Using continuation passing style with ES6 generators along with node's callback style functions as an alternative to promises for escaping callback hell.

continuable-generators.js

"use strict";

const GeneratorFunction = function*(){}.constructor;
const GeneratorFunctionPrototype = GeneratorFunction.prototype;
const GeneratorPrototype = GeneratorFunctionPrototype.prototype;

const slice = Array.prototype.slice;
const concat = Array.prototype.concat;
const toString = Object.prototype.toString;


/**
 * A user callback function that is raised when the result of an async function
 * is available.
 *
 * @param Any error
 *        If truthy indicates an error result. Will be thrown from the
 *        generator.
 * @param Any result
 *        If the callbaxk is successful, will be the yielded value.
 */
function Callback(error, result){}


/**
 * The type of function that must be yielded from a generator being run.
 *
 * @param Callback callback
 *        The function which will handle the error or result of the
 *        continuation.
 */
function Continuation(callback){}


/**
 * A GeneratorFunction where every value yielded must either itself be a
 * Generator or a Continuation.
 *
 * @return Generator
 */
function Continuable(/*...args*/){}



/**
 * A no-operation function.
 */
const noop = function(){};


/**
 * Coerce a value to an object.
 *
 * @param any value
 *        The value to coerce.
 * @return Object
 */
const toObject = function(value){
  if (value == null) {
    throw new TypeError('Cannot coerce to object.');
  }
  return Object(value);
};


/**
 * Coerce a value to an unsigned 32 bit integer;
 *
 * @param any value
 *        The value to coerce.
 * @return Number
 */
const toUint32 = function(value){
  return value >>> 0;
};


/**
 * Wrap a function so it is only called the first time the wrapper is called.
 *
 * @param Function fn
 *        The function to wrap.
 * @return Function
 *         The wrapper.
 */
const once = function(fn){
  return function(){
    if (fn) {
      const f = fn;
      fn = null;
      return f.apply(this, arguments);
    }
  };
};


/**
 * Check whether a given value is a GeneratorFunction.
 *
 * @param Any value
 *        The value to check.
 * @return Boolean
 */
const isGeneratorFunction = function(value){
  return value instanceof GeneratorFunction
      || typeof value === 'function'
      && !!value.constructor
      && value.constructor.name === 'GeneratorFunction';
};


/**
 * Check whether a given value is a Generator.
 *
 * @param Any value
 *        The value to check.
 * @return Boolean
 */
const isGenerator = function(value){
  return toString.call(value) === '[object Generator]';
};


// `send` was removed after an early iteration in V8's implementation
const send = 'send' in GeneratorPrototype ? 'send' : 'next';

/**
 * Uses continuation passing style to iterate through each yield in a Generator.
 *
 * @param Generator generator
 *        The generator object to pump.
 * @param Continuation continuation
 *        The continuation which will be passed the callback that dispatches to
 *        the generator.
 */
const pump = function(generator, continuation){
  if (typeof continuation !== 'function') {
    throw new TypeError("Yielded a non-function");
  }

  continuation(function(err, result){
    if (err) {
      generator.throw(err);
    } else {
      const next = generator[send](result).value;
      next && pump(generator, next);
    }
  });
};


/**
 * Handles either a Continuable function or a Generator. If passed a Generator a
 * nested event loop is run to completion to obtain the final value.
 *
 * @param Continuable|Continuation
 *        The thing to pump a single turn.
 * @param Callback cb
 *        The Callback to be passed to the Continuable.
 */
const resolve = function(value, cb){
  if (typeof value === 'function') {
    value(cb);
  } else if (isGenerator(value)) {
    run(function*(){
      try {
        cb(null, yield* value);
      } catch (e) {
        cb(e);
      }
    });
  } else {
    throw new TypeError('Must return a Continuation or Continuable');
  }
};


/**
 * Creates a generator from a generator function and begins executing it.
 *
 * @param GeneratorFunction generatorFn
 *        The generator function to execute. Every yielded value should be a
 *        Continuable.
 */
const run = exports.run = function run(generatorFn){
  const generator = generatorFn();
  const first = generator.next();
  if (!first.done) {
    pump(generator, first.value);
  }
};


/**
 * Wrap an async function as a GeneratorFunction so it can be used with
 * delegating yield (yield*).
 *
 * @param Function fn
 *        Function to wrap. This function must accept a Callback as its last
 *        parameter.
 * @return Continuable
 *         Wrapped version of the function that can be yielded to in a
 *         generator executed using `run`.
 */
const wrap = exports.wrap = function wrap(fn){
  return function*(){
    const args = slice.call(arguments);
    const receiver = this;
    return yield function(cb){
      fn.apply(receiver, args.concat(cb));
    };
  };
};


const _sleep = wrap(function(ms, cb){
  const start = Date.now();
  setTimeout(function(){
    cb(null, Date.now() - start);
  }, ms);
});

