On 10 December 2013 18:24, larry google groups <lawrencecloj...@gmail.com>wrote:
>
> > As an aside, this doesn't look like a good use-case for macros
> >. You'd probably be better defining a map with keys for :
> > worker, :channel, etc. Either that or a protocol.
>
> Thank you much for your help. But I thought this is exactly what macros
> were for? In another language I would end up just copying-and-pasting the 4
> defs, and then changing "update" to "remove" so I could have both an update
> channel and a remove channel. I thought that macros were suppose to be good
> at removing that kind of redundancy?
>
Macros should be considered a "last resort" feature. They are usually the
worst way of solving a problem, unless they are the *only* way of solving a
problem.
Let's consider the functions your macros produce (ignoring the channel for
now):
(add-to-update-channel document)
(update-mongo-worker)
(start-mongo-channel)
These functions are very specific, but we can make them more
general-purpose by adding an additional argument:
(add-to-channel mongo-updates document)
(update-worker mongo-updates)
(start-channel mongo-updates)
Since these are generic functions, any specific functionality must be
defined by the new "mongo-updates" argument. In your example there are two
things specific to the mongo update job: the channel, and the function to
execute.
We could place these two piece of information in a data structure:
(def mongo-updates
{:channel (channel)
:to-execute println})
Or we could define a type or protocol. For now the map seems good enough,
and we can use it to build generic versions of the functions generated by
your macro:
(defn add-to-channel [{:keys [channel]} data]
(enqueue channel document))
(defn update-worker [{:keys [channel to-execute]}]
(loop [document @(read-channel channel)]
(to-execute document)
(recur @(read-channel channel))))
(defn start-channel [worker-info]
(dotimes [_ 6]
(future (update-worker worker-info))))
- James
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