Freeze Model

Foldkit depends on immutable Model updates. TypeScript's readonly checks source code, but it cannot stop a write at runtime. If code mutates model.items in place, reference equality can no longer tell which parts changed. A Subscription may keep stale dependencies, and the view may leave stale DOM on screen.

During development, Foldkit deep-freezes the Model after init and after every update. An accidental write then throws a TypeError at the write site instead of producing a later rendering or Subscription bug.

Freezing runs only when Vite HMR is active. Set freezeModel to false to disable it, but do not use that option to hide a mutation the guardrail found.

Foldkit freezes values selectively:

• Plain objects and arrays are frozen recursively.

• Effect values such as Option, Result, DateTime, HashSet, HashMap, and Chunk are not frozen. They may cache a hash on first use, which requires writing to the instance.

• Date, Map, Set, File, and class instances are not frozen.

Option.some has one extra rule. Its wrapper remains writable for Effect's hash cache, but Foldkit freezes a plain object or array inside it. Mutating the array in Option.some({ items: [...] }) still throws.

Messages are not frozen. The guardrail applies only to the Model.

Already-frozen values are skipped. Because evo preserves unchanged branches by reference, each update freezes only the newly created plain objects and arrays.