如何在ExceptT和List中使用遍歷，只有在所有項都是Left時才返回Left？

Question

例如，我有這個功能

foo :: Int -> ExceptT String (State Int) Int 
foo x = if x == 0 then throwError "x == 0" else return x 

，如果我用traverse

evalState (runExceptT $ traverse foo [0,1,2]) 0 

返回

Left "x == 0" 

但我想Right [1,2] 和它應該返回Left只有當所有列表項目爲零

Answer 1

這是一個關於traverse的問題？或者您僅是想收集foo的結果，此投票不引起異常？

後者的直接方式是在「try-catch」塊中運行foo的每個調用，該塊返回Maybe Int，然後返回列表。

例子：

import Data.Maybe 
import Control.Monad 
import Control.Monad.Except 
import Control.Monad.State 

foo :: Int -> ExceptT String (State Int) Int 
foo x = if x == 0 then throwError "x == 0" else return x 

orNothing p = catchError (fmap return p) (\e -> return Nothing) 

collectFoos xs = do ys <- fmap catMaybes $ mapM (orNothing . foo) xs 
        case ys of 
         [] -> throwError "no successful call to foo" 
         _ -> return ys 

doit xs = runState (runExceptT (collectFoos xs)) 100