Myujth

Completely new to prolog. Interesting journey so far in trying to change how I think, so appreciate any help here.

I am trying to assert facts for a pre-defined set of names. For example, assume I have a a set of people [alice, bob, ...] in one file. I would like to assert facts about these folks in other files, but want to make sure that these folks exist and that is checked when the facts are loaded/compiled(?).

For example, assume I don't have 'chuck' in the list and I make an assertion:

user: swipl app.pl

?- full_name(chuck, "Charlie Steel").

should result in an error.

What is the best way I can do this?

So, here's the code I came up with:

person(deborah).
person(tony).

read_my_file(Filename) :-
 open(Filename, read, In),
 read_my_file1(In),
 close(In).

read_my_file1(In) :-
 read(In, Term),
 ( Term == end_of_file
 -> true
 ; assert_or_abort(Term),
 read_my_file1(In)
 ).

assert_or_abort(Term) :-
 ( full_name(Person, Name) = Term
 -> ( person(Person)
 -> assertz(full_name(Person, Name))
 ; format(user, '~w is not a person I recognize~n', [Person])
 )
 ; format(user, '~w is not a term I know how to parse~n', [Term])
 ).

The trick here is using read/2 to obtain a Prolog term from the stream, and then doing some deterministic tests of it, hence the nested conditional structure inside assert_or_abort/1. Supposing you have an input file that looks like this:

full_name(deborah, 'Deborah Ismyname').
full_name(chuck, 'Charlie Steel').
full_name(this, has, too, many, arguments).
squant.

You get this output:

?- read_my_file('foo.txt').
chuck is not a person I recognize
full_name(this,has,too,many,arguments) is not a term I know how to parse
squant is not a term I know how to parse
true.

?- full_name(X,Y).
X = deborah,
Y = 'Deborah Ismyname'.