donn:
>
> On Mar 8, 2008, at 10:54 PM, Don Stewart wrote:
>
> [... replying to my poorly informed rant about exceptions ... ]
>
> >
> >I don't understand this complaint -- you can handle all these with
> >Control.Exception.
> >
> >xmonad catches all these things for example, in user code, to prevent
> >poorly written modules throwing a pattern match exception, or calling
> >'error' and making the window manager unstable.
> >
> >Handling exceptions generated from pure code is just another part of
> >making systems more robust -- and of course you can do it in Haskell.
>
> OK, I tried this out and found that it does work, and I thought to
> myself,
> `no more posting rants to haskell-cafe after late nights out with too
> much
> wine!' But then I changed my test error from a pattern match, to a
> `head',
> and that gets past my exception handler:
>
> module Main (main) where
> import System (getArgs)
>
> ax = getArgs >>= print . head
>
> px = catch ax (\ e -> putStrLn ("caught this one: " ++ show e))
>
> main = px
>
> Is there a way to catch it, that I'm missing? What is the essential
> difference between these errors?
That's the difference between Prelude.catch and Control.Exception.catch.
You almost always want Control.Exception.catch.
Prelude.catch:
$ runhaskell A.hs
"A.hs: Prelude.head: empty list
Control.Exception.catch
$ runhaskell A.hs
"caught this one: Prelude.head: empty list
As the docs for Control.Exception say:
-- Note that 'catch' catches all types of exceptions, and is generally
-- used for \"cleaning up\" before passing on the exception using
-- 'throwIO'.
-- Also note that the "Prelude" also exports a function called
-- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
-- except that the "Prelude" version only catches the IO and user
-- families of exceptions (as required by Haskell 98).
--
-- We recommend either hiding the "Prelude" version of 'Prelude.catch'
-- when importing "Control.Exception"
There's a number of other useful exception handlers:
handle
finally
bracket
All useful, all have their place.
-- Don
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