## 4.28 Built-in list operations

Most list operations are defined in the library `library(lists)` described in section A.11. Some that are implemented with more low-level primitives are built-in and described here.

is_list(+Term)
  True if Term is bound to the empty list (`[]`) or a term with functor ```.``' and arity 2 and the second argument is a list.53In versions before 5.0.1, is_list/1 just checked for `[]` or `[_|_]` and proper_list/1 had the role of the current is_list/1. The current definition is conform the de-facto standard. Assuming proper coding standards, there should only be very few cases where a quick-and-dirty is_list/1 is a good choice. Richard O'Keefe pointed at this issue. This predicate acts as if defined by the definition below on acyclic terms. The implementation is fails safely if Term represents a cyclic list.

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

memberchk(?Elem, +List)
  Equivalent to member/2, but leaves no choice point.
length(?List, ?Int)
  True if Int represents the number of elements of list List. Can be used to create a list holding only variables.
sort(+List, -Sorted)
  True if Sorted can be unified with a list holding the elements of List, sorted to the standard order of terms (see section 4.6). Duplicates are removed. The implementation is in C, using natural merge sort.54Contributed by Richard O'Keefe. The sort/2 predicate can sort a cyclic list, returning a non-cyclic version with the same elements.
msort(+List, -Sorted)
  Equivalent to sort/2, but does not remove duplicates. Fails with a `type_error` if List is not a list or a cyclic list.
keysort(+List, -Sorted)
  List is a proper list whose elements are `Key-Value`, that is, terms whose principal functor is (-)/2, whose first argument is the sorting key, and whose second argument is the satellite data to be carried along with the key. keysort/2 sorts List like msort/2, but only compares the keys. It is used to sort terms not on standard order, but on any criterion that can be expressed on a multi-dimensional scale. Sorting on more than one criterion can be done using terms as keys, putting the first criterion as argument 1, the second as argument 2, etc. The order of multiple elements that have the same Key is not changed. The implementation is in C, using natural merge sort. Fails with a `type_error` if List is not a list or a cyclic list or one of the elements of List is not a pair.
predsort(+Pred, +List, -Sorted)
  Sorts similar to sort/2, but determines the order of two terms by calling Pred(-Delta, +E1, +E2) . This call must unify Delta with one of ``<``, ``>`` or ``=``. If built-in predicate compare/3 is used, the result is the same as sort/2. See also keysort/2.55Please note that the semantics have changed between 3.1.1 and 3.1.2
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.
merge_set(+Set1, +Set2, -Set3)
  Set1 and Set2 are lists without duplicates, sorted to the standard order of terms. Set3 is unified with an ordered list without duplicates holding the union of the elements of Set1 and Set2.