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feat: core.async Phase 3 — channel transducers + dropping/sliding buffers

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(chan n xform) applies a transducer on the put side: a jolt transducer is a
directly-callable closure, composed over a reducing fn whose step gives each
output value into the channel (honoring its buffer kind). One put may yield zero
or more values; a reduced result (e.g. from take) closes the channel; close!
runs the transducer completion arity to flush stateful remainders. Works with
map/filter/mapcat/take/comp/etc.

Buffers: (buffer n) fixed, (dropping-buffer n) drops new values when full,
(sliding-buffer n) drops the oldest. Implemented via a non-blocking give —
(ev/select [ch v] closed-chan) detects a full buffer without parking.

harness: run-spec flushes per suite. spec: core.async/channel-transducers (5),
core.async/buffers (3). jpm test green.

Note: distinct/dedupe/partition-all/partition-by still lack a 0-coll transducer
arity in core (separate gap), so they can't yet be used as channel xforms.
This commit is contained in:
Yogthos 2026-06-05 15:40:24 -04:00
parent e8cb4605ac
commit 54db79e927
5 changed files with 94 additions and 9 deletions
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29
test/spec/core-async-spec.janet
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@ -4,7 +4,7 @@
(use ../support/harness)
(def REQ
"(require '[clojure.core.async :refer [go go-loop chan <! >! close! alts! timeout put! take! chan?]]) ")
"(require '[clojure.core.async :refer [go go-loop chan <! >! close! alts! timeout put! take! chan? buffer dropping-buffer sliding-buffer]]) ")
(defn- a [body] (string "(do " REQ body ")"))
(defspec "core.async / go & channels"
@ -69,3 +69,30 @@
["go's own binding shadows conveyed"
":inner"
(d "(<! (binding [*x* :outer] (go (binding [*x* :inner] (<! (timeout 5)) *x*))))")])
# Channel transducers (Phase 3): a transducer is applied on the put side, so one
# put may yield zero or more values; `take` closes the channel early.
(defn- drain [setup]
(string "(do " REQ setup
" (<! (go-loop [o []] (let [v (<! c)] (if (nil? v) o (recur (conj o v)))))))"))
(defspec "core.async / channel transducers"
["map transducer"
"[2 3 4]" (drain "(def c (chan 10 (map inc))) (go (>! c 1) (>! c 2) (>! c 3) (close! c))")]
["filter transducer"
"[0 2 4]" (drain "(def c (chan 10 (filter even?))) (go (doseq [x (range 6)] (>! c x)) (close! c))")]
["mapcat expands"
"[1 1 2 2]" (drain "(def c (chan 10 (mapcat (fn [x] [x x])))) (go (>! c 1) (>! c 2) (close! c))")]
["take closes early"
"[:a :b]" (drain "(def c (chan 10 (take 2))) (go (>! c :a) (>! c :b) (>! c :c) (>! c :d) (close! c))")]
["comp of transducers"
"[10 30 50]" (drain "(def c (chan 10 (comp (filter odd?) (map (fn [x] (* x 10)))))) (go (doseq [x (range 6)] (>! c x)) (close! c))")])
# Buffers: fixed (default), dropping (drops new when full), sliding (drops oldest
# when full). Filled synchronously on this fiber (dropping/sliding never park).
(defn- fill [bufexpr]
(string "(do " REQ "(def c (chan " bufexpr ")) (doseq [x [1 2 3 4 5]] (>! c x)) (close! c)"
" (<! (go-loop [o []] (let [v (<! c)] (if (nil? v) o (recur (conj o v)))))))"))
(defspec "core.async / buffers"
["dropping-buffer keeps first" "[1 2]" (fill "(dropping-buffer 2)")]
["sliding-buffer keeps last" "[4 5]" (fill "(sliding-buffer 2)")]
["fixed (buffer n) holds all" "[1 2 3 4 5]" (fill "(buffer 5)")])