Question

我试图逐项排除因阅读功能造成的例外情况:

run :: CurrentData -> IO ()
run current = do
{
    x <- (getCommandFromUser) `E.catch` handler;
    updated <- executeCommand x current;
    run updated;    
} where handler :: E.IOException -> IO Command
    handler e = do putStrLn "wrong command format" >> return (DoNothing);

In this code function getCommandfrom user gets some string from user and then tries to read some data from this string using "read" function

If read fails there is exception thrown:

*** Exception : prelude.read : no parse

and program exits... I can t catch this exception - what is type of this exception???

我还尝试了E.SomeException,而不是E.IOException...。

E is from import Control.Exception As E

Answer 1

"what is type of this exception?" The type is ErrorCall, also available from Control.Exception. An ErrorCall is what is thrown when the error function is called. Just change the type of handler and it will work. A last resort to get things working is to catch E.SomeException, but that s almost always the wrong thing to do.

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