Validation with liftp and toPromise

validation.js

// I've been writing validation utilities for many times, 
// never satisfied until I realize that the combination of `liftp` and `toPromise`
// is just perfect. 
// It could work with any number of arguments. Leaving 
// the choices to you of what error to return and how to check if an argument is valid.

var P = require('bluebird-promisell');
var R = require('ramda');

// Number -> Number -> Promise Number
var addAsync = function(a, b) {
  return P.purep(a + b);
};

// String -> () -> Error
var toError = function(fieldname) {
  return function() {
    return new Error('invalid field: ' + fieldname);
  };
};

// Number -> Number -> Promise Number
exports.addAsync = function(a, b) {
  return P.liftp(addAsync)(
    P.toPromise(R.is(Number), toError('a'))(a),
    P.toPromise(R.is(Number), toError('b'))(b));
};

exports.addAsync(1, 2); // Resolved Promise: 3
exports.addAsync("1", 2); // Rejected Promise: Error "invalid field: a"