Mapper constructor parameters must be manually deep-frozen (no docs/error)

[hubyrod]

Bug: Mutable mapper parameters cause silent data inconsistencies

Version: @skipruntime/core@0.0.19

Problem

When passing arrays (or other mutable objects) as constructor parameters to Mapper implementations, the reactive graph produces subtle, hard-to-debug data inconsistencies. There is no error, no warning, and no documentation of this requirement.

Reproduction

class PhotoVisibilityFilter implements Mapper<string, Photo, string, Photo> {
  constructor(
    private userId: string,
    private groupIds: string[] // mutable array
  ) {}

  mapEntry(key: string, values: Values<Photo>): Iterable<[string, Photo]> {
    const photo = values.getUnique();
    if (this.groupIds.includes(photo.groupId)) {
      return [[key, photo]];
    }
    return [];
  }
}

// Later, when instantiating:
const groupIds = ["group-1", "group-2"];

// This SILENTLY causes issues — groupIds is mutable
const filtered = collection.map(PhotoVisibilityFilter, userId, groupIds);

Impact

• Manifests as intermittent, non-deterministic data inconsistencies in the reactive graph
• Extremely difficult to debug — no error message points to mutability as the cause
• Discovered only through trial and error after ruling out other possible causes

Current workaround

Freezing all array/object parameters before passing them to mappers:

const groupIds = Object.freeze(["group-1", "group-2"]);
const filtered = collection.map(PhotoVisibilityFilter, userId, groupIds);

And marking the types as readonly to prevent accidental mutation:

class PhotoVisibilityFilter implements Mapper<string, Photo, string, Photo> {
  constructor(
    private userId: string,
    private groupIds: readonly string[]
  ) {}
  // ...
}

Expected behavior

One of:

1. Auto-freeze parameters internally when they are captured by the reactive graph
2. Throw a clear error at construction time if mutable objects are passed as mapper parameters (e.g., TypeError: Mapper parameters must be frozen — call Object.freeze() on arrays/objects before passing them)
3. Document the requirement prominently in the Mapper API docs with examples

Option 1 or 2 would be strongly preferred over documentation alone, since the failure mode is completely silent.