A comparison function decides the relative order of two values. Its numeric result is negative when the first value comes before the second, zero when they compare equally, and positive when it comes after. Test the sign, not an exact value: (compare "a" "z") returns -25, not -1. section 6.4
Clojure's compare is the default comparison function for sorting and ordered collections. It accepts two arguments and can compare numbers, nil, and compatible values implementing java.lang.Comparable. A custom comparison function supplies a different order; Clojure functions can also serve as java.util.Comparator objects for sorted-set-by. section 6.4 section 13.3
Reverse a sorted set
(sorted-set-by #(compare %2 %1) "t" "d" "j") ;; #{"t" "j" "d"} Reversing the arguments reverses the order. sorted-map-by likewise accepts a comparison function for its keys; without one, sorted-map and sorted-set use the default order. section 11.1.3 section 13.3
In a sorted map or set, zero has a consequence beyond ordering: the collection treats the compared keys or elements as the same. Comparing vectors only by count therefore collapses different vectors of equal length. Add a tie-breaker if both must remain present. section 11.1.3 section 13.3
Keep equal-length vectors distinct
(def by-count (fn [a b] (compare [(count b) a] [(count a) b]))) (sorted-set-by by-count [1 :a] [:b] [3 :c] [:v]) ;; #{[1 :a] [3 :c] [:b] [:v]} Length determines the primary order; the vectors themselves break ties. section 13.3
Check edge cases before relying on an order. Incompatible values can cause ClassCastException. compare treats comparisons involving Double/NaN as zero, which can make sorting results surprising. The practical rule is to choose a comparison that both orders the values and distinguishes every value the collection must retain. section 6.4 section 13.3
