Denys Duchier writes:
> People who use `dis' often do it to simulate logic programming
> non-determinism. That is probably the worst, most inefficient way to
> use Oz. Oz is _not_ a logic programming language, it is a concurrent
> constraint programming language: it shines at constraint propagation.
This is putting things a bit too strongly, in my view, because it
identifies 'logic programming' with 'Prolog-style logic programming'.
In fact, there are two things one can say:
1. Prolog-style logic programming is inefficient when search is needed.
This is independent of Oz.
2. Oz was not designed to support this inefficient kind of logic
programming, but rather to support other, more efficient kinds of
logic programming.
Prolog-style generate-and-test is a *terrible* way to write most
programs, and (hindsight tells us that) the inclusion of depth-first
search as the *default* strategy in Prolog is one of the biggest design
errors of the language. Most general-purpose programs in Prolog don't
need generate-and-test, and so have to fight against this default
(e.g., a single missing cut can cause an enormous memory leak, slowing
down a program by orders of magnitude).
On the other hand, 'logic programming' means just programming with
logical relations, with some kind of logical model giving the
semantics. This is a perfectly reasonable general approach to
programming, even though Prolog-style nondeterminism is a terrible way
to do it. There are two much better ways to do logic programming, and
Oz supports both well:
1. Programs that need search: Oz does concurrent constraint programming,
as explained by Denys.
2. Programs that are specified logically, but don't need search:
many of the standard language operations of Oz (if, case,
procedures, data structures, variable binding) have a simple
logical semantics.
So, strictly speaking, Oz *is* a logic programming language. It even
allows Prolog-style generate-and-test. But this is not recommended for
Oz (and not for Prolog either). Finally, Oz supports well a much
better style of logic programming, using the two approaches outlined
above. This better style is not limited to Oz, and should be used in
modern versions of Prolog as well.
See the 'Models book' (previous message) for more information.
Peter Van Roy
Universite catholique de Louvain
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