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upghost • 06/15/2025 • 2 replies • view on HN

Datalog is a syntactic subset of Prolog[1], which this is... not.

I think the most misunderstood thing about Prolog (and Datalog, the functor-free subset of pure Prolog) is that the syntax is really, really important.

It's like, the whole gimmick of the language. It is designed to efficiently and elegantly query and transform itself. If you lose the syntax you lose all of intermediate and advanced Prolog (and Datalog).

[1]: https://en.m.wikipedia.org/wiki/Datalog

Replies

kragen • 06/16/2025

Semantics are more important than syntax. Prolog's flexible syntax is a nice-to-have rather than essential when you're in Lisp. And Datalog is purely first-order, so the advanced Prolog you're talking about doesn't exist in it.

However, syntax does matter, and this is not acceptable

    (dl-find 
     (fresh-vars 1 
      (lambda (?id) 
       (dl-findo dl
        ((,?id reachable ,?id)))))))
as a way to ask

    reachable(Id, Id).
I think you could, however, write a bit more Scheme and be able to ask

    (?id reachable ?id)
which would be acceptable.

However, the ordering betrays a deeper semantic difference with orthodox Datalog, which is about distinct N-ary relations, like a relational database, not binary relations. This implementation seems to be specific to binary relations, so it's not really Datalog for reasons that go beyond mere syntax.

On the other hand, this (from the initial goal) would be perfectly fine:

    (dl-rule! dl (reachable ,?x ,?y) :- 
                     (edge ,?x ,?z) (reachable ,?z ,?y))
The orthodox Datalog syntax is:

    reachable(X, Y) :- edge(X, Z), reachable(Z, Y).
➕ show 2 replies
j-pb • 06/16/2025

Most database literature simply uses Datalog to mean the query language fragment of conjunctive queries + recursion/fixpoint-iteration and potentially stratified negation.

Yes it started out as a Prolog subset, but the definition as the fragments it supports has become much more prevalent, mainly to contrast it to non-recursive fragments with arbitrary negation (e.g. SQL).

This usage dates back to database literature of the 80s by Ullman et. al.