Hello world in Prolog

24
votes

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?

4
prologread-eval-print-loop
Where are your virtues of declarative programming now? MWHAHAHA :) - Karmic Coder
I dunno, I think most real implementations of Prolog in an application use it as an embedded language for what it's good at, rather than write the entire app (UI, file I/O, hardware control, etc...) in Prolog. - FrustratedWithFormsDesigner

4 Answers

20
votes

Using GNU Prolog:

$ cat hello.pl 
:- initialization(main).
main :- write('Hello World!'), nl, halt.

$ gplc hello.pl $ ./hello
Hello World!

11
votes

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
$
3
votes

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.
1
votes
writeln('hello world').