Promise 로 래핑하면서 오히려 실수할만한 내용.

child.js

process.on('message', function(message) {
  console.log("message received", message);
  process.send(message.toUpperCase(), function(err) {
    console.log("replied",err);
  });
});

execution.md

Promise는 Haskell의 모나드와 달라서 이미 'fire'된 상태.

JooyungsMacBook:js jooyung.han$ node parent.js
sent
sent
message received abc
message received cde
received
replied null
received
replied null
[ 'ABC', 'ABC' ]
done
^C
JooyungsMacBook:js jooyung.han$ 

이를 해결하려면 interpreter패턴 혹은 Free monad를 사용해야.

parent.js

var cp = require('child_process');
var child = cp.fork('child.js');

function write(message) {
  return new Promise((resolve, reject) => {
    child.send(message, err => {
      console.log("sent")
      if (err) {
        reject(err); return;
      } else {
        child.once('message', message => {
          console.log("received");
          resolve(message);
        });
      }
    });
  });
}

Promise.all([write("abc"), write("cde")])
.then(result => console.log(result))
.then(()=> console.log("done"));

parent_async.js

var cp = require('child_process');
var async = require('async');

var child = cp.fork('child.js');

function write(message, cb) {
  child.send(message, err => {
    console.log("sent");
    if (err) {
      cb(err);
    } else {
      child.once('message', message => {
        console.log("received");
        cb(null, message);
      });
    }
  });
}

async.mapSeries(["abc", "cde"], write, (err, result) => {
  console.log(result);
  console.log("done");
});