How do you append an element to a list in place in Prolog?

Question

If I have a list in Prolog such as X = [1, 2, 3, 4], how do I add the element 5 to the end of the list to have X = [1, 2, 3, 4, 5]?

The append function needs two lists

Answer 1

You're worrying about the wrong end of the problem. Structure sharing can only happen by consing an element onto the beginning of the list. That method has the performance characteristics you want. Because of the way lists are defined, when you append two lists the entire first list will be copied. In this case, that's going to be the whole list. The garbage generated by a one-item list is obviously going to be much smaller than that.

If you really must append, consider building the list backwards and then reversing it once at the end, which is much cheaper, or use difference lists, which enable efficient appending to the end.