Hi all!
I'm current working on a project where I am ingesting events off a stream
and processing them. There are many many steps involved in the processing
component, so I am choosing to write the steps as a series of transducers
(because, hey, they're super cool!). Here's the problem though, after
writing the first 2 processing steps I'm noticing that all of them are
going to look very similar:
(defn a-step-transducer
[]
(fn [reducing-fn]
(fn
([] (reducing-fn))
([result] (reducing-fn result))
([[guid-state processed-events :as result] event]
;; step-specific logic
))))
Given how many steps I am planning to write, this is a ton of boilerplate!
So, my first thought was to use a macro to abstract away all this
boilerplate. Now, I have to admit that Clojure is my first Lisp, so I'm
really not sure I fully understand when or why to use macros to do things.
My current understanding is that macros are a kind of "template" for code,
so something like this where I don't want to write the same function
structure over and over seems like a decent use case for macros (feel free
to correct me if I'm totally off on this). Here is my first attempt:
(defmacro deftransducer
[body]
`(fn [reducing-fn]
(fn
([] (reducing-fn))
([result] (reducing-fn result))
([[guid-state processed-events :as result] event]
~@body))))
The idea here being that in body I can reference the variables defined by
the macro like reducing-fn, result, event, etc. Of course, I quickly found
out that this doesn't work:
storm-etl.entry-processing> (deftransducer "something")
CompilerException java.lang.RuntimeException: Can't use qualified name as
parameter: storm-etl.entry-processing/reducing-function
Some quick googling tells me that the solution to this is to use gensyms
for these variable names, but that would defeat the whole purpose of this
because I want to be able to reference those variables from within the code
that I pass to my macro. Is this an appropriate use case for macros or am I
way off base? Is there an alternative approach that would be recommended?
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