如何在`Yesod Handler'中捕捉異常？

Question

當我嘗試：

foos <- (return $ map (encode .> cs .> jsonToFoo body) `catch` r400 

我有以下類型的錯誤：

/path/to/File.hs:47:78: error: 
    • Couldn't match type ‘(->) e0’ with ‘IO’ 
     arising from a functional dependency between: 
      constraint ‘MonadBase IO ((->) e0)’ arising from a use of ‘r400’ 
      instance ‘MonadBase ((->) r) ((->) r)’ at <no location info> 
    • In the second argument of ‘catch’, namely ‘r400’ 
     In a stmt of a 'do' block: 
     foos <- (return $ map (encode .> cs .> jsonToFoo) body) 
       `catch` r400 
     In the expression: 
     do { (body :: [Value]) <- requireJsonBody; 
      foos <- (return $ map (encode .> cs .> jsonToFoo) body) 
        `catch` r400; 
      .... } 

如果有一種方法，它涉及liftIO，不是嗎？

Answer 1

我只是缺少論據catch：

foos <- (return $ map (encode .> cs .> jsonToFoo body) 
    `catch` (\(e :: SomeException) -> r400) 

太糟糕了，這不是對我來說更明顯的錯誤類型。