Component Pascal type system is unsound
To: Types List <firstname.lastname@example.org>
Subject: Component Pascal type system is unsound
From: Fergus Henderson <email@example.com>
Date: Fri, 28 Dec 2001 02:49:13 +1100
In-Reply-To: <200112171946.fBHJk4C03239@saul.cis.upenn.edu>; from firstname.lastname@example.org on Mon, Dec 17, 2001 at 01:18:22PM -0600
While we're on the topic of languages with unsound type
systems, let me mention another one: Component Pascal.
According to www.oberon.ch, Component Pascal is a refined version of
Pascal, Modula-2, and Oberon, and moreover it is advertised as being
However, in July this year, in response to some mail from John Gough,
I constructed an example which demonstrates a hole in Component Pascal's
The basic problem arises due to the parameter compatibility rules for
"OUT"-mode parameters (which use pass-by-reference) and "extended types"
(Component Pascal's name for derived classes).
These rules allow, for instance, "OUT"-mode parameters whose type is
a pointer to an extended type to be bound to variables whose type is
a pointer to the base type. This can lead to problems when such
parameters are aliased.
Here's an example which shows the problem.
base = RECORD
x : INTEGER;
derived1 = RECORD(base)
y : INTEGER;
derived2 = RECORD(base)
z : INTEGER;
b : POINTER to base;
PROCEDURE bar(OUT a1 : POINTER TO derived1;
OUT a2 : POINTER TO derived2);
a2^.z := 42;
Component Pascal allows this sort of code, but this program ends up
assigning to the `z' member of an object of type `derived1', which
does not have any `z' member.
(I probably have some of the details of this program wrong, since I don't
have a Component Pascal implementation around to check it. But John Gough
has verified that the problem I describe did occur for his Component
Pascal implementation, and last I heard the authors of the Component
Pascal told him that they are planning to revise the language to correct
this type loophole... in particular removing OUT parameter variance.)
Fergus Henderson <email@example.com> | "I have always known that the pursuit
The University of Melbourne | of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp.