gistfile1.hs

{-# LANGUAGE RankNTypes #-}
module Main where

type Bool' = forall a. a -> a -> a

true :: Bool'
true = const

false :: Bool'
false = const id

class Eq' a where
  (.==) :: a -> a -> Bool'

data Foo = Hoge | Piyo | Fuga deriving Show

instance Eq' Foo where
  Hoge .== Hoge = true
  Piyo .== Piyo = true
  Fuga .== Fuga = true
  _ .== _ = false

--それっぽく書くための小細工
_if :: a -> a
_if = id

main :: IO ()
main = do
  putStrLn $ _if (Hoge .== Hoge) "○" "✕"
  putStrLn $ _if (Piyo .== Fuga) "○" "✕"
  putStrLn $ _if (Fuga .== Fuga) "○" "✕"

{- 実行結果
○
✕
○
-}