Prolog, building list with conditional clauses

Question

I need to do this homework assignment using prolog (SWI-flavor) and cant get my head around some things.

For example, if i want to iterate through a list and add it

Answer 1

you can use the if structure:

 -> (do_something) ; (do_something else)

in this case:

goal(Stuff):-
  do_something(X),
  if_something(Y)-> do_this(Y) ; true,
  always_do_this(Z).

or you simply write two clauses like:

goal(Stuff):-
  do_something(X),
  conditional_stuff(Y),
  always_do_this(Z).

conditional_stuff(Y):-
  condition(Y),
  do_this(Y).

conditional_stuff(_).