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This is a followup to Clojure: pre post functions

Goal

For every Clojure function, I want to have a pre and post function that gets executed:

  • right before the function is evaluated and

  • right after the function returns

    Now, I want to do this all functions in my *.clj files.

    I would prefer (this is also an learning exercise) to do this at the Clojure Compiler level.

Question:

How do I get started on this? What part of the Clojure Compiler source code should I be reading? What documentation / tutorials on the internals of the Clojure Compiler I should be aware of?

Thanks!

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First off, this sounds like a slightly crazy thing to do in general. There are almost certainly better ways to achieve any sensible objective (i.e. this is screaming "XY Problem"). But as long as you say it is just for a learning exercise, that is fine :-)

I can think of a couple of strategies you might want to consider before hacking the compiler:

  • Create your own defn macro that does the wrapping when functions are created. Obviously you'll need to make sure your own version of defn is used rather than the built-in one. Probably the simplest solution.
  • Walk your namespaces at runtime (after they are loaded) and redefine all functions to a wrapped version of the same function. Could get a bit messy but will certainly enhance your understanding of namespaces :-)

If you really want to hack the compiler, the easiest place to make this change would probably be just by hacking defn in core.clj

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4 Comments

I'd probably wrap fn in its own macro, since defn is just a convenience macro for defining functions and assigning them to symbols?
Yes you could wrap fn, but if so I think you'd need to redefine defn as well since it refers directly to clojure.core/fn rather than your own version....
Thanks! I really appreciate the insightful + helpful solutions (rather than the waves of "Why would you want to do this???" I was expecting :-).
@mikera: you should go answer my new question, so you can be 3/3 :-).

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