So, the Haskell98 solution to this is: class StringableList a where listToString :: [a] -> String -- now [a] is of the proper form; T = [], a is a type variable instance StringableList a => Stringable [a] where toString = listToString -- now to make an instance for Stringable [Char] -- we just make an instance for StringableList Char instance StringableList Char where listToString = id I think "FlexibleInstances" just makes the compiler jump through these hoops instead of you. -- ryan On Wed, Oct 15, 2008 at 3:20 AM, George Pollard <porges@porg.es> wrote:
I'm a little confused. Why is this allowed:
data Blah = Blah
instance Eq Blah where x == y = True
But not this:
class Stringable a where toString :: a -> String
instance Stringable [Char] where toString = id
(Resulting in:)
Illegal instance declaration for `Stringable [Char]' (All instance types must be of the form (T a1 ... an) where a1 ... an are distinct type *variables* Use -XFlexibleInstances if you want to disable this.) In the instance declaration for `Stringable [Char]'
'Blah' isn't a type variable, is it? Is my brain just not working right today?
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