Promise.hs

-- User.signup = function(user) {
--     return new Promise(function(resolve, reject) {
--         User.validate(user).then(function() {
--             return Bcrypt.genSalt(10);
--         }).then(function(salt) {
--             return Bcrypt.hash(user.password, salt, null);
--         }).then(function(saltedHashedPassword) {
--             return User.insertIntoDatabase(user, saltedHashedPassword);
--         }).then(function(userRecord) {
--             resolve(userRecord);
--         }).catch(function(error) {
--             reject(error);
--         });
--     });
-- };


type Promise a = Either String a

-- in normal bind form
signup :: User -> Promise ()
signup user =
  validate >>
  Bcrypt.genSalt 10 >>=
  \salt -> Bcrypt.hash (user^.password) salt Nothing >>=
  User.insertIntoDatabase user

-- in 'do' notation form
signup :: User -> Promise ()
signup user = do
  validate
  salt <- Bcrypt.genSalt 10
  saltedHashedPassword <- Bcrypt.hash (user^.password) salt Nothing
  userRecord <- User.insertIntoDatabase user saltedHashedPassword
  return userRecord

-- mixed style (how I would write it)
signup :: User -> Promise ()
signup user = do
  validate
  salt <- Bcrypt.genSalt 10
  Bcrypt.hash (user^.password) salt Nothing >>= User.insertIntoDatabase user