Re: Ann: SWI-Prolog 7.1.9

On 02/27/2014 06:39 PM, Jose F. Morales wrote:
>     It is not optional (well, there is --traditional). I think that the Ciao
>     modular and strict architecture is a great piece of engineering. I
>     prefer to keep things `just available' though. That approach surely has
>     disadvantages, but quite likely it contributed a lot in the popularity
>     of the system.
>
>
> You are definitely right that 'just available' is a great idea to push
> the language forward. However, that implies that each SWI version is
> potentially implementing a slightly different language/dialect.

To some extend, yes.  In practice the backward compatibility seems to
be pretty reasonable, even with the major changes of V7.

> I wonder if there is any best practice for specifying the expected
> dialect, system version, etc. Per-module declarations are precise but
> too noisy for medium/large projects. Is it possible to specify the
> requirements to the 'pack' definition in SWI?

You refer to these packs [1]?  They can specify requirements, but not
in enough detail to be very meaningful.  In my experience, :- if does
a good job writing programs and libraries that run over multiple
versions that are not compatible.  In most cases

:- if(\+current_predicate(xyz/2)).

xyz(X, Y) :-
	<write a replacement>.

:- endif.

That nicely deals with the the introduction of xyz in version V. The
only issue is that you cannot deal with code that is syntactically
invalid. That can be a problem if you want to write code that runs in V7
using dicts or quasi quotations and other Prolog systems. In such cases,
I guess the only way is not to use dicts or use another syntactic
abstraction and implement that with and without dicts. Possibly
ECLiPSe's idea of having a hook that provides term rewriting after
read_term/3 is a good idea after all. When I heart about it I was
sceptical.

p.s. We could consider ignoring syntax errors in excluded conditional
code? The problem is of course that it needs to find the :- endif. or :-
elif or :- else. I guess we can define some way around, but it would be
harder to get that accepted by all systems providing conditional
compilation.

	Cheers --- Jan

[1] http://www.swi-prolog.org/pack/list