约束以查看一种类型是否“满足”另一种类型

Question

Is there a way to write a constraint:

C t1 t2 :: Constraint

Such that it "satisfies" (is that the right word) if t2 matches against t1 .

For example:

C (forall a. Num a => a -> a -> a) (forall a. Num a => a -> a -> a) -- good
C (forall a. Num a => a -> a -> a) (Int -> Int -> Int) -- good
C (forall a. Num a => a -> a -> a) (Bool -> Bool -> Bool) -- fails
C (Int -> Int -> Int) (forall a. Num a => a -> a -> a) -- fails

Answer 1

The basic answer is no. Haskell's type system is predicative : a type variable can only be instantiated to a monotype. So if you had

class C t u

or

type family C t u

the t and u simply can't be instantiated to the polymorphic types you want to talk about.