Question

Trying to append a tree structure into a list. In Erlang I would use the ++ operator for appending but I keep getting syntax errors when doing this in Prolog.

I will include my error code, its all about the appending of lists that I think is missing.

tree2list(leaf(X),L++[X]).
tree2list(branch(Branch1,Branch2),L):-
tree2List(B1,L1),
tree2List(B2,L2),
L++L1++L2.

Any ideas how to solve my appending issue?

Answer 1

Sure, use difference lists:

tree2list(leaf(X),[X|A]-A).
tree2list(branch(B1,B2),L-Z):-
        tree2list(B1,L-A),
        tree2list(B2,A-Z).

/* 7 ?- tree2list(branch(leaf(1),leaf(2)), L-[]).
L = [1, 2] ;
No
8 ?- */

But your tree doesn’t seem to have empty branches. Add them thus:

tree2list(empty,A-A).

/* 10 ?- tree2list(branch(branch(leaf(1),leaf(2)),empty), L-[]).
L = [1, 2] ;
No
11 ?- */