一起使用Maybe和Writer(Using Maybe and Writer together)

 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
 
 似乎工作得很好，我可以很好地链接add功能。 
 
 但我想记录每一步添加多少鸡蛋的日志。 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，我似乎失去了像我之前做的那样链接add函数的能力： 
 
let c = pure(Carton 2) >>= add 4 -- works
let c = pure(Carton 2) >>= add 4 >>= add 2 -- does not work
 
 我怎样才能链接我的新作家功能的add功能？ 有没有更好的方法来做到这一点？ 

Here is my egg packing factory:
 
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
 
Seems to work well, I can nicely chain add functions.
 
But I want to record a log of how many eggs were added at every step. So I do this:
 
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
 
This gives me what I want: I can see the resulting carton and the record for 4 eggs being added.
 
But I seem to have lost the ability to chain add functions like I did before:
 
let c = pure(Carton 2) >>= add 4 -- works
let c = pure(Carton 2) >>= add 4 >>= add 2 -- does not work
 
How can I chain my new writer-enabled add functions? Is there a better way of doing this?

原文：https://stackoverflow.com/questions/38194378