Is there a way to peek into Clojure async channels without consuming their values?

Question

I'm trying to debug something and I'd like to see what's on the channel before consuming it.

Answer 1

Only for the sake of debugging, you may use something similar to function below to observe a value on a channel without actually take!ing it (warning: this relies heavily on core.async implementation details):

(defn peek [ch]
  (if (and (.buf ch)
           (pos? (count (.buf ch))))
    (if (instance? clojure.core.async.impl.buffers.PromiseBuffer
                   (.buf ch))
      (a/<!! ch)
      (last (.buf (.buf ch))))
    (some-> ch .puts first second)))

core.async itself doesn't provide public API for peeking value from channel.

As noted by @amalloy, the function above may fail for other kinds of buffers, in particular for those that don't have buf field declared.