React State Migration for TMNL

Overview

TMNL enforces a strict separation: useState for local UI state, effect-atom for shared/service-scoped state. Migrating from useState to atoms eliminates prop drilling, enables reactive subscriptions, and integrates with Effect services.

Critical Doctrine: When React consumes Effect services via effect-atom, Atom.make() is the primary state mechanism—not Effect.Ref.

Canonical Sources

TMNL Implementations

DataManagerTestbed — /src/components/testbed/DataManagerTestbed.tsx (BEFORE/AFTER example)
- BEFORE: 4 useState hooks for results, status, stats, isIndexing
- AFTER: All state via useDataManager() hook consuming atoms
- Demonstrates progressive stream updates without setter soup
DataManager Service — /src/lib/data-manager/v1/DataManager.ts
- State lives in Atom.make() atoms, NOT Effect.Ref
- Service methods mutate atoms directly via Atom.set()
- React subscribes via useAtomValue()
SliderTestbed — /src/components/testbed/SliderTestbed.tsx
- useState acceptable for controlled slider value (local UI state)
- Slider behavior injectable via runtime atoms

Reference Documentation

Pattern Recognition

When useState Is ACCEPTABLE

Use useState for:

Pure UI state — Input bindings, hover states, toggle visibility
Single-component scope — State used only within one component
No async dependencies — Simple synchronous state
Controlled component values — Parent controls value, component notifies via onChange

tsx

// ✅ ACCEPTABLE - Local toggle
function Accordion() {
  const [isOpen, setIsOpen] = useState(false)
  return <div onClick={() => setIsOpen(!isOpen)}>...</div>
}

// ✅ ACCEPTABLE - Input binding
function SearchBox({ onSearch }: { onSearch: (q: string) => void }) {
  const [query, setQuery] = useState('')
  return (
    <input
      value={query}
      onChange={(e) => setQuery(e.target.value)}
      onKeyDown={(e) => e.key === 'Enter' && onSearch(query)}
    />
  )
}

// ✅ ACCEPTABLE - Controlled slider value
function VolumeControl() {
  const [volume, setVolume] = useState(75)
  return <Slider value={volume} onChange={setVolume} />
}

When Atoms Are REQUIRED

Use effect-atom when:

Crosses component boundaries — Shared state between sibling/distant components
Derives from async operations — API calls, streams, Effects
Needs reactive subscriptions — Multiple consumers of same state
Benefits from service scoping — Lifecycle tied to a service, not component mount
Integrates with Effect services — DataManager, SearchKernel, etc.

tsx

// ❌ BANNED - useState for cross-component state
function SearchResults() {
  const [results, setResults] = useState<SearchResult[]>([])
  const [status, setStatus] = useState<'idle' | 'loading' | 'complete'>('idle')
  // Other components can't access this!
}

// ✅ CORRECT - Atoms for shared state
const resultsAtom = Atom.make<SearchResult[]>([])
const statusAtom = Atom.make<'idle' | 'loading' | 'complete'>('idle')

function SearchResults() {
  const results = useAtomValue(resultsAtom)
  const status = useAtomValue(statusAtom)
  // Other components can subscribe to same atoms
}

Migration Patterns

Pattern 1: Simple State Migration

BEFORE (useState pollution):

tsx

function SearchUI() {
  const [results, setResults] = useState<SearchResult[]>([])
  const [status, setStatus] = useState<'idle' | 'loading' | 'complete'>('idle')
  const [isIndexing, setIsIndexing] = useState(false)

  const handleSearch = async (query: string) => {
    setStatus('loading')
    setResults([])

    const newResults = await fetchResults(query)

    setResults(newResults)
    setStatus('complete')
  }

  return (
    <div>
      {status === 'loading' && <Spinner />}
      {results.map((r) => <Result key={r.id} result={r} />)}
    </div>
  )
}

AFTER (effect-atom):

tsx

// In atoms/index.ts
export const resultsAtom = Atom.make<SearchResult[]>([])
export const statusAtom = Atom.make<'idle' | 'loading' | 'complete'>('idle')
export const isIndexingAtom = Atom.make<boolean>(false)

// In hooks/useDataManager.ts
export function useDataManager() {
  const results = useAtomValue(resultsAtom)
  const status = useAtomValue(statusAtom)
  const isIndexing = useAtomValue(isIndexingAtom)

  const search = useCallback(async (query: string) => {
    Atom.set(statusAtom, 'loading')
    Atom.set(resultsAtom, [])

    const newResults = await fetchResults(query)

    Atom.set(resultsAtom, newResults)
    Atom.set(statusAtom, 'complete')
  }, [])

  return { results, status, isIndexing, search }
}

// In component
function SearchUI() {
  const { results, status, search } = useDataManager()

  return (
    <div>
      {status === 'loading' && <Spinner />}
      {results.map((r) => <Result key={r.id} result={r} />)}
    </div>
  )
}

