{-# LANGUAGE CPP #-}
#if __GLASGOW_HASKELL__ >= 702
{-# LANGUAGE Safe #-}
{-# LANGUAGE DeriveGeneric #-}
#endif
#if __GLASGOW_HASKELL__ >= 710 && __GLASGOW_HASKELL__ < 802
{-# LANGUAGE AutoDeriveTypeable #-}
#endif
-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Monad.Trans.State.Lazy
-- Copyright   :  (c) Andy Gill 2001,
--                (c) Oregon Graduate Institute of Science and Technology, 2001
-- License     :  BSD-style (see the file LICENSE)
--
-- Maintainer  :  R.Paterson@city.ac.uk
-- Stability   :  experimental
-- Portability :  portable
--
-- Lazy state monads, passing an updatable state through a computation.
-- See below for examples.
--
-- Some computations may not require the full power of state transformers:
--
-- * For a read-only state, see "Control.Monad.Trans.Reader".
--
-- * To accumulate a value without using it on the way, see
--   "Control.Monad.Trans.Writer".
--
-- In this version, sequencing of computations is lazy, so that for
-- example the following produces a usable result:
--
-- > evalState (sequence $ repeat $ do { n <- get; put (n*2); return n }) 1
--
-- For a strict version with the same interface, see
-- "Control.Monad.Trans.State.Strict".
-----------------------------------------------------------------------------

module Control.Monad.Trans.State.Lazy (
    -- * The State monad
    State,
    state,
    runState,
    evalState,
    execState,
    mapState,
    withState,
    -- * The StateT monad transformer
    StateT(..),
    evalStateT,
    execStateT,
    mapStateT,
    withStateT,
    -- * State operations
    get,
    put,
    modify,
    modify',
    modifyM,
    gets,
    -- * Lifting other operations
    liftCallCC,
    liftCallCC',
    liftCatch,
    liftListen,
    liftPass,
    -- * Examples
    -- ** State monads
    -- $examples

    -- ** Counting
    -- $counting

    -- ** Labelling trees
    -- $labelling
  ) where

import Control.Monad.IO.Class
import Control.Monad.Signatures
import Control.Monad.Trans.Class
#if MIN_VERSION_base(4,12,0)
import Data.Functor.Contravariant
#endif
import Data.Functor.Identity

import Control.Applicative
import Control.Monad
#if MIN_VERSION_base(4,9,0)
import qualified Control.Monad.Fail as Fail
#endif
import Control.Monad.Fix
#if __GLASGOW_HASKELL__ >= 704
import GHC.Generics
#endif

-- ---------------------------------------------------------------------------
-- | A state monad parameterized by the type @s@ of the state to carry.
--
-- The 'return' function leaves the state unchanged, while @>>=@ uses
-- the final state of the first computation as the initial state of
-- the second.
type State s = StateT s Identity

-- | Construct a state monad computation from a function.
-- (The inverse of 'runState'.)
state :: (Monad m)
      => (s -> (a, s))  -- ^pure state transformer
      -> StateT s m a   -- ^equivalent state-passing computation
state :: forall (m :: * -> *) s a. Monad m => (s -> (a, s)) -> StateT s m a
state s -> (a, s)
f = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT (forall (m :: * -> *) a. Monad m => a -> m a
return forall b c a. (b -> c) -> (a -> b) -> a -> c
. s -> (a, s)
f)
{-# INLINE state #-}

-- | Unwrap a state monad computation as a function.
-- (The inverse of 'state'.)
runState :: State s a   -- ^state-passing computation to execute
         -> s           -- ^initial state
         -> (a, s)      -- ^return value and final state
runState :: forall s a. State s a -> s -> (a, s)
runState State s a
m = forall a. Identity a -> a
runIdentity forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT State s a
m
{-# INLINE runState #-}

-- | Evaluate a state computation with the given initial state
-- and return the final value, discarding the final state.
--
-- * @'evalState' m s = 'fst' ('runState' m s)@
evalState :: State s a  -- ^state-passing computation to execute
          -> s          -- ^initial value
          -> a          -- ^return value of the state computation
evalState :: forall s a. State s a -> s -> a
evalState State s a
m s
s = forall a b. (a, b) -> a
fst (forall s a. State s a -> s -> (a, s)
runState State s a
m s
s)
{-# INLINE evalState #-}

-- | Evaluate a state computation with the given initial state
-- and return the final state, discarding the final value.
--
-- * @'execState' m s = 'snd' ('runState' m s)@
execState :: State s a  -- ^state-passing computation to execute
          -> s          -- ^initial value
          -> s          -- ^final state
execState :: forall s a. State s a -> s -> s
execState State s a
m s
s = forall a b. (a, b) -> b
snd (forall s a. State s a -> s -> (a, s)
runState State s a
m s
s)
{-# INLINE execState #-}

