Ok, let's say i have these two facts:
data1([50, 20, 15, 20, 25, 20, 84]). data2([50, 30, 15, 5, 10, 18, 60]).
I want to make a query that asks whether the N-th element of the list defined as a part of those facts is some value.
query([L1|Tail]) :- L1 is 50. This would work but only if the query is like that - query([50, 30, 20]).. I want to ask something like query(data1).
CapelliC's answer is correct, and he discusses why you should use the built-in nth/3 or however it is called (nth1/3 is actually not ISO, and not available in some implementations, like GNU-Prolog). I just want to add some clarification:
Your first issue here is that to ask a question, "does the Nth element of a list have a certain value", you need two arguments already: N and Value. Also, is is a predicate for evaluating arithmetic expressions and cannot be used for your purpose.
As for the query(data1), you need some form of meta-calling....
In summary:
% query(Fact, N, Value) is true
% when the Nth element of the list argument of Fact is Value
query(Fact, N, Value) :-
call(Fact, List),
nth(N, List, Value).
and it can be called like this:
?- query(data1, 3, 15).
to query all data1 facts that have one argument, a list.
Here nth should be defined as:
nth(N, List, Value) is true when Value is the Nth element of List
but you should really rather use the built-in available in the implementation you are using.