr f z = Prelude.foldr f z . elems
>
> with, of course,
>
> > instance Ix i => Foldable (UArray i) where
> > foldr f z = Prelude.foldr f z . elems
>
> which did not work yielding the following type message
>
> Could not deduce (IArray UArray a) arising from a use of `elems'
> from the context (Ix i) bound by the instance declaration at
> ... Possible fix: add an instance declaration for (IArray UArray
> a) In the second argument of `(.)', namely `elems' In the
> expression: Data.List.foldr f z . elems In an equation for
> `foldr': foldr f z = Data.List.foldr f z . elems
>
> I clearly do not understand something because I cannot make this work,
> and I am not sure why.
>
> With the Haskell type system or even with ghc extensions, can one even
> make a Data.Foldable UArray? If so, how?
>
> Thanks in advance,
> - Marcus
>
>
> You could create an instance of MonoFoldable for UArray, similar to how the instance for unboxed vectors works[1]. There's no inherent reason why the Array instances don't exist there yet, I simply didn't get around to adding them yet.
>
> [1] https://github.com/snoyberg/mono-traversable/blob/d81bf2fe5ef4ee5957f3a5c54af07a00637c932f/src/Data/MonoTraversable.hs#L502
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