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Hi. I realize that the first rule of Macro Club is Don't Use Macros, so the following question is intended more as an exercise in learning Clojure than anything else (I realize this isn't necessarily the best use of macros).

I want to write a simple macro which acts as a wrapper around a regular (defn) macro and winds up adding some metadata to the defined function. So I'd like to have something like this:

(defn-plus f [x] (inc x))

...expand out to something like this:

(defn #^{:special-metadata :fixed-value} f [x] (inc x))

In principle this doesn't seem that hard to me, but I'm having trouble nailing down the specifics of getting the [args] and other forms in the defined function to be parsed out correctly.

As a bonus, if possible I'd like the macro to be able to handle all of the disparate forms of defn (ie, with or without docstrings, multiple arity definitions, etc). I saw some things in the clojure-contrib/def package that looked possibly helpful, but it was difficult to find sample code which used them.

+5  A: 
Brian Carper
Interesting! But is there a way to do it more functionally? Wrapping a `defn` in a `do` and then destructively modifying its metadata seems kind of weird to me. Then again, my experiments with using the `#^{:k :v}` syntax from inside a `defmacro` have all been utter failures so far...
Tim Gilbert
Doing any kind of `def` is not a very functional thing to do, since it destructively mutates the state of a global dispatch table. :) But you're right, and I've updated my answer. My previous answer was dropping the #^Integer tag metadata that a `def` would normally take.
Brian Carper
Thanks, this is a lot more understandable and those links look very helpful, although I'm still a bit mystified about why (macroexpand-1 '(defn-plus foo [bar] (baz))) doesn't display the with-meta tag.
Tim Gilbert
Try (set! *print-meta* true) and then look at the macro expansion. You'll see the metadata has been applied to the symbol that names the function. `with-meta` is executed in the body of the macro, before the macro returns. Note that it's un-quoted in the macro.
Brian Carper