gistfile1.js

var less = require("less"),
    Promise = require("promise");

module.exports = function(options) {
  return function(inputs) {
    return Promise.all(inputs.map(lessp)).then(function(css) { return css.join(''); });
  };
};

function lessp(input) {
  return input.getContents().then(compile); 
};

function compile(buffer) {
  return new Promise(function(resolve, reject) {
    less.compile(buffer.toString(), function(err, css) { 
      if(err) reject(err); else resolve(css); 
    });    
  });
}