Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!bellcore!decvax!decwrl!pyramid!hplabs!hpfcdc!hpfcla!hpfcmt!chan
From: chan@hpfcmt
Newsgroups: net.lang
Subject: Re: and if you put this in your language ...
Message-ID: <9100002@hpfcmt>
Date: Wed, 2-Apr-86 15:51:00 EST
Article-I.D.: hpfcmt.9100002
Posted: Wed Apr  2 15:51:00 1986
Date-Received: Wed, 9-Apr-86 09:30:23 EST
References: <1187@mmintl>
Lines: 48

>	BLOCK label_name {
>	  .....
>	  }

>	LOOP label_name {
>	  .....
>	  }

>	FRAGMENT label_name {
>	  .....
>	  }

>A FRAGMENT is essentially an internal procedure without arguments;
>a BLOCK is a labelled block of code; and a LOOP is a labelled (forever)
>loop.  The command statement that I want is:

>	escape [(block/loop)_label_name] [via fragment_name]

>This statement transfers control to the next statement after the
>block (loop) but first executes the named fragment.  If there is
>no block/loop label name the escape leaves the current block/loop.
>If there is no via clause control is transferred directly to the
>next statement.  Every once in a while I find that I would really
>like to have something like this available.

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.

			-- Chan Benson
			{ihnp4 | hplabs}!hpfcla!chan
			Hewlett-Packard Company
			Fort Collins, CO

As usual, HP has nothing to do with what I say here.