I've been going through Stuart Halloway's book, _Programming Clojure_,
and thinking about his deftarget macro has brought up some questions
as to how macros and special forms interact in clojure. The deftarget
macro needs to produce a "def" form that has metadata on its first
argument, which is a symbol. Typically this is done with a reader
macro, e.g. (def #^{:foo "bar"} x ...), but reader macros are
problematic inside of (regular) macros, as they are executed before
macro expansion. Stuart's solution involves doing some gensyms, but I
thought it would be nice to just have a form of def what was more
macro friendly. So I defined
(defmacro def-with-md [md sym & optional-init]
`(def ~(with-meta sym md) ~...@optional-init))
which means I can now write things like
(defmacro foo [...]
`(let [a# (...)] (def-with-md {:a a#} x (...))))
This got me thinking about whether I could attack the problem even
more directly. I defined another macro:
(defmacro wmd [s md] (with-meta s md))
Trying it out:
(macroexpand '(wmd x {:a 1}))
==> x
But
(def (wmd x {:a 1}) 37)
gives me an error: "Second argument to def must be a Symbol". This
confuses me. Doesn't macro expansion happen entirely before
evaluation? So before the def special form is evaluated, (wmd x {:a
1}) should have already been macro expanded to give a symbol. (And
why is it talking about the *second* argument, instead of the *first*?)
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