[m-rev.] for review: implement undo for declarative debugger

[Ian MacLarty]

On Tue, 26 Jul 2005, Mark Brown wrote:

> On 26-Jul-2005, Ian MacLarty <maclarty at cs.mu.OZ.AU> wrote:
> >
> >
> > On Tue, 26 Jul 2005, Mark Brown wrote:
> >
> > > On 26-Jul-2005, Ian MacLarty <maclarty at cs.mu.OZ.AU> wrote:
> > > > On Tue, 26 Jul 2005, Mark Brown wrote:
> > > >
> > > > > On 26-Jul-2005, Ian MacLarty <maclarty at cs.mu.OZ.AU> wrote:
> > > > > > --- doc/user_guide.texi	17 Jun 2005 10:13:55 -0000	1.439
> > > > > > +++ doc/user_guide.texi	26 Jul 2005 02:04:16 -0000
> > > > > > @@ -4100,6 +4100,11 @@
> > > > > >  @item skip
> > > > > >  Skip this question and ask a different one if possible.
> > > > > >  @sp 1
> > > > > > + at item undo
> > > > > > +Reset the state of the declarative debugger to the state it was in before the
> > > > > > +last `yes', `no', `inadmissible', `trust' or `skip' answer was given or
> > > > > > +the last subterm was marked, whichever was most recent.
> > > > > > + at sp 1
> > > > >
> > > > > What happens if you give the `undo' command twice in succession?  The
> > > > > documentation should be clearer on this point.
> > > > >
> > > >
> > > > Isn't it clear that if the state of the declarative debugger has been reset
> > > > to what it was before the previous answer, then issuing another undo will
> > > > reset the state of the declarative debugger to what it was before the
> > > > previous-previous answer?
> > >
> > > Not entirely.  It depends on how you interpret the word "last" in the
> > > description.  Is the sequence of commands from which we select the "last"
> > > considered part of the state of the debugger (and is therefore also reset),
> > > or not?  If you implement a "redo" command, for example, you could say that
> > > it reverses the effect of the last undo command, but that would be "last"
> > > in a different sense.
> > >
> >
> > Are you saying that people might interpret the description as meaning
> > undo is idempotent?
>
> Yes.  Or at least they might think of that possibility and wonder whether
> that was the actual behaviour, which is what I did.
>

How about if I add the following line:
Successive undos will reset the state to what it was before previous answers.

Ian.

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