Question

I’m new in Prolog , I am trying to set rule on SWI-Prolog shell e.g –

listensToMusic(X) :- happy(X).

But it prompts –

ERROR: Undefined procedure: (:-)/2

I use SWI-Prolog version 6.2.6

Answer 1

SWI-Prolog does not accept new rules and facts on the top-level, it only accepts queries.

Rules are typically added by writing them in a text file (for example rules.pl), and load it into SWI-Prolog using:

?- [rules].

Absolute paths to files can be used like this:

?- ['C:/Program Files/pl/demo/likes'].

You can type rules by issuing [user]., typing your rule and ending with EOF (typically Ctrl-D):

?- [user].
|: listensToMusic(X) :- happy(X).
|: <EOF> 
true.

There is an elaborate FAQ on this subject: ERROR: Undefined procedure: (:-)/1 | (:-)/2 | (?-)/1