> >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
> "IO String":
>
> 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
> value.
>
> I'm not sure what to make of this...
This is just an artifact of the laziness of 'return'. One of the monad
laws is
return a >>= k = k a
so therefore
return undefined >>= (\_ -> m) = m
so return can't be strict. Since return isn't strict,
return undefined `seq` a = a
So that's really why we have Exception.evaluate: it's a strict version
of return.
Cheers,
Simon
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