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Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

2026-05-20 23:52:15 +09:00

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title

상태 머신 (State Machine) 모델링 및 Redux 액션/리듀서 설계

매 한 줄

"매 impossible state 의 unrepresentable 화". 매 finite state machine (FSM) 는 explicit state + event + transition 의 enumerate. Redux reducer 는 매 (state, action) → state 의 pure function — 매 FSM 와 isomorphic. 2026 년 매 XState 5 + RTK 의 combine 패턴 의 standard.

매 핵심

매 FSM 5-tuple

States (Q): enumerable set — idle | loading | success | error.
Events (Σ): external triggers — FETCH | RESOLVE | REJECT.
Transitions (δ): Q × Σ → Q — 매 partial function.
Initial state (q₀): idle.
Final states (F): optional — terminal nodes.

매 Redux 와 mapping

Reducer = δ: (state, action) => newState 의 pure transition.
Action = Σ event: typed discriminated union.
Store state = Q: 매 status field 의 enum.
Selector: 매 derived view — isLoading = state.status === 'loading'.

매 응용

Form wizard — multi-step flow 의 state 의 explicit.
Data fetching — idle/loading/success/error 의 4-state.
Auth flow — anonymous/authenticating/authenticated/expired.

💻 패턴

Discriminated union state

type FetchState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error };

// 매 impossible state ('loading' + data) 의 unrepresentable.

RTK slice = FSM reducer

import { createSlice, PayloadAction } from '@reduxjs/toolkit';

const userSlice = createSlice({
  name: 'user',
  initialState: { status: 'idle' } as FetchState<User>,
  reducers: {
    fetch: (state) => {
      if (state.status === 'idle' || state.status === 'error') {
        return { status: 'loading' };
      }
      return state; // ignore invalid transition
    },
    resolve: (_, action: PayloadAction<User>) => ({
      status: 'success',
      data: action.payload,
    }),
    reject: (_, action: PayloadAction<Error>) => ({
      status: 'error',
      error: action.payload,
    }),
  },
});

Transition guard table

const TRANSITIONS = {
  idle:    { FETCH: 'loading' },
  loading: { RESOLVE: 'success', REJECT: 'error' },
  success: { FETCH: 'loading' },
  error:   { FETCH: 'loading' },
} as const;

function transition<S extends keyof typeof TRANSITIONS>(
  state: S,
  event: string
): string {
  return (TRANSITIONS[state] as any)[event] ?? state;
}

XState 5 + Redux integration

import { createMachine, createActor } from 'xstate';

const fetchMachine = createMachine({
  id: 'fetch',
  initial: 'idle',
  states: {
    idle: { on: { FETCH: 'loading' } },
    loading: {
      invoke: {
        src: 'fetchUser',
        onDone: { target: 'success', actions: 'storeData' },
        onError: 'error',
      },
    },
    success: { on: { REFRESH: 'loading' } },
    error: { on: { RETRY: 'loading' } },
  },
});

const actor = createActor(fetchMachine).start();
actor.subscribe((snapshot) => store.dispatch({ type: 'fsm/sync', payload: snapshot.value }));

Hierarchical (nested) state

// 매 auth flow — 매 sub-state machine.
const authMachine = createMachine({
  initial: 'unauthenticated',
  states: {
    unauthenticated: {
      initial: 'idle',
      states: {
        idle: { on: { LOGIN: 'submitting' } },
        submitting: { on: { SUCCESS: '#auth.authenticated', FAIL: 'idle' } },
      },
    },
    authenticated: {
      on: { LOGOUT: 'unauthenticated' },
    },
  },
}, { id: 'auth' });

Parallel state regions

const editorMachine = createMachine({
  type: 'parallel',
  states: {
    document: { initial: 'clean', states: { clean: {}, dirty: {} } },
    network:  { initial: 'online', states: { online: {}, offline: {} } },
  },
});
// 매 state = (document, network) 의 cross product — explicit.

Test as transition table

test.each([
  ['idle',    'FETCH',   'loading'],
  ['loading', 'RESOLVE', 'success'],
  ['loading', 'REJECT',  'error'],
  ['success', 'FETCH',   'loading'],
])('transition %s --%s--> %s', (from, event, to) => {
  expect(transition(from, event)).toBe(to);
});

매 결정 기준

상황	Approach
Simple async (fetch)	RTK `createAsyncThunk` + discriminated union
Multi-step wizard	XState machine
Complex auth	XState hierarchical machine
Parallel concerns	XState parallel states
Pure UI toggle	useState (not Redux)

기본값: RTK + discriminated union state. 복잡 시 XState 5.

🔗 Graph

부모: 프론트엔드_및_UIUX_표준
변형: XState
응용: Data Fetching
Adjacent: Discriminated Union · Pure Function · Event Sourcing

🤖 LLM 활용

언제: state 가 3+ 개 + transition 의 explicit rule 의 필요 시. 언제 X: simple boolean toggle 또는 single-shot async — 매 overkill.

❌ 안티패턴

Boolean explosion: isLoading + isError + isSuccess — impossible state (true+true) 의 representable.
Implicit transition: reducer 안 valid state check 의 X — 매 invalid transition 의 silently 발생.
God reducer: 매 모든 logic 의 한 reducer — 매 split.
Side effect in reducer: fetch() 의 reducer 내부 — 매 thunk/saga/middleware 사용.

🧪 검증 / 중복

Verified (Hopcroft & Ullman Automata Theory; Redux docs; XState v5 docs 2026).
신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — FSM↔Redux mapping + XState 5 patterns

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