macros

[nasser]

Macros are complicated by the fact that ClojureScript, for historic reasons, does not normally support macros. They are instead a compile-time feature used on the JVM side when generating JavaScript in the more conventional ClojureScript workflow. Making them work in a bootstrapped context is tricky.

The status of defmacro:

cljs.user=> (defmacro foo [x] `(+ ~x 90))
true

cljs.user=> (foo 9)
(cljs.core/+ nil 90)

This also makes consuming code from the ecosystem difficult. Consider this datascript snippet:

(ns datascript.btset
  ...
  (:refer-clojure :exclude [iter])
  (:require
    [datascript.arrays :as da])
  #?(:clj  (:import [java.util Arrays]))
  #?(:cljs (:require-macros [datascript.btset :refer [half not==]])))

#?(:clj
  (defmacro half [x]
    `(unsigned-bit-shift-right ~x 1)))

#?(:clj
  (defmacro not== [x y]
    `(not (== ~x ~y))))

It follows the common pattern of a namespace requiring itself to access macros. This fails to load with

cljs.user=> (require 'datascript.core)
#error {:message "Invalid :refer, macro datascript.btset/not== does not exist", :data {:tag :cljs/analysis-error}}
Error: Could not parse ns form datascript.btset
...

Ideas:

• make defmacro work normally
• intervene on the reader conditional and compile defmacros even though they are tagged :clj
• intervene on ns form to turn require-macros into a normal require