Flatten a list in Prolog

Question

I\'ve only been working with Prolog for a couple days. I understand some things but this is really confusing me.

I\'m suppose to write a function that takes a list

Answer 1

I didn't find a solution using findall, so I'll add it. (it will work if the list is ground)

First, we define how to test for a list:

list(X) :- var(X), !, fail.
list([]).
list([_|_]).

and the transitive closure of member, we call it member*:

'member*'(X, Y) :- member(X, Y).
'member*'(X, Y) :- member(Z, Y), 'member*'(X, Z).

The flattened list is all the solution of member* which are not lists:

flatten(X, Y) :- findall(Z, ('member*'(Z, X), \+ list(Z)), Y).

Example:

?- flatten([[4],[[5,6],[7,[8],[9,[10,11]]]]],Y).
Y = [4, 5, 6, 7, 8, 9, 10, 11].