Example
Lists are a special kind of compound term. Lists are defined inductively:
- the atom
[]is a list, denoting the empty list. - if
Lsis a list, then the term'.'(L, Ls)is also a list.
There is a special syntax for denoting lists conveniently in Prolog:
- The list
'.'(a, '.'(b, '.'(c, [])))can also be written as[a,b,c]. - 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]]].
