Q:

How do I include a .pl file in Prolog?

I'd like to include code from another source file. Does anyone know how to do that?

A:

If your file is called foo.pl, you can include it using
:- [foo].

or, equivalently and a bit more explicit
:- consult(foo).

or, if you're worried it may be loaded several times in a larger app
:- ensure_loaded(foo).

or, if you're using full-blown modules
:- use_module(foo).

though the exact name of the last predicate differs between Prolog versions.

A:

If you want to include the file literally - similar to #include, use
:- include('file.pl').
Most of the time it is preferable to structure your program using
modules, though.

