Change the Return response of Prolog

Question

I am a novice at programming in prolog.

I want to change the value returned by a prolog program such that it returns true / false instead of the standard yes or no.

Consider a very simple example : E.g. simple.P

node(1).

isNode(X) : node(X)

on the prolog command line if I type isNode(1) it returns with yes like:

isNode(1).

yes

My question is :

How do i change this from yes to true?

Strictly speaking, that is not a part that you control with your program, but the user interface of the Prolog system you are using. Such interface, often called `REPL` (Read,Eval,Print,Loop), or more often `console`, it's the simpler way to allow a programmer to control the 'inner working' of Prolog. As a programmer, you should define *your* own interface to your program, thus answering with true/false or whatever you think is better to appropriate user input. — CapelliC, Mar 03 '13 at 21:17
This would depend on the Prolog dialect you are using. For instance, SWI Prolog sometimes does not report anything, sometimes it says "true". — Alexander Serebrenik, Mar 03 '13 at 21:22
@AlexanderSerebrenik: When does SWI not report anything? To my understanding it always produces an answer. — false, Mar 03 '13 at 22:06
@false: it produces an answer but it does not necessarily add "yes" or :true" once the answer has been shown. — Alexander Serebrenik, Mar 06 '13 at 13:08

score 1 · Accepted Answer · edited Mar 04 '13 at 03:45

Prolog attempts to find a proof of your query. If your query has variables, it prints a value that makes them true.

Q: Are there any prime numbers that are even? A: Yes - 2 is even and prime

It'll keep giving you more proofs as long as you type ; Eventually it'll run out, and respond false.

Q: Are there any prime numbers that are even? A: Yes - 2 is even and prime Q: Are there any more? A: false.

What you want is for your program to perform output. There's a number of library predicates to do this. The most flexible is format/2

myprogram :-
   my_old_program, !,
   format('yup, that sure is right!~n', []).
myprogram :-
   format('nope, nope, no way in heck!~n', []).

1 Answers1