out.hs

-- Monad の (>>=) に相当
comb m n = let (a, log1) = m
               (b, log2) = n a
           in  (b, log1 ++ log2)

-- Monad の (>>) に相当
comb_ m n = m `comb` \x -> n

-- Monad の return に相当
ret x = (x, "")

-- 出力のための関数
out x    = ((), show x ++ ",")
outStr x = ((), x)

main = print $ outStr "begin: " `comb_`
               ret (1 + 2)      `comb`  \a -> 
               out a            `comb_`
               ret (a * 3)      `comb`  \b ->
               out b            `comb_` 
               ret (b - 4)      `comb`  \c ->
               out c            `comb_`
               outStr " :end"   `comb_`
               ret c