4.29 List Manipulation

is_list(+Term)

Succeeds if Term is bound to the empty list () or a term with functor `.' and arity 2 and the second argument is a list. (28) This predicate acts as if defined by the following definition:

is_list(X) :- var(X), !, fail. is_list([]). is_list([_|T]) :- is_list(T).

append(?List1, ?List2, ?List3)

Succeeds when List3 unifies with the concatenation of List1 and List2. The predicate can be used with any instantiation pattern (even three variables).

member(?Elem, ?List)

Succeeds when Elem can be unified with one of the members of List. The predicate can be used with any instantiation pattern.

memberchk(?Elem, +List)

Equivalent to member/2, but leaves no choice point.

delete(+List1, ?Elem, ?List2)

Delete all members of List1 that simultaneously unify with Elem and unify the result with List2.

select(?Elem, ?List, ?Rest)

Select Elem from List leaving Rest. It behaves as member/2, returning the remaining elements in Rest. Note that besides selecting elements from a list, it can also be used to insert elements. (29)

nth0(?Index, ?List, ?Elem)

Succeeds when the Index-th element of List unifies with Elem. Counting starts at 0.

nth1(?Index, ?List, ?Elem)

Succeeds when the Index-th element of List unifies with Elem. Counting starts at 1.

last(?Elem, ?List)

Succeeds if Elem unifies with the last element of List. If List is a proper list last/2 is deterministic. If List has an unbound tail, backtracking will cause List to grow.

reverse(+List1, -List2)

Reverse the order of the elements in List1 and unify the result with the elements of List2.

flatten(+List1, -List2)

Transform List1, possibly holding lists as elements into a `flat' list by replacing each list with its elements (recursively). Unify the resulting flat list with List2. Example:

?- flatten([a, [b, [c, d], e]], X). X = [a, b, c, d, e]

length(?List, ?Int)

Succeeds if Int represents the number of elements of list List. Can be used to create a list holding only variables.

merge(+List1, +List2, -List3)

List1 and List2 are lists, sorted to the standard order of terms (see section 4.6). List3 will be unified with an ordered list holding both the elements of List1 and List2. Duplicates are not removed.