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Type Modelling Review

Reshapes types so invalid states cannot be represented, rather than validating them at runtime and hoping.

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CategoryCodingForDevelopersTested onClaudeChatGPT

Running it, start to finish

  1. List every invalid state the current types allow.
  2. Replace optional-plus-flag shapes with discriminated unions.
  3. Move validation to the boundary and parse into a guaranteed type.

What you get back

The output this produces, every time.

  • Lists the invalid states your current types permit, which is usually more than expected.
  • Moves validation to the boundary so downstream code is guaranteed by construction.
  • Checks each proposed change against bugs you actually had, rather than theoretical ones.

Getting better results

Where this usually goes wrong, and how to avoid it.

  • Look hardest at optional pairs. Two optionals where exactly one should be set is the most common representable-invalid-state, and a union fixes it outright.
  • Split the meanings of null. Absent, not-applicable and empty collapsed into one value produces bugs nobody can reproduce.
  • Brand your identifiers. An order id and a user id are both strings, and swapping them type-checks perfectly today.

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Written for The AI University. Every prompt in this library is original work — authored, tested and revised here, not collected from elsewhere. 365 of them, free with an account.