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|Built-in list operations|
Most list operations are defined in the library
described in section A.21.
Some that are implemented with more low-level primitives are built-in
and described here.
) or a compound term with name‘
’126The traditional list functor name 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.127In 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 safely fails 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. If List is cyclic and Elem is not a member of List, memberchk/2 eventually raises a
typeerror.128Eventually here means it will scan as many elements as the longest list that may exist given the current stack usage before raising the exception.
?- length(List,4). List = [_27940,_27946,_27952,_27958]. ?- length(List,Length). List = , Length = 0 ; List = [_24698], Length = 1 ; List = [_24698,_25826], Length = 2 ...
It raises errors if Length is bound to a non-integer or a negative integer or if List is neither a list nor a partial list. This error condition includes cyclic lists:129ISO demands failure here. We think an error is more appropriate.
?- A=[1,2,3|A], length(A,L). ERROR: Type error: `list' expected ...
Covering an edge case, the predicate fails if the tail of List is equivalent to Length:130This 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.
?- List=[1,2,3|Length],length(List,Length). false. ?- length(Length,Length). false.
Note that List may contain non-ground terms. If Sorted
is unbound at call-time, for each consecutive pair of elements in
Sorted, the relation
E1 @< E2 will hold.
However, unifying a variable in Sorted may cause this
relation to become invalid,
even unifying a variable in Sorted with another
(older) variable. See also section
for the Order argument but this is likely to change. SWI-Prolog extends this predicate to deal with dicts.
If Key is the integer zero (0), the entire term is used to compare two elements. Using Key=0 can be used to sort arbitrary Prolog terms. Other values for Key can only be used with compound terms or dicts (see section 5.4). An integer key extracts the Key-th argument from a compound term. An integer or atom key extracts the value from a dict that is associated with the given key. A type_error is raised if the list element is of the wrong type and an existence_error is raised if the compound has not enough argument or the dict does not contain the requested key.
Deeper nested elements of structures can be selected by using a list of keys for the Key argument.
The Order argument is described in the table below:133For
compatibility with ECLiPSe, the values
are allowed as synonyms.
The sort is stable, which implies that, if duplicates are kept, the order of duplicates is not changed. If duplicates are removed, only the first element of a sequence of duplicates appears in Sorted.
This predicate supersedes most of the other sorting primitives, for example:
sort(List, Sorted) :- sort(0, @<, List, Sorted). msort(List, Sorted) :- sort(0, @=<, List, Sorted). keysort(Pairs, Sorted) :- sort(1, @=<, Pairs, Sorted).
The following example sorts a list of rows, for example resulting from csv_read_file/2) ascending on the 3th column and descending on the 4th column:
sort(4, @>=, Rows0, Rows1), sort(3, @=<, Rows1, Sorted).
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 4.6.1). 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).
. Duplicates are removed (i.e. equivalence classes of elements as defined by Pred are collapsed to a single element in Sorted) If the built-in predicate compare/3 is used, the result is the same as sort/2. See also keysort/2.