Examples of multiple constraints? about barbies

[jcpetruzza]

IMO it would also be completely reasonable to actually define and export it in barbies itself if the language extensions aren't too problematic.

Done in 2.0.5.0!

[jcpetruzza]

Good question. I think one could use a higher-kinded version of the (&) class in the constraints package. Something like:

{-# LANGUAGE ConstraintKinds, UndecidableSuperClasses #-}
module Foo where

import GHC.Generics
import Barbies

class (c a, d a) => (c & d) a where
instance (c a, d a) => (c & d) a where

blah :: (Eq a, Show a, Enum a) => Maybe a -> [a]
blah = maybe [] pure

convert :: Foo Maybe -> Foo []
convert = bmapC @(Eq & Show & Enum) blah

data Foo f = Foo (f Int)
  deriving Generic
  deriving (FunctorB, ConstraintsB)

deriving instance AllBF Show f Foo => Show (Foo f)

Would that work for you?

[MichaelXavier]

That worked great, thank you! I had tried the one from the constraints package but it wasn't clicking with me that I needed a fully applied version. It may be worth adding this somewhere in the docs and mentioning that you can use it in the constraint-oriented parts of the barbies API. IMO it would also be completely reasonable to actually define and export it in barbies itself if the language extensions aren't too problematic.