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Re: ‘with-exception-handler’ rationale

From: Mark H Weaver
Subject: Re: ‘with-exception-handler’ rationale
Date: Sun, 09 Mar 2014 15:47:37 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Nikita Karetnikov <address@hidden> 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.


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