1HaskellADay: collectReverse

collectReverse.hs

import Control.Applicative

{- We still have yesterday's room
-}
data Room a = Room {doors :: (Bool, Bool), content :: a}
  deriving (Eq)

{- | Given a list of rooms, we want to go as far as we can and collect the elements
  in he room, **starting from the last room**.

  Examples:

  >>> collectReverse [Room (True, True) 1, Room (True, False) 2, Room (True, False) 3]
  []

  >>> collectReverse [Room (False, True) 1, Room (True, False) 2, Room (True, True) 3]
  [3]

  >>> collectReverse  [Room (True, True) 1, Room (False, True) 2, Room (True, True) 3]
  [3,2]
-}

collectWith :: ((Bool, Bool) -> Bool) -> [Room a] -> [a]
collectWith p = map content . uncurry (++) . (fmap $ filter (p . doors) . take 1) . span (uncurry (&&) . doors)

collect :: [Room a] -> [a]
collect = collectWith fst

collectReverse :: [Room a] -> [a]
collectReverse = collectWith snd . reverse