(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.
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