:- use_module(library(pairs)).(can be autoloaded)
?- group_pairs_by_key([a-2, a-1, b-4, a-3], X). X = [a-[2,1], b-, a-]
Sorting the list of pairs before grouping can be used to group all values associated with a key. For example, finding all values associated with the largest key:
?- sort(1, @>=, [a-1, b-2, c-3, a-4, a-5, c-6], Ps), group_pairs_by_key(Ps, [K-Vs|_]). K = c, Vs = [3, 6].
In this example, sorting by key only (first argument of sort/4 is 1) ensures that the order of the values in the original list of pairs is maintained.
|Joined||List of Key-Group, where Group is the list of Values associated with equivalent consecutive Keys in the same order as they appear in Pairs.|