ctfp-14.3.6-wrong-solution

ctfp-14.3.6-wrong-solution.hs

{-# LANGUAGE DeriveFunctor, TypeFamilies #-}

module Main where

data Pair a = Pair a a
  deriving (Eq)

getFst :: Pair a -> a
getFst (Pair x1 _) = x1

getSnd :: Pair a -> a
getSnd (Pair _ x2) = x2

class Representable f where
  type Rep f :: *
  tabulate :: (Rep f -> x) -> f x
  index :: f x -> Rep f -> x

instance Representable Pair where
  type Rep Pair = ()
  tabulate f = Pair (f ()) (f ()) -- fmap f Pair((),())
  index (Pair x1 x2) = \u -> x1

-- index (tabulate f)
f :: () -> String
f x = "AAA"

tabulatef :: Pair String
tabulatef = tabulate f

indextabulatef :: () -> String
indextabulatef = index tabulatef

check1 = indextabulatef () == "AAA"

-- tabulate (index Pair x1 x2)
pair  :: Pair String
pair = Pair "A" "B"

indexpair :: ()->String
indexpair = index pair

tabulateindexpair :: Pair String
tabulateindexpair = tabulate indexpair

check2 = tabulateindexpair == Pair "A" "B"

main = do
  putStrLn "test"
  putStrLn (show check1)
  putStrLn (show check2)
  putStrLn (show (getFst tabulateindexpair)) 
  putStrLn (show (getSnd tabulateindexpair)) 
  
--GHCi, version 8.6.5
   
--test
--True
--False
--"A"
--"A"