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