niuk

data Op = Plus | Minus | Concat deriving Show

f l = concatMap (\r -> map (: r) l)

step :: Int -> Op -> (Int, Int) -> (Int, Int)
step digit op (carry, sum) = case op of
    Concat -> (digit * 10 ^ ceiling (logBase 10 (fromIntegral carry)) + carry, sum)
    Plus -> (digit, sum + carry)
    Minus -> (digit, sum - carry)