{-# LANGUAGE
    GADTs
  , KindSignatures
  , DataKinds
  , TypeOperators
  #-}

module Data.ConstrainedList where

import Data.Foldable (Foldable())
import Data.Traversable (Traversable())

data FoldableList (fs :: [* -> *]) where
    FNil :: FoldableList '[]
    FCons :: (Functor f, Foldable f) => FoldableList fs -> FoldableList (f ': fs)

class Foldables (fs :: [* -> *]) where
    frep :: FoldableList fs

instance Foldables '[] where
    frep = FNil

instance (Functor f, Foldable f, Foldables fs) => Foldables (f ': fs) where
    frep = FCons frep


data TraversableList (fs :: [* -> *]) where
    TNil :: TraversableList '[]
    TCons :: Traversable f => TraversableList fs -> TraversableList (f ': fs)

class Traversables (fs :: [* -> *]) where
    trep :: TraversableList fs

instance Traversables '[] where
    trep = TNil

instance (Traversable f, Traversables fs) => Traversables (f ': fs) where
    trep = TCons trep