+flow-multi-msg-pass.js

// @flow

type Msg<Type, V> = {|
  type: Type,
  val: V
|};

type AcceptFn =
  & (<A, B, OV>('val', Msg<'val', A> => B) =>
    (Msg<'val', A> | Msg<'err', OV>) => (B | Msg<'err', OV>))
  & (<A, B, OV>('err', Msg<'err', A> => B) =>
    (Msg<'err', A> | Msg<'val', OV>) => (B | Msg<'val', OV>));

declare var accept: AcceptFn;

function accept(type, fn) {
  return msg => msg.type === type
    ? fn(msg)
    : msg;
}

const pipe = <A, B, C>(fn1: A => B, fn2: B => C): (A => C) => (v: A) => fn2(fn1(v));

const createMsg = <Type, V>(type: Type, val: V): Msg<Type, V> => ({
  type,
  val
});

const fn1 = accept('val', (msg: Msg<'val', number>): Msg<'val', string> =>
  createMsg('val', msg.val.toString()));

const fn2 = accept('err', (msg: Msg<'err', string>): Msg<'err', string> =>
  createMsg('err', msg.val.toString()));

const fn3 = accept('val', (msg: Msg<'val', string>): Msg<'val', string> =>
  createMsg('val', msg.val.toString()));

const fn4 = accept('err', (msg: Msg<'err', string>): Msg<'err', string> =>
  createMsg('err', msg.val.toString()));

const fn = pipe(pipe(pipe(fn1, fn2), fn3), fn4);
fn(createMsg('val', 23));

.flowconfig

[ignore]

[include]

[libs]

[lints]

[options]

[strict]

package-lock.json

{
  "name": "flow-multi-msg-pass",
  "version": "1.0.0",
  "lockfileVersion": 1,
  "requires": true,
  "dependencies": {
    "flow-bin": {
      "version": "0.61.0",
      "resolved": "https://registry.npmjs.org/flow-bin/-/flow-bin-0.61.0.tgz",
      "integrity": "sha512-w6SGi5CDfKLNGzYssRhW6N37qKclDXijsxDQ5M8c3WbivRYta0Horv22bwakegfKBVDnyeS0lRW3OqBC74eq2g==",
      "dev": true
    }
  }
}

package.json

{
  "name": "flow-multi-msg-pass",
  "version": "1.0.0",
  "description": "",
  "main": "-flow-msg-msg-pass.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "flow-bin": "^0.61.0"
  }
}