stdin.txt

{-# LANGUAGE TypeFamilies #-}

type family StateFor a
type instance StateFor Integer = ()
type instance StateFor (a, b) = (StateFor a, StateFor b)
type instance StateFor [a] = (Integer, [StateFor a])

class ReadObj a where
  readObj :: StateFor a -> [[String]] -> (a, [[String]])

consNE :: [a] -> [[a]] -> [[a]]
consNE [] t = t
consNE h t = h:t

-- grab an integer off of the stream
instance ReadObj Integer where
  readObj _ ((h:t):t') = (read h, consNE t t')

-- read a pair of objects, make sure they are on the same line.
instance (ReadObj a, ReadObj b) => ReadObj (a,b) where
  readObj (pa,pb) (h:rest) = let (a,[t]) = readObj pa [h]
                                 (b,[t']) = readObj pb [t] in
                             ((a,b),consNE t' rest)

-- should assume that the entire list is going to be on one line
instance ReadObj a => ReadObj [a] where
  readObj (0, _) l =  ([], l)
  readObj  (n, (ph:pt)) l =
    let (retH, listTail) = (readObj) (ph) l
        (retT, realTail) = readObj (n-1, pt) listTail in
    (retH:retT, realTail)