    {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
    module Numeric.Algebra.Hopf
      ( HopfAlgebra(..)
      ) where

    import Numeric.Algebra.Unital

    -- | A HopfAlgebra algebra on a semiring, where the module is free.
    --
    -- When @antipode . antipode = id@ and antipode is an antihomomorphism then we are an InvolutiveBialgebra with @inv = antipode@ as well

    class Bialgebra r h => HopfAlgebra r h where
      -- > convolve id antipode = convolve antipode id = unit . counit
      antipode :: (h -> r) -> h -> r

    -- incoherent
    -- instance (UnitalAlgebra r a, HopfAlgebra r h) => HopfAlgebra (a -> r) h where antipode f h a = antipode (`f` a) h
    -- instance HopfAlgebra () h where antipode = id

    -- TODO: check this
    -- instance InvolutiveSemiring r => HopfAlgebra r () where antipode = adjoint

    instance (HopfAlgebra r a, HopfAlgebra r b) => HopfAlgebra r (a, b) where
      antipode f (a,b) = antipode (\a' -> antipode (\b' -> f (a',b')) b) a

    instance (HopfAlgebra r a, HopfAlgebra r b, HopfAlgebra r c) => HopfAlgebra r (a, b, c) where
      antipode f (a,b,c) = antipode (\a' -> antipode (\b' -> antipode (\c' -> f (a',b',c')) c) b) a

    instance (HopfAlgebra r a, HopfAlgebra r b, HopfAlgebra r c, HopfAlgebra r d) => HopfAlgebra r (a, b, c, d) where
      antipode f (a,b,c,d) = antipode (\a' -> antipode (\b' -> antipode (\c' -> antipode (\d' -> f (a',b',c',d')) d) c) b) a

    instance (HopfAlgebra r a, HopfAlgebra r b, HopfAlgebra r c, HopfAlgebra r d, HopfAlgebra r e) => HopfAlgebra r (a, b, c, d, e) where
      antipode f (a,b,c,d,e) = antipode (\a' -> antipode (\b' -> antipode (\c' -> antipode (\d' -> antipode (\e' -> f (a',b',c',d',e')) e) d) c) b) a