Small PVs are never chunked

Question

A PersistenVector that has <=32 elements and is {{seq}}'ed will return an IndexedSeq. Though IndexedSeq always fails the chunked-seq? check.

This means a:

(def xv (vec (range 32))) (reduce + (map inc xv))

is about 4x slower than a map over a vector with 33 elements.

Options:
1. Return a ChunkedCons with the "rest" set to nil in PersistentVector.seq()
2. Implement IChunkedSeq for IndexedSeq:

`
(extend-type IndexedSeq

IChunkedSeq
(-chunked-first [x] x)
(-chunked-rest [x] ())
IChunkedNext
(-chunked-next [x] nil)
IChunk
(-drop-first [coll]
  (if-some [n (-next coll)]
    n
    (throw (js/Error. "-drop-first of empty chunk")))))

`

I think option #2 is better since IndexedSeq is used quite a bunch throughout the code base, so the chunking will also kick in for many other code paths.

Answer 1

Comment made by: aralo

Note:

This is different from Clojure (which does not consider an IndexedSeq a ChunkedSeq) since we use IndexedSeq a lot more where Clojure often uses a ChunkedSeq. For instance:

(apply (fn [& a] (type a)) [1 2 3 4 8])

will return IndexedSeq in CLJS but ChunkedCons in CLJ.

Since these IndexedSeq's will be passed around wildly in a normal CLJS app it makes sense to extend them as a IChunkedSeq since otherwise all these will never be chunked and get a slow first/next treatment.

Answer 2

Comment made by: mfikes

See CLJS-2926

Answer 3

Comment made by: mfikes

CLJS-2467.patch passes in CI and Canary. (/)

Answer 4

Comment made by: mfikes

CLJS-2467.patch added to Patch Tender (i)

Answer 5

Reference: https://clojure.atlassian.net/browse/CLJS-2467 (reported by aralo)