React Hook Composition for TMNL

Overview

Custom hooks encapsulate stateful logic, effects, and event handlers into reusable functions. In TMNL, hooks bridge Effect services (via effect-atom) with React components, providing ergonomic interfaces for complex systems.

Key Insight: Hooks compose behavior, not UI. They return data and operations, letting components decide presentation.

Canonical Sources

TMNL Implementations

useSlider — /src/lib/slider/v1/hooks/useSlider.ts (comprehensive hook interface)
- Local atom state via Atom.make()
- Event dispatch with reducer pattern
- Derived values via useMemo
- Handler composition with useCallback
useDataManager — /src/lib/data-manager/v1/hooks/useDataManager.ts (Effect service integration)
- Atom subscriptions via useAtomValue
- Operation setters via useAtomSet
- Derived atoms for computed state
- Promise-based operations
useVariable — /src/lib/variables/hooks/useVariable.ts (registry pattern)
- Service-scoped state management
- Type-safe variable access
- Reactive updates via atoms
useDrawer — /src/lib/drawer/hooks/useDrawer.ts (UI state hook)
- Atom-based drawer state
- Imperative open/close API
- Content registration

Reference Documentation

Pattern Variants

Pattern 1: Atom Subscription Hook

Use when consuming reactive state from effect-atom atoms.

tsx

import { useAtomValue, useAtomSet } from '@effect-atom/atom-react'
import { useCallback } from 'react'
import { resultsAtom, searchOps } from '../atoms'
import type { SearchQuery, SearchResult } from '../types'

// ─────────────────────────────────────────────────────────────────────────
// Hook Interface
// ─────────────────────────────────────────────────────────────────────────

export interface UseDataManagerResult<T = unknown> {
  // State (from atoms)
  readonly results: readonly SearchResult<T>[]
  readonly isSearching: boolean
  readonly hasResults: boolean

  // Operations
  readonly search: (query: SearchQuery) => Promise<readonly SearchResult<T>[]>
}

// ─────────────────────────────────────────────────────────────────────────
// Hook Implementation
// ─────────────────────────────────────────────────────────────────────────

export function useDataManager<T = unknown>(): UseDataManagerResult<T> {
  // Atom subscriptions (plain atoms return values directly)
  const results = useAtomValue(resultsAtom) as readonly SearchResult<T>[]
  const isSearching = useAtomValue(isSearchingAtom)
  const hasResults = useAtomValue(hasResultsAtom)

  // Operation setters (fn atoms return promises)
  const doSearch = useAtomSet(searchOps.search, { mode: 'promise' })

  // Wrapped operations
  const search = useCallback(
    async (query: SearchQuery): Promise<readonly SearchResult<T>[]> => {
      const result = await doSearch(query)
      return result as readonly SearchResult<T>[]
    },
    [doSearch]
  )

  return {
    results,
    isSearching,
    hasResults,
    search,
  }
}

Canonical source: src/lib/data-manager/v1/hooks/useDataManager.ts:98-174

Key patterns:

useAtomValue for reactive state subscriptions
useAtomSet for mutation operations
useCallback to stabilize function references
Type generics for consumer-provided types

Pattern 2: Local State Hook with Reducer

Use when managing complex local state with dispatched events.

tsx

import { useCallback, useMemo, useRef } from 'react'
import { Atom } from '@effect-atom/atom-react'
import type { SliderState, SliderEvent, SliderBehaviorShape } from '../types'
import { sliderReducer } from '../atoms'
import { LinearBehavior } from '../services/SliderBehavior'

// ─────────────────────────────────────────────────────────────────────────
// Hook Interface
// ─────────────────────────────────────────────────────────────────────────

export interface UseSliderOptions {
  value?: number
  onChange?: (value: number) => void
  behavior?: SliderBehaviorShape
  debug?: boolean
}

export interface UseSliderReturn {
  // State
  state: SliderState
  value: number
  normalizedValue: number

  // Event handlers
  dispatch: (event: SliderEvent) => void
  handlePointerDown: (e: React.PointerEvent) => void
  handleKeyDown: (e: React.KeyboardEvent) => void

  // Ref for container
  containerRef: React.RefObject<HTMLDivElement>
}

// ─────────────────────────────────────────────────────────────────────────
// Hook Implementation
// ─────────────────────────────────────────────────────────────────────────

export function useSlider(options: UseSliderOptions = {}): UseSliderReturn {
  const {
    value: initialValue = 0,
    onChange,
    behavior = LinearBehavior.shape,
    debug = false,
  } = options

  // Local atom for state
  const stateAtom = useMemo(
    () => Atom.make<SliderState>({
      value: initialValue,
      isDragging: false,
      isFocused: false,
      isEditing: false,
      activeSensitivity: 1.0,
      activeModifiers: [],
    }),
    []
  )

  const state = Atom.get(stateAtom)

