Clojure REPL-Driven Development — The Inner Loop That Defines Clojure | Episode 21

The REPL isn't where you test Clojure code. It's where you *write* it. In this episode we grow `parse-person` one form at a time — splitting, trimming, destructuring, assembling — all from a live session that never restarts. The rich-comment block is your scratchpad, and the REPL holds state across everything you try. We also force a crash — `NumberFormatException` on `"thirty"` — and watch the REPL shrug it off. Prompt comes right back. That resilience is what makes the inner loop feel different from every other language's REPL. Student code: https://github.com/GoCelesteAI/clojure-for-beginners/tree/main/episode21 Every keystroke is shown on screen with generous pauses so you can follow along at your own pace. What You'll Learn: - `(comment …)` rich-comment blocks — the REPL scratchpad pattern - `(require 'ns)` and `(in-ns 'ns)` — load once, then live inside the namespace - Growing a function incrementally: split → trim → destructure → assemble - Why REPL state survives errors — stack trace prints, prompt returns - `(require 'app.core :reload)` — pick up file edits without restarting - When the REPL beats "edit → save → re-run" every time Timestamps: 0:00 - Intro 0:15 - Preview: the inner loop 0:52 - The finished file and its comment block 1:08 - Start the REPL 1:16 - require + in-ns — live inside app.core 1:32 - Step 1 — split on commas 1:43 - Step 2 — trim each piece 1:55 - Step 3 — destructure into names 2:08 - Step 4 — the whole function together 2:18 - A different person 2:27 - Force a crash — "thirty" as a word 2:38 - The REPL survives 2:51 - Control-D 2:57 - Recap 3:34 - What's next: Episode 22 Key Takeaways: 1. The REPL is the workspace, not the afterthought — keep one session alive. 2. `(comment …)` blocks are ignored at load time but eval fine from the REPL — use them as scratchpads committed to source control. 3. Errors print a stack trace; they don't end the session. Redefine and retry. 4. `(require 'app.core :reload)` re-loads a namespace after you've edited its file. 5. Growing a function one form at a time, with live feedback, is faster than any edit-save-rerun cycle. Phase 4 continues. Next up: testing with `clojure.test` — turning the functions you just grew into a green test suite. Taught by CelesteAI. Like and subscribe for more Clojure tutorials!