Simple helpers for tagged unions in typescript

tagged-unions.ts

// library code for non-generic discriminated unions
export type ReplaceReturnType<F, R> = F extends (...args: infer Args) => any
  ? (...args: Args) => R
  : never;

export type TaggedUnionFactory<TUF, R> = {
  [K in keyof TUF]: ReplaceReturnType<TUF[K], R>;
};

// for monomorphic types this just works well
const _actionResult = {
  failure: (message: string) => ({ kind: "failure", message } as const),
  value: (value: number) => ({ kind: "result", value } as const),
};

export type ActionResult = ReturnType<
  typeof _actionResult[keyof typeof _actionResult]
>;

export const actionResult: TaggedUnionFactory<
  typeof _actionResult,
  ActionResult
> = _actionResult;

// For generic types you have to type the usage sites but it's still pretty good
export const status = {
  loading: () => ({ kind: "loading" } as const),
  success: <V>(value: V) => ({ kind: "success", value } as const),
  error: <E>(error: E) => ({ kind: "error", error } as const),
};

// This probably requires a recent typescript version.
export type Status<V, E> = ReturnType<
  | typeof status.loading
  | typeof status.success<V>
  | typeof status.error<E>
  | never
>;