When I evaluate the following core.async clojurescript code I get an error: "Uncaught Error: <! used not in (go ...) block"
(let [chans [(chan)]]
(go
(doall (for [c chans]
(let [x (<! c)]
x)))))
What am I doing wrong here? It definitely looks like the <! is in the go block.
because go blocks can't cross function boundaries I tend to fall back on loop/recur for a lot of these cases. the (go (loop pattern is so common that it has a short-hand form in core.async that is useful in cases like this:
user> (require '[clojure.core.async :as async])
user> (async/<!! (let [chans [(async/chan) (async/chan) (async/chan)]]
(doseq [c chans]
(async/go (async/>! c 42)))
(async/go-loop [[f & r] chans result []]
(if f
(recur r (conj result (async/<! f)))
result))))
[42 42 42]
Why dont you use alts! from Core.Async?
This function lets you listen on multiple channels and know which channel you read from on each data.
For example:
(let [chans [(chan)]]
(go
(let [[data ch] (alts! chans)]
data)))))
You can ask of the channel origin too:
...
(let [slow-chan (chan)
fast-chan (chan)
[data ch] (alts! [slow-chan fast-chan])]
(when (= ch slow-chan)
...))
From the Docs:
Completes at most one of several channel operations. Must be called inside a (go ...) block. ports is a vector of channel endpoints, which can be either a channel to take from or a vector of [channel-to-put-to val-to-put], in any combination. Takes will be made as if by !. Unless the :priority option is true, if more than one port operation is ready a non-deterministic choice will be made. If no operation is ready and a :default value is supplied, [default-val :default] will be returned, otherwise alts! will park until the first operation to become ready completes. Returns [val port] of the completed operation, where val is the value taken for takes, and a boolean (true unless already closed, as per put!) for put
Doumentation ref
goblocks :<!won't work inside aforfor instance. See the accepted answer for a similar question.