[m-dev.] Re: proposed command set for the internal trace based debugger

[Fergus Henderson]

On 03-Jun-1998, Lee Naish <lee at cs.mu.OZ.AU> wrote:
> Peter Schachte <pets at students.cs.mu.oz.au> writes:
> 
> >On Wed, 3 Jun 1998, Fergus Henderson wrote:
> 
> >[Re: the distinction between spy and break points]
> 
> >But I'm not so sure that break points correspond very closely to break
> >points in imperative language debuggers.
> 
> I agree. 

Why?

> You could argue the convention break points stop the execution
> when the flow of control reaches a certain point.  You can argue a
> similar thing for spy points.

I remain unconvinced by such arguments.  Spy points correspond to the
four-port model.  Every predicate or subgoal has four ports associated
with it.  Spy points are a high level notion closely related to the
proof tree.  Break points, on the other hand, are a more low level notion,
relating to the flow of control through a procedure.

At least that's how I see it.

> A major difference is that control can
> reach a certain point in a greater variety of ways: call+fail and
> exit+redo are the same program points but how we get there is different.
> Of course spypoints actually specify two program points (those above),
> but even if only one was specified, it would correspond to two ports.

OK, so you agree that there is a "major difference" between break
points and spy points.  This sounds more like you are agreeing with
my position rather than Peter's ;-)

I don't see why you say that "call+fail" are the same program point.
Perhaps you meant "call+redo" and "exit+fail"?

But even this is wrong from the (low-level) flow-of-control perspective
offered by break-points: "call" and "redo" are different points in the
flow of control (even though they are both "entry" ports of the same
predicate), so for the purposes of break points they are separate.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.