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Re: lisp unwind-protect question


From: Kevin Rodgers
Subject: Re: lisp unwind-protect question
Date: Wed, 06 Mar 2002 11:12:49 -0700

address@hidden wrote:
> 
> The doc string for unwind-protect says:
> 
>     Do BODYFORM, protecting with UNWINDFORMS.
>     Usage looks like (unwind-protect BODYFORM UNWINDFORMS...).
>     If BODYFORM completes normally, its value is returned
>     after executing the UNWINDFORMS.
>     If BODYFORM exits nonlocally, the UNWINDFORMS are executed anyway.

Yeah, the usage would be better described as

(unwind-protect PROTECTED-FORM UNWIND-FORM ...) or
(unwind-protect PROTECTED-FORM UNWIND-FORM-1 ... UNWIND-FORM-N)

since the unwind forms comprise an implicit progn.

> So if you say
> 
>   (unwind-protect
>       (foo)
>     (bar)
>     (baz))
> 
> What's supposed to happen if (bar) signals an error?  Does (baz) still
> get run?

No, only the BODYFORM (foo) is protected.

> What if bar doesn't signal an error by itself, but some asynchronous
> signal arrives (e.g. user hits C-g) while bar is running?

Same thing: evaluation is terminated.

> How do Lisp systems in general (not just Emacs Lisp) deal with this
> situation?  I guess it's easy for me to check Emacs Lisp's behavior by
> experiment, but it's the general question about other Lisps that
> interests me most.

Common Lisp defines the unwind-protect special form with the same syntax
and semantics as the Emacs Lisp built-in.

> I hope this isn't too off-topic for this list.

Followup-To: gnu.emacs.help

You should probably post your general question to comp.lang.lisp

-- 
Kevin Rodgers <address@hidden>



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