Idempotent Channel Creation

[Wai Lee Chin Feman]

Hi everyone,

I have a situation where I am creating channels and wiring them up dynamically:

--------------------------------------------------------------------------

(def dumping-ground (named-channel "dumping-ground"))

(defn build-and-wire [name]

  (let [created-channel (named-channel name)]

    (siphon (map* inc created-channel) dumping-ground)))

--------------------------------------------------------------------------

Now, this works great. (build-and-wire "thomas-mann")  will give me what I want.

The problem is that this is not idempotent.  If I call build-and-wire twice with the same name, I will wind up with more stuff in the dumping ground than I was hoping for.  In the following console session, I hope to only see "100" printed once:

-------------------------------------------------------------------------

moose.core=> (receive-all dumping-ground prn)

true

moose.core=> (build-and-wire "thomas-mann")

true

moose.core=> (build-and-wire "thomas-mann")

true

moose.core=> (receive-all dumping-ground prn)

true

moose.core=> (enqueue (named-channel "thomas-mann") 99)

100

100

-------------------------------------------------------------------------

Can you all think of a way to make this function idempotent?  Can I examine the channel named "thomas-mann", and check if it is already wired up?  If I can check whether or not it's connected to dumping-ground, then I'm able to know whether or not to call siphon.

Any ideas?

Thanks!

[Zach Tellman]

Siphon is idempotent, actually. The problem is the intermediate transformation creates new channels each time. Is there any reason not to do the wiring in the on-create callback of named-channel?

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[Wai Lee Chin Feman]

A-ha.  I didn't see that there was a callback for the function.  That solves the problem, thanks!

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