/**
 * Helper that can be used to pause execution.
 *
 * @param Number ms
 *        Time to pause.
 * @return Continuation
 *         Function that can be be yielded to in a generator executed using
 *         `run`.
 */
exports.sleep = function sleep(ms){
  return _sleep(ms);
};


const _tick = wrap(function(){
  if (typeof process === 'undefined') {
    return function(cb){
      setTimeout(cb, 0);
    };
  }
  return process.nextTick;
}());

/**
 * Helper that waits until the next event loop tick.
 *
 * @return Continuation
 *         Function that can be be yielded to in a generator executed using
 *         `run`.
 */
exports.tick = function tick(){
  return _tick();
};


/**
 * Uses a Continuable function to sequentially map an array of values.
 *
 * @param Array array
 *        The set of values to map over.
 * @param Continuable cb
 *        The function to be applied to each value.
 * @param Any [receiver]
 *        The |this| value in the callback.
 * @return Array
 *         The mapped array.
 */
exports.forEach = function* forEach(array, itemcb, receiver){
  const obj = toObject(array);
  const len = toUint32(obj.length);

  for (let i = 0; i < len; i++) {
    if (i in obj) {
      yield* itemcb.call(receiver, obj[i], i, array);
    }
  }
};


/**
 * Uses a Continuable function to sequentially map an array of values.
 *
 * @param Array array
 *        The set of values to map over.
 * @param Continuable itemcb
 *        The function to be applied to each value.
 * @param Any [receiver]
 *        The |this| value in the callback.
 * @return Array
 *         The mapped array.
 */
exports.map = function* map(array, itemcb, receiver){
  const obj = toObject(array);
  const len = toUint32(obj.length);
  const result = new Array(len);

  for (let i = 0; i < len; i++) {
    if (i in obj) {
      result[i] = yield* itemcb.call(receiver, obj[i], i, array);
    }
  }

  return result;
};


/**
 * Uses a Continuable function to sequentially filter a set of values.
 *
 * @param Array array
 *        The set of values to map over.
 * @param Continuable itemcb
 *        The function to be applied to each value.
 * @param Any [receiver]
 *        The |this| value in the callback.
 * @return Array
 *         The filtered array.
 */
exports.filter = function* filter(array, itemcb, receiver){
  const obj = toObject(array);
  const len = toUint32(obj.length);
  const result = [];

  for (let i = 0; i < len; i++) {
    const item = obj[i];
    if (i in obj && (yield* itemcb.call(receiver, item, i, array))) {
      result[result.length] = item;
    }
  }

  return result;
};


/**
 * Uses a Continuable function to reduce a set of values.
 *
 * @param Array array
 *        The set of values to map over.
 * @param Continuable itemcb
 *        The function to be applied to each value.
 * @param Any [receiver]
 *        The |this| value in the callback.
 * @return Any
 *         The final accumulated value.
 */
exports.reduce = function* reduce(array, itemcb, initial){
  const obj = toObject(array);
  const len = toUint32(obj.length);
  let start = arguments.length < 3 ? 1 : 0;
  let accum = start ? obj[0] : initial;

  for (let i = start; i < len; i++) {
    if (i in obj) {
      accum = yield* itemcb(accum, obj[i], array);
    }
  }

  return accum;
};


/**
 * Returns true the first time a Continuable function returns a truthy value
 * against a set of values, otherwise returns false.
 *
 * @param Array array
 *        The set of values to map over.
 * @param Continuable itemcb
 *        The function to be applied to each value.
 * @param Any [receiver]
 *        The |this| value in the callback.
 * @return Boolean
 */
exports.some = function* some(array, itemcb, receiver){
  const obj = toObject(array);
  const len = toUint32(obj.length);

  for (let i = 0; i < len; i++) {
    if (i in obj && (yield* itemcb.call(receiver, obj[i], i, array))) {
      return true;
    }
  }

  return false;
};


/**
 * Returns false the first time a Continuable function returns a falsey value
 * against a set of values, otherwise returns true.
 *
 * @param Array array
 *        The set of values to map over.
 * @param Continuable itemcb
 *        The function to be applied to each value.
 * @param Any [receiver]
 *        The |this| value in the callback.
 * @return Boolean
 */
exports.every = function* every(array, itemcb, receiver){
  const obj = toObject(array);
  const len = toUint32(obj.length);

  for (let i = 0; i < len; i++) {
    if (i in obj && !(yield* itemcb.call(receiver, obj[i], i, array))) {
      return false;
    }
  }

  return true;
};


