def vs defn in Clojure

In Clojure, def creates a var in the current namespace. A var can hold any value: a number, a map, a function, a Java object, etc.

defn is the idiomatic way to define a named function. Conceptually, it is a def whose value is an fn, with some extra conveniences (name, docstring, arglists).

1(def answer 42)
2(defn add [a b] (+ a b))

Why this matters in practice

• You will see a lot of “top-level defs” in Clojure namespaces; that is how APIs are defined.
• At the REPL, redefining vars is normal and is part of the workflow.

Java mental model: vars are closer to namespace-scoped bindings than to class fields. They support interactive development and late binding.

In this section

• Using def for Namespace Definitions
  Learn what def creates, when it belongs at namespace scope, and why it is not the same as a mutable Java local variable.
• Defining Functions with defn
  Use defn as the normal way to publish named Clojure functions, and understand what it adds beyond def plus fn.
• Scope and Immutability in Clojure
  Distinguish namespace Vars from local bindings, and learn where Clojure's immutability guarantees apply when values are named.