Typed visitor discriminated union

safe-switch.ts

function calculateArea(shape: Shape): number {
  switch(shape.__typename) {
    case 'Circle':
      return Math.PI * (shape.radius ^ 2);
    case 'Rectangle':
      return shape.width * shape.height;
    case 'Square':
      return shape.side ^ 2;
    case 'Triangle':
      return (shape.base / 2) * shape.height;
    default:
      const nope: never = shape;
      return nope;
  }
}

switch.ts

function calculateArea(shape: Shape): number {
  switch(shape.__typename) {
    case 'Circle':
      return Math.PI * (shape.radius ^ 2);
    case 'Rectangle':
      return shape.width * shape.height;
    case 'Square':
      return shape.side ^ 2;
    case 'Triangle':
      return (shape.base / 2) * shape.height;
  }
}

const shape: Shape = {
  __typename: 'Circle',
  radius: 5
};

const area = calculateArea(shape);

union-map.ts

/**
 * TUnion           The union type
 * TDiscriminator   The discriminator property name
 */
type UnionKeys<TUnion, TDisciminator extends keyof TUnion> = 
  TUnion[TDisciminator] extends string | number | symbol ? 
    TUnion[TDisciminator] : never;

/**
 * TUnion The union type
 * TDiscriminator The discriminator property name
 * TDiscriminatorValue The discriminator value to retrieve the union part
 */
type UnionPartForKey<
  TUnion, 
  TDisciminator extends keyof TUnion, 
  TDiscriminatorValue extends UnionKeys<TUnion, TDisciminator>
  > = 
    TUnion extends any ?
      TDiscriminatorValue extends TUnion[TDisciminator] ? 
        TUnion 
      : never
    : never;

/**
 * TUnion The union type
 * TDiscriminator The discriminator property name
 * TReturn Return type for the strategy functions 
 */
type UnionMap<
  TUnion, 
  TDisciminator extends keyof TUnion, 
  TReturn = void
  > = { 
  [K in UnionKeys<TUnion, TDisciminator>]: (part: UnionPartForKey<TUnion, TDisciminator, K>) => TReturn 
};

union.ts

type Circle = {
  __typename: "Circle";
  radius: number;
};

type Rectangle = {
  __typename: "Rectangle";
  width: number;
  height: number;
};

type Square = {
  __typename: "Square";
  side: number;
};

type Triangle = {
  __typename: "Triangle";
  base: number;
  height: number;
};

type Shape = Circle | Square | Rectangle | Triangle;

visitor.ts

const areaCalculation: UnionMap<Shape, '__typename', number> = {
  Circle: shape => Math.PI * (shape.radius ^ 2),
  Rectangle: shape => shape.width * shape.height,
  Square: shape => shape.side ^ 2,
  Triangle: shape => (shape.base / 2) * shape.height
};

const shape: Shape = {
  __typename: 'Circle',
  radius: 5
};

const area = areaCalculation[shape.__typename](shape);