Making Impossible States Impossible
summary
Shows concrete techniques in Elm to make invalid UI states unrepresentable: handling CSS at-rules (the @charset rule must be first) by avoiding lists of interchangeable declarations, replacing validation-and-sort with a typed model so invalid sheets simply won’t compile; modeling surveys with paired records, a non-empty list (first+others), and a zip list (previous/current/remaining) so current is always valid; and keeping a stable API via a single-constructor union that makes internal upgrades nonbreaking. Results: far fewer tests, compile-time errors instead of runtime mystery bugs, and simpler runtime logic.
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chapters (12)
- 0:00Introduction and setup
- 1:17Elm CSS and invalid states
- 5:07Validating and sorting CSS rules
- 6:12Making invalid states impossible
- 8:58Survey app example
- 11:04Coupling prompts and responses
- 13:08Ensuring at least one question
- 14:30Zip list for navigation
- 15:53Upgrade experience without breaking changes
- 19:45Hiding implementation details
- 21:29Status bar with undo functionality
- 23:38Key considerations and summary
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