TypeScript 中的可辨识联合(Discriminated Unions)设计模式

type Square = {
  kind: 'square';
  size: number;
};

type Rectangle = {
  kind: 'rectangle';
  width: number;
  height: number;
};

type Circle = {
  kind: 'circle';
  radius: number;
};

type Shape = Square | Rectangle | Circle;

function calculateArea(shape: Shape): number {
  switch (shape.kind) {
    case'square':
      return shape.size * shape.size;
    case'rectangle':
      return shape.width * shape.height;
    case 'circle':
      return Math.PI * shape.radius * shape.radius;
  }
}

function calculateAreaIncomplete(shape: Shape): number {
  switch (shape.kind) {
    case'square':
      return shape.size * shape.size;
    case'rectangle':
      return shape.width * shape.height;
    // 缺少 case 'circle' 分支
  }
}

type CounterState = {
  kind: 'initial';
} | {
  kind: 'incremented';
  count: number;
} | {
  kind: 'decremented';
  count: number;
};

function updateCounter(state: CounterState, action: 'increment' | 'decrement'): CounterState {
  switch (state.kind) {
    case 'initial':
      return action === 'increment'? { kind: 'incremented', count: 1 } : { kind: 'decremented', count: -1 };
    case 'incremented':
      return action === 'increment'? { kind: 'incremented', count: state.count + 1 } : { kind: 'decremented', count: state.count - 1 };
    case 'decremented':
      return action === 'increment'? { kind: 'incremented', count: state.count + 1 } : { kind: 'decremented', count: state.count - 1 };
  }
}

type ApiResult<T> = {
  kind: 'success';
  data: T;
} | {
  kind: 'error';
  message: string;
};

async function fetchData(): Promise<ApiResult<string>> {
  try {
    const response = await fetch('https://example.com/api/data');
    const data = await response.text();
    return { kind:'success', data };
  } catch (error) {
    return { kind: 'error', message: error.message };
  }
}

async function processData() {
  const result = await fetchData();
  switch (result.kind) {
    case'success':
      console.log('Data:', result.data);
      break;
    case 'error':
      console.error('Error:', result.message);
      break;
  }
}