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 <[EMAIL PROTECTED]> 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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