Nikita Karetnikov <nik...@karetnikov.org> writes:
> I’ve expected ‘with-exception-handler’ to behave like ‘catch’, but it
> doesn’t.
>
> scheme@(guile-user)> ,use (srfi srfi-34)
> scheme@(guile-user)> (with-exception-handler (const "got it") (lambda ()
> (raise "boom!")))
> ice-9/boot-9.scm:106:20: In procedure #<procedure 94b0e40 at
> ice-9/boot-9.scm:97:6 (thrown-k . args)>:
> ice-9/boot-9.scm:106:20: Throw to key `srfi-34' with args `("boom!")'.
>
> Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
> scheme@(guile-user) [1]>
> scheme@(guile-user)> (catch 'misc-error (lambda () (error "boom!")) (const
> "got it"))
> $1 = "got it"
>
> Is there an explanation that doesn’t involve diving into the land of
> continuations and dynamic wind? I mean, when should I use the one but
> not the other?
Neil's response was exactly correct, but it used some terminology that
might be difficult for non-experts to understand, so I'm going to try my
hand at a simpler explanation.
When an exception is caught, there are two things that typically need to
be done:
1. Run the user-specified handler.
2. Do a non-local exit back to the catch/guard/with-exception-handler,
returning the value produced by the handler.
'catch' and 'guard' handle both of these steps for you.
'with-exception-handler' is a more powerful but lower-level mechanism.
It handles step 1, but leaves step 2 to you. If you use it, it is your
responsibility to handle the non-local exit yourself, typically by
invoking a continuation. That's what the 'call/cc' and the call to 'k'
was about in Neil's response.
Having said all this, I think the shorter answer is that you should
probably be using 'guard' instead.
Regards,
Mark
