(Ab)using constant Boolean's for conditional compilation

elide.md

The goal is to have the spec-related code conditionally elided from the below for prod builds:

(ns my-app.core
  (:require [clojure.spec.alpha :as s]
            [my-app.config :as config]))

(defn new-machine
  [opts]

  [(merge {:state/name "A"}
     (when config/debug
       {:state/spec (s/keys :req [::foo ::bar])}))

   (merge {:state/name "B"}
     (when config/debug
       {:state/spec (s/keys :req [::baz])}))

   ])

(when config/debug
  (s/def ::foo int?)
  (s/def ::bar int?)
  (s/def ::baz int?))

(new-machine nil)

The my-app.config/debug symbol is defined as a constant Boolean in a namespace which is loaded from one path for dev and another for prod (where the only difference is that the value is true or false):

(ns my-app.config)

(def ^:const debug false)

We use a constant Boolean in order to take advantage of aggressive elision that occurs owing to CLJS-2267. Since this occurs at the ClojureScript level, it can even eliminate code in cases where using goog.DEBUG is not effective.

With the above, under :advanced, this compiles down to the same JavaScript as if the code were:

(ns my-app.core)

(defn new-machine
  [opts]

  [(merge {:state/name "A"}
     nil)

   (merge {:state/name "B"}
     nil)

   ])

(new-machine nil)

One disadvantage of this approach is that it may be susceptible to compilation caching. In other words, you may need to either do a clean build when switching between dev and prod builds, or ensure that :output-dir differs between dev and prod configuration.