How do I rewrite the following so it uses if_?

I am doing some easy exercises to get a feel for the language.

is_list([]).
is_list([_|_]).

my_flatten([],[]).
my_flatten([X|Xs],RR) :-
   my_flatten(Xs,R),
   (is_list(X), !, append(X,R,RR); RR = [X | R]).

Here is a version using cut, for a predicate that flattens a list one level.

my_flatten([],[]).
my_flatten([X|Xs],RR) :-
   my_flatten(Xs,R),
   if_(is_list(X), append(X,R,RR), RR = [X | R]).

Here is how I want to write it, but it does not work. Neither does is_list(X) = true as the if_ condition. How am I intended to use if_ here?

Solution

In Prolog, the equivalen of an if … then … else … in other languages is:

(condition -> if-true; if-false)

With condition, if-true and if-false items you need to fill in.

So in this specific case, you can implement this with:

my_flatten([],[]).
my_flatten([X|Xs],RR) :- 
    my_flatten(Xs,R),
    (  is_list(X)
    -> append(X,R,RR)
    ; RR = [X | R] ).

or we can flatten recursively with:

my_flatten([],[]).
my_flatten([X|Xs],RR) :- 
    my_flatten(Xs,R),
    (  flatten(X, XF)
    -> append(XF,R,RR)
    ; RR = [X | R] ).