  // Dispatch function
  const dispatch = useCallback((event: SliderEvent) => {
    const newState = sliderReducer(Atom.get(stateAtom), event, behavior)
    Atom.set(stateAtom, newState)

    // Fire onChange if value changed
    if (newState.value !== Atom.get(stateAtom).value && onChange) {
      onChange(newState.value)
    }
  }, [stateAtom, onChange, behavior])

  // Derived values
  const normalizedValue = useMemo(
    () => behavior.toNormalized(state.value),
    [state.value, behavior]
  )

  // Event handlers
  const containerRef = useRef<HTMLDivElement>(null)

  const handlePointerDown = useCallback((e: React.PointerEvent) => {
    dispatch({ type: 'POINTER_DOWN', event: e })
  }, [dispatch])

  const handleKeyDown = useCallback((e: React.KeyboardEvent) => {
    dispatch({ type: 'KEY_DOWN', event: e })
  }, [dispatch])

  return {
    state,
    value: state.value,
    normalizedValue,
    dispatch,
    handlePointerDown,
    handleKeyDown,
    containerRef,
  }
}

Canonical source: src/lib/slider/v1/hooks/useSlider.ts:98-300

Key patterns:

useMemo to create stable atom reference (once per component)
Atom.make() for local reactive state
useCallback for stable event handlers
useRef for DOM node access

Pattern 3: Derived State Hook

Use when exposing computed values from atoms.

tsx

import { useAtomValue } from '@effect-atom/atom-react'
import { useMemo } from 'react'
import { resultsAtom, statusAtom } from '../atoms'

export function useSearchMetrics() {
  const results = useAtomValue(resultsAtom)
  const status = useAtomValue(statusAtom)

  // Derived values
  const resultCount = useMemo(() => results.length, [results])
  const hasResults = useMemo(() => resultCount > 0, [resultCount])
  const isSearching = useMemo(() => status === 'streaming', [status])

  return {
    resultCount,
    hasResults,
    isSearching,
  }
}

Key patterns:

useMemo for derived computations
Multiple atom subscriptions
Pure computation, no side effects

Pattern 4: Hook with useDebugValue

Use when debugging complex hook state.

tsx

import { useAtomValue } from '@effect-atom/atom-react'
import { useDebugValue } from 'react'
import { stateAtom } from '../atoms'

export function useSliderDebug() {
  const state = useAtomValue(stateAtom)

  // Show in React DevTools
  useDebugValue(
    state,
    (s) => `value: ${s.value}, dragging: ${s.isDragging}`
  )

  return state
}

Key patterns:

useDebugValue for DevTools visibility
Optional formatter function for readable labels

Pattern 5: Registry Pattern Hook

Use when accessing service-scoped registries.

tsx

import { useAtomValue, useAtomSet } from '@effect-atom/atom-react'
import { useCallback } from 'react'
import { variablesAtom, registerOps } from '../atoms'
import type { Variable } from '../types'

export function useVariableRegistry() {
  const variables = useAtomValue(variablesAtom)
  const doRegister = useAtomSet(registerOps.register)

  const register = useCallback(
    (variable: Variable) => {
      doRegister(variable)
    },
    [doRegister]
  )

  const get = useCallback(
    (id: string) => variables.find((v) => v.id === id),
    [variables]
  )

  return {
    variables,
    register,
    get,
  }
}

Canonical source: src/lib/variables/hooks/useVariable.ts

Key patterns:

Registry lookup via find()
Imperative register API
Service-scoped state

Decision Tree

Need reusable stateful logic?
│
├─ Consuming Effect service atoms?
│  └─ Use: Atom Subscription Hook
│     (e.g., useDataManager with useAtomValue)
│
├─ Complex local state with events?
│  └─ Use: Local State Hook with Reducer
│     (e.g., useSlider with Atom.make + dispatch)
│
├─ Computed values from multiple atoms?
│  └─ Use: Derived State Hook
│     (e.g., useSearchMetrics with useMemo)
│
└─ Accessing a registry or collection?
   └─ Use: Registry Pattern Hook
      (e.g., useVariableRegistry)

Examples

Example 1: useSlider Full Interface

Comprehensive hook with state, events, derived values, and handlers.

tsx

function SliderComponent({ value, onChange }: { value: number; onChange: (v: number) => void }) {
  const {
    state,
    normalizedValue,
    displayValue,
    handlePointerDown,
    handlePointerMove,
    handlePointerUp,
    handleKeyDown,
    handleWheel,
    handleDoubleClick,
    containerRef,
  } = useSlider({
    value,
    onChange,
    behavior: DecibelBehavior.shape,
    debug: true,
  })

  return (
    <div
      ref={containerRef}
      onPointerDown={handlePointerDown}
      onPointerMove={handlePointerMove}
      onPointerUp={handlePointerUp}
      onKeyDown={handleKeyDown}
      onWheel={handleWheel}
      onDoubleClick={handleDoubleClick}
    >
      <div style={{ width: `${normalizedValue * 100}%` }} />
      <span>{displayValue}</span>
    </div>
  )
}

