[Haskell-cafe] List containing different types but all implementing the same class

[Keean Schupke]

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 at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe