Most list operations are defined in the library
described in section A.13. Some
that are implemented with more low-level primitives are built-in and
) or a term with functor `
'[|]''102The traditional list functor is the dot (
'.'). This is still the case of the command line option --traditional is given. See also section 5.1. and arity 2 and the second argument is a list.103In 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 conforms to 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 fails safely if Term represents a cyclic list.
is_list(X) :- var(X), !, fail. is_list(). is_list([_|T]) :- is_list(T).
typeerror if scanning List encounters a non-list. Note that memberchk/2 does not perform a full list typecheck. For example,
memberchk(a, [a|b])succeeds without error and memberchk/2 loops on a cyclic list if Elem is not a member of List.
This predicate fails if the tail of List is equivalent to
is logically correct. An exception would be more appropriate, but to our
best knowledge, current practice in Prolog does not describe a suitable
candidate exception term.
type_errorif List is a cyclic list or not a list.
Key-Valuepairs, terms whose principal functor is (-)/2. List is sorted on Key according to the standard order of terms (see section 4.7.1). Duplicates are not removed. Sorting is stable with regard to the order of the Values, i.e., the order of multiple elements that have the same Key is not changed.
predicate is often used together with library
library(pairs). It can be used to sort lists on different
or multiple criteria. For example, the following predicates sorts a list
of atoms according to their length, maintaining the initial order for
atoms that have the same length.
:- use_module(library(pairs)). sort_atoms_by_length(Atoms, ByLength) :- map_list_to_pairs(atom_length, Atoms, Pairs), keysort(Pairs, Sorted), pairs_values(Sorted, ByLength).
. If the built-in predicate compare/3 is used, the result is the same as sort/2. See also keysort/2.