Hello world in Prolog

Question

I'm tearing my hair out trying to find how to just write a Hello World program in Prolog. I just want to create a program that runs like so:

> ./hw
Hello, world!
>

The problem is that every single example I can find works in a REPL, like so:

?- consult(hello_world).
% hello compiled 0.00 sec, 612 bytes

Yes
?- hello_world.
Hello World!

Yes

This is the same even with examples of compiled Prolog: the program still just drops into a REPL. This is obviously not much use for a "general-purpose" language. So, how do I write the traditional Hello World?

Answer 1

Prolog is not really a general purpose language. We use it to design artificial intelligence systems at university.

You'd have to define a fact, that answers "hello world".

hello('hello world').

Then, inquire the fact:

?- hello(X).

However, depending on the PROLOG compiler, you probably have a write() rule, that you could use:

?- write('hello world'), nl.

Answer 2

writeln('hello world').

Answer 3

You can write your source file to both launch the Prolog interpreter and to quit it when your code is done running. Here is an example using SWI-Prolog:

#!/usr/bin/swipl -q -t hello_world -f

hello_world :- write('Hello World'), nl, 
               halt.

Assuming you put this in a file named 'hw', and set the executable permission, you can call it like you want to:

$ ./hw
Hello World
$