/**
 * Create a parallelized function that works over an array of values.
 *
 * @param Function transform
 *        The callback that transforms each value set in the returned array.
 * @param Function finalize
 *        The callback that transforms the completed array as a whole.
 * @return Continuable
 */
const parallelFunction = function(transform, finalize){
  return wrap(function(array, itemcb, receiver, cb){
    const obj = toObject(array);
    const len = toUint32(obj.length);
    const result = new Array(len);

    if (!len) {
      return void cb(null, finalize(result));
    }

    let remaining = len;
    cb = once(cb);

    const handle = function(input, index){
      return function(err, value){
        if (err) {
          cb(err);
        } else if (!(index in result)) {
          result[index] = transform(input, value);
          if (!--remaining) {
            cb(null, finalize(result));
          }
        }
      };
    };

    for (let i=0; i < len; i++) {
      if (i in obj) {
        const item = obj[i];
        resolve(itemcb.call(receiver, item, i, array), handle(item, i));
      } else {
        remaining--;
      }
    }
  });
};


const _parallelForEach = parallelFunction(noop, noop);

/**
 * Uses a Continuable function to map an array of values in parallel.
 *
 * @param Array array
 *        The set of values to map over.
 * @param Continuable itemcb
 *        The function to be applied to each value.
 * @param Any [receiver]
 *        The |this| value in the callback.
 * @return Array
 *         The filtered array.
 */
exports.parallelForEach = function parallelForEach(array, itemcb, receiver){
  return _parallelForEach(array, itemcb, receiver);
};


const _parallelMap = parallelFunction(function(input, result){
  return result;
}, function(array){
  return array;
});

/**
 * Uses a Continuable function to map an array of values in parallel.
 *
 * @param Array array
 *        The set of values to map over.
 * @param Continuable itemcb
 *        The function to be applied to each value.
 * @param Any [receiver]
 *        The |this| value in the callback.
 * @return Array
 *         The filtered array.
 */
exports.parallelMap = function parallelMap(array, itemcb, receiver){
  return _parallelMap(array, itemcb, receiver);
};


const EMPTY = {};

const _parallelFilter = parallelFunction(function(input, result){
  return result ? input : EMPTY;
}, function(array){
  return array.filter(function(item){
    return item !== EMPTY;
  });
});

/**
 * Uses a Continuable function to filter a set of values in parallel.
 *
 * @param Array array
 *        The set of values to map over.
 * @param Continuable itemcb
 *        The function to be applied to each value.
 * @param Any [receiver]
 *        The |this| value in the callback.
 * @return Array
 *         The filtered array.
 */
exports.parallelFilter = function parallelFilter(array, itemcb, receiver){
  return _parallelFilter(array, itemcb, receiver);
};


const _join = wrap(function(items, cb){
  const result = [];

  if (!remaining) {
    return void cb(null, result);
  }

  let remaining = items.length;
  cb = once(cb);

  const handle = function(index){
    return function(err, value){
      if (err) {
        cb(err);
      } else if (!(index in result)) {
        result[index] = value;
        if (!--remaining) {
          cb(null, result);
        }
      }
    };
  };

  for (let i=0, item; item = items[i]; i++) {
    resolve(item, handle(i));
  }
});

/**
 * Take multiple Continuables and combine them into a single Continuable
 * that returns an array of the completed values, or errors if any error.
 *
 * @param ...Continuable args
 *        Any amount of Continuables or arrays of Continuables to join.
 * @return Continuable
 *         The combined Continuable which will yield the results as an array.
 */
exports.join = function join(/* ...args */){
  return _join(concat.apply([], arguments));
};

usage.js

"use strict";

const path = require('path');
const gen = require('../');

function wrapAll(obj, names){
  const out = {};
  names.forEach(function(name){
    out[name] = gen.wrap(obj[name]);
  });
  return out;
}

const fs = wrapAll(require('fs'), [
 'rename', 'ftruncate', 'truncate', 'chown', 'fchown', 'lchown', 'chmod','fchmod',
 'lchmod', 'stat', 'lstat', 'fstat', 'link', 'symlink', 'readlink', 'realpath',
 'unlink', 'rmdir', 'mkdir', 'readdir', 'close', 'open', 'utimes', 'futimes',
 'fsync', 'write', 'read', 'readFile', 'writeFile', 'appendFile'
]);



function* fulldir(dir){
  return (yield* fs.readdir(dir)).map(function(child){
    return path.resolve(dir, child);
  });
}


function* statdir(dir){
  const children = yield* fulldir(dir);

  return yield* gen.map(children, function(filepath){
    return fs.stat(filepath);
  });
}


function* sizedir(dir){
  return (yield* statdir(dir)).reduce(function(total, child){
    return child.isFile() ? child.size + total : total;
  }, 0);
}


function* x10(value){
  yield* gen.sleep(20);
  return value * 10;
}

gen.run(function*(){
  console.time('sequential');
  console.log(yield* gen.map([1, 2, 3, 4, 5], x10));
  console.timeEnd('sequential');

  console.time('parallel');
  console.log(yield* gen.parallelMap([1, 2, 3, 4, 5], x10));
  console.timeEnd('parallel');

  console.log(yield* gen.join(fs.stat('.'), fs.stat('..')));
  console.log(yield* sizedir('..'));
});