[Haskell-beginners] From Functional Dependencies to Type Families

[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 ?


Thanks

-
Adolfo Builes
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