Macrovich

Because any macros problem can be solved by another level of macros, Macrovich is a set of four macros to ease writing *.cljc supporting Clojure, Clojurescript and self-hosted Clojurescript.

Excerpt from Being John Macrovich script:

Usage

Clojurescript >= 1.9.293 is required. This means Planck 2.0.0 or later is required. Lumo 1.0.0 is ok.

Add [net.cgrand/macrovich "0.2.1"] to your dependencies.

Macrovich exposes four macros:

• macros/deftime and macros/usetime to clearly demarcate regions of code that should be run in the macro-definition stage or in the macro-usage stage. (In Clojure there's no distinction; in pure Clojurescript it's easy: just wrap the first stage in #?(:clj ...) and the latter one in #?(:cljs ...); in self-hosted Clojurescript it's messy or everything gets evaluated twice; supporting the three at the same time is Macrovich's raison d'être.)
• macros/case is a macro to use instead of reader conditionals in macros or macros-supporting fns. This solves a problem with regular Clojurescript where macros are Clojure code and thus are read by taking the :clj branch of conditionals. So macros/case is like reader conditionals except the branch is picked at expansion time and not at definition time.
• macros/replace is a macro to avoid repeating similar reader conditionals, see https://github.com/cgrand/xforms/blob/d4f0280bb50d8cc53c3a5dfe24b17fe7701b4e43/src/net/cgrand/xforms.cljc#L276 for an example.

Sample

Below is a sample being-john.cljc file:

(ns being-john
  #?(:clj
     (:require [net.cgrand.macrovich :as macros])
     :cljs
     (:require-macros [net.cgrand.macrovich :as macros]
        [net.cgrand.being :refer [add]]))) ; cljs must self refer macros

(macros/deftime
  ; anything inside a deftime block will only appear at the macro compilation stage.

  (defmacro add
    [a b]
    `(+ ~a ~b)))

(macros/usetime
  ; anything inside a usetime block will not appear at the macro compilation stage.

  (defn sum
    [a b]
    (add a b)))
    
; anything outside these block is always visible as usual

case allows to select which form to emit in a macro based on the target language rather than the macro language. Consider these two macros:

(defmacro broken []
  #?(:clj "clojure" :cljs "clojurescript"))

(defmacro correct []
  (macros/case :clj "clojure" :cljs "clojurescript"))

; or
(defmacro correct []
  `(macros/case :clj "clojure" :cljs "clojurescript")) ; this works too, so no need to unquote in the middle of a syntax quotation and mess with gensyms

In regular (Clojure-hosted) Clojurescript (broken) expands to "clojure" while (correct) expands to "clojurescript".

Example

The xforms lib has been converted to cljc:

• https://github.com/cgrand/xforms/blob/cljc/src/net/cgrand/xforms.cljc
• https://github.com/cgrand/xforms/blob/cljc/src/net/cgrand/xforms/rfs.cljc
• https://github.com/cgrand/xforms/blob/cljc/test/net/cgrand/xforms_test.clj

License

Copyright © 2016-2017 Christophe Grand

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.