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From: broman%bugs@NOSC.ARPA (Vincent Broman)
Newsgroups: net.lang.mod2
Subject: extensions, a modest question
Message-ID: <8603210013.AA00587@bugs.ARPA>
Date: Thu, 20-Mar-86 19:13:27 EST
Article-I.D.: bugs.8603210013.AA00587
Posted: Thu Mar 20 19:13:27 1986
Date-Received: Sun, 30-Mar-86 07:59:14 EST
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Organization: The ARPA Internet
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Twice (or more) I've bumped heads with a small shortcoming of M2 that may
eventually be best cured by a small extension of M2 syntax.

Both in writing an i/o buffering scheme and a command interpreter the problem
arose of accessing components of a big structure which needs to be viewed as
having two different types: "buft=ARRAY range1 OF WORD", say, and
"datat=ARRAY range2 OF somerecord". Unfortunately, type casts are syntactically
expressions and not "designators", and only designators may be called,
dereferenced, subscripted, or selected. So, "datat(buff)[sub].field" bums out.
The only methods I've found which don't copy the whole durned structure are:
	TYPE dataptr = POINTER TO datat; VAR datap: dataptr
1.	datap := dataptr(ADR(buff));
	use(datap^[sub].field);		(* not beautiful *)
2. deceiving the compiler with untagged variant records (which I am too
honest to do :-).

Has anyone a suggestion on how to type cast AND select components?
My preference would be to allow wild-and-wooly syntax like

ComponentValue ::=
	Factor { "." Ident | "[" ExpList "]" | "^" | ActualParameters }.
ProcedureCall ::=
	ComponentValue [ ActualParameters ].
Factor ::=
	Set | Number | etc... | ComponentValue.


vincent broman, code 632,  naval ocean systems center, san diego, ca 92152, usa
phone: +1 619 225 2365     starship: 32d 42m 22s n/ 117d 14m 13s w
arpa: broman@bugs.nosc.mil uucp: {sdcsvax,gould9,bonnie,hp-sdd}!noscvax!broman