4.1.3.1 Lists

Lists form an important class of data structures in Prolog. They are essentially the same as the lists of Lisp: a list is either the atom [], representing the empty list, or else a compound term with functor . and two arguments, which are the head and tail of the list respectively, where the tail of a list is another list. Thus a list of the first three natural numbers is the structure

             .
            / \
           1    .
               / \
              2    .
                  / \
                 3   []

which could be written using the standard syntax, as (A) but which is normally written in a special list notation, as (B). Two examples of this list notation, as used when the tail of a list is a variable, are (C), which represent the structure in (D).

.(1,.(2,.(3,[])))  (A)

[1,2,3]  (B)

[X|L]     [a,b|L]  (C)

             .               .
            / \             / \
          X     L         a     .
                               / \
                             b     L  (D)

Note that the notation [X|L] does not add any new power to the language; it simply improves readability. These examples could be written equally well as (E).

 .(X,L) .(a,.(b,L))  (E)