How do return both a variable result and a true/false in Prolog?

Question

It sounds silly, but lets say my predicate largest/2 returns the largest element in a list...the output should look like this:

?- largest([1,2,3,4,5], X).
X = 5.
false.

I implemented largest, and it works like above except it doesn't output "false". How do I make it so it also outputs this "false." value? This is for an annoying assignment I have to finish. :(

Answer 1

That extra false. or No just means that the person running the program asked to get all possible solutions for X, not just the first possible solution.

On most interactive Prolog interpreters, you check to see if there is another solution by pressing the semicolon (;) key.

Answer 2

sounds like impossible, as if predicate fails, no binding of free variables happens, see

 ?- A=5.
A = 5.

 ?- A=5,false.
false.

however

 ?- A=5;false.
A = 5 ;
false.

To achieve this you should make your predicate "largest" non-deterministic. But to me this seems pretty silly.