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Type Systems — Senior

Use types at architectural boundaries to encode ownership, nullability, errors, units, state transitions, and protocol phases. Avoid abstractions that require callers to fight inference or scatter casts. Combine static guarantees with runtime validation for untrusted input.

Test yourself

  1. Which invariant belongs in a type?
  2. Where must runtime validation remain?
  3. How do you migrate a public type safely?

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