- Domain/state machines (payments/fulfillment-style workflows): modeling states + transitions so “impossible” states literally can’t be represented. - Parsers/DSLs & config tooling: log parsers, small interpreters, schema validation, migration planners. - Internal CLIs / automation: batch jobs, release helpers, data shapers, anything you want to be correct and easy to refactor later. - Small backend services when the domain is gnarly (Servant / Yesod style) rather than huge monoliths.

If you’re learning it beyond CS exposure, I’d start with a CLI + a parser (JSON/CSV/logs), then add property-based tests (QuickCheck). That combo teaches types, purity, effects, and testing in one project without needing to “go full web stack” on day 1.