#!/bin/runghc

import qualified Data.ByteString.Lazy as L
import qualified Data.ByteString.Lazy.Char8 as L8
import Data.Char (isSpace)

-- let's see if we can write a parser for this silly string
sample = L8.pack "123 789"

-- the results will go here
data Duo = Duo {
    alpha :: Int,
    beta :: Int
}

instance Show Duo where
    show (Duo a b ) = "Duo " ++ show a ++ " and " ++ show b

-- parse a number
pInt :: L.ByteString -> Maybe (Int, L.ByteString)
pInt s = case L8.readInt s of
    Nothing -> Nothing
    Just (num,rem) -> Just (num,rem)

-- skip white space
spaces :: L.ByteString -> Maybe (Int, L.ByteString)
spaces s
    | isSpace $ L8.head s = Just (0, L8.dropWhile isSpace s)
    | otherwise = Nothing

parse :: L.ByteString -> Maybe (Duo)
parse arg =
        pInt arg >>=
        \(n1,s1) -> spaces s1 >>=
        \(_,s2) -> pInt s2 >>=
        \(n2,_) -> Just (Duo n1 n2)

wrap :: L.ByteString -> String
wrap arg = case parse arg of
        Nothing -> "trouble"
        Just x -> show x

main = do
        let rv = wrap sample
        putStrLn rv

--  THE END