Question

I wonder if it’s possible to do something like the following in prolog

L = [1,2,3,4,5,_,_,7].

i want to do something like

getElements(L, R)

that returns
R = [1,2,3,4,5,7]

and ignore the wildcards in the list

Answer 1

If your Prolog has library(apply), you can write

getElements(L, R) :- 
   include(nonvar, L, R).

or, a bit more compact

getElements(L, R) :- 
   exclude(var, L, R).

Otherwise, this should be a working predicate:

getElements([], []).
getElements([H|T], [H|R]) :-
  nonvar(H), !,
  getElements(T, R).
getElements([_|T], R) :-
  getElements(T, R).

edit as highlighted by @false, the latter getElements/2 is buggy, due to matching with the anonymous variable in last clause. Here a correction

getElements([], []).
getElements([H|T], R) :-
  var(H), !,
  getElements(T, R).
getElements([H|T], [H|R]) :-
  getElements(T, R).