TEA, but in Haskell

tea.hs

{-# LANGUAGE GADTs #-}
module Main where

newtype Login = Login String
newtype Password = Password String

data LoginState = Auth {
  login :: Login,
  password :: Password
} | Nop

data LoginMutation where
  Input :: Either Login Password -> LoginMutation
  LoginClicked :: LoginMutation

data LoginEffect where
  TryLogin :: Login -> Password -> LoginEffect

update :: LoginState -> LoginMutation -> (LoginState, [LoginEffect])
update s (Input (Left l)) = (s { login = l }, [])
update s (Input (Right p)) = (s { password = p }, [])
update s LoginClicked = (s, [TryLogin (login s) (password s)])

tea2.hs

-- TEA with transactional-like mutations

type Upd s m e = s -> m -> (s, [e])

data TMutation s e = TMutation {
  state :: s -> s,
  effs :: s -> [e]
}

type TUpd s e = Upd s (TMutation s e) e