Discriminated unions voor veilige pattern matching

type Circle = { kind: 'circle'; radius: number };
type Square = { kind: 'square'; side: number };
type Shape = Circle | Square;

function area(shape: Shape): number {
  if (shape.kind === 'circle') {
    return Math.PI * shape.radius ** 2;
  }
  return shape.side ** 2; // narrowed to Square
}

function describe(shape: Shape): string {
  switch (shape.kind) {
    case 'circle': return `Circle r=${shape.radius}`;
    case 'square': return `Square s=${shape.side}`;
  }
}

function area(shape: Shape): number {
  switch (shape.kind) {
    case 'circle': return Math.PI * shape.radius ** 2;
    case 'square': return shape.side ** 2;
    default:
      const _exhaustive: never = shape;
      throw new Error('Unhandled shape');
  }
}

type Ok<T> = { ok: true; value: T };
type Err = { ok: false; error: string };
type Result<T> = Ok<T> | Err;

type Action =
  | { type: 'INCREMENT'; amount: number }
  | { type: 'DECREMENT'; amount: number }
  | { type: 'RESET' };

type AdminUser = { role: 'admin'; permissions: string[] };
type RegularUser = { role: 'user'; credits: number };
type User = AdminUser | RegularUser;

type StringOrArr = string | string[];
function flatten(val: StringOrArr): string[] {
  return Array.isArray(val) ? val : [val];
}

type ApiState<T> =
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; message: string };