liftEff影響排式統一

Question

我有得到一個問題，下面來編譯：

module ContrivedExample where 

import Prelude 
import Data.Either (Either(..)) 
import Control.Monad.Eff (Eff) 
import Control.Monad.Eff.Exception (EXCEPTION, throw) 
import Control.Monad.Eff.Console (CONSOLE, log) 
import Control.Monad.Eff.Class (liftEff) 
import Control.Monad.Aff (launchAff) 
import Control.Monad.Aff.Console (log) as A 

contrivedExample :: forall e. Either String String -> Eff (exception :: EXCEPTION, console :: CONSOLE | e) Unit 
contrivedExample a = do 
     _ <- launchAff do 
     _ <- A.log "yay" 
     liftEff $ case a of 
      Left e -> log e 
      Right a -> throw a 
     pure unit 

我得到這個錯誤：

Could not match type 

    (console :: CONSOLE 
    | e3 
    ) 

    with type 

    (exception :: EXCEPTION 
    , console :: CONSOLE 
    | t2 
    ) 

如果我從影響行刪除異常，我得到在Either的另一端出現錯誤。有沒有更好的選擇liftEff或某種方式我可以統一類型？

Answer 1

按照launchAff的documentation：

Converts the asynchronous computation into a synchronous one. All values are ignored, and if the computation produces an error, it is thrown.

Catching exceptions by using catchException with the resulting Eff computation is not recommended, as exceptions may end up being thrown asynchronously, in which case they cannot be caught.

If you do need to handle exceptions, you can use runAff instead, or you can handle the exception within the Aff computation, using catchError (or any of the other mechanisms).

我相信你無法拋出異常裏面launchAff，除非你也追上他們相同的計算內。否則，您只能在launchAff計算內執行其他效果。