defnk addition to c.c.def

[Meikel Brandmeyer]

Hello Stephen,

the following is a macro defining a function with keyword
arguments. It is called like this:

(defnk foo
  [a b c :x 1 :z 2]
  (println x z))

a, b and c denote positional arguments, 1 and 2 are default
values for x resp. z. This pattern came up several times now,
and there were always some "aha" effects. So it may be a
nice addition to c.c.def.

Here's the macro:
(defmacro defnk

  values of the keyword arguments are available in the function body by

  [fn-name & fn-tail]
  (let [[fn-name [args & body]] (name-with-attributes fn-name fn-tail)
        [pos kw-vals]           (split-with symbol? args)
        kw-vals                 (apply hash-map kw-vals)
        de-map                  {:keys (vec (keys kw-vals))
                                 :or   kw-vals}]
    `(defn ~fn-name
       [...@pos & options#]
       (let [~de-map (apply hash-map options#)]
         ~...@body))))

The docstring is a bit contorted but I'm too sleepy now,
to get that right...

Sincerely
Meikel

"Define a function accepting keyword arguments. Symbols up to the first keyword in the parameter list are taken as positional arguments. Then an alternating sequence of keywords and defaults values is expected. The virtue of the symbol corresponding to the keyword (cf. :keys destructuring). defnk accepts an optional docstring as well as an optional metadata map."

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