Hi Andrew, Andrew U. Frank wrote:
here a simplistic case (i know that A could be reduced to [], my real cases are more complicated).
data A b = A b [b]
data Asup x ab y = Asup x ab y
class X a b where push :: b -> a b -> a b
instance X A Int where push b' (A b bs) = A b' (b:bs)
instance X Asup Char Int Float where push b' (Asup a b c) = Asup a (push b' b) c
If I understand you correctly, what you want here are type level lambdas. Abusing notation: instance X (\t -> Asup Char t Float) Int where push b' (Asup a b c) = Asup a (push b' b) c However, type level lambdas introduce lots of ambiguities and are therefore AFAIK not supported in haskell[1].
if i try with a type
type A_2 b = Asup Char (A b) Float
instance X A_2 Int where push b' (Asup a b c) = Asup a (push b' b) c
(and --TypeSynonymInstances) i get:
Type synonym `A_2' should have 1 argument, but has been given 0 In the instance declaration for `X A_2 Int'
However, this error message looks strange. I tried to reduce this to a simpler case[1] and got the same message. Does anyone know why it complains just about the number of type arguments (which is correct) ? -- Steffen [1] http://www.mail-archive.com/haskell-cafe@haskell.org/msg69579.html [2] http://ideone.com/9BAj7MG7 (note that ideone is using ghc-6.8)