Canonical source: src/components/testbed/DataManagerTestbed.tsx:463-638

Pattern 2: Setter Soup → Service Methods

BEFORE (setter soup in callbacks):

tsx

function StreamingSearch() {
  const [results, setResults] = useState<SearchResult[]>([])
  const [status, setStatus] = useState<StreamStatus>('idle')
  const [stats, setStats] = useState<StreamStats>({ chunks: 0, items: 0, ms: 0 })

  const handleSearch = async (query: string) => {
    setStatus('streaming')
    setResults([])
    setStats({ chunks: 0, items: 0, ms: 0 })

    for await (const chunk of searchStream(query)) {
      setResults((prev) => [...prev, ...chunk.results])
      setStats((prev) => ({
        chunks: prev.chunks + 1,
        items: prev.items + chunk.results.length,
        ms: Date.now() - startTime,
      }))
    }

    setStatus('complete')
  }

  // 20 lines of setter calls scattered across component
}

AFTER (service-scoped atoms):

tsx

// In DataManager.ts (Effect service)
class DataManager extends Effect.Service<DataManager>()("tmnl/DataManager", {
  effect: Effect.gen(function* () {
    const search = (query: SearchQuery) =>
      Effect.gen(function* () {
        // Service mutates atoms directly
        Atom.set(statusAtom, 'streaming')
        Atom.set(resultsAtom, [])
        Atom.set(statsAtom, { chunks: 0, items: 0, ms: 0 })

        for await (const chunk of searchStream(query)) {
          Atom.set(resultsAtom, (prev) => [...prev, ...chunk.results])
          Atom.set(statsAtom, (prev) => ({
            chunks: prev.chunks + 1,
            items: prev.items + chunk.results.length,
            ms: Date.now() - startTime,
          }))
        }

        Atom.set(statusAtom, 'complete')
      }).pipe(Effect.withSpan('DataManager.search'))

    return { search } as const
  }),
}) {}

// In component
function StreamingSearch() {
  const { results, status, stats, search } = useDataManager()

  const handleSearch = async (query: string) => {
    await search({ query, limit: 100 })
    // Atoms update automatically via service
  }

  return <div>...</div>
}

Canonical source: src/lib/data-manager/v1/DataManager.ts:73-200

Pattern 3: Derived State Migration

BEFORE (useState + useEffect for derived values):

tsx

function SearchMetrics() {
  const [results, setResults] = useState<SearchResult[]>([])
  const [resultCount, setResultCount] = useState(0)
  const [hasResults, setHasResults] = useState(false)

  // Derived state in useEffect
  useEffect(() => {
    setResultCount(results.length)
    setHasResults(results.length > 0)
  }, [results])

  // Now have 3 state hooks for 1 source of truth!
}

AFTER (derived atoms):

tsx

// In atoms/index.ts
export const resultsAtom = Atom.make<SearchResult[]>([])

export const resultCountAtom = Atom.make((get) => {
  const results = get(resultsAtom)
  return results.length
})

export const hasResultsAtom = Atom.make((get) => {
  const count = get(resultCountAtom)
  return count > 0
})

// In component
function SearchMetrics() {
  const results = useAtomValue(resultsAtom)
  const resultCount = useAtomValue(resultCountAtom)
  const hasResults = useAtomValue(hasResultsAtom)

  // No useEffect needed - atoms auto-derive!
}

Canonical source: src/lib/data-manager/v1/atoms/index.ts:82-104

Pattern 4: Progressive Stream Updates

BEFORE (flicker from array recreation):

tsx

function ProgressiveResults() {
  const [results, setResults] = useState<SearchResult[]>([])

  useEffect(() => {
    const stream = searchStream(query)

    for await (const chunk of stream) {
      // Creates new array on every chunk - causes grid flicker!
      setResults((prev) => [...prev, ...chunk.results])
    }
  }, [query])

  return <AgGridReact rowData={results} />
}

AFTER (atom updates with transaction batching):

tsx

// In DataManager service
const search = (query: SearchQuery) =>
  Effect.gen(function* () {
    const stream = yield* searchKernel.queryStream(query)

    for await (const chunk of stream) {
      // Atom update batches efficiently
      Atom.set(resultsAtom, (prev) => [...prev, ...chunk.results])
    }
  })

// In component
function ProgressiveResults() {
  const { results, search } = useDataManager()

  useEffect(() => {
    search({ query, limit: 100 })
  }, [query, search])

  return <AgGridReact rowData={results} />
  // No flicker - AG-Grid receives stable updates
}

