content: {-# LANGUAGE DataKinds          #-}
  {-# LANGUAGE DefaultSignatures  #-}
  {-# LANGUAGE DeriveAnyClass     #-}
  {-# LANGUAGE DeriveGeneric      #-}
  {-# LANGUAGE FlexibleContexts   #-}
  {-# LANGUAGE MonoLocalBinds     #-}
  {-# LANGUAGE StandaloneDeriving #-}
  {-# LANGUAGE TypeApplications   #-}
  
  module Main (main) where
  
  import Optics.Core
  import           Control.Monad (void)
  import Data.Generics.Product
  import GHC.Generics
  import Test.HUnit
  
  main :: IO ()
  main = void $ runTestTT $
    bimap (* 2) show mytree ~=? mytreeBimapped
  
  data Tree a w = Leaf a
                | Fork (Tree a w) (Tree a w)
                | WithWeight (Tree a w) w
         deriving (Show, Eq, Generic)
  
  instance Bifunctor Tree where
    bimap = gbimap
  
  mytree :: Tree Int Int
  mytree = Fork (WithWeight (Leaf 42) 1)
                (WithWeight (Fork (Leaf 88) (Leaf 37)) 2)
  
  mytreeBimapped :: Tree Int String
  mytreeBimapped = Fork (WithWeight (Leaf 84) "1")
                        (WithWeight (Fork (Leaf 176) (Leaf 74)) "2")
  
  --------------------------------------------------------------------------------
  
  class Bifunctor p where
    bimap :: (a -> c) -> (b -> d) -> p a b -> p c d
  
  gbimap ::
      ( HasParam 0 (p a b) (p a d) b d
      , HasParam 1 (p a d) (p c d) a c
      ) => (a -> c) -> (b -> d) -> p a b -> p c d
  gbimap f g s = s & param @0 %~ g & param @1 %~ f
  