Type Narrowing at I/O Boundaries
Many type-related bugs begin not in core logic, but at the edges of the system. API payloads, queue events, environment variables, and user input do not become safe just because a type exists nearby. The key is how early the system narrows uncertain values.
A healthy flow
- accept input broadly and skeptically
- validate at the boundary
- convert into narrow domain types
- keep internal logic working only with validated shapes
This pushes failure toward the edge and keeps the core simpler.
Common mistakes
- skipping validation with
ascasts - spreading raw payloads across the codebase
- mixing business rules and type guards in one place
Conclusion
The real power of a type system is not pretending every input is known. It is rejecting unknown input early and clearly.
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