Exhaustive Type Checking with TypeScript

exhaustive-type-checking.ts

function area(s: Shape) {
  switch (s.kind) {
    case "square":
      return s.size * s.size;
    case "rectangle":
      return s.width * s.height;
    case "circle":
      return Math.PI * s.radius * s.radius;
    default: 
      return assertExhaustive(s, 'Reached unexpected case')
  }
}

function assertExhaustive(
  value: never,
  message: string
): never {
  throw new Error(message);
}