Question

I believe I understand that during macro expansion, a macro does not have access to the things a function has access to, because expansion happens before compile time.

But, I am having trouble understanding is how to perform checks at macro expansion time.

For example:

(defn gen-class [cl-nam]
    (fn [cmd & args]
        (condp = cmd
            :name (name cl-nam))))

(defmacro defnclass [cl-nam]
    `(def ~cl-nam (gen-class '~cl-nam)))

I would like to check to see that cl-nam is not a sequence. I would like to use count and find out of its length is > 1.

I understand I can unquote the println in the following macro, so that I can get an expansion-time message.

(defmacro defnclass_info [cl-nam]
    `(do
        ~(println cl-nam)
        (def ~cl-nam (gen-class '~cl-nam))))

But, I am not sure how to go about checking to see what was passed for cl-nam.

I’m reading a lot of Clojure macro descriptions from several books, and am stumped.
Thanks.

Answer 1

A macro is really just a function.

(defmacro defnclass-info
  [cl-name]
  (when (seq? cl-name)
    (throw (Exception. "cl-name must not be a sequence")))
  `(def ~cl-name (gen-class '~cl-name)))

Edit: One could also switch behaviour and recursively call the macro again for the whole list. What you do, depends on your requirements.

(defmacro defnclass-info
  [cl-name]
  (if (seq? cl-name)
    `(do ~@(for [cn cl-name] `(defnclass-info ~cn)))
    `(def ~cl-name (gen-class '~cl-name))))

The macro expansion is just what the “function” returns. So you have full flexibility on what you do with the macro arguments.