Make Range, Repeat and Cycle implement Indexed

Question

Currently, Cycle, Range and Repeat do not implement Indexed, which means that "nth" is O( n ) on average.

The proposed change is to implement Indexed for these classes, so that "nth" becomes an O( 1 ) operation.

Answer 1

Comment made by: alexmiller

This is an expansion of capabilities and commitments beyond what these functions have done in the past. We've already committed to more than we really wanted to with them, so I'm not sure we want to add yet more commitments. In any case, we won't do it for 1.7.

FYI, that Range you're patching is not currently used for anything - the current impl uses a chunked seq definition in core.clj. CLJ-1515 will (likely) replace the Range class with an all new impl. In any case, patching Range here probably isn't useful until CLJ-1515 is resolved.

Answer 2

Comment made by: mikera

Understood re 1.7. Though I personally think this is small enough that you could squeeze it in. Your call.

However I still think this approach is useful though: nth is a very common operation and I note you aren't benchmarking it yet in CLJ-1515. Whatever new Range implementation is used will benefit from implementing Indexed.

Answer 3

Comment made by: alexmiller

None of these functions currently promises to return something Indexed. If we add that and people come to rely on it, we can never change the implementation in a way that removes it. So I'm not sure that's a promise we want to make.

Answer 4

Comment made by: mikera

I am not proposing that we make any "promise" of an indexed return value, simply that such classes implement the interface as an implementation detail (where it makes sense).

This then causes the fast path in functions like RT.nth to be taken, so we get O(1) instead of O( n) for the most common indexed lookup cases.

TBH, my assumption was that this was the main purpose of the "Indexed" interface, i.e. to allow concrete collection types to participate in Clojure's indexed access functions with an efficient implementation.

Answer 5

Comment made by: gshayban

People regularly rely on implementation details, promise or no promise. If clojure were to add Indexed then remove it, people's code would either break or get slower.

Implementation behavior (whether overt or implicit) is necessarily treated as future constraints (shackles), so it is considered carefully.

Answer 6

Reference: https://clojure.atlassian.net/browse/CLJ-1690 (reported by mikera)