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Path: utzoo!linus!decvax!cca!g-rh
From: g-rh@cca.UUCP (Richard Harter)
Newsgroups: net.lang
Subject: Re: and if you put this in your language ...
Message-ID: <7078@cca.UUCP>
Date: Tue, 8-Apr-86 01:03:44 EST
Article-I.D.: cca.7078
Posted: Tue Apr  8 01:03:44 1986
Date-Received: Fri, 11-Apr-86 20:38:39 EST
References: <1187@mmintl> <>
Reply-To: g-rh@cca.UUCP (Richard Harter)
Organization: Computer Corp. of America, Cambridge
Lines: 31
Summary: 

In article <> chan@hpfcmt writes:
>
>Common Lisp (as well as other dialects) has a capability similar to
>what you desire, that is CATCH and THROW, and UNWIND-PROTECT.
>
>(CATCH <label> <form>*) says evaluate the <form>s and if any throws to the
>same label occur during that evaluation, return the thrown value as the
>value of the CATCH. There may however be intervening catches with the
>same label that will "intercept" the throw. A throw looks like
>(THROW <label> <value>).
>
>If there is cleanup code that needs to be executed, you need to set up
>an UNWIND-PROTECT.
>
>(UNWIND-PROTECT <protected-form> <cleanup-form>*) "guarantees" that the
>cleanup forms will be evaluated, even if there is a non-local exit such
>as a THROW from the protected form. This is useful for instance for
>making sure that a file that was opened in the protected form gets closed.
>
	Sorry, this is a little obscure to the non-Lisp user.  I follow
the idea of the THROW; however what happens to the value of the CATCH.
How do I tell one CATCH from another?  Is (CATCH ...) an expression 
that returns a value in other expressions?  If so, what does it return
if nothing is thrown to it?  The (UNWIND-PROTECT ...) looks like a
demon, i.e. it executes whenever there is an exit from the protected
form as an interjection.  Also, what happens when there is a THROW?
Does flow control transfer to the CATCH or does it continue?  These
may seem like dumb questions, but if one is not a LISP programmer it
seems a little obscure.

		Richard Harter, SMDS Inc.