core.async

Completion arity of transducer is called twice

Details

  • Type: Defect Defect
  • Status: Open Open
  • Priority: Major Major
  • Resolution: Unresolved
  • Affects Version/s: None
  • Fix Version/s: None
  • Component/s: None
  • Labels:

Description

completion arity (1) of a transducer function is called twice in a channel

(let [xf (fn [rf]
           (fn
             ([] (println "INIT") (rf))
             ([result] (println "COMPLETING") (rf result))
             ([result input] (println "STEP") (rf result input))))
      c (chan 10 xf)]
  (close! c)
  (println "RESULT" (<!! c)))

will print

COMPLETING
COMPLETING
RESULT nil

According to https://clojure.org/reference/transducers, it is probably a bug:

> A completing process must call the completion operation on the final accumulated value exactly once

Looks like completing fn is called at there places in core.async: https://github.com/clojure/core.async/blob/master/src/main/clojure/cljs/core/async/impl/channels.cljs#L122 https://github.com/clojure/core.async/blob/master/src/main/clojure/cljs/core/async/impl/channels.cljs#L146

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Dates

  • Created:
    Updated: