Equality constraints in Constraint kinds
My question is about how to put an equality constraint in an associated type constraint (ie type of kind Constraint)
The specific use case is a class parameterized by a partially applied type:
class Foo c where -- c has kind *->*->*
type Ctx c m r :: Constraint
f :: (Ctx c m r) => c m r -> c m r
In a particular instance, I would like to write:
data Bar m r = ...
instance Foo Bar where
type Ctx Bar m r = (m~Maybe b)
-- m must be a Maybe, I don't care what its parameter is
f = ...
However, GHC complains: 'Not in scope: type variable b'. I don't see any other way to express this constraint. Not every instance requires that 'm~Maybe b', so I cannot move this constraint to the type signature of f. b is not in scope in the instance anywhere (perhaps this is what GHC is complaining about), but there is no need for it to be. The function
f :: (a ~ Maybe b) => a -> a -> a
is valid, I see no reason why I can't do this with Constraints. This problem occurs with top-level constraints, as well as with associated type constraints.
Possibly related is this question, except I need equality with a variable NOT in scope.
Here is another way to express this constraint:
class IsMaybe m {- where ...whatever operations you need to do on Maybe values -}
instance IsMaybe (Maybe b) {- where ...implement those operations -}
instance Foo Bar where
type Ctx Bar m r = IsMaybe m
However, given Haskell naming conventions, I would be kind of surprised if this is actually what you want -- is m really a type variable of kind * and not of kind * -> * ? In case it's the latter, you'd just want
instance Foo Bar where
type Ctx Bar m r = m~Maybe
...and possibly a kind annotation in the class declaration for Foo .
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