-- | Map both the return value and final state of a computation using
-- the given function.
--
-- * @'runState' ('mapState' f m) = f . 'runState' m@
mapState :: ((a, s) -> (b, s)) -> State s a -> State s b
mapState :: forall a s b. ((a, s) -> (b, s)) -> State s a -> State s b
mapState (a, s) -> (b, s)
f = forall (m :: * -> *) a s (n :: * -> *) b.
(m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b
mapStateT (forall a. a -> Identity a
Identity forall b c a. (b -> c) -> (a -> b) -> a -> c
. (a, s) -> (b, s)
f forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall a. Identity a -> a
runIdentity)
{-# INLINE mapState #-}

-- | @'withState' f m@ executes action @m@ on a state modified by
-- applying @f@.
--
-- * @'withState' f m = 'modify' f >> m@
withState :: (s -> s) -> State s a -> State s a
withState :: forall s a. (s -> s) -> State s a -> State s a
withState = forall s (m :: * -> *) a. (s -> s) -> StateT s m a -> StateT s m a
withStateT
{-# INLINE withState #-}

-- ---------------------------------------------------------------------------
-- | A state transformer monad parameterized by:
--
--   * @s@ - The state.
--
--   * @m@ - The inner monad.
--
-- The 'return' function leaves the state unchanged, while @>>=@ uses
-- the final state of the first computation as the initial state of
-- the second.
newtype StateT s m a = StateT { forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT :: s -> m (a,s) }
#if __GLASGOW_HASKELL__ >= 704
    deriving (forall a.
(forall x. a -> Rep a x) -> (forall x. Rep a x -> a) -> Generic a
forall s (m :: * -> *) a x. Rep (StateT s m a) x -> StateT s m a
forall s (m :: * -> *) a x. StateT s m a -> Rep (StateT s m a) x
$cto :: forall s (m :: * -> *) a x. Rep (StateT s m a) x -> StateT s m a
$cfrom :: forall s (m :: * -> *) a x. StateT s m a -> Rep (StateT s m a) x
Generic)
#endif

-- | Evaluate a state computation with the given initial state
-- and return the final value, discarding the final state.
--
-- * @'evalStateT' m s = 'liftM' 'fst' ('runStateT' m s)@
evalStateT :: (Monad m) => StateT s m a -> s -> m a
evalStateT :: forall (m :: * -> *) s a. Monad m => StateT s m a -> s -> m a
evalStateT StateT s m a
m s
s = do
    ~(a
a, s
_) <- forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m a
m s
s
    forall (m :: * -> *) a. Monad m => a -> m a
return a
a
{-# INLINE evalStateT #-}

-- | Evaluate a state computation with the given initial state
-- and return the final state, discarding the final value.
--
-- * @'execStateT' m s = 'liftM' 'snd' ('runStateT' m s)@
execStateT :: (Monad m) => StateT s m a -> s -> m s
execStateT :: forall (m :: * -> *) s a. Monad m => StateT s m a -> s -> m s
execStateT StateT s m a
m s
s = do
    ~(a
_, s
s') <- forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m a
m s
s
    forall (m :: * -> *) a. Monad m => a -> m a
return s
s'
{-# INLINE execStateT #-}

-- | Map both the return value and final state of a computation using
-- the given function.
--
-- * @'runStateT' ('mapStateT' f m) = f . 'runStateT' m@
mapStateT :: (m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b
mapStateT :: forall (m :: * -> *) a s (n :: * -> *) b.
(m (a, s) -> n (b, s)) -> StateT s m a -> StateT s n b
mapStateT m (a, s) -> n (b, s)
f StateT s m a
m = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ m (a, s) -> n (b, s)
f forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m a
m
{-# INLINE mapStateT #-}

-- | @'withStateT' f m@ executes action @m@ on a state modified by
-- applying @f@.
--
-- * @'withStateT' f m = 'modify' f >> m@
withStateT :: (s -> s) -> StateT s m a -> StateT s m a
withStateT :: forall s (m :: * -> *) a. (s -> s) -> StateT s m a -> StateT s m a
withStateT s -> s
f StateT s m a
m = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m a
m forall b c a. (b -> c) -> (a -> b) -> a -> c
. s -> s
f
{-# INLINE withStateT #-}

instance (Functor m) => Functor (StateT s m) where
    fmap :: forall a b. (a -> b) -> StateT s m a -> StateT s m b
fmap a -> b
f StateT s m a
m = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s ->
        forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap (\ ~(a
a, s
s') -> (a -> b
f a
a, s
s')) forall a b. (a -> b) -> a -> b
$ forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m a
m s
s
    {-# INLINE fmap #-}

