[Haskell-cafe] Flexible instances

George Pollard porges at porg.es
Tue Oct 14 22:20:26 EDT 2008

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

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