Re: [Haskell-cafe] Data.Foldable UArray

23 Feb 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 mailto:marcus@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/d81bf2fe5ef4ee5957f3a5c54a...