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^{125For compatibility with ECLiPSe, the values <, =<, > and >= 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).

See also sort/2 (ISO), msort/2, keysort/2, predsort/3 and order_by/2.