Misuse Resistance

Interfaces must be easy to use correctly and hard to use incorrectly (Scott Meyers; the "pit of success"). A design that relies on developer discipline — remembering to call something, reading a comment, knowing the convention — is a defect, because a competent-but-rushed developer will eventually get it wrong. Hunt for:

• False affordances: a signature that implies safety it doesn't provide.

  function createUser(name, email, role) { return { name, email, role }; }
  

  Callers reasonably assume a create function validates or enforces something; this one is an object literal in a trench coat, and its existence discourages callers from validating. Give it a real job (validate, normalize, apply invariants) or delete it and use a literal openly.

• Temporal coupling: correctness depends on call order that nothing enforces.

  verifySession(token);                 // step 1 — easy to forget
  const ctx = buildContext(req.body);  // step 2 — also accepts unverified data!
  

  Fix by fusing the sequence (contextFromToken(token) verifies internally) or by making the later step require unforgeable proof of the earlier one (it accepts only the type that step 1 returns, and nothing else can produce that type).

• Validation separated from construction: "parse, don't validate" (Alexis King). Don't check a string is a valid email and keep passing the string — parse it once into an EmailAddress type whose only constructor does the check, then require EmailAddress downstream. Possession of the value is proof of validity; unvalidated data becomes unrepresentable ("make illegal states unrepresentable"). Full treatment — branded domain types, deriving types from constants, correlated fields — in type-driven-design.