Xref: utzoo comp.lang.modula2:3795 comp.lang.modula3:213
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!think.com!mintaka!bloom-beacon!eru!hagbard!sunic!mcsun!cernvax!chx400!chx400!bernina!neptune!inf.ethz.ch!brandis
From: brandis@inf.ethz.ch (Marc Brandis)
Newsgroups: comp.lang.modula2,comp.lang.modula3
Subject: Re: Two Oberon questions
Message-ID: <28054@neptune.inf.ethz.ch>
Date: 17 Apr 91 07:23:14 GMT
References: <1991Apr12.104304.4194@jyu.fi>
Sender: news@neptune.inf.ethz.ch
Reply-To: brandis@inf.ethz.ch (Marc Brandis)
Organization: Departement Informatik, ETH, Zurich
Lines: 34

In article <1991Apr12.104304.4194@jyu.fi> sakkinen@jytko.jyu.fi (Markku Sakkinen) writes:
>How is the type equivalence of two objects really defined in Oberon?
>Modula-2 has "name equivalence", but Modula-3 has gone over to
>structural equivalence (in my opinion: unfortunately).

Oberon uses name equivalence as does Modula-2. There are two special cases in
which structural equivalence is used: Open arrays and procedure variables.

>Can function procedures in Oberon return structured values?

No. There are good reasons not to allow this. It is rather complicated to 
implement and it is hard to find good examples where you need it. If you want
to return a structured value, you would have to first assemble it into a local
variable, which would then be copied to the final destination. In an efficient
implementation, this would be an implicit VAR parameter. The common idea in
Wirth's languages is not to hide what is going on, so in this respect it is
better to declare the VAR parameter explicitly and to assemble the result 
directly into it.
Note that you cannot use the result of such a function in an expression. All
what you could do with it would be to assign it to another variable or to
pass it to a procedure as a parameter. If you look at the intended programming
style in Oberon, you will see that structure assignments and passing of
structures as value-parameters occurs rarely.

>Do different implementations of Oberon differ?

The implementations of Oberon implemented at ETH do not. I cannot comment on 
other implementations, but I guess that the answer is also no.


Marc-Michael Brandis
Computer Systems Laboratory, ETH-Zentrum (Swiss Federal Institute of Technology)
CH-8092 Zurich, Switzerland
email: brandis@inf.ethz.ch