Prolog: Doubling the value of each element in a list of lists and returning a single list -

i need write set of clauses take list of integer lists , return single list elements doubled.

for example:

?- double([[1,2],[3]], x). yes x = [2,4,6] 

i have set of clauses called mega_append return single list list of lists.

for example:

?- mega_append([[1,2],[3]], x). yes x = [1,2,3] 

here progress (m_a short mega_append):

double([],[]). double(list,[h1|t1]) :-     m_a(list,[h2|t2]),     h1 2 * h2,     double(t2, t1). 

i'll try explain how thought work. flatten first list , split head , tail (h2 , t2). split second list head , tail (h1 , t1). check make sure h1 (the doubled value) equal 2 times h2 (the original value). if check rest of list. if match correctly should left 2 empty lists should match first clause , return yes.

it works when there single value (for example: double([[1]], x)). can offer insight doing wrong? logic or code incorrect?

your problem t2 single list list after recursive call not list of lists.

to solve can first use mega_append flatten list , use auxiliary predicate work on flattened list. i.e. double this:

double([],[]). double(list,x) :-     m_a(list,flist),     double_aux(list, flist). 

edit: here way use 1 clause since want see one. recommend using auxiliary predicate.

double([],[]). double([[]],[]). double(list,[h1|t1]) :-     mega_append(list,[h2|t2]),     h1 2 * h2,     double([t2], t1).