You can do this like:
>data TTrue = TTrue >data TFalse = TFalse > >data Nil = Nil >data Cons a l = Cons a l > >class Constrain c a b | c a -> b where > constrain :: c -> a -> b > >data ZConstraint = ZConstraint >instance Z a b => Constrain ZConstraint a b > >class List c l >instance List c Nil >instance (Constrain c a TTrue,List c l) => List c (Cons a l)
Now define a class 'Z' using a fundep such that 'b' becomes TTrue if 'a' is in Z.
>class Z a b | a -> b where >instance (TypeEq a Int b,TypeEq a Float b,TOr a b c) => Z a c > -- Int and Float are members of Z, TypeEq and TOr come from the > -- HList library.
Finally you can constrain the list:
>f :: List ZConstraint l -> List ZConstraint l >f x = x
Regards, Keean.
Bo Herlin wrote:
Hi everyone. Being new to Haskell I wonder how I can make a list contain different types but all implementing the same class, like this:
data X = X data Y = Y
class Z a where f :: a -> Int
instance Z X where f x = 0 instance Z Y where f y = 1
test1 :: Z a => [a] test1 = [X,Y]
test2 = map f test1
Is it possible to make this work?
/Bo Herlin _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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