haskell - 一起使用 Maybe 和 Writer

Question

这是我的鸡蛋包装厂:

type Eggs = Int
data Carton = Carton Eggs deriving Show

add :: Eggs -> Carton -> Maybe Carton
add e (Carton c)
    | c + e <= 12 = Just (Carton $ c + e)
    | otherwise = Nothing

main = do
    print $ pure(Carton 2) >>= add 4 >>= add 4 >>= add 3

似乎运作良好，我可以很好地链接 add职能。

但是我想记录每一步添加了多少鸡蛋的日志。所以我这样做:

import Control.Monad.Writer

type Eggs = Int
data Carton = Carton Eggs deriving Show

add :: Eggs -> Carton -> Writer [String] (Maybe Carton)
add e (Carton c)
    | c + e <= 12 = do
        tell ["adding " ++ show e]
        return (Just (Carton $ c + e))
    | otherwise = do
        tell ["cannot add " ++ show e]
        return Nothing

main = do
    let c = add 4 $ Carton 2
    print $ fst $ runWriter c
    mapM_ putStrLn $ snd $ runWriter c

这给了我想要的东西:我可以看到生成的纸箱和添加 4 个鸡蛋的记录。

但是我好像失去了上链的能力add像我以前一样的功能:

let c = pure(Carton 2) >>= add 4 -- works
let c = pure(Carton 2) >>= add 4 >>= add 2 -- does not work

如何链接我的新启用作家的 add职能？有没有更好的方法来做到这一点？

Answer 1

只需撰写 add与 MaybeT :

import Control.Trans.Monad.Maybe

test = pure (Carton 2) >>= MaybeT . add 3
                       >>= MaybeT . add 4
                       >>= MaybeT . add 5

runTest = do
  print $ fst $ runWriter (runMaybeT test)

完整示例位于:http://lpaste.net/169070