Write precise TypeScript types for my data and APIs

Help me design precise, maintainable TypeScript types for my code. What I'm typing: [e.g. "API responses", "a reducer's state and actions", "a form library", "a config object"] Current code or types: [PASTE] TypeScript version and settings: [VERSION, strict mode on or off, framework] Pain points: [e.g. "too many any types", "impossible states are allowed", "generics are unreadable", "slow type-checking"] Aim for: - Impossible states can't be represented: use discriminated unions instead of optional flags (e.g. `{ status: "loading" } | { status: "success"; data: T } | { status: "error"; error: E }`). - Narrow, don't cast: replace `as` assertions with type guards, `satisfies`, or validation at the boundary. For untrusted data (APIs, user input), recommend runtime validation with a schema library and derive the types from the schema. - Generics with purpose: constrain type parameters, name them clearly, and avoid generics callers must spell out by hand. - Advanced types only when they pay off: mapped, conditional, and template literal types and `infer`, each non-obvious one with a short comment. - Exhaustiveness: `never` checks in switch statements over unions. - Compiler performance: avoid deeply recursive types that slow type-checking. Return: 1. The revised types in a code block, with brief comments. 2. A table: Problem | Fix | Why it's safer. 3. Usage examples that should compile and ones that should fail, marked with `// @ts-expect-error`. 4. Any changes needed in calling code. If something can't be typed safely without a runtime check, say so instead of forcing it.