Prolog: Filter a list into a list

Question

I want to create a filter in Prologue from a list that filters out all elements that match the filter.

Example: filter(list, element, result)
?- filter([1,2,3,2,3,1,5],3,X).
X=[3,3]

But I have created a filter that so far only outputs true or false.

filter([L|Y],X,OUT) :- 
    filter(Y,X,OUT).

I don't know how to create a list now.

Answer 1

You actually do not need to implement a filter yourself. There is a filter predicate include/3 [swi-doc] :

filter(L, X, R) :-
    , L, R.

You can also implement this yourself with recursion. Here the base case is an empty list:

filter([],_,[]).

for the recursive case, you can make an if-then-else expression that depending on whether the condition is satisfied, make a list that is (not) prepended with that element:

filter([Y|T], X, R) :-
    (  X == Y
    -> …
    ;  …
    ),
    filter(T, X, S).

here you still need to fill in … yourself.