In a static language like Scala or Haskell you usually work with a type-class, which is pretty cool because you can provide implementations for types you don't control (not implying that Scala's Ordering is perfect). Instead of Java's Comparable interface I was expecting a protocol, which are almost equivalent to type-classes, or a multi-method.

Of course, in many implementations you usually also get a version of a constructor that uses a provided comparison function. However certain things, like plain numbers or strings, have a natural ordering to them, hence the need to have the sorted-set in Clojure also work with Java's Comparable. But again, I was expecting a Clojure specific protocol.

The reason for why this happens, I believe, is because protocols weren't there from the beginning, being a feature introduced in Clojure 1.2. From what I understood, in ClojureScript protocols are "at the bottom" as they say, so there's hope :-)

Thanks! So if I read your comment correctly there is nothing inherently wrong with the Comparable interface it's just that Clojure's sorted sets and maps could have used protocols instead. I can see why that would be useful for extending existing types (as you mentioned).

OTOH it’s not often I’ve seen third-party types that I wanted to be comparable but were not. In general I think that if a type does not implement a core interface you have to consider the possibility that the designer chose not to implement it for a reason.

Protocols were added in Clojure 1.2, well after the Clojure collections or stdlib were created. In a perfect world, Clojure itself could leverage protocols more widely across the stdlib. For practical reasons, this is difficult now.

Maybe a mitigating factor is that any Clojure 2-arg function extends AFunction which implements Comparator. So any Clojure function that returns -1/0/1 will work transparently. Example, use - as a comparator:

=> (sorted-set-by - 2 9 3 5 4) #{2 3 4 5 9}