Namespaces and Imports

Namespaces in Clojure are like Java packages, but they hold vars (functions and values), not classes. You usually configure a file’s namespace once at the top with ns.

The ns Form You’ll See Everywhere

1(ns myapp.core
2  (:require [clojure.string :as str]
3            [myapp.util :as util])
4  (:import [java.time Instant]))

What this means in practice:

• :require loads other Clojure namespaces (your code or libraries).
• :as gives you a short alias so calls are explicit: str/join, util/parse-int, etc.
• :import brings Java classes into scope so you can write Instant/now instead of java.time.Instant/now.

You can always skip :import and use the fully-qualified class name:

1(java.time.Instant/now)

:refer (Use Sparingly)

1(:require [clojure.string :refer [join split]])

refer can be convenient for a small, stable set of names, but aliases are usually clearer. Avoid :refer :all in production code because it hides where names come from and makes conflicts harder to spot in review.

REPL Reload Tip

During development you’ll often reload a namespace after changes:

1(require 'myapp.core :reload)

Most editors also provide a “reload namespace” command.

File Path Rule of Thumb

Namespace names map to file paths:

• myapp.core → src/myapp/core.clj
• hyphens in namespace segments become underscores in filenames:
  • my-app.core → src/my_app/core.clj

Knowledge Check: Namespaces