加載錯誤時怎么做無法匹配預期類型`IO（可能是字符串）'

Question

module REPL(REPL(..), repl) where
import qualified Control.Exception as E
import System.Console.Readline(readline, addHistory)

data REPL s = REPL {
    repl_init :: IO (String, s),        -- prompt and initial state
    repl_eval :: s -> String -> IO (Bool, s),       -- quit flag and new state
    repl_exit :: s -> IO ()
    }

repl :: REPL s -> IO ()
repl p = do
    (prompt, state) <- repl_init p
    let loop s = (do
        mline <- readline prompt
        case mline of
        Nothing -> loop s
        Just line -> do
            (quit, s') <- repl_eval p s line
            if quit then
            repl_exit p s'
             else do
            addHistory line
            loop s'
        ) E.catch undefined (\(e :: E.SomeException) -> putStrLn "Handled exception!"
        )
    loop state

輸出：

REPL.hs:21:5:
    Couldn't match expected type `IO (Maybe String)'
           against inferred type `t -> Maybe String'
    In a stmt of a 'do' expression: mline <- readline prompt
    In the expression:
        (do { mline <- readline prompt;
              case mline of {
                Nothing -> loop s
                Just line
                  -> do { (quit, s') <- repl_eval p s line;
                          .... } } })
          E.catch
          undefined
          (\ (e :: E.SomeException) -> putStrLn "Handled exception!")
    In the definition of `loop':
        loop s = (do { mline <- readline prompt;
                       case mline of {
                         Nothing -> loop s
                         Just line -> do { ... } } })
                   E.catch
                   undefined
                   (\ (e :: E.SomeException) -> putStrLn "Handled exception!")

Answer 1

自上一個問題以來，你已經刪除了反引號，`，圍繞你的E.catch語句。 我對這個問題的回答沒有使用反引號，因為我沒有調用catch綴。 你可以使用catch post-fix而不需要反引號：

let loop s = E.catch (do ...) (\(e :: E.SomeException) -> ...)

或者你可以使用catch infix和反引號：

let loop s = (do ...) `E.catch` (\(e :: E.SomeException) -> ...)

另外， undefined是我只是為了GHCi REPL上的一個例子而強制異常 - 它取代了代碼中的(do ..)語句。