---
id: fiber-type
name: Define Fiber and Effect types
status: pending
depends_on: [review-key-field]
created: 2026-05-18T16:22:57.140011417Z
modified: 2026-05-18T16:22:57.140011873Z
scope: narrow
risk: medium
impact: phase
level: implementation
---

# Description

Define the `Fiber<I>` interface and `Effect` type in a new file `src/host/fiber.ts`. The fiber tree is the reconciler's internal state — it maps `UElement` positions to host instances across renders. This is the foundational type that all reconciler work builds on.

The `Fiber` type is generic over `I` (matching `HostConfig`'s `Instance` type) so each fiber carries a reference to its host instance. This is the bridge: fiber → host instance → host-specific state.

## Acceptance Criteria

- [ ] `Fiber<I>` interface defined with all fields: `instance`, `tag`, `props`, `key`, `children`, `parent`, `effect`, `signalDisposers`, `prevProps`
- [ ] `Effect` union type defined with variants: `update`, `insert`, `move`, `remove`
- [ ] Types are exported from `src/host/fiber.ts`
- [ ] Types are re-exported from `src/mod.ts` barrel
- [ ] TypeScript compiles (`npm run build:tsc`)
- [ ] No runtime behavior changes — this is type-only

## References

- docs/architecture/reconciler.md — Fiber Node, Effect Types sections
- docs/architecture/lifecycle.md — Fiber Tree Disposal (signalDisposers field)

## Notes

> To be filled by implementation agent

## Summary

> To be filled on completion