[Haskell-cafe] classes with types which are wrapped in
Steffen Schuldenzucker
sschuldenzucker at uni-bonn.de
Fri Jan 22 12:42:04 EST 2010
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)
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