Yes, returning a value that will be created in some sort of async fashion,
directly from a function is not idiomatic. Instead, you should be returning
channels and calling <! on them. Inside your functions you can use put! to
keep from having to decide between >! and >!!. As an example, here is how
you would use http-kit's http async client from core.async:
(require '[org.httpkit.client :as http])
(defn http-get [url]
(let [c (chan)]
(http/get url
(fn [r] (put! c r)))
c))
Now you can do both:
(<!! (http-get "www.google.com"))
(go (<! (http-get "www.google.com")))
Timothy Baldridge
On Thu, Oct 31, 2013 at 4:34 AM, Leon Grapenthin
<grapenthinl...@gmail.com>wrote:
> Hello everyone,
> (I am posting here because I could not find a dedicated core.async group)
>
> recently I made a rather obscure bug-report (ASYNC-29). As a comment Rich
> Hickey stated that <!! and >!! should not be used in library code. I can
> now reproduce the same problem without <!!/>!! by simply derefing 50
> undelivered promises in dedicated go-blocks (dotimes [_ 50] (go
> @(promise))). After that it is not possible to take from channels inside
> go-blocks.
>
> Now I can imagine that using promises in go-blocks is not exactly doing
> what they are designed for. I suppose that using promises within go is not
> supported. However I wonder what my options as a library creator are when I
> want to write a function that is designed to wait until it has taken a
> value from a channel. The only way I can see is to write a macro instead of
> a function, two versions. One using <!, designed for use in go-blocks, one
> using <!!, designed for use outside of go blocks. Otherwise, I don't see
> any possible way to get a value from a channel and returning it. Or am I
> missing something?
>
> Is it simply unidiomatic to return a value from a channel from a function
> body?
>
> Best regards,
> Leon.
>
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