Tagged Unions in Flow

MaybeFruit.js

// @flow
type NoneType = {|
  type: 'None',
|};
type AppleType<AppleInner> = {|
  type: 'Apple',
  value: AppleInner,
|};
type BananaType = {|
  type: 'Banana',
  value: number,
|};

type DisjointUnion<AppleInner> = NoneType | AppleType<AppleInner> | BananaType;

export default class MaybeFruit<AppleInner> {
  type: $PropertyType<DisjointUnion<AppleInner>, 'type'>;
  value: $PropertyType<DisjointUnion<AppleInner>, 'value'>;

  constructor(data: DisjointUnion<AppleInner>) {
    Object.assign(this, data);
  }

  static None: MaybeFruit<any> = new MaybeFruit({
    type: 'None',
  });

  static Apple = <OtherAppleInner>(
    value: OtherAppleInner
  ): MaybeFruit<OtherAppleInner> =>
    new MaybeFruit({
      type: 'Apple',
      value,
    });

  static Banana = (value: number): MaybeFruit<any> =>
    new MaybeFruit({type: 'Banana', value});

  match<NoneResult, AppleResult, BananaResult, DefaultResult>(
    // You need to specify either a handler for every case,
    // or a default handler and a handler for some cases.
    matchObj:
      | {|
          None: () => NoneResult,
          Apple: (value: AppleInner) => AppleResult,
          Banana: (value: number) => BananaResult,
        |}
      | {|
          None?: () => NoneResult,
          Apple?: (value: AppleInner) => AppleResult,
          Banana?: (value: number) => BananaResult,
          _: (...args: Array<mixed>) => DefaultResult,
        |}
  ): NoneResult | AppleResult | BananaResult | DefaultResult {
    const self = ((this: any): DisjointUnion<AppleInner>);

    if (self.type === 'None') {
      return (matchObj.None || matchObj._)();
    } else if (self.type === 'Apple') {
      return (matchObj.Apple || matchObj._)(self.value);
    } else if (self.type === 'Banana') {
      return (matchObj.Banana || matchObj._)(self.value);
    } else {
      (self: empty);
    }
  }
}

usage.js

function analyzeFruit<AppleInner>(fruit: MaybeFruit<AppleInner>) {
  const phrase1 = fruit.match({
    Apple(inner) {
      return 'it was an apple with inner: ' + String(inner);
    },
    _() {
      return 'idk';
    },
  });

  const phrase2 = fruit.match({
    None() {
      return 'no fruit :(';
    },
    Apple() {
      return 'yup still an apple';
    },
    Banana(num) {
      return 'it was a banana with number: ' + num;
    },
  });

  return [phrase1, phrase2];
}

analyzeFruit(MaybeFruit.Apple('yellow')); // ['it was an apple with inner: yellow', 'yup still an apple']
analyzeFruit(MaybeFruit.None); // ['idk', 'no fruit :(']
analyzeFruit(MaybeFruit.Banana(5)); // ['idk', 'it was a banana with number: 5']