continuable.js

exports.create = function() {
  var queue = [],
      queueIndex = 0;

  // I don't know if we want to save state, but it is easy to implement.
  // I am just worried about overhead
  var cachedValue = null;

  var enqueue = function enqueue(func) {
    queue.push(func);

    if( cachedValue !== null ) {
      fulfill(cachedValue);
    }

    return enqueue;
  };

  enqueue.isContinuable = true; // the one hack. so you can know if a returned
                                // value is itself another continuable

  var fulfill = function fulfill(val) {
    if( queueIndex < queue.length ) {
      // check the return type from the val
      if( val instanceof Error ) {
        var error = true;
      } 
      else {
        var error = false;
      }

      // in case this was a delayed response, set the cachedValue to null
      cachedValue = null;

      var returned = queue[queueIndex++](val, !error);
      if( typeof returned === 'function' && returned.isContinuable ) {
        // need to queue up our function in the continuable
        returned(function(val, succeeded) {
            fulfill(val);
          });
      }
      // should we make a check for Promises?
      else {
        fulfill(returned || val);
      }
    }
    else {
      cachedValue = val;
    }
  };

  return { 'enqueue': enqueue, 'fulfill': fulfill };
};

example.js

var async_function = function(val) {
  var cont = continuable.create();

  process.nextTick(function() {
      cont.fulfill(val);
    });

  return cont.enqueue;
};

function loadConfig(startVal) {
  // uses what you pass in to loadConfig so you can test
  // different continuable fulfillment values and how they
  // affect the chain
  return async_function(startVal)
    (function(val, succeeded) {
      return succeeded ? val : async_function('{"hello": "world"}');
    })
    (function(val, succeeded) {
      return succeeded ? JSON.parse(val) : val;
    });
};

loadConfig(new Error())( function(obj, succeeded) { sys.p(obj) } );
/* Outputs
{
 "hello": "world"
}
*/

loadConfig('{"hi":"there"}')( function(obj, succeeded) { sys.p(obj) } );
/* Outputs
{
 "hi": "there"
}
*/