by sirwhinesalot | 2025-06-16 13:56:21+00:00
Apologies for the wall of text below, but it's required to explain it.

It's not that cuts themselves are a problem, they're a symptom of a larger problem, of which unrestricted IO is another. They both exist because a Prolog implementation is intimately tied to a specific search strategy. One that you, as a programmer, must learn and exploit.

And because you can use cuts and IO, the search strategy must remain what it is. It's an ouroboros. A snake eating its own tail. This enforced search strategy makes Prolog a poor declarative language (despite all the neat parlor tricks you can do with it).

Why? As we climb the abstraction ladder, we relinquish control. This is a double-edged sword. We can express problems orders of magnitude more succinctly, but we must trust the machine to arrive at the solution efficiently and with limited guidance.

For example, C might be considered low level in comparison to most languages, but it is extremely high level compared to assembly.

C compilers can generate efficient machine code precisely because certain details are abstracted away: register allocation, function inlining, loop unrolling, constant-folding, dead-code elimination, etc.

These are only possible thanks to various higher-level concepts being available (variables, constants, functions, loops, conditionals, etc.) and lower-level concepts being unavailable (e.g., unrestricted goto). You have inline assembly, but it's restricted to function bodies, and you have to tell the compiler which registers you touch, or you'd prevent it from applying many optimizations entirely.

Many of the supposedly higher-level languages (like Java, C#, etc.), aren't really higher-level than C. You can express the same things in C, just slightly more verbose. Garbage Collection? You can bolt a garbage collector to C. Generators (yield)? I can implement that with macros. Generics? If void* is too cumbersome, a template file and some include tricks is a few lines of code away. They're just a pile of admittedly nice conveniences.

A pure functional language though? That could allow us to go a level higher. It's not the pattern matching or the higher order functions, however, those are still things C can express. No, the key is the referential transparency and lack of sequencing. Referential transparency means order of execution no longer matters. And if order of execution no longer matters, then we can parallelize everything.

We don't have a good strategy for automating this in a compiler yet, but the capability is there. Large scale MapReduce is only possible because we've abstracted away loops into those higher-level operations (map & reduce) calling referentially transparent functions.

Prolog didn't relinquish enough control. We can't express certain problems without resorting to weird dances to appease the poor search algorithm, same as how I might have to contort the way I write a C program so the compiler can vectorize the code.

If we relinquish enough control to arrive at Datalog (not even Turing-complete), we can make use of all the tools available to the relational model. The sort of dynamic query planning and optimization database engines use. A human can write a better query plan than a database engine, but a human cannot write a tailored query plan every few microseconds.

In Datalog I don't have to worry much about how I write my rules. I can have them be mutually recursive, I can have multiple rules with the same pattern in the head, I can write the body in a really inefficient order. A Datalog engine is free to optimize all of that out. I can't use higher-order predicates, but I can in ASP, which also has massively more efficient search than Prolog.

Even in other Turing-complete logic programming languages, like Mercury and Curry, I have a lot more freedom in how I express the problem because they have the freedom to optimize the code in a way that allows the solution to be arrived at efficiently. Curry can guarantee a solution will be arrived at if one exists, Prolog cannot.

The extra-logical control Prolog provides (like cuts) only serves as a tool to work around the poor search algorithm it uses, one it cannot change because of the control it gives. To me it's just an evolutionary dead-end, albeit one of great historical importance.