Learn Prolog Now! - Chapter 6 - Practical Session - Programming Exercise 3 - Flatten a list

lpn-6-practical-3.pl

% flatten/2 - Checks whether the second argument is a fully flattened form of the first, both of which are lists
% Caveat - This solution doesn't make use of append/3 as the exercise page explicitly stated but uses append/2 instead (probably not what the author intended) :p

flatten([], []).
flatten([X | List], [X | Flat]) :- \+ is_list(X), flatten(List, Flat).
flatten([X | List], NewFlat) :- is_list(X), flatten(X, XFlat), flatten(List, Flat), append([XFlat, Flat], NewFlat).