Ndtyjky

So I am new to Prolog and have implemented my own implementation of member.

#contains element

mymember(A, [A|T]).

mymember(A, [X|T]) :-  mymember(A, T).

After looking up this should be the classical implementation, but when I test it, it fails:

[trace]  ?- mymember(2, [1, 2, 3]).

   Call: (8) mymember(2, [1, 2, 3]) ? 

   Call: (9) mymember(2, [2, 3]) ? 

   Call: (10) mymember(2, [3]) ? 

   Call: (11) mymember(2, ) ? 

   Fail: (11) mymember(2, ) ? 

   Fail: (10) mymember(2, [3]) ? 

   Fail: (9) mymember(2, [2, 3]) ? 

   Fail: (8) mymember(2, [1, 2, 3]) ? 

false.

Now what confuses me is that Prolog only uses the second case until it fails but never tries to match my first case. Does anybody know the reason of this behavior?

(I'm using SWI-Prolog Version 7.6.4)

EDIT: Well now I know what my problem was. Since I don't have syntax highlighting I didn't notice that I actually didn't use the correct character for one line comments. After replacing '#' with '%' my first case now matches in the second call. A typical beginner mistake I guess...

Your implementations works for me.

mymember(2,[1,2,3]).

    Call: (8) mymember(2, [1, 2, 3]) ? creep

    Call: (9) mymember(2, [2, 3]) ? creep

    Exit: (9) mymember(2, [2, 3]) ? creep

    Exit: (8) mymember(2, [1, 2, 3]) ? creep

true .

Though you could lose some of the variables as they are singletons, like this:

mymember(A, [A|_]).

mymember(A, [_|T]) :-  mymember(A, T).