RFC: Type-class type signatures (:: Constraint)

Data types can be defined:

data A a b
-- or
data A (a :: Bool) (b :: Type)
-- or
data A (a :: Bool) (b :: Type) :: Type
-- or
data A :: Bool -> Type -> Type

data B :: Type

while type classes can't:

class A a b
-- or
class A (a :: Bool) (b :: Type)
-- but not
-- class A (a :: Bool) (b :: Type) :: Constraint
-- nor
-- class A :: Bool -> Type -> Constraint

-- class B :: Constraint

I foresee problems with how to bind a and b, but adding a final :: Constraint should be legal:

class A a b :: Constraint

class A (a :: Bool) (b :: Type) :: Constraint

class B :: Constraint

Trac metadata

Trac field	Value
Version	8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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