Nesting macros in Clojure

Question

Consider this pseudo code:

(defrc name
  "string"
    [a :A]
    [:div a])

Where defrc would be a macro, that would expand to the following

(let [a (rum/react (atom :A))]
  (rum/defc name < rum/reactive []
    [:div a]))

Where rum/defc is itself a macro. I came up with the code below:

(defmacro defrc
          [name subj bindings & body]
          (let [map-bindings# (apply array-map bindings)
                keys# (keys map-bindings#)
                vals# (vals map-bindings#)
                atomised-vals# (atom-map vals#)]
               `(let ~(vec (interleave keys# (map (fn [v] (list 'rum/react v)) (vals atomised-vals#))))
                     (rum/defc ~name < rum/reactive [] ~@body))))

Which almost works:

(macroexpand-all '(defrc aname
       #_=>   "string"
       #_=>   [a :A]
       #_=>   [:div a]))
(let* [a (rum/react #object[clojure.lang.Atom 0x727ed2e6 {:status :ready, :val nil}])] (rum/defc aname clojure.core/< rum/reactive [] [:div a]))

However when used it results in a syntax error:

ERROR: Syntax error at (clojure.core/< rum.core/reactive [] [:div a])

Is this because the inner macro is not being expanded?

Ah, seems this comes from rum itself, in parse-defc, investigating.. — Pete
– Pete, Commented Dec 19, 2015 at 18:00
sorry, having a hard time picturing the expected behaviour, can you provide a few examples? — Valentin Waeselynck
– Valentin Waeselynck, Commented Dec 19, 2015 at 23:33

Pete · Accepted Answer · 2015-12-20 11:36:40Z

1

Turns out the macro was working correctly but the problem occurred because < was inside the syntax quote it got expanded to clojure.core/<, and Rum simply looks for a quoted <, relevant snippet from Rum's source:

...(cond
        (and (empty? res) (symbol? x))
          (recur {:name x} next nil)
        (fn-body? xs)        (assoc res :bodies (list xs))
        (every? fn-body? xs) (assoc res :bodies xs)
        (string? x)          (recur (assoc res :doc x) next nil)
        (= '< x)             (recur res next :mixins)
        (= mode :mixins)
          (recur (update-in res [:mixins] (fnil conj []) x) next :mixins)
        :else
          (throw (IllegalArgumentException. (str "Syntax error at " xs))))...

answered Dec 20, 2015 at 11:36

Pete

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