Path: utzoo!attcan!uunet!mcvax!cernvax!ethz!marti
From: marti@ethz.UUCP (Robert Marti)
Newsgroups: comp.lang.modula2
Subject: Re: Recursive data types
Message-ID: <867@ethz.UUCP>
Date: 12 May 89 07:50:45 GMT
References: <5734@cs.Buffalo.EDU>
Organization: ETH Zuerich
Lines: 37

In articles <5734@cs.Buffalo.EDU> and <5760@cs.Buffalo.EDU>,
ugprussa@cs.Buffalo.EDU (Michal Prussak) writes:

> Wirth describes in his book "Algorithms and Data Structures" (from 1986)
> a recursive data type (p 173).
> [ ... ]
> Does anybody know of a compiler that implements this? Or is this example
> for future versions of Modula?

and

> I agree that this should not work, but...
> Wirth not only seems to imply that this should work, but he also gives
> a diagram of internal representation of such record.


I don't think he implies that it should work.  In fact, p.175 of the
very same book states (quoted without permission):

  The characteristic property of recursive structures [ ... ] is their
  ability to vary in size.  Hence, it is impossible to assign a fixed
  amount of storage to a recursively defined structure, and as a
  consequence a compiler cannot associate specific addresses to the
  components of such variables.  The technique most commonly used to
  master this problem involves _dynamic allocation_ of storage [ ... ].

I don't think there is a Modula-2 compiler that implements recursive
data types _directly_, although it could be done.  Indeed, someone
in Wirth's group has developed an experimental language, Mona, which
supports recursive data types (TREE types).  Mona is _not_ a superset
of Modula-2, though.

-- 
Robert Marti                      Phone:      +41 1 256 52 36
Institut fur Informationssysteme
ETH-Zentrum                       CSNET/ARPA: marti%inf.ethz.ch@relay.cs.net
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