Path: utzoo!utgpu!water!watmath!clyde!rutgers!sdcsvax!ucbvax!ihuxy.UUCP!nowlin
From: nowlin@ihuxy.UUCP
Newsgroups: comp.lang.icon
Subject: Re: variables
Message-ID: <8712222141.AA19282@megaron.arizona.edu>
Date: 22 Dec 87 21:06:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: inet
Organization: The ARPA Internet
Lines: 67

> Seeing as variables are passed by value and not reference, it is difficult
> to perform the same operation on a series of variables with any degree of
> elegance or economy.  I often end up doing things like
> 
>         a := doito(a)
>         b := doito(b)
>         c := doito(c)
>         etc...
> 
> I wonder if it would eventually be possible to include some mechanism in
> ICON for manipulating variables themselves (and not just their values).
> 
> Or is there some basic problem with doing things this way...?
> 
>                                                        -Richard Goerwitz

It's easy to have doito() perform it's operation on a list of variables and
return a new list with modified variables.  I'd call that fairly elegant. 
You could call doito() with an already defined list:

	l1 := [a,b,c]
	l2 := doito(l1)

or just pass the list of variables as a constant:

	l2 := doito([a,b,c])

or go even further and make doito() a generator:

	every i := doito([a,b,c]) do ...

It sounds like what you want is procedures with side effects instead
of procedures that act like functions, kind of like subroutines in
FORTRAN.  This is possible but the way that data structures like lists
are normally operated on in Icon creates new copies of the list.

The following procedure modifies the contents of a list and the
modifications stay in effect in the main procedure.  In order to avoid
making a copy of the list during the processing you have to use
subscripts. YECH!

	procedure main (args)
		dub(args)
		every write(!args)
	end
	procedure dub(lst)
		every i := 1 to *lst do
			lst[i] *:= 2
	end

The normal way I'd do this in Icon would be:

	procedure main (args)
		every write(dub(args))
	end
	procedure dub(lst)
		every suspend !lst *:= 2
	end

This is elegant to me.

Jerry Nowlin
(...!ihnp4!ihuxy!nowlin)

P.S. I tried to reply to the message about regular expressions last week
     but it didn't get out correctly.  I have a grep written in Icon if
     anyone is interested.