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Path: utzoo!watmath!watnot!watdragon!cdshaw
From: cdshaw@watdragon.UUCP (Chris Shaw)
Newsgroups: net.lang
Subject: Re: Escape structures (retitled)
Message-ID: <865@watdragon.UUCP>
Date: Sun, 13-Apr-86 16:39:33 EST
Article-I.D.: watdrago.865
Posted: Sun Apr 13 16:39:33 1986
Date-Received: Mon, 14-Apr-86 06:18:32 EST
References: <1187@mmintl.UUCP> <6925@cca.UUCP> <> <7040@cca.UUCP>
Reply-To: cdshaw@watdragon.UUCP (Chris Shaw)
Organization: U of Waterloo, Ontario
Lines: 47

The most beautiful way I have seen for directed escape is with the
Waterloo System Language's (WSL)  GUESS-ADMIT structure.
The following details are probably wrong, but you will hopefully get the gist:

	GUESS : LABEL1 
		quif( some_condition ) : LABEL1	  /* quit guess completely*/
		quif( some_other_cond )	          /* goto admit */
		< processing >
	ADMIT : LABEL2			/* label optional */
		< different processing >
	ENDGUESS

"quif( cond )" means quit the local block if "cond" is true. If a label
follows in the quit statement, then the quif goes to the label. One can 
only go to labels (I recall) at your control statement level or "outer". 
For example, you can't quit into the body of a non-enclosing WHILE statement.

The semantics of quits and quifs are basically gotos which can only branch
DOWNWARDS. Therefore, no spaghetti code. This also makes for object code
efficiency, since there is rarely a case where one would have to do complicated
control structures. The GUESS-ADMIT structure is more or less a thinking
person's case statement. ADMITs can be added at will in a GUESS statement.
Thus the following could happen (code omitted between each GUESS or ADMIT):

	GUESS
	ADMIT :LABEL1
	ADMIT :LABEL2
	ADMIT :LABEL3
	ENDGUESS

Each ADMIT block handles one particular situation and no other. One can easily
tell by looking what is being done, and it is clear at a glance where control
passes to when processing for any ADMIT block is complete. The ADMIT labels
allow for quitting past ADMIT blocks that will fail anyway. 

Hopefully I haven't been too unclear here, but the point I'm trying to make
is this:
GOTOs are "evil" because they are generally unconstrained in their "scope".
The quif/quit structure answers this difficulty by allowing limited scope 
to where they can transfer control. All control function can be determined at
compile time, and the maximum scope available is the surrounding procedure.

(I used (and liked) this language a longish while ago. email me for more detail)

Chris Shaw    watmath!watrose!cdshaw  or  cdshaw@watmath
University of Waterloo
Bogus as HELL !!!