[Haskell-cafe] Re: Haskell stacktrace
wren ng thornton
wren at freegeek.org
Wed Sep 10 01:37:08 EDT 2008
Pieter Laeremans wrote:
> This :
> Prelude> let f = (\x -> return "something went wrong") :: IOError -> IO
> Prelude> let t = return $ show $ "too short list" !! 100 :: IO String
> Prelude> catch t f
> "*** Exception: Prelude.(!!): index too large
> doesn't work.
As others've said, the right answer is to correct the bug rather than
doing exception handling, but in as far as knowing how to do exception
handling for "pure" functions, consider:
The trickiest thing to watch out for is that you must strictly evaluate
the expression or else the return/evaluate function will just lazily
thunk up the expression. Which means that when you finally run the IO,
you'll have already stripped off the IO wrapper that can catch the
exception prior to evaluating the expression (which then throws an
exception out past the catcher).
Another thing to watch out for is that Prelude.catch doesn't do what you
want because the H98 spec declares these exceptions to be uncatchable.
The Control.Exception.catch function does what you want and is portable
even though it's not H98.
If you're doing the unsafePerformIO trick to purify your exception
handling be sure to give a NOINLINE pragma to prevent potentially buggy
inlining. You should also be sure that the handler is something which is
actually safe to unsafePerformIO.
Finally, to ensure that other optimizations or evaluation orders don't
accidentally mess you up, you should take the higher-order approach of
`safely` to ensure that you don't accidentally apply the function prior
to wrapping it up in a catcher.
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