Java’s `Comparable` interface gives instances a natural ordering through `compareTo`. Clojure’s `compare` uses that ordering for compatible values, and functions such as `sort` and `sort-by` use `compare` by default. A comparison returns a negative number, zero, or a positive number—not necessarily −1, 0, or 1. section 6.4
The ordering must be usable for the values being compared:
- `compareTo` on the first value must accept the second value’s type; implementing `Comparable` alone does not make every pair compatible. An incompatible pair can cause a `ClassCastException`. section 6.4
- `compare` also handles `nil` and pairs of numbers. Clojure vectors, keywords, ratios, refs, and symbols provide their own comparison behavior. section 6.4
Make records sortable
A `Point` record can implement `Comparable` with `compareTo` that calls `compare` on each point’s distance from an origin. A `GasStation` record can then compare its locations; `(sort gas-stations)` orders stations by distance rather than brand. The same ordering can govern keys in a `sorted-map`. Use `sorted-map-by` when a different comparator is needed. section 6.4 section 11.1.3
(defn- sq [x] (* x x))
(defn- distance [x1 y1 x2 y2] ; #1
(Math/sqrt (+ (sq (- x1 x2)) (sq (- y1 y2)))))
(defrecord Point [x y distance-origin] ; #2
Comparable
(compareTo [this other]
(compare (distance-origin (:x this) (:y this)) ; #3
(distance-origin (:x other) (:y other)))))
(defn relative-point [x1 y1 x2 y2] ; #4
(->Point x1 y1 (partial distance x2 y2)))
(defrecord GasStation [brand location] ; #5
Comparable
(compareTo [this other]
(compare (:location this) (:location other)))) ; #6
The record definitions show `compareTo` delegating to `compare`, so each record’s ordering follows the value chosen for comparison. section 6.4
