Clojure Threading Macros — thread-first and thread-last | Episode 15

Nested function calls read inside-out. That's backwards. Threading macros flip the flow so you read left-to-right, top-to-bottom — the way you think about the data moving through each step. In this episode we cover the nested mess, thread-last for sequence pipelines, thread-first for map updates, and the rule of thumb for picking between them based on API shape. Student code: https://github.com/GoCelesteAI/clojure-for-beginners/tree/main/episode15 Every keystroke is shown on screen with generous pauses so you can follow along at your own pace. What You'll Learn: - Why nested calls are hard to read - Thread-last — perfect for map, filter, reduce - Thread-first — perfect for assoc, update, get - Picking between them by API shape (coll-last vs coll-first) - Rewriting Episode 14's nested pipeline as a single threaded expression Timestamps: 0:00 - Intro 0:12 - Preview: pipelines, not nesting 0:32 - Start the REPL 0:40 - Nested vs threaded: reverse, drop, take 0:48 - Thread-last 1:06 - Thread-first 1:18 - Exit REPL 1:23 - Write threading.clj in Neovim 1:46 - Nested mess section 2:23 - Thread-last section 4:22 - Thread-first section 5:38 - Ep14 pipeline, threaded 7:32 - Run with clj -M 7:36 - Output walked through 8:32 - Review 8:36 - Recap 9:11 - What's next: Episode 16 Key Takeaways: 1. Threading macros turn nested calls into readable pipelines. 2. Thread-last passes the value as the LAST argument — use it for sequence transforms. 3. Thread-first passes the value as the FIRST argument — use it for map and string transforms. 4. Pick by API shape: if the function takes the collection last, reach for thread-last. Phase 3 continues. Next up, Episode 16 — a deep dive on `reduce`. Taught by CelesteAI. Like and subscribe for more Clojure tutorials!