Effect Days 2025: Building Effect 4

Michael describes Effect 4 as a review of choices made in Effect 3, driven by production feedback. The work aims to remove duplicate concepts, reduce bundle size, and improve core data types. 04:21 05:09 06:17

A fiber is independently running work. Batching groups operations. Transactional operations commit together.

He connects faster fibers with batching and transactional operations becoming ordinary Effect facilities rather than separate execution models. 22:56 28:03

Sources

  1. Building Effect 4.0 | Michael Arnaldi (Effect Days 2025) - YouTube. Michael Arnaldi. Recording; used for Effect 4’s redesign from production feedback. Accessed 2026-10-01.

Last changed 2026-10-07 in 0e2d656. Source on GitHub. Change log.

Linked from

  1. Agent guide. An Effect stack so pure (aspirational) Kit Langton will blush.

    Effect Days 2025: Building Effect 4 — Michael Arnaldi presents Effect 4 as a redesign shaped by production feedback.

  2. Change log. What changed in the files served here, newest first.

    Effect Days 2025: Building Effect 4 — edited: feat(learn): teach cards 23 to 42 with plain lines, diagrams and snippets

  3. Full agent guide. An Effect stack so pure (aspirational) Kit Langton will blush.
  4. Rat Stack lore | rat-stack. Short, source-grounded notes on the ideas and decisions behind rat-stack.

    Effect Days 2025: Building Effect 4 — Michael Arnaldi presents Effect 4 as a redesign shaped by production feedback.

  5. Source change history (log.md). The generated change log as Markdown.

    Effect Days 2025: Building Effect 4 — edited: feat(learn): teach cards 23 to 42 with plain lines, diagrams and snippets