[Haskell-cafe] classes with types which are wrapped in
Andrew U. Frank
frank at geoinfo.tuwien.ac.at
Fri Jan 22 11:08:15 EST 2010
i encounter often a problem when i have a class with some operations (say
class X with push) applied to a type A b. I then wrap A in a type A_sup, with
some more type parameters and i cannot write a instance of class A_sup because
i have a kind mismatch. any suggestions? (reordering of the type parameters of
A_sup is not a solution, because another class operates on this parameter)
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
-- this does not compile because the number of type arguments for X is wrong.
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'
what is the solution? thank you!
andrew
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