Canonical source: src/lib/slider/v1/hooks/useSlider.ts:50-92

Example 2: useDataManager with Progressive Results

Hook that subscribes to streaming search results.

tsx

function SearchUI() {
  const {
    results,
    isSearching,
    hasResults,
    resultCount,
    search,
    indexData,
  } = useDataManager<MovieItem>()

  useEffect(() => {
    indexData(movies, { fields: ['title', 'cast', 'genres'] })
  }, [movies, indexData])

  const handleSearch = async (query: string) => {
    await search({ query, limit: 100 })
  }

  return (
    <div>
      {isSearching && <Spinner />}
      <div>Results: {resultCount}</div>
      {results.map((r) => (
        <Result key={r.item.id} result={r} />
      ))}
    </div>
  )
}

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

Anti-Patterns (BANNED)

useState for Cross-Component State

tsx

// BANNED - Use atoms for shared state
function useSharedData() {
  const [data, setData] = useState([])
  // Other components can't access this!
  return { data, setData }
}

// CORRECT - Use atoms
const dataAtom = Atom.make([])

function useSharedData() {
  const data = useAtomValue(dataAtom)
  const setData = useCallback((newData) => {
    Atom.set(dataAtom, newData)
  }, [])
  return { data, setData }
}

Returning JSX from Hooks

tsx

// BANNED - Hooks return data, not UI
function useCard() {
  return <Card>Content</Card>
}

// CORRECT - Return data and operations
function useCard() {
  const [isOpen, setIsOpen] = useState(false)
  return { isOpen, setIsOpen }
}

Unstable Function References

tsx

// BANNED - Creates new function on every render
function useSearch() {
  const search = (query: string) => { /* ... */ }
  return { search }
}

// CORRECT - Stable with useCallback
function useSearch() {
  const search = useCallback((query: string) => { /* ... */ }, [])
  return { search }
}

Conditional Hook Calls

tsx

// BANNED - Violates Rules of Hooks
function useData(shouldFetch: boolean) {
  if (shouldFetch) {
    const data = useAtomValue(dataAtom) // ← WRONG!
  }
}

// CORRECT - Always call hooks
function useData(shouldFetch: boolean) {
  const data = useAtomValue(dataAtom)
  return shouldFetch ? data : null
}

Implementation Checklist

When creating a custom hook:

Name: Prefix with use (e.g., useSlider)
Interface: Define UseXOptions and UseXReturn types
Atoms: Use useAtomValue for subscriptions, useAtomSet for operations
Callbacks: Wrap handlers with useCallback
Memoization: Use useMemo for derived values
Refs: Use useRef for DOM nodes or mutable values
Debug: Add useDebugValue for complex hooks
TypeScript: Export hook interface types
Documentation: Add JSDoc with usage example

Composition Patterns

Composing Multiple Hooks

tsx

function useSearchWithMetrics() {
  const { search, results } = useDataManager()
  const { resultCount, hasResults } = useSearchMetrics()
  const { logEvent } = useAnalytics()

  const searchWithLogging = useCallback(async (query: string) => {
    logEvent('search_started', { query })
    const results = await search({ query, limit: 100 })
    logEvent('search_completed', { resultCount: results.length })
    return results
  }, [search, logEvent])

  return {
    search: searchWithLogging,
    results,
    resultCount,
    hasResults,
  }
}

Hook Factories

tsx

function createUseSlider(defaultBehavior: SliderBehaviorShape) {
  return function useSliderWithDefaults(options: UseSliderOptions = {}) {
    return useSlider({
      ...options,
      behavior: options.behavior ?? defaultBehavior,
    })
  }
}

const useDecibelSlider = createUseSlider(DecibelBehavior.shape)
const useLinearSlider = createUseSlider(LinearBehavior.shape)

Related Patterns

effect-patterns — Hooks consume Effect services via atoms
react-state-migration — Migrate useState to atoms consumed by hooks
react-compound-components — Hooks often power compound component context

Filing New Patterns

When you discover a new hook pattern:

Implement in src/lib/<domain>/hooks/ with full TypeScript types
Add usage example in testbed
Update this skill with canonical source references
Document atom subscriptions and service integrations

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SKILL.md

React Hook Composition for TMNL

Overview

Canonical Sources

TMNL Implementations

Reference Documentation

Pattern Variants

Pattern 1: Atom Subscription Hook

Pattern 2: Local State Hook with Reducer

Pattern 3: Derived State Hook

Pattern 4: Hook with useDebugValue

Pattern 5: Registry Pattern Hook

Decision Tree

Examples

Example 1: useSlider Full Interface

Example 2: useDataManager with Progressive Results

Anti-Patterns (BANNED)

useState for Cross-Component State

Returning JSX from Hooks

Unstable Function References

Conditional Hook Calls

Implementation Checklist

Composition Patterns

Composing Multiple Hooks

Hook Factories

Related Patterns

Filing New Patterns