TypeScript tagged union matching with exahaustive checking

tagged-union.ts

/**
 * Map discriminated union based on its tag value.
 * Possible tag values are mapped to expressions
 * that return a transformation of the original value.
 * Each value is exhaustive checked such that a mapping
 * has to be provided for each one. 
 * 
 * e.g.
 * 
 * type Union = 
 *   | { tag: 'aaa', value: string }
 *   | { tag: 'bbb', createdAt: Date }
 * 
 * declare const union: Union;
 * 
 * // number | Date
 * const result = match(union, {
 *   aaa: ({ value }) => value.length,
 *   bbb: ({ createdAt }) => createdAt
 * })
 */
function match<
  T extends { tag: string },
  M extends { [P in T['tag']]: T extends { tag: P } ? (t: T) => any : never },
>(
  value: T,
  map: M
): ReturnType<M[keyof M]> {
  return map[value.tag as keyof M](value)
}


/** Example usage */

type Union =
  | { tag: 'aaa', value: string }
  | { tag: 'bbb', createdAt: Date }

declare const union: Union;

// number | Date
const result = match(
  union,
  {
    aaa: ({ value }) => value.length,
    bbb: ({ createdAt }) => createdAt
  }
)