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From: rcd@nbires.UUCP (Dick Dunn)
Newsgroups: net.lang.mod2
Subject: Re: structured function values
Message-ID: <127@nbires.UUCP>
Date: Tue, 15-Oct-85 02:09:38 EDT
Article-I.D.: nbires.127
Posted: Tue Oct 15 02:09:38 1985
Date-Received: Sat, 19-Oct-85 03:55:56 EDT
References: <53@noscvax.UUCP>
Distribution: net
Organization: NBI,Inc, Boulder CO
Lines: 40

>...   Modula-2's restriction of function procedure results to be of 
> unstructured types seems to have no solid justification...
Agree.

>...   Function calls are usually implemented with values returned in the 
> machine's fast registers, providing they fit in that much space...
The "...providing..." here is an important point:  As long as it's
necessary to make provision, in some architectures, for returning results
somewhere other than a register,
	1.  There's no justification for excluding those processors from
	the language, nor introducing a processor dependency on the result
	type of a function, hence
	2.  "Returnable in a register" is not a reasonable argument for the
	restriction on result type.

>...   Allowing structured function values will not introduce strange notations 
> like func(arg).field or func(arg)[sub] because the components of a structure 
> are accessed with the syntax: "designator=qualident{selector}", and a function
> call is not a qualident.  However, Modula allows at present, as far as I can 
> see, such curiosities as "ffunc(arg)(arg)" if ffunc is a function returning 
> a function, or funcarray[sub](arg) for arrays of functions. Not allowed at 
> present is pointerfunc(arg)^...

There's no particular reason to disallow the new constructs once a function
can return a value of a type which makes them reasonable.  The restriction
against pointerfunc(...)^ also existed in Pascal.  I have never understood
why it should exist; in fact, we removed this restriction in a Pascal-based
language I worked on some years ago.  (That same language also allowed
functions to return structured results, with no particular problems.)  The
overall effect on the language, if anything, is to make it more uniform.

>      I have never written a compiler myself, so I invite correction if my 
> suggestion would create unacceptable difficulties...

Again, talking about the Pascal-based language that I worked on, my notes
say that it added about 150 lines to the compiler (which was a variant of
the ETH compiler).  That's small change.
-- 
Dick Dunn	{hao,ucbvax,allegra}!nbires!rcd		(303)444-5710 x3086
   ...Simpler is better.