Macro debugging in ClojureScript

The following gist is the summary and solution from the discussions of https://groups.google.com/forum/#!topic/clojurescript/BEGF_AjSjmQ.

1. Problem

1.1. My question

I wrote a macro in ClojureScript and wanted to test the macro by using println function like this,

;; qna/macro.clj
(ns qna.macro)

(defmacro my-add [a b]
  (println "a =" a "b =" b)   ; <-- Here
  `(+ ~a ~b))


;; qna/core.cljs
(ns qna.core
  (:require-macros [qna.macro :refer [my-add]]))

(defn ^:export main []
  (my-add 2 3))

The source of project.clj and resources/public/index.html are like the following.

;; project.clj
(defproject qna "0.1.0-SNAPSHOT"
  :dependencies [[org.clojure/clojure "1.9.0"]
                 [org.clojure/clojurescript "1.10.339"]
                 [philoskim/debux "0.4.12"]]
  :min-lein-version "2.6.0"
  :plugins [[lein-cljsbuild "1.1.7"]
            [lein-figwheel "0.5.16"]]
  :clean-targets ^{:protect false}
  ["target"
   "resources/public/js/out"
   "resources/public/js/core.js"]
  :cljsbuild
  {:builds
   [{:id "dev"
     :source-paths ["src"]
     :compiler {:main      qna.core
                :output-to "resources/public/js/core.js"
                :output-dir "resources/public/js/out/"
                :asset-path "js/out/"
                :optimizations :none
                :source-map true
                :pretty-print true} }]})


;; resources/public/index.html
<!doctype html>
<html lang="en">
  <head>
    <meta charset='utf-8'>
    <title>QnA Demo</title>
  </head>

  <body>
    <h2>QnA Demo</h1>
    <div id="app"></div>

    <script src="js/core.js"></script>
    <script>qna.core.main();</script>
  </body>
</html>

When I open the above .html file in the browser, I encounter the following error.

Uncaught SyntaxError: Unexpected identifier in core.js:4

And the compiled core.js file is like this.

// Compiled by ClojureScript 1.10.339 {}
goog.provide('qna.core');
goog.require('cljs.core');
a = 2 b = 3  // <-- embedded printed result
qna.core.main = (function qna$core$main(){
return ((2) + (3));
});
goog.exportSymbol('qna.core.main', qna.core.main);

//# sourceMappingURL=core.js.map

To sum up, whenever I use println function within a macro for ClojureScript, the printed result doesn’t go to the REPL but is embedded into the compiled .js file.

If this is a bug of ClojureScript, I will register this error in the ClojureScript JIRA.

1.2. The answers

1.2.1. Thomas Heller

You can use cljs.util/debug-prn if you need to print during macro expansion.

Whether or not this should be called a bug I don’t know.

1.2.2. David Nolen

The compiler emits to *out* during compilation. You will need to print
to some other output stream like *err* for now.

David

2. Solutions

2.1. Using `cljs.util/debug-prn`

;; qna/macro.clj
(ns qna.macro
  (:require [cljs.util :as ut]))

(defmacro my-add [a b]
  (ut/debug-prn "a =" a "b =" b)  ; <-- Here
  `(+ ~a ~b))


;; qna/core.cljs
(ns qna.core
  (:require-macros [qna.macro :refer [my-add]]))

(defn ^:export main []
  (my-add 2 3))

$ lein cljsbuild once
Compiling ClojureScript...
Compiling ["resources/public/js/core.js"] from ["src"]...
a = 2 b = 3
Successfully compiled ["resources/public/js/core.js"] in 0.496 seconds.

2.2. Using `debux.core/dbg-prn`

If you use my debux library, use dbg-prn function like this.

Caution

All the macros for ClojureScript are compiled by the Clojure compiler, not by the ClojureScript compiler. So you have to use debux.core, not debux.cs.core, like the following.

;; qna/macro.clj
(ns qna.macro)

(use 'debux.core)

(defmacro my-add [a b]
  (dbg-prn "a =" a "b =" b)   ;; Use dbg-prn in debux
  `(+ ~a ~b))


;; qna/core.cljs
(ns qna.core
  (:require-macros [qna.macro :refer [my-add]]))

(defn ^:export main []
  (my-add 2 3))

You can see the printed output of dbgm only in the ClojureScript compiling process like this.

$ lein cljsbuild once
Compiling ClojureScript...
Compiling ["resources/public/js/core.js"] from ["src"]...

dbg-prn: a = 2 b = 3
Successfully compiled ["resources/public/js/core.js"] in 0.495 seconds.

philoskim/macro-debugging-in-clojurescript.adoc