Question

I am a bit confused about this function definition in Prolog:

sample(X,[X|Tail]) :- member(X,Tail).

this function checks if X is at the first position of the given list and if X is also found in the tail of the list.

sample(1,[1,2,3]).
false.
% because 1 is not found in the tail

sample(1,[1,2,1]).
true.

But how does it work? X is the parameter given by the user but it seems to be overwriten by the head|tail extraction from the list. So it seems X new value is the first element in the list.

Answer 1

You’re thinking too hard. In the first case:

sample(1, [1,2,3])

Prolog is able to bind X, because 1 is 1:

sample(1, [1,2,3])
X=1, Tail=[2,3]

Prolog then checks the body of the clause:

member(X, Tail) -> member(1, [2,3])

This is obviously false, so sample(1, [1,2,3]) doesn’t succeed.

In the next case, the binding still works, but Tail is bound to something else:

sample(1, [1,2,1])
X=1, Tail=[2,1]

So we check the body:

member(1, [2,1])

which is obviously true, so sample(1, [1,2,1]) succeeds.

Now, if you were to try this call:

sample(1, [2,3,1])

You’ll still get false, because in this case, 1 isn’t equal to 2, so there’s no way to bind X at the beginning, so the body never gets checked. sample(1, [2,3,1]) is false even though 1 appears in both places, because it didn’t appear at the beginning of the list as well.

I have no idea why you’d want such a function, but it is working as one would expect. There’s no overwriting of variables happening here.