pattern-matching.ts

type Shape = 
  { kind: "circle", radius: number} |
  { kind: "rectangle", w: number, h: number} |
  { kind: "square", size: number}

function assertNever(obj: never) {
  throw new Error("unexpected object");
}

function getArea(shape: Shape) {
  switch (shape.kind) {
    case "circle": return Math.PI * shape.radius ** 2;
    //case "rectangle": return shape.w * shape.h; // commenting this leads to compiler error!! :)
    case "square": return shape.size ** 2;
  }
  assertNever(shape);
}

const shape: Shape = {kind: "circle", radius: 0};
const area = getArea(shape);