Clojure `for`, `doseq` & List Comprehensions — Generate and Iterate with Clarity | Episode 19

`for` is Clojure's list comprehension — give it one or more bindings and it hands back a lazy sequence of results. Add `:when` to filter, `:let` to name intermediate values, and you have a compact language for "generate me everything shaped like X." `doseq` has the exact same shape, but it's eager and returns nil. You reach for it when you want the side effects — printing, writing, logging — not a collection. In this episode we build chess-board coordinates with `for`, filter dark squares with `:when`, find Pythagorean triples with `:let`, and print a multiplication table with `doseq`. Student code: https://github.com/GoCelesteAI/clojure-for-beginners/tree/main/episode19 Every keystroke is shown on screen with generous pauses so you can follow along at your own pace. What You'll Learn: - `for` as a list comprehension — bindings in, lazy seq out - Multiple bindings — Cartesian product, rightmost varies fastest - `:when` — inline filter over the bindings - `:let` — name intermediate values mid-comprehension - `doseq` — eager, nil-returning sibling for side effects - The rule of thumb: `for` for values, `doseq` for effects Timestamps: 0:00 - Intro 0:15 - Preview: generate & iterate with clarity 0:38 - Start the REPL 0:52 - `for` with a single binding 1:05 - Multiple bindings 1:30 - `:when` filters 1:43 - `doseq` in the REPL 1:59 - Exit REPL 2:12 - Write for_doseq.clj in Neovim 2:40 - `for` section 3:15 - Multiple bindings section 3:55 - Chess board coordinates 5:00 - `:when` filters section 5:40 - `:let` bindings & Pythagorean triples 6:50 - `doseq` section 7:45 - Multiplication table with `doseq` 8:40 - Run with clj -M 8:45 - Output walked through 9:55 - Review 10:10 - Recap 10:45 - What's next: Episode 20 Key Takeaways: 1. `for` returns a lazy sequence — it describes what to generate. 2. Multiple bindings nest — the rightmost binding varies fastest. 3. `:when` filters bindings inline, `:let` names intermediate values. 4. `doseq` shares `for`'s shape but runs eagerly and returns nil. 5. If you find yourself throwing a `for` result away, you want `doseq`. Phase 3 — Functional Core — is now complete. Next up, Phase 4 kicks off with Episode 20: Namespaces and `require`. Taught by CelesteAI. Like and subscribe for more Clojure tutorials!