.CLJ File Format

Introduction

Clojure is a dialect of Lisp created by Rich Hickey and released in 2007. It runs on the Java virtual machine and makes immutable data the default. ClojureScript is the same language compiled to JavaScript.

The four extensions are related. .clj is Clojure for the JVM, .cljs is ClojureScript and .cljc is code shared between the two. .edn is not code but data: Extensible Data Notation, the subset of Clojure’s syntax used for configuration files and data exchange, such as a project’s deps.edn.

Structure

A Clojure file is UTF-8 text made of forms. It usually starts with an ns form that names its namespace and its dependencies. A namespace maps to a path, with dots as folders and hyphens as underscores, so my-app.core lives in my_app/core.clj.

Beyond lists in parentheses, the syntax has vectors in [ ], maps in { }, sets in #{ } and keywords such as :name. Function parameters are a vector. Comments start with ;, and #_ makes the reader skip the next form. In a .cljc file, reader conditionals such as #?(:clj a :cljs b) choose code for each platform.

(ns example.greet
  (:require [clojure.string :as string]))

;; A function with a documentation string and one parameter.
(defn greet
  "Return a greeting for a person."
  [person]
  (str "Hello, " (string/capitalize person)))

(println (greet "world"))

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