[Haskell-cafe] Flexible instances

[Henning Thielemann]

George Pollard schrieb:
> 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?

http://www.haskell.org/haskellwiki/List_instance