1)For your 1st example,look at https://stackoverflow.com/questions/9875760/sum-of-elements-in-list-in-prolog#:~:text=The%20answer%20is%20simple%3A,lists%20with%20that%20specific%20sum.

"it does not allow you to generate lists with that specific sum"

2)For your 1st example, if you look at the signature, 2nd argument is - i.e. only output, so a result!

3)If you want a bidirectional workaround, think about member(+,+) for example

As per a remark made on stack overflow, this predicate's name

• expresses what it does "sum the list"
• but does not express the order of its parameters, otherwise it would be called list_sum/2.

The predicate exhibits a bruti dataflow :

?- sum_list([1,2,X],6).
ERROR: Arguments are not sufficiently instantiated

This is also the same as

list_to_sum(L,Sum) :- foldl([X,FL,TR]>>(TR is X+FL),L,0,Sum).

There is no mult_list/2 but one can easily add it:

list_to_product(L,Product) :- foldl([X,FL,TR]>>(TR is X*FL),L,1,Product).