Comment by YeGoblynQueenne

>> The main issue is that Prolog is not truly declarative; how you write the rules has a major impact on how the interpreter behaves. You might end up with repeated answers for a query or it might enter an infinite loop. Prolog also allows IO, so the order in which things get executed is critical. It’s Turing-complete, for better or worse.

Yeah, sounds like a real show-stopper. Prolog is not 100% declarative, which is really what every programmer wants from a programming language.

Better use a language like C# or Java, or Python, which are 0% declarative. Or try to implement datalog (only not really) so that you can't even use lists because that's what real-word programmers are looking for in a real-world programming language: 100% declarative, 0% real-world uses.

Ah well, at least the author has heard of SLD-Resolution which is more than one can say about the authors of SICP. But they haven't heard of SLG Resolution with tabulation (a.k.a. "tabling") which is a way to execute Prolog programs while preserving the Turing-completeness but without getting stuck in left-recursions. Among other things.

https://www.swi-prolog.org/pldoc/man?section=tabling

Edit:

>> Prolog also allows IO, so the order in which things get executed is critical.

I paid €7 to make this post because I'm on a boat in the middle of the Adriatic sea and my Vodafone contract is AWOL, so listen closely: no, Prolog's IO is not what makes it Turing Complete. That comes from definite logic, i.e. a fragment of the First Order Predicate Calculus (a.k.a. First Order Logic) restricted to formulae in clausal form with at most one positive literal a.k.a Horn clauses, that nevertheless retains the full expressivity of full FOL. Definite logic is semi-decidable and when evaluated by Resolution, as SLD-Resolution (restricted to definite program clauses and Horn goals), it is refutation-complete (and sound, to boot). Meaning, in summary, that any true sentence can be proved true but if a sentence is false then it cannot always be proved false, by SLD-Resolution. So far, so theoretically good.

What Prolog does which is really dangerous and makes the order of execution "critical" is that it allows the programmer to edit the program database i.e. the database of facts and rules, in other words, the program itself, programmatically, i.e from the program itself. This means that not all your program's state is known before the program runs and database operations are completed. That sucks hairy balls but it is also a very useful mechanism that requires experience and understanding of the language, its core concepts, and programming in general, to use right.

But Prolog's Turing completeness has nothing to do with IO, or database ops. That was not worth €7.