degenerator

Turns sync functions into async generator functions

Sometimes you need to write sync looking code that's really async under the hood. This module takes a String to one or more synchronous JavaScript functions, and returns a new String that with those JS functions transpiled into ES6 Generator Functions.

So this:

function foo () {
  return a('bar') || b();
}

Gets compiled into:

function* foo() {
    return (yield a('bar')) || (yield b());
}

From there, you can provide asynchronous thunk-based or Generator-based implementations for the a() and b() functions, in conjunction with any Generator-based flow control library to execute the contents of the function asynchronously.

Installation

Install with npm:

$ npm install degenerator

Example

You must explicitly specify the names of the functions that should be "asyncified". So say we wanted to expose a get(url) function that did and HTTP request and returned the response body.

The user has provided us with this implementation:

function myFn () {
  var one = get('https://google.com');
  var two = get('http://nodejs.org');
  var three = JSON.parse(get('http://jsonip.org'));
  return [one, two, three];
}

Now we can compile this into an asyncronous generator function, implement the async get() function, and finally evaluate it into a real JavaScript function instance with the vm module:

var co = require('co');
var vm = require('vm');
var degenerator = require('degenerator');
 
// the `get()` function is thunk-based (error handling omitted for brevity) 
function get (endpoint) {
  return function (fn) {
    var mod = 0 == endpoint.indexOf('https:') ? require('https') : require('http');
    var req = mod.get(endpoint);
    req.on('response', function (res) {
      var data = '';
      res.setEncoding('utf8');
      res.on('data', function (b) { data += b; });
      res.on('end', function () {
        fn(null, data);
      });
    });
  };
}
 
// convert the JavaScript string provided from the user (assumed to be `str` var) 
str = degenerator(str, [ 'get' ]);
 
// at this stage, you could use a transpiler like `facebook/regenerator` 
// here if desired. 
 
// turn the JS String into a real GeneratorFunction instance 
var genFn = vm.runInNewContext('(' + str + ')', { get: get });
 
// use a generator-based flow control library (`visionmedia/co`, `jmar777/suspend`, 
// etc.) to create an async function from the generator function. 
 
var asnycFn = co(genFn);
 
// NOW USE IT!!! 
asyncFn(function (err, res) {
  // ... 
});

API

degenerator(String jsStr, Array functionNames) → String

Returns a "degeneratorified" JavaScript string, with ES6 Generator functions transplanted.

License

(The MIT License)

Copyright (c) 2013 Nathan Rajlich <nathan@tootallnate.net>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.