Reader conditionals

Reader conditionals allow a single source file to contain code for multiple Clojure platforms. clojurust evaluates the :cljrs branch.

:cljrs is the canonical platform key. The older key :rust is still accepted as an alias and selects the same branch, but new code should use :cljrs.

Syntax

Non-splicing form

#?(:cljrs  expr-cljrs
   :clj    expr-jvm
   :cljs   expr-clojurescript
   :default expr-fallback)

Exactly one branch is selected based on the current platform. Clauses are tried left to right and the first one whose key is :cljrs (or its alias :rust) or :default wins, as in Clojure. If no clause matches, the entire form is skipped (reads as nothing).

The selected branch is a single expression; the whole #?(...) form evaluates to that expression.

(def platform #?(:cljrs  "clojurust"
                 :clj    "JVM Clojure"
                 :cljs   "ClojureScript"
                 :default "unknown"))

Splicing form

#?@(:cljrs  [a b c]
    :clj    [x y z]
    :default [])

The splicing form #?@(...) selects a vector from the active platform and splices its elements into the surrounding form. It is only valid inside a list, vector, map, or set literal.

;; Adds platform-specific items to a vector
(def features [#?@(:cljrs  [:gc :cranelift]
                   :clj    [:jvm :hotspot]
                   :default [])])
; => [:gc :cranelift]  (on clojurust)

;; Platform-specific require in an ns form
(ns myapp.core
  (:require [clojure.string :as str]
            #?@(:cljrs [[:clojurust.system :as sys]]
                :clj   [[:java.lang.System :as sys]])))

File-extension behaviour

Extension	Platform dispatch
`.cljrs`	Always `:cljrs`. Reader conditionals are still supported but `:cljrs` is always the active platform.
`.cljc`	Cross-platform. Reader conditional branches are stored as-is; the evaluator selects `:cljrs`.

Notes

The reader stores all branches of a #?(...) form; only the evaluator discards non-matching branches. This means reader-conditional forms can be inspected programmatically without losing the other branches.
Order within a reader conditional matters: keys are checked left-to-right. :default should come last.
:cljr is ClojureCLR’s key, not clojurust’s; the clojurust key is :cljrs.
:rust is a legacy alias for :cljrs. Both keys match, so a form that lists both selects whichever comes first. Prefer :cljrs in new code.