Clojure Namespaces & `require` — Organizing Code Across Files | Episode 20

Every Clojure file starts with a namespace. A namespace is how Clojure says "this code lives here" — and `require` is how one file reaches into another. In this episode we leave the single-file world behind and build our first real two-file project: `app.core` requiring `app.util`. We show the three moves you'll use every day: `:require` with `:as` for an alias, `:refer` to pull specific names in unqualified, and the file-to-namespace mapping that ties it all together (`src/app/util.clj` ↔ `app.util`). This kicks off Phase 4 — Namespaces, Projects, Testing — the practical phase that lets you actually ship Clojure code beyond a single script. Student code: https://github.com/GoCelesteAI/clojure-for-beginners/tree/main/episode20 Every keystroke is shown on screen with generous pauses so you can follow along at your own pace. What You'll Learn: - `*ns*` and `in-ns` — what a namespace is, at the REPL - `ns` declarations — every file starts with one - `:require` with `:as` — the default, readable form - `:require` with `:refer` — pulling specific names in unqualified - File-to-namespace mapping — `src/app/util.clj` ↔ `app.util` - `deps.edn` with a `:run` alias for `clj -M:run` Timestamps: 0:00 - Intro 0:15 - Preview: organizing code across files 0:38 - Start the REPL 0:52 - `*ns*` — the current namespace 1:10 - `in-ns` and fully-qualified names 1:50 - Exit REPL 2:05 - Write deps.edn 2:30 - Project layout 2:45 - Write src/app/util.clj 4:30 - Write src/app/core.clj with `:require` 6:15 - Run `clj -M -m app.core` 6:35 - Run via `:run` alias with an argument 7:00 - Recap 7:25 - What's next: Episode 21 Key Takeaways: 1. A namespace is Clojure's unit of code organization — every file has one. 2. `ns` at the top of a file declares its namespace and its dependencies. 3. `:as` gives an alias you write at every call site (`u/shout`). 4. `:refer` pulls specific names in unqualified (`join` instead of `str/join`). 5. File path under `src/` must match the namespace (`app.util` ↔ `src/app/util.clj`). 6. A `deps.edn` `:aliases` block lets you run the app with `clj -M:run`. Phase 3 — Functional Core — is behind us. Phase 4 begins here: the practical work of namespaces, projects, and testing that turns your Clojure from REPL snippets into shippable code. Taught by CelesteAI. Like and subscribe for more Clojure tutorials!