Prelude.catch vs. Exception.catch
Mon, 13 May 2002 22:07:11 -0700
At 2002-05-13 08:44, Simon Marlow wrote:
>Prelude.catch catches IO exceptions only, because this is what the
>Haskell report specifies.
>The idea is
>that if you want to use Exceptions in their full glory, you:
> import qualified Exception
I've noticed something a bit unusual about Exception.catch. It seems it
can't catch "return undefined" by itself. Consider these values of type
iouPure :: IO String;
iouPure = undefined;
iouError :: IO String;
iouError = error "error";
These aren't even an IO actions, they're simply bottom. Straightforward
enough. But they _will_ be caught by Exception.catch.
iouFail :: IO String;
iouFail = fail "failure";
iouEvaluate :: IOString;
iouEvaluate = Exception.evaluate undefined;
These two are IO actions that "fail" when executed. They will also be
caught by Exception.catch.
iouReturn :: IO String;
iouReturn = return undefined;
This one is an IO action that "succeeds" when executing. It _won't_ be
caught by Exception.catch, which will instead simply return the undefined
I'm not sure what to make of this...
Ashley Yakeley, Seattle WA