[Haskell-cafe] Why distinct tyvars in instance declarations?
robdockins at fastmail.fm
Mon Jun 27 12:08:13 EDT 2005
>>but GHC complains:
>>> Illegal instance declaration for `Foo (Either b b)'
>>> (The instance type must be of form (T a b c)
>>> where T is not a synonym, and a,b,c are distinct type variables)
>>> In the instance declaration for `Foo (Either b b)'
> unless I'm totally mistaken, your problem isn't the distinction thingy, but
> rather an error like supplying an Int for where you need (Int -> Int -> Int).
> That is, you're trying make (Either String String) an instance of Foo,
> (Either String String) already being a fully constructed type; Foo, on the
> other hand, seems to require a type constructor that is yet to parameterize
> over three more types (e.g. StateT).
I think that you are mistaken. The OP listed:
> class Foo a
> instance Foo (Either b b)
Without further information, Haskell compilers will assume that the
type(s) in a class declaration has/have kind * (Report section 4.6).
Either b b does have kind *, so that's not the problem.
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