序言：附加到列表的开头

Question

How do I do the above? Specifically, if I have a list like this:

List = [8, 9, 10]. % I know this using matching to assign the say that [8, 9, 10] = [8, 9, 10], thus making List contain [8, 9, 10]

Now if I try to append another variable to this existing list, how do I do this? I tried this:

List = [7|List].

This will obviously not work (although I wish it did). Then I tried this:

List is [7|List].

Still nothing. What can I do to append a variable to the head of a list? If there is a way without using another function, but by manipulating this list, I will prefer, but all solutions are welcome.

Thanks

---

Say I have a stub like so:

stub(List):-
    List2 = [7|List],
    %% Do something recursively
    List = List2 %% How do I do thi part?

Found a solution, doesn't look very elegant but it works!

stub(List, List2):-
    stub(List, [7|List2]), %% Some base case causes this recursion to stop
    %% Do something in the recursion
    List = List2 %% Finally do what I wanted to do

Answer 1

You need a new variable. Thus List2 = [7|List] .

This is very odd compared to traditional, command oriented programming languages (also known as imperative pls), but it is the very essence of a declarative programming language, in particular a logic programming language.

There are a lot of implications behind this, it permits - for instance - to reason about a program in a much easier manner.

_{In any case, please do consult an introductory Prolog book like the Art of Prolog.}