On Wed, Aug 5, 2009 at 2:25 AM, Michael P Mossey
Peter Verswyvelen wrote:
Note however that the following is correct:
instance Functor ((,) a) where fmap f (x,y) = (x, f y)
Thanks. What's curious to me about these instances is that they have a type variable a which is never referenced in the definition.
Is there ever a case in which you would refer to the type variable 'a' somewhere in the definition of an instance? I know that the types of the "member functions" of the instance are given in the class definition, so there is no place to put a type definition in the instance, I don't think.
Good question, I don't know. You can use it to add constraints, e.g. instance Show a => Functor ((,) a) where fmap f (x,y) = (x, f y) but the "a" type does not seem to be available in the definitions, it's scope seems to be limited, e.g. instance Functor ((,) a) where fmap f (x,y) = (x::a, f y) doesn't compile. But I'm just guessing here, don't know the details :-(
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