Minimal continuation passing construct

future.js

function thenable(action) {
  var self = {
    fork: function(succ, err) {
      action(succ || function() {}, err || function() {});
    },
    then: function(fn) {
      return thenable(function(resolve, reject) {
        self.fork(
          function(data) { 
            var step = fn(data);

            if (step && step.then) {
              step.fork(resolve, reject);
            } else {
              resolve(step);
            }
          }, 
          function(e) { 
            reject(e); 
          }
        );
      });
    }
  };

  return self;
};