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Question

Why isn't father/2 recognized and why can't I load a file that calls father/2 ?

theogony.pl

father(kronos, zeus).
father(zeus, ares).

mythos.pl

consult('theogony.pl').

%% --
%% X is an ancestor of Y
%% --

ancestor(X,Y) :-
    father(X,Y).

ancestor(X,Y) :-
    ancestor(X,Z),
    ancestor(Z,Y).

swipl

?- consult('mythos.pl'). 
   false.
?- consult('theogony.pl').
   true.
?- father(X,zeus).
   ERROR: Unknown procedure: father/2 (DWIM could not correct goal)

Answer 1

As noted there are two Prolog files with the file type pl . This code works with both files in the same directory, eg 'C:/Users/Groot/Example_01'. You can use another directory but be consistent with the directory name.

Directory: 'C:/Users/Groot/Example_01'
File: 'theogony.pl'

:- module(theogony,
    [
        father/2
    ]).

father(kronos, zeus).
father(zeus, ares).

Directory: 'C:/Users/Groot/Example_01'
File: 'mythos.pl'

:- module(mythos,
    [
        ancestor/2
    ]).

ancestor(X,Y) :-
    father(X,Y).

ancestor(X,Y) :-
    ancestor(X,Z),
    ancestor(Z,Y).

Start SWI-Prolog

Welcome to SWI-Prolog (threaded, 64 bits, version 8.5.15)
...

?- 

I know, there is a newer version but this is so basic even the really old versions should work.

Change the working directory.

?- working_directory(_,'C:/Users/Groot/Example_01').
true.

Use consult which is also done using [] to load the Prolog files.

?- [theogony].
true.

?- [mythos].
true.

Run your query.

?- father(X,zeus).
X = kronos.