quiz.hs

-- Haskell Quiz http://blog.hackers-cafe.net/2010/06/haskell-quiz.html の答えっぽいもの
-- Preludeの$とコンフリクトするので隠す
import Prelude hiding (($))
-- Showのような独自型クラスMyShowを定義
class MyShow a where
  myShow :: a -> String
instance MyShow Bool where
  myShow arg = "True"
instance MyShow Int where
  myShow arg = show arg

infixr 0 $
-- $を再定義してないとは書いてない
($) :: (MyShow a) => (String -> IO()) -> a -> IO()
f $ x = putStrLn (myShow x)

a :: Int
a = 0
b :: Int
b = 1
c :: Int
c = 0
d :: Int
d = 0

main = do
  -- ここから
  print $ a + c == 0 -- True.
  print $ a == c     -- True. a == c == 0.
  print $ c == 0     -- True. Of course.
  print $ a + d == 1 -- True. d == 1.
  print $ b + c == 1 -- True. b == 1.
  print $ b + d == 0 -- True. What happened?!
  print $ b == d     -- True. ?!
  print $ d == 0     -- True. ?!?!
  -- ここまではある意味ダミー？
  -- hints to make answer unique (perhaps...)
  -- 実は以下の部分だけで答えが[a, b, c, d] = [0, 1, 0, 0] に確定する
  print $ sum([a, b, c])  -- 1
  print $ sum([a, b, d])  -- 1
  print $ [a, b, c] !! a  -- 0
  print $ [a, b, c] !! b  -- 1
  print $ [a, b, c] !! c  -- 0