Did you know ... Search Documentation:
Predicate predsort/3
Availability::- use_module(library(sort)).(can be autoloaded)
Sourcepredsort(+Pred, +List, -Sorted)
Sorts similar to sort/2, but determines the order of two terms by calling Pred(-Delta, +E1, +E2) . This call must unify Delta with one of <, > or =. 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.
Tags are associated to your profile if you are logged in|Report abuse
Tags:
  • doc-needs-help
Anne Ogborn said (2018-03-23T16:38:04):1 upvotes 1 0 downvotes
Picture of user Anne Ogborn.

predsort/3 removes duplicates, based on the predicate saying =

This bit me when I wanted to sort on a field in a dict.

LogicalCaptain said (2020-04-03T12:25:28):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

This absolutely needs to be extended to

predsort(+Pred, +List, -Sorted, +Flags)

Where Flags should at least accept "collapse_equivalence_classes(true)", "collapse_equivalence_classes(false)"

Here is another example of predsort in a rather disconcerting duplicate-killing action:

https://stackoverflow.com/questions/61006643/how-to-sort-a-list-of-strings-a-bcd-ef-and-ghij-in-descending-order-o/

Example of hierarchical sorting

An example of hierarchical sorting using predsort/3 which takes care to not lose records via "duplicate elimination":

hierarchical_sorting_example.pl

See also

order_by/2 from library(solution_sequences)