[m-rev.] For review: deconstruct non-canonical terms in pretty_printer.m

[Ian MacLarty]

On Mon, Oct 08, 2007 at 03:54:21PM +1000, Mark Brown wrote:
> On 08-Oct-2007, Ralph Becket <rafe at csse.unimelb.edu.au> wrote:
> > Index: library/pretty_printer.m
> > ===================================================================
> > RCS file: /home/mercury1/repository/mercury/library/pretty_printer.m,v
> > retrieving revision 1.6
> > diff -u -r1.6 pretty_printer.m
> > --- library/pretty_printer.m	3 Sep 2007 16:41:41 -0000	1.6
> > +++ library/pretty_printer.m	8 Oct 2007 04:05:09 -0000
> > @@ -645,7 +645,18 @@
> >          Doc = set_formatting_limit_correctly(!.Limit,
> >              Formatter(Univ, ArgTypeDescs))
> >        else
> > -        deconstruct(univ_value(Univ), canonicalize, Name, _Arity, Args),
> > +        % We justify this promise on the grounds that
> > +        % (a) observers of views of non-canonical values should give
> > +        % them the same denotation,
> > +        % (b) practically it is far more useful to see the construction
> > +        % of non-canonical values (e.g., when debugging programs), and
> > +        % (c) propagating cc_multi all the way up into the interface could
> > +        % make use of the pretty printer very awkward.
> > +        %
> > +        promise_equivalent_solutions [Name, Args] (
> > +            deconstruct(univ_value(Univ), include_details_cc,
> > +                Name, _Arity, Args)
> > +        ),
> 
> In view of the followup discussion on the developers list, i think you
> should:
> 
>     - Add the promise here, and add a comment to say that the promise is
>       valid because of the way that we interpret equality on the doc type.
> 
>     - Document in the interface that the intended interpretation of equality
>       on type doc is that docs are equal if they describe the same logical
>       document, and in particular that docs created with format/1 are equal
>       if the terms are equal.
> 
>     - Also document that we don't actually implement this version of
>       equality, and enjoin users to avoid it.  It would help to give the
>       example of two docs created from equal non-canonical terms being equal
>       according to the intended interpretation, but failing a test
>       unification.
> 
>     - Make format/8 cc_multi.  Use cc_multi_equal at an appropriate point.
> 
>     - I would suggest adding format_io/8, which is specialised to the io
>       stream, and which is det.  This will need a promise too: document
>       that this promise is safe because we can never test io streams for
>       equality.
> 
>     - If you actually do need a string stream, we can discuss that next.
> 

Isn't the above call to deconstruct inside the implementation of
format/8?  If so why not just let the cc_multiness propogate up to
format/8, instead of adding this promise here and then using
cc_multi_equal in format/8?  From looking at pretty_printer.m it seems
the only exported predicate that calls deconstruct is format/8, but
perhaps I'm missing something?

Ian.
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