Login.hs

module Login where

import qualified Data.Text as T

type LoginMessage = T.Text

data LoginState =
  -- This user is logged in
    LoggedIn User

  -- The user is not logged in. Show a form and let them do stuff
  -- The message allows for failed logins to display something
  | LoggedOut (Maybe LoginMessage)

  -- We have submitted this user, and are waiting for a response
  -- from the server
  | LogginInProcess
  deriving (Show)

data LoginAction =
    LogOutA
  | SubmitFormA      User
  | ResponseFailureA LoginMessage
  | ResponseSuccessA User
  deriving (Show)

loginTransition :: LoginState -> LoginAction -> LoginState
loginTransition LoggedIn         LoginLogoutA        = LoggedOut Nothing
loginTransition LoggedOut       (SubmitFormA u)      = LoginInProgress u
loginTransition LoginInProgress (ResponseFailureA m) = LoggedOut m
loginTransition LoginInProgress (ResponseSuccessA u) = LoggedIn u
loginTransition state action =
  error $ "Invalid transition. From " ++ show state ++ " with " ++ show action