StT.hs

{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE KindSignatures #-}
-- | The 'StT' type family.
module Control.Monad.Trans.Lift.StT (StT) where

import qualified Control.Monad.Trans.Except        as E
import qualified Control.Monad.Trans.Identity      as I
import qualified Control.Monad.Trans.List          as L
import qualified Control.Monad.Trans.Maybe         as M
import qualified Control.Monad.Trans.Reader        as R
import qualified Control.Monad.Trans.RWS.Lazy      as RWS.Lazy
import qualified Control.Monad.Trans.RWS.Strict    as RWS.Strict
import qualified Control.Monad.Trans.State.Lazy    as S.Lazy
import qualified Control.Monad.Trans.State.Strict  as S.Strict
import qualified Control.Monad.Trans.Writer.Lazy   as W.Lazy
import qualified Control.Monad.Trans.Writer.Strict as W.Strict

-- | Internal state of a monad transformer.
--   Same as @StT@ from the @monad-control@ package.
type family StT (t :: (* -> *) -> (* -> *)) (a :: *) :: *

type instance StT (E.ExceptT e) a = Either e a
type instance StT I.IdentityT a = a
type instance StT L.ListT a = [a]
type instance StT M.MaybeT a = Maybe a
type instance StT (R.ReaderT r) a = a
type instance StT (RWS.Lazy.RWST r w s) a = (a, s, w)
type instance StT (RWS.Strict.RWST r w s) a = (a, s, w)
type instance StT (S.Lazy.StateT s) a = (a, s)
type instance StT (S.Strict.StateT s) a = (a, s)
type instance StT (W.Strict.WriterT w) a = (a, w)
type instance StT (W.Lazy.WriterT w) a = (a, w)