Question

I’m a newbie prolog programmer, and for an assignment, I have to have a basic program that succeeds if and only if list X is a list of two elements, with the first as the same as the second.

From my view of prolog, programs seem to be pretty small, so I typed this in:

firstPair(x,x).

When I run it under swipl, I get this as output:

Syntax error: Operator expected

Is there something more that needs to be done? I thought that if I executed this with say, firstPair(1,2). this would be all it would need to know that it is false.

Answer 1

First, lowercase x is not a variable, it’s an atom. Make x uppercase to fix the problem:

firstPair(X,X).

Second, you do not type this into the interpreter. Rather, you write it into a file firstPair.pl, and then read that file into Prolog.

At the command prompt, type this:

['firstPair.pl'].

Press enter. Now you can use your firstPair/2 rule.

Finally, since the assignment talks about lists, I think the instructor wanted you to write firstPair/1, not firstPair/2:

firstPair([X,X]).