[Haskell-cafe] Problem with a sample from RWH

[Ivan Lazar Miljenovic]

On 30 September 2010 16:46, C K Kashyap <ckkashyap at gmail.com> wrote:
>
> Could you please review the change I've done to Don Stewart's
> scripting example -
>
> run s = handle (fail . show) $ do
>    (ih,oh,eh,pid) <- runInteractiveCommand s
>    so <- hGetContents oh
>    se <- hGetContents eh
>    hClose ih
>    ex <- waitForProcess pid
>    case ex of
>        ExitFailure e      -> fail $ "Failed with status: " ++ show e
>        _  | not (null se) -> fail se
>           | otherwise     -> return so
>
>
> My change -
>
> run s = handle (\e@(SomeException{}) -> return (show e)) $ do
>    (ih,oh,eh,pid) <- runInteractiveCommand s
>    so <- hGetContents oh
>    se <- hGetContents eh
>    hClose ih
>    ex <- waitForProcess pid
>    case ex of
>        ExitFailure e      -> fail $ "Failed with status: " ++ show e
>        _  | not (null se) -> fail se
>           | otherwise     -> return so
>

The change looks good; note, however, that you're doing something
subtly different if an error occurs: in the original, if an error
occured then "fail" was used to forcibly terminate the program.  Your
variant however returns the shown error message.  Depending on the
situation, this is usually a bad thing (better off to use Maybe or
Either).

Also, here's the official rationale about why SomeException is not
recommended: http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/Control-Exception.html#4
.  My main summation of this is that it even catches when you try to
use Ctrl-c to kill a program.

-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
IvanMiljenovic.wordpress.com