function lessAST(filename, options) {
    options = options || {};
    options.filename = path.resolve(filename);
    return new Promise(function (res, rej) {
        fs.readFile(filename, 'utf8', function (err, data) {
            if (err) {
                rej(err);
            } else {
                res(data);
            }
        });
    }).then(function (data) {
        return new Promise(function (res, rej) {
            less.parse(data, options, function (err, tree) {
                if (err) {
                    rej(err);
                } else {
                    res(tree);
                }
            });
        });
    });
}
lessAST(filename).then(function (tree) {
  fs.writeFile('tmp.json', JSON.stringify(tree, null, 2));
});