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?
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