Am Donnerstag 03 Dezember 2009 04:06:43 schrieb Adolfo Builes:
{-# OPTIONS -fglasgow-exts #-}
module VarArg where import Data.FiniteMap -- for an example below
class BuildList a r | r-> a where build' :: [a] -> a -> r
instance BuildList a [a] where build' l x = reverse$ x:l
instance BuildList a r => BuildList a (a->r) where build' l x y = build'(x:l) y
--build :: forall r a. (BuildList a r) => a -> r build x = build' [] x
I'm trying to replace the code below to work with type families, I started out replacing the definition of class with :
class BuildList r where type Build r build' :: [Build r] -> Build r -> r
follow by the instance for [a] resulting in
instance BuildList [a] where type Build [a] = a build' l x = reverse $ x:l
Until here, everything is working, and I'm able to do
build' [2,3,4] 1 :: [Integer] [4,3,2,1]
then I move on to the next instance (a -> r) with
instance BuildList r => BuildList (a-> r) where type Build (a -> r) = a build' l x = \ y -> build'(x:l) y
But I get the following error
Couldn't match expected type `Build r' against inferred type `a' `a' is a rigid type variable bound by the instance declaration at /home/adolfo/foo.hs:347:35 In the first argument of `(:)', namely `x' In the first argument of `build'', namely `(x : l)' In the expression: build' (x : l) y
then I try with :
instance BuildList r => BuildList (a-> r) where type Build (a -> r) = Build r build' l x = \ y -> build'(x:l) y
And I get
Couldn't match expected type `a' against inferred type `Build r' `a' is a rigid type variable bound by the instance declaration at /home/adolfo/foo.hs:347:35 Expected type: [Build r] Inferred type: [Build (a -> r)] In the second argument of `(:)', namely `l' In the first argument of `build'', namely `(x : l)'
I have been trying to figure out, which type should it be, but I haven't found the correct one, any ideas ?
I think instance (BuildList r, Build r ~ a) => BuildList (a -> r) where type Build (a -> r) = a build' l x = \y -> build' (x:l) y should work. You need to tell the compiler explicitly that a and Build r should be the same type.
Thanks
- Adolfo Builes