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From: jgk@osc.COM (Joe Keane)
Newsgroups: comp.theory,comp.lang.functional
Subject: Re: Why not multiple out parameters? [again]
Keywords: aliasing
Message-ID: <3788@osc.COM>
Date: 11 Sep 90 09:44:45 GMT
References: <1990Aug28.203643.11214@zaphod.mps.ohio-state.edu> <4019@rex.cs.tulane.edu> <2501@l.cc.purdue.edu> <4060@rex.cs.tulane.edu> <3352@stl.stc.co.uk>
Reply-To: jgk@osc.COM (Joe Keane)
Followup-To: comp.theory
Organization: Versant Object Technology, Menlo Park, CA
Lines: 26

I think returning a record just gets around the issue.  We don't use a single
record to pass all the in parameters to a function, so why should we do it for
out parameters?

The original `problem' with multiple problems was really with pointer
aliasing, or more generally, modifier aliasing.  If objects are returned,
there is no possibility of conflict.  The caller may actually allocate space
on the stack for the returned objects, but this is an implementation issue and
there is still no possibility of conflict.

There are two ways to deal with aliased modifiers.  One way is to say that the
modifications are actually sequential.  Therefore, if two modifications are
done to the same object, one of the modifications is lost, or at least doesn't
determine the most current version.  The order may be pre-defined, so say the
textually earlier parameter is the one that is lost.  Or the order may be
undefined, so that which ever modifier gets there first loses.

The other way is to insist that the modifications are done together, which is
more like what we'd expect in a functional language.  Then each expression
which writes a return value expects to be the last one before the function
exits.  This works fine as long as they don't both want to write to the same
place.  What happens if they are aliases?  In that case each one has to go
before the other.  Therefore the problem is deadlock.  This makes sense if you
think about it; the two problems are really closely related.  Two modifiers
want exclusive rights to determine a single value, the current state of the
returned object.  They can't both have it.