Basically, given {-# LANGUAGE PolymorphicKinds, ConstraintKinds, TypeFamilies #-} (and more, if necessary), does the (~) type-level operator work on type-level expressions of kind Constraint? I tried googling the answer, but had no luck.
8
votes
The question is weird, but asked well. Out of curiosity, what would you do with a constraint equality?
- Daniel Wagner
@Daniel Wagner I'm writing a cross-paradigm EDSL that uses a generic function system, and, for various reasons, I can't make use of type classes directly. I could get away with not using this, but it would lead to extremely clunky types and possibly some unnecessary restrictions
- Ptharien's Flame
1 Answers
6
votes
Yes, it is possible. Because types of kind Constraint are finite sets of atomic type constraints, you can test their equality very easily.
The PolyKinds extension is not necessary, however. Also, there's very few situations when this kind equality would actually be useful, because I don't see a practical way of passing polymorphic constraints as the arguments c1, c2 to Bla, so the constraint equality would be a tautology in every case (Show ~ Show here):
{-# LANGUAGE ConstraintKinds, TypeFamilies #-}
type Bla c1 c2 a = (c1 a, c2 a, c1 ~ c2)
foo :: Bla Show Show a => a -> IO ()
foo = print
main = foo "Bla"