Canonical source: src/components/testbed/DataManagerTestbed.tsx:536-638

Decision Tree

Need state in React component?
│
├─ State used only in this component?
│  │
│  ├─ Pure UI state (toggle, hover, input)?
│  │  └─ Use: useState
│  │
│  └─ Derives from async operation?
│     └─ Use: effect-atom
│
├─ State shared across components?
│  └─ Use: effect-atom
│
├─ State integrates with Effect service?
│  └─ Use: effect-atom (Atom.make, NOT Effect.Ref)
│
└─ Derived from other state?
   └─ Use: Derived atom (Atom.make with getter)

Migration Checklist

When migrating from useState to atoms:

Identify shared state — Find useState that crosses component boundaries
Create atoms — Define atoms in atoms/index.ts with Atom.make()
Update service — Service methods use Atom.set(), not Effect.Ref
Create hook — Wrap atoms in custom hook (e.g., useDataManager())
Migrate components — Replace useState with useAtomValue() subscriptions
Remove setters — Delete setState calls, let service handle mutations
Add derived atoms — Replace useEffect derivations with derived atoms
Test reactivity — Verify subscriptions update correctly

Anti-Patterns (BANNED)

Effect.Ref for React State

tsx

// BANNED - Effect.Ref creates bridge complexity
class DataManager extends Effect.Service<DataManager>()("tmnl/DataManager", {
  effect: Effect.gen(function* () {
    const stateRef = yield* Ref.make<State>(initial) // ← WRONG!

    // Now need polling, SubscriptionRef, streams-to-consume-streams...
  }),
}) {}

// CORRECT - Atom.make as primary state
const resultsAtom = Atom.make<SearchResult[]>([])

class DataManager extends Effect.Service<DataManager>()("tmnl/DataManager", {
  effect: Effect.gen(function* () {
    const search = () => Effect.gen(function* () {
      Atom.set(resultsAtom, newResults) // ← Direct mutation
    })
    return { search } as const
  }),
}) {}

useState for Cross-Component State

tsx

// BANNED - Prop drilling for shared state
function Parent() {
  const [results, setResults] = useState([])
  return <Child results={results} setResults={setResults} />
}

function Child({ results, setResults }) {
  return <GrandChild results={results} setResults={setResults} />
}

// CORRECT - Atoms for shared state
const resultsAtom = Atom.make([])

function Parent() {
  return <Child />
}

function Child() {
  const results = useAtomValue(resultsAtom)
  // No props needed
}

Derived useState + useEffect

tsx

// BANNED - Manual derived state
const [results, setResults] = useState([])
const [count, setCount] = useState(0)

useEffect(() => {
  setCount(results.length) // ← Fragile, extra render
}, [results])

// CORRECT - Derived atom
const resultsAtom = Atom.make([])
const countAtom = Atom.make((get) => get(resultsAtom).length)

Mixing useState and Atoms

tsx

// BANNED - State split across useState and atoms
function Component() {
  const [localResults, setLocalResults] = useState([])
  const atomResults = useAtomValue(resultsAtom)

  // Which is the source of truth?
}

// CORRECT - Pick one pattern
function Component() {
  const results = useAtomValue(resultsAtom)
  // Single source of truth
}

Examples

Example 1: DataManagerTestbed Migration

Full migration from 4 useState hooks to service-scoped atoms.

BEFORE:

tsx

const [results, setResults] = useState<SearchResult[]>([])
const [status, setStatus] = useState<StreamStatus>('idle')
const [stats, setStats] = useState<StreamStats>({ chunks: 0, items: 0, ms: 0 })
const [isIndexing, setIsIndexing] = useState(false)

const handleSearch = async () => {
  setStatus('streaming')
  setResults([])
  // ... 30 more lines of setter soup
}

AFTER:

tsx

const {
  results,
  status,
  stats,
  isIndexing,
  search,
} = useDataManager<MovieItem>()

const handleSearch = async () => {
  await search({ query, limit: 100 })
  // Atoms update automatically
}

Canonical source: src/components/testbed/DataManagerTestbed.tsx:54-638

Example 2: Slider Acceptable useState

Local UI state for controlled component.

tsx

function VolumeControl() {
  const [volume, setVolume] = useState(75)

  return (
    <Slider
      value={volume}
      onChange={setVolume}
      behavior={LinearBehavior.shape}
      config={{ min: 0, max: 100 }}
    />
  )
}

Canonical source: src/components/testbed/SliderTestbed.tsx

Related Patterns

effect-patterns — Atom-as-State doctrine (NO Effect.Ref)
react-hook-composition — Hooks wrap atoms for ergonomic API
react-performance-patterns — Atoms enable fine-grained subscriptions

Filing New Patterns

When you discover a new migration pattern:

Document BEFORE/AFTER in testbed
Update this skill with canonical source references
Add to .edin/EFFECT_PATTERNS.md registry
Create migration guide in project docs

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