Prolog program- implementing rules and list matching

Question

I am trying to write a program in Prolog that would recognize 'is a' statements and apply the transitive property in inquired. For example:

Input: Mary is a girl.
Output: ok.
Input: A girl is a human.
Output: ok.
Input: Is Mary a human?
Output: yes.

Here's my code:

begin :-
    begin(Input).

begin(Input) :-
    write('Begin.. '),
    write('\n'),
    readln(Input),
    tokenize_atom(Atom, List),
    rules(List).
    begin.

rules(['A', Subj, is, a, What]) :-
    asserta(a(Subj, What)),
    write('ok'),
    write('\n').

rules([Subj, is, a, What]) :-
    asserta(is(Subj, What)),
    write('ok'),
    write('\n').

rules(['Is', Subj, a, What]) :-
    (is(Subj, Z) ; a(Z, What)) -> (write('Yes.'), nl) 
    ; (write('Unknown.'), nl).

It doesn't go into any of the cases, it will just say true and terminate when given a statement. What am I doing wrong?

Please take a look at [these](http://stackoverflow.com/questions/16173977/prolog-error-when-querying-a-false-statement) [three](http://stackoverflow.com/questions/12733874/read-input-in-prolog-and-print-result/12734394) [answers](http://stackoverflow.com/questions/16220467/in-prolog-swi-how-to-build-a-knowledge-base-of-user-supplied-pairs-and-assert) and see if any of them help. — Daniel Lyons, Apr 29 '13 at 16:13

score 1 · Answer 1 · answered Apr 29 '13 at 16:48

Prolog itself wants to help you. Look at the helpful messages you get:

|:     begin :-
|:      begin(Input).
Warning: user://1:9:
        Singleton variables: [Input]

This is pointing out that this rule is no different from:

begin :- begin(_).

This is highlighting that the role of Input in begin/1 is ambiguous. Is it input or output? If you can use it like this, it must be output, and that's consistent with how it's used in its definition, but look at the other problem you have there:

|:     begin(Input) :-
|:     write('Begin.. '),
|:     write('\n'),
|:     readln(Input),
|:     tokenize_atom(Atom, List),
|:     rules(List).
Warning: user://1:14:
        Singleton variables: [Atom]

Where did Atom and List come from? Presumably you wanted one of those to show up from readln/1. What's actually going on here is that you're asking Prolog to input a value, which you then return, having done nothing with it; meanwhile Prolog materializes Atom and List from thin air and uses them with your rules/1 predicate. So clearly something isn't hooked up here that should be.

Then you have an obvious typo:

|:         begin.
Warning: user://1:22:
        Clauses of begin/0 are not together in the source-file

I suspect you meant a comma after rules(List) instead of a period.

Try fixing those problems and see if you make some progress.

Thanks for the quick response. I resolved those problems, but it's still not going into the rules. Any statement I give it just returns false.?- begin(Input). Begin.. |: Mary is a girl. false. — Terri Brasher, Apr 29 '13 at 17:04
Don't debug it from the very top. Test each piece separately, like you would any other program. Trace in if you don't get the result you expect. — Daniel Lyons, Apr 29 '13 at 17:07

Prolog program- implementing rules and list matching

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