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From: rcd@opus.UUCP (Dick Dunn)
Newsgroups: net.lang.c
Subject: Re: YAAO and `with'
Message-ID: <985@opus.UUCP>
Date: Wed, 19-Dec-84 01:01:13 EST
Article-I.D.: opus.985
Posted: Wed Dec 19 01:01:13 1984
Date-Received: Thu, 20-Dec-84 05:22:01 EST
References: <209@cmu-cs-k.ARPA> <529@vu44.UUCP> <6616@brl-tgr.ARPA>
Organization: NBI,Inc, Boulder CO
Lines: 31

> > Dean Rubine asked for a ->= operator,...

(Sorry about the grandparental nature of the reference for the followup.)

The desire for ->= (assign from subfield) comes from one case of a more
general desire--working on a referenced object in some convenient way.
Consider as an alternative something like the Pascal `with' clause.  (No,
put down the torch, I don't mean to consider adding it to C!  Just consider
it for a hypothetical language.)  The semantics of `with', somewhat
simplified, is that in the form
	with <reference>
	do   <statement>
<reference> must be a reference to a structure, and within <statement> the
fields of that structure are accessible without prefixing them with the
structure reference.  For example (here, "^" is Pascal's pointer-de-
reference), one Pascal idiom is"
	while p<>nil do with p^ do
		begin
		. . .
			p := next;
		end
where "next" is a field of the record referenced by p; without the `with'
the statement would have been written p := p^.next
The `with' is useful in a lot of other situations, such as filling in
several fields of the same record.  The usual implementation is to grab a
pointer to the base of the structure, taking care of any address
calculations in the <reference>, and try to keep the pointer in a
register.
-- 
Dick Dunn	{hao,ucbvax,allegra}!nbires!rcd		(303)444-5710 x3086
   ...Nothing left to do but smile, smile, smile.