5_xx.hs

5_xx.hs

data Queue a = DeQue [a] [a] deriving (Show)

head' :: Queue a -> a
head' (DeQue (x:_) _) = x

checkf :: [a] -> [a] -> Queue a
checkf [] r = DeQue (reverse r) []
checkf l r = DeQue l r

tail' :: Queue a -> Queue a
tail' (DeQue (_:ls) r) = checkf ls r

snoc' :: Queue a -> a -> Queue a
snoc' (DeQue l r) x = checkf l (x : r)

-- balanceみたいなのを書く

main :: IO()
main = do
  let q = DeQue [1, 2, 3] [6, 5, 4]
  print q
  print "---------"
  print (head' q)

  print "---------"
  print (tail' q)

  print "---------"
  print (snoc' q 10)