A few days ago, Chouser and I had a discussion on IRC about the
viability of named arguments (a la Smalltalk) for Clojure. In clojure-
contrib, there is the macro "defnk" which provides this sort of
capability, but it's performance characteristics are worse than than
normal function call. I wanted to try my hand at a better performing
macro and here is what I came up with:
(defmacro named-call [fn & named-args]
(let [named-args-map (apply hash-map named-args)
meta-data (eval `^(var ~fn))
arg-names (:arg-names meta-data)
arg-list (for [arg-key arg-names] (get named-args-map arg-
key))]
`(~fn ~...@arg-list)))
It is used on the call site instead of the definition like so:
(defn subtract [from take] (- from take))
(call subtract :from 10 :take 2)
The macro assumes some meta data on the function variable which
describes the argument list keyed by :arg-names like so:
(def #^{:arg-names [:from :take]} subtract subtract)
The macro isn't as sophisticated as I'd like yet. It doesn't deal
with multiple function signatures and I'd like a way to define default
values. I'd also like it to return useful error messages if the
caller's signature is wrong.
Thanks to Chouser, Rich and others from IRC for helping me with this
stuff. Even if the macro isn't all that valuable, I learned a lot
about Clojure in the process. If anyone has any suggestions, I'd love
to hear them.
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