+flow-maybe-monad.js

// @flow
type Nothing = {|
  type: 'nothing'
|};

type Just<A> = {|
  type: 'just',
  value: A
|};

type Maybe<A> = Just<A> | Nothing;

const just = <A>(value: A): Just<A> => ({
  type: 'just',
  value
});

const nothing: Nothing = {
  type: 'nothing',
};

const ret = <A>(a: A): Maybe<A> => just(a);

const bind = <A, B>(a: Maybe<A>, fn: (A => Maybe<B>)): Maybe<B> =>
  a.type === 'just'
    ? fn(a.value)
    : nothing;

const evenOnly = (a: number): Maybe<number> =>
  a % 2 === 0
    ? just(a)
    : nothing;

const dbl = (a: number): Maybe<number> =>
  just(a * 2);

console.log(bind(bind(ret(22), evenOnly), dbl));  // ret 22 >>= evenOnly >>= dbl
console.log(bind(bind(ret(23), evenOnly), dbl));  // ret 23 >>= evenOnly >>= dbl

.flowconfig

[ignore]

[include]

[libs]

[lints]

[options]

[strict]

package-lock.json

{
  "name": "flow-maybe-monad",
  "version": "1.0.0",
  "lockfileVersion": 1,
  "requires": true,
  "dependencies": {
    "babylon": {
      "version": "6.18.0",
      "resolved": "https://registry.npmjs.org/babylon/-/babylon-6.18.0.tgz",
      "integrity": "sha512-q/UEjfGJ2Cm3oKV71DJz9d25TPnq5rhBVL2Q4fA5wcC3jcrdn7+SssEybFIxwAvvP+YCsCYNKughoF33GxgycQ==",
      "dev": true
    },
    "flow-bin": {
      "version": "0.62.0",
      "resolved": "https://registry.npmjs.org/flow-bin/-/flow-bin-0.62.0.tgz",
      "integrity": "sha1-FLymaabj+VwLwMLR61XsTpjLHYM=",
      "dev": true
    },
    "flow-remove-types": {
      "version": "1.2.3",
      "resolved": "https://registry.npmjs.org/flow-remove-types/-/flow-remove-types-1.2.3.tgz",
      "integrity": "sha512-ypq/U3V+t9atYiOuSJd40tekCra03EHKoRsiK/wXGrsZimuum0kdwVY7Yv0HTaoXgHW1WiayomYd+Q3kkvPl9Q==",
      "dev": true,
      "requires": {
        "babylon": "6.18.0",
        "vlq": "0.2.3"
      }
    },
    "vlq": {
      "version": "0.2.3",
      "resolved": "https://registry.npmjs.org/vlq/-/vlq-0.2.3.tgz",
      "integrity": "sha512-DRibZL6DsNhIgYQ+wNdWDL2SL3bKPlVrRiBqV5yuMm++op8W4kGFtaQfCs4KEJn0wBZcHVHJ3eoywX8983k1ow==",
      "dev": true
    }
  }
}

package.json

{
  "name": "flow-maybe-monad",
  "version": "1.0.0",
  "description": "",
  "main": "+flow-maybe-monad.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "flow-bin": "^0.62.0",
    "flow-remove-types": "^1.2.3"
  }
}