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Component Pascal type system is unsound

While we're on the topic of languages with unsound type
systems, let me mention another one: Component Pascal.
<http://www.oberon.ch/docu/language_report.html>.

According to www.oberon.ch, Component Pascal is a refined version of
Pascal, Modula-2, and Oberon, and moreover it is advertised as being
"type-safe".

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
type checking.

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.

	MODULE Foo;
	TYPE
		base = RECORD 
			x : INTEGER;
		END;
		derived1 = RECORD(base)
			y : INTEGER;
		END;
		derived2 = RECORD(base)
			z : INTEGER;
		END;

	VAR
		b : POINTER to base;

	PROCEDURE bar(OUT a1 : POINTER TO derived1;
	              OUT a2 : POINTER TO derived2);
	BEGIN
		NEW(a2);
		NEW(a1);
		a2^.z := 42;
	END bar;

	BEGIN
		bar(b, b);
	END Foo.

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 <fjh@cs.mu.oz.au>  |  "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.