Why Data.Set (containers) doesn't have Functor and Monad instance?

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You could do it like this:

{-# LANGUAGE ConstraintKinds, TypeFamilies, FlexibleInstances #-}

import qualified Data.Set as Set
import Data.Set (Set)
import qualified Data.List as List

import Data.Kind

class Mappable m where
  type C m a :: Constraint
  type C m a = () -- no constraint by default
  mmap :: (C m a, C m b) => (a -> b) -> m a -> m b

instance Mappable Set where
  type C Set a = Ord a -- overwrite default with the Ord constraint
  mmap = Set.map

instance Mappable [] where
  mmap = List.map

@rae also did a talk about partial type constructors which is another stab at this problem using constraints (solving the constrained functor problem is actually more of a useful side effect of solving a more general problem in that talk).

I can’t tell you anything about the usability of either approach, I don’t have experience with them.

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