No Invalid States

Review and refactor a codebase so that correctness constraints are carried by the strongest mechanism the language provides. Scattered defensive checks are not that mechanism.

The core principle:

Validate at boundaries, convert into stronger representations, and make invalid internal states difficult or impossible to construct.

This is the "parse, don't validate" discipline:

• Each fact is checked once, at the edge.
• The result is encoded in a representation the rest of the program can trust.
• Runtime validation is not deleted: checks on external, dynamic, or untrusted data stay.

When to Apply

Use this skill when the code shows any of these signals: