CODEWITHSUNDEEP
prolog
OneMoreError
OneMoreError

Reputation: 7738

Unable to define predicate in Prolog

I have just started learning Prolog and so pardon me if this is a bit naive, or rather a lot naive. I am trying to define this predicate

| ?- times(M,N,Product) :- Product is M*N.

which gives me this error

uncaught exception: error(existence_error(procedure,(:-)/2),top_level/0)

I am using GNU Prolog. What is the fault here ?

Upvotes: 3

Views: 2565

Answers (2)

harvyS
harvyS

Reputation: 648

Check user manual 8.7 Dynamic clause management especially: asserta/assertz.

Next will help you to define new predicate in gprolog:

asserta( ( times(M,N,Product) :- Product is M*N ) ).

Upvotes: 0

Rubens
Rubens

Reputation: 14778

As false said, If you want to define a predicate or function during the execution of the interpreter, you must enter the "user" mode of the interpreter, by typing:

['user'].

After typing your predicates, you'll just need to leave this "zone", as a confirmation to the interpreter that you're finished with the coding, and it can start to compile your predicates to bytecodes -- which you'll be, then, allowed to access through your function names.

To leave the "user" mode simply press ctrl + d.

Tested in both swipl and gprolog:

SWI-Prolog version 5.10.4
(GNU Prolog) 1.3.0

Regards!

Upvotes: 6

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