[YeGoblynQueenne]

Ahem. "Prolog underneath" is doing SLD-Resolution implemented as a Depth-First Search with backtracking. Saying it's "doing backtracking" is really fudging quite a bit.

Datalog, instead, is "underneath" implementing a TP-Operator, a procedure that finds the fix-point of a Datalog program which happens to be the same as its Least Herbrand Model, which is what SLD-Resolution also finds, except that SLD-Resolution allows functions and does not guarantee termination, like Datalog does. The big advantage of a TP Operator is the termination guarantees and it can be implemented so that it's efficient, but it's still limited, and the fact that there are many different flavours of Datalog (with or without stratified negation, arithmetic, lists etc) is testament to the difficulty of improving on Prolog's efficiency without breaking its soundness or completeness. Or, like I always say, "sound, complete, efficient: choose two".

[gobdovan]

By the same logic, would you say that the fact that there's only one mainstream Rust is testament to the simplicity of implementing a macro-heavy, borrow checked language without breaking safety, performance and expressiveness?

[YeGoblynQueenne]

I... don't really have an opinion on Rust. I have no idea, honest.

Mostly what I mean above is that Prolog is a hard balancing act, specifically balancing efficient execution with the practicalities of programming (e.g. lists, database asserts/retracts, I/O etc) and the theoretical underpinnings of Horn logic and SLD-Resolution. There's a lot of stuff that went into Prolog and its development process is quite distinct to any other language I'm aware of, where there wasn't like a central committee or an enlightened dictator, or a small band of hard-headed academics (...Haskell...) and so on, but instead there was a decades-long academic research process of going from First-Order Logic to Herbrand semantics, to definite logic, to Resolution, to Linear Resolution, to Linear Resolution with a Selection Rule, to Linear Resolution with a Selection Rule restricted to Definite clauses, to Negation-As-Failure under a Closed World Assumption, and finally to a language with an automated theorem prover as an interpreter and a clunky mockery of FOL syntax and terminology that isn't even complete (despite Resolution being complete); a process with no centralised structure instead split across multiple acadmics in several universities in the UK, in France, and in Japan.

Another way to see Prolog is that it's the result of a very peculiar academic process that is hard to replicate and that yielded a result that is difficult to outdo, with all its compromises; because of its compromises. Because its particular compromises were made to solve hard problems and were arrived at after a long, collaborative process that won't be easily outdone. Not that attempts haven't been made. For example, in logic programming circles Prolog is now considered a little bit quaint, even outdated. Most of the activity has shifted to Answer Set Programming (ASP). And that makes sense, Prolog is old news. ASP is the new kid on the block, it's only 30 years old! And like normal programming languages it was designed by a couple of people before being adopted by a wider community.

I just really don't know how any of that maps to Rust.