SWI-Prolog 5.11.18 Reference Manual: Section A.17

## A.17 library(pairs): Operations on key-value lists

author
  Jan Wielemaker
See also
  keysort/2, library(assoc)

This module implements common operations on Key-Value lists, also known as Pairs. Pairs have great practical value, especially due to keysort/2 and the library `assoc.pl`.

This library is based on disussion in the SWI-Prolog mailinglist, including specifications from Quintus and a library proposal by Richard O'Keefe.

[det]pairs_keys_values(?Pairs, ?Keys, ?Values)
  True if Keys holds the keys of Pairs and Values the values.

  Deterministic if any argument is instantiated to a finite list and the others are either free or finite lists. All three lists are in the same order.

  See also
    pairs_values/2 and pairs_keys/2.
[det]pairs_values(+Pairs, -Values)
  Remove the keys from a list of Key-Value pairs. Same as pairs_keys_values(Pairs, _, Values)
[det]pairs_keys(+Pairs, -Keys)
  Remove the values from a list of Key-Value pairs. Same as pairs_keys_values(Pairs, Keys, _)
[det]group_pairs_by_key(+Pairs, -Joined:list(Key-Values))
  Group values with the same key. Pairs must be a key-sorted list. For example:

      ?- group_pairs_by_key([a-2, a-1, b-4], X).

      X = [a-[2,1], b-[4]]

  Pairs | Key-Value list, sorted to the standard order of terms (as keysort/2 does)
  Joined | List of Key-Group, where Group is the list of Values associated with Key.

[det]transpose_pairs(+Pairs, -Transposed)
  Swap Key-Value to Value-Key and sort the result on Value (the new key) using keysort/2.
map_list_to_pairs(:Function, +List, -Keyed)
  Create a key-value list by mapping each element of List. For example, if we have a list of lists we can create a list of Length-List using

              map_list_to_pairs(length, ListOfLists, Pairs),