Comment by jodrellblank

Do you mean Northern Conservative Baptist Great Lakes Region Council of 1879 standard Prolog?[2]

SWI Prolog (specifically, see [2] again) is a high level interpreted language implemented in C, with an FFI to use libraries written in C[1], shipping with a standard library for HTTP, threading, ODBC, desktop GUI, and so on. In that sense it's very close to Python. You can do everyday ordinary things with it, like compute stuff, take input and output, serve HTML pages, process data. It starts up quickly, and is decently performant within its peers of high level GC languages - not v8 fast but not classic Java sluggish.

In other senses, it's not. The normal Algol-derivative things you are used to (arithmetic, text, loops) are clunky and weird. It's got the same problem as other declarative languages - writing what you want is not as easy as it seemed like it was going to be, and performance involves contorting your code into forms that the interpreter/compiler is good with.

It's got the problems of functional languages - everything must be recursion. Having to pass the whole world state in and out of things. Immutable variables and datastructures are not great for performance. Not great for naming either, temporary variable names all over.

It's got some features I've never seen in other languages - the way the constraint logic engine just works with normal variables is cool. Code-is-data-is-code is cool. Code/data is metaprogrammable in a LISP macro sort of way. New operators are just another predicate. Declarative Grammars are pretty unique.

The way the interpreter will try to find any valid path through your code - the thing which makes it so great for "write a little code, find a solution" - makes it tough to debug why things aren't working. And hard to name things, code doesn't do things it describes the relation of states to each other. That's hard to name on its own, but it's worse when you have to pass the world state and the temporary state through a load of recursive calls and try to name that clearly, too.

This is fun:

    countdown(0) :-
      write("finished!").   

    countdown(X) :-
      writeln(X),
      countdown(X-1).
    

It's a recursive countdown. There's no deliberate typos in it, but it won't work. The reason why is subtle - that code is doing something you can't do as easily in Python. It's passing a Prolog source code expression of X-1 into the recursive call, not the result of evaluating X-1 at runtime. That's how easy metaprogramming and code-generation is! That's why it's a fun language! That's also how easy it is to trip over "the basics" you expect from other languages.

It's full of legacy, even more than Python is. It has a global state - the Prolog database - but it's shunned. It has two or three different ways of thinking about strings, and it has atoms. ISO Prolog doesn't have modules, but different implementations of Prolog do have different implementations of modules. Literals for hashtables are contentious (see [2] again). Same for object orientation, standard library predicates, and more.

[1] https://news.ycombinator.com/item?id=26624442