I'd like to make synonyms for a class constraint like the following (very much simplified) example
{-# LANGUAGE ConstraintKinds #-}
type Foo = Functor
data MyFunctor a = MyFunctor
instance Foo MyFunctor where
fmap _ _ = MyFunctor
but I get fmap is not a (visible) method of class Foo.
It seems like I should be able to do this because the GHC documentation says "... standard constraints, tuples and type synonyms for those two sorts of constraint are permitted in instance contexts and superclasses". I'm on GHC 7.6.
Your case isn't covered by those statements in the GHC documentation. An instance context using Foo is something like:
instance Foo a => Bar a
A superclass would be something like:
class Foo a => Bar a
i.e. you're defining some other class or instance and Foo/Functor is one of the requirements. Here you're actually trying to define Functor itself and you have to use the real class name.
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