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To: python-list@python.org
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject: Re: Python 3.x adoption
Date: Fri, 17 Jan 2014 20:01:53 -0500
Organization: IISS Elusive Unicorn
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On Fri, 17 Jan 2014 18:03:45 -0500, Terry Reedy <tjreedy@udel.edu>
declaimed the following:


>Not mentioned in the wiki article is the change in calling convention 
>from call by value to call by reference (or maybe the opposite). I 
>remember a program crashing because of this when I tried it with F77.
>
	Maybe because there was NO change in calling convention. FORTRAN was
call by reference in all versions I've worked with. DEC did add special
"functions" to force a parameter to be passed as "value", "reference", or
"descriptor" -- mostly to interface with other languages (descriptor is
what was used for character strings -- basically a structure containing the
length of the string/buffer, and the address of said buffer).

	However, one may have encountered an implementation that did some
checking for duplicated parameter addresses -- or may have behaved
internally as a value/return system, wherein the by-reference parameters
were copied to local/stack storage, manipulated, and only written back to
the reference address on exit. That would result in differences:

	a = 1
	call xyz(a, b, a)

	subroutine xyz(x, y, z)

	x = x + 1
	z = z * 2

...
	In a true by-reference system, "a" would turn into 2 for the first
assignment, but then turn into 4 after the second.

	In value/return, the second assignment still sees "1" and results in
"2", and thereby "a" becomes "2", not "4"...

	I'd still consider a value/return scheme to be a misbehaving FORTRAN.
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
    wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/