Prolog Language Lists


Example

Lists are a special kind of compound term. Lists are defined inductively:

  • the atom [] is a list, denoting the empty list.
  • if Ls is a list, then the term '.'(L, Ls) is also a list.

There is a special syntax for denoting lists conveniently in Prolog:

  1. The list '.'(a, '.'(b, '.'(c, []))) can also be written as [a,b,c].
  2. The term '.'(L, Ls) can also be written as [L|Ls].

These notations can be combined in any way. For example, the term [a,b|Ls] is a list iff Ls is a list.

Creating lists

A list consisting of literals unified with the variable List:

?- List = [1,2,3,4].
List = [1, 2, 3, 4].

Building a list by consing:

?- Tail = [2, 3, 4], List = [1|Tail].
Tail = [2, 3, 4],
List = [1, 2, 3, 4].

Building a list of unknown values using the built-in length/2:

?- length(List,5).
List = [_G496, _G499, _G502, _G505, _G508].

Since in Prolog everything is in essence a Term, lists behave heterogeneous:

?- List = [1, 2>1, this, term(X), 7.3, a-A].
List = [1, 2>1, this, term(X), 7.3, a-A].

This means a list can also contain other lists, also called inner lists:

List = [[1,2],[3,[4]]].