On Thu, Feb 20, 2014 at 11:12 PM, Marcus D. Gabriel
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...