[mercury-users] [petdr at miscrit.be: Report on the ICFP 2000 prog contest]

[Thomas Conway]

On Wed, Sep 06, 2000 at 11:46:26AM EST, David Overton wrote:
> On Wed, Sep 06, 2000 at 10:57:17AM +1100, schachte at cs.mu.OZ.AU wrote:
> > > It is instructive to note that one of the bugs that took the longest to
> > > find was in the following code:
> > > 
> > >     inside_sphere(Point, Trans) :-
> > > 	Point = point_to_object_space(Trans, Point),
> > > 	mag2(Point) =< 1.0.
> > 
> > That's an interesting bug; it's such an easy mistake to make, and so
> > hard to see.  The interesting thing about it is that the code doesn't
> > say what you intended, but you *could* have intended what you wrote, so
> > the language really can't find it for you.
> 
> One way this bug could have been avoided would be by using different
> types for the object space points and world space points.

Note that I did something like this with terms changing them from:

:- type term
	--->	variable(var)
	;	functor(atom, list(term), context)
	.

to

:- type term(T)
	--->	variable(var(T))
	;	functor(atom, list(term(T)), context)
	.

and

:- type var. % abstractly == int.

to

:- type var(T). % abstractly ---> var(int).

That way we made type variables and program variables have different
types in the compiler so that we would get errors when they got mixed.
Of course, I didn't find any errors when I introduced the change because
DJ (dgj) had already sweated over the code long enough to find the
several that were there.

We could easily have made point be:

:- type point(T)
	--->	point(float, float, float).

:- type worldPointType ---> worldPointType.

:- type objectPointType ---> objectPointType.

:- type worldPoint == point(worldPointType).

:- type objectPoint == point(objectPointType).

This technique of using "place holder" type parameters to distinguish
objects whose type is the "same" is very useful and I've used it lots
of times.
-- 
 Thomas Conway              Unicode for terrorists: U+0001 300C
 <conway at cs.mu.oz.au>         "Tie his hands behind his back"
 Mercurian )O+  
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