Question

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Prolog delete: doesn't delete all elements that unify with Element

In Prolog if you write this:

delete([(1,1),(1,2),(1,1),(3,4)],(1,_),L).

the result will be:

L = [ (1, 2), (3, 4)].

What is normal because the _ variable binds with 1 in the first element and it searches for further elements of (1,1) and deletes them.

Is there a way to prevent this unification from happening and deleting all members of the form (1,_).
In that case the result must be:
L = [ (3, 4)].

Answer 1

delete_pattern([], _, []).
delete_pattern([H|T], P, O) :-
    (    H \= P
    ->   O = [H|O1],
         delete_pattern(T, P, O1)
    ;    delete_pattern(T, P, O) ).

You may like to use other predicates for filtering that would result in slightly different semantics as ==/2 or =@=/2.