Why is hmatrix Matrix not an instance of 'functor'?

[theobat]

A relevant discussion is here:

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

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 …

In particular this solution (Why Data.Set (containers) doesn't have Functor and Monad instance? - #19 by jaror) is, I believe, something that could be in base but … who knows