Custom Comparator Sets in Clojure

Alpha A custom Clojure/Script type for custom comparator sets.

Rationale

When dealing with domain data in maps this data often has unique identifiers. To make merging easy and guarantee that there are no two items with the same identifier people often store those models in maps of the following format:

{(:id model) model
 ,,,}

While this certainly is a solution it complicates the handling of the data in every place imaginable. Often you just want (vals my-models) instead of the actual map. Adding new items requires code that is tightly coupled to the underlying map that you only chose to guarantee uniqueness of a certain property ((assoc my-models (:id model) model)).

Custom Sets

This library provides a custom type CustomComparatorSet which allows users to manage data in a set-like datastructure while using a custom function as uniqueness comparator.

;; (import [org.martinklepsch.cc_set.impl CustomComparatorSet])            ; Clojure
;; (require '[org.martinklepsch.cc-set :refer [custom-comparator-set]]) ; ClojureScript
(def x (custom-comparator-set :id))
(conj x {:id 1} {:id 2})
;; => #CustomComparatorSet{{:id 1} {:id 2}}

While a keyword :id is used as comparator here you can also use more complex functions.

A constructor set-by is provided which additionally wraps the passed comparator in a nil check, throwing if nil is returned.

(require '[org.martinklepsch.cc-set :refer [set-by]])
(set-by :id {:id "a"} {:id "x"} {:id "b"})

If you don't want the nil guard behavior you can implement your own constructor like this:

(defn my-set-by [comparator & keys]
  (reduce conj (CustomComparatorSet. {} comparator) keys)))

Simple!

Low level constructors

Clojure

(import [org.martinklepsch.cc_set.impl CustomComparatorSet])
(CustomComparatorSet. {} :id)

ClojureScript

(require '[org.martinklepsch.cc-set.impl :refer [CustomComparatorSet]])
(CustomComparatorSet. nil {} :id)

Notice the different arity, resulting of the differences in how Clojure and ClojureScript handle metadata.

Differences to regular sets

get

Regular set: (get regular-set v) looks for entries where v == entry

Custom-comparator set: (get ccset v) look for entries where (comparator v) == (comparator entry)

conj

Regular set: (conj regular-set v) will add the value only if the set does not already contain it, maintaining uniqueness by value equality

Custom-comparator set: (conj ccset v) will behave like assoc with the key (comparator v) and the value v, maintaining uniqueness by comparator. This means that values will be updated if the comparator result is equal but the value is different

Building and Testing

boot build-jar                   # install to ~/.m2
boot testing test-cljc           # run tests once
boot testing watch test-cljc     # run tests after changes

Contributing

Feedback and PRs welcome, especially generative testing seems like a good fit.

License

MPLv2, see LICENSE.