ski.hs

-- Sxyz -> xz yz
-- Kxy  -> x
-- Ix   -> x

data Combinator = S | K | I | E 
                deriving (Show, Eq)

char2ski :: Char -> Combinator
char2ski c = case c of
  'S' -> S
  'K' -> K
  'I' -> I
  _   -> E

ski2char :: Combinator -> Char
ski2char c = case c of
  S -> 'S'
  K -> 'K'
  I -> 'I'
  _ -> 'E'

s x y z = x z (y z)
k x y = x
i x = x

isempty :: [a] -> Bool
isempty [] = True
isempty _  = False

list :: a -> [a]
list a = [a]

myeval :: [Combinator] -> [Combinator]
myeval [] = []
myeval (com:coms) =
  if isempty coms then [com]
  else
    case com of
      S -> do x <- list $ head coms
              y <- list $ head $ tail coms
              z <- tail $ tail coms
              x ++ z ++ y ++ z
      K -> list $ head coms
      I -> coms

str2ski :: String -> [Combinator]
str2ski [] = []
str2ski (c:cs) = char2ski c : str2ski cs

ski2str :: [Combinator] -> String
ski2str [] = []
ski2str (c:cs) = ski2char c : ski2str cs

main :: IO ()
main = do cs <- getContents
          putStrLn $ ski2str $ myeval $ str2ski cs