WhatsBetter.hs

registerUser :: UserName -> EmailAddress -> RegistrationAction ()
registerUser uname email = do
  mst <- gets aggState

  case mst of
    Nothing -> raise (UserRegistered uname email)
    Just st -> do
      when (userExists st) (throwError UserAlreadyRegistered)
      when (emailExists st) (throwError EmailAddressExists)
      raise (UserRegistered uname email)

  where
    userExists = member uname
    emailExists users = email `elem` fmap emailAddress (elems users)

registerUser' :: UserName -> EmailAddress -> RegistrationAction ()
registerUser' uname email = do
  st <- fromMaybe mempty <$> gets aggState

  when (userExists st) (throwError UserAlreadyRegistered)
  when (emailExists st) (throwError EmailAddressExists)
  raise (UserRegistered uname email)

  where
    userExists = member uname
    emailExists users = email `elem` fmap emailAddress (elems users)