Practical Examples of Immutability

Immutability becomes real when you stop thinking “how do I change this object?” and start thinking “what new value should this function produce?”

Here is the basic shape you will use constantly:

1(def order {:id 1 :status :new :items [{:sku "A" :qty 1}]})
2
3(update order :status (constantly :paid))

What to focus on

• Prefer update, assoc, dissoc for shallow changes.
• Use update-in / assoc-in for nested structures.
• Keep transformations pure so they are easy to test and reuse.

Java mental model: instead of “mutate a DTO”, treat state changes like producing a new immutable DTO (but without boilerplate constructors and setters).

In this section

• Transforming Collections
  Move from mutable loops to value-oriented collection pipelines with `map`, `filter`, `reduce`, and threading macros.
• Immutability in Application State
  Model state changes as pure transitions between immutable values, and keep atoms or other references at the boundary.
• Refactoring Imperative Code
  Use a repeatable recipe to move from mutable Java-style workflows to pure Clojure functions over immutable data.