Most list operations are defined in the library
described in section A.11. Some
that are implemented with more low-level primitives are built-in and
) or a term with functor `
' and arity 2 and the second argument is a list.55In versions before 5.0.1, is_list/1 just checked for
[_|_]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).
type_errorif List is not a list or a cyclic list.
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_errorif List is not a list or a cyclic list or one of the elements of List is not a pair.
. If built-in predicate compare/3 is used, the result is the same as sort/2. See also keysort/2.57Please note that the semantics have changed between 3.1.1 and 3.1.2