instance (Functor m, Monad m) => Applicative (StateT s m) where
    pure :: forall a. a -> StateT s m a
pure a
a = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> forall (m :: * -> *) a. Monad m => a -> m a
return (a
a, s
s)
    {-# INLINE pure #-}
    StateT s -> m (a -> b, s)
mf <*> :: forall a b. StateT s m (a -> b) -> StateT s m a -> StateT s m b
<*> StateT s -> m (a, s)
mx = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> do
        ~(a -> b
f, s
s') <- s -> m (a -> b, s)
mf s
s
        ~(a
x, s
s'') <- s -> m (a, s)
mx s
s'
        forall (m :: * -> *) a. Monad m => a -> m a
return (a -> b
f a
x, s
s'')
    {-# INLINE (<*>) #-}
    StateT s m a
m *> :: forall a b. StateT s m a -> StateT s m b -> StateT s m b
*> StateT s m b
k = StateT s m a
m forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
>>= \a
_ -> StateT s m b
k
    {-# INLINE (*>) #-}

instance (Functor m, MonadPlus m) => Alternative (StateT s m) where
    empty :: forall a. StateT s m a
empty = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
_ -> forall (m :: * -> *) a. MonadPlus m => m a
mzero
    {-# INLINE empty #-}
    StateT s -> m (a, s)
m <|> :: forall a. StateT s m a -> StateT s m a -> StateT s m a
<|> StateT s -> m (a, s)
n = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> s -> m (a, s)
m s
s forall (m :: * -> *) a. MonadPlus m => m a -> m a -> m a
`mplus` s -> m (a, s)
n s
s
    {-# INLINE (<|>) #-}

instance (Monad m) => Monad (StateT s m) where
#if !(MIN_VERSION_base(4,8,0))
    return a = StateT $ \ s -> return (a, s)
    {-# INLINE return #-}
#endif
    StateT s m a
m >>= :: forall a b. StateT s m a -> (a -> StateT s m b) -> StateT s m b
>>= a -> StateT s m b
k  = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> do
        ~(a
a, s
s') <- forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m a
m s
s
        forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT (a -> StateT s m b
k a
a) s
s'
    {-# INLINE (>>=) #-}
#if !(MIN_VERSION_base(4,13,0))
    fail str = StateT $ \ _ -> fail str
    {-# INLINE fail #-}
#endif

#if MIN_VERSION_base(4,9,0)
instance (Fail.MonadFail m) => Fail.MonadFail (StateT s m) where
    fail :: forall a. String -> StateT s m a
fail String
str = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
_ -> forall (m :: * -> *) a. MonadFail m => String -> m a
Fail.fail String
str
    {-# INLINE fail #-}
#endif

instance (MonadPlus m) => MonadPlus (StateT s m) where
    mzero :: forall a. StateT s m a
mzero       = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
_ -> forall (m :: * -> *) a. MonadPlus m => m a
mzero
    {-# INLINE mzero #-}
    StateT s -> m (a, s)
m mplus :: forall a. StateT s m a -> StateT s m a -> StateT s m a
`mplus` StateT s -> m (a, s)
n = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> s -> m (a, s)
m s
s forall (m :: * -> *) a. MonadPlus m => m a -> m a -> m a
`mplus` s -> m (a, s)
n s
s
    {-# INLINE mplus #-}

instance (MonadFix m) => MonadFix (StateT s m) where
    mfix :: forall a. (a -> StateT s m a) -> StateT s m a
mfix a -> StateT s m a
f = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> forall (m :: * -> *) a. MonadFix m => (a -> m a) -> m a
mfix forall a b. (a -> b) -> a -> b
$ \ ~(a
a, s
_) -> forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT (a -> StateT s m a
f a
a) s
s
    {-# INLINE mfix #-}

instance MonadTrans (StateT s) where
    lift :: forall (m :: * -> *) a. Monad m => m a -> StateT s m a
lift m a
m = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> do
        a
a <- m a
m
        forall (m :: * -> *) a. Monad m => a -> m a
return (a
a, s
s)
    {-# INLINE lift #-}

instance (MonadIO m) => MonadIO (StateT s m) where
    liftIO :: forall a. IO a -> StateT s m a
liftIO = forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (m :: * -> *) a. MonadIO m => IO a -> m a
liftIO
    {-# INLINE liftIO #-}

#if MIN_VERSION_base(4,12,0)
instance Contravariant m => Contravariant (StateT s m) where
    contramap :: forall a' a. (a' -> a) -> StateT s m a -> StateT s m a'
contramap a' -> a
f StateT s m a
m = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \s
s ->
      forall (f :: * -> *) a' a.
Contravariant f =>
(a' -> a) -> f a -> f a'
contramap (\ ~(a'
a, s
s') -> (a' -> a
f a'
a, s
s')) forall a b. (a -> b) -> a -> b
$ forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m a
m s
s
    {-# INLINE contramap #-}
#endif

-- | Fetch the current value of the state within the monad.
get :: (Monad m) => StateT s m s
get :: forall (m :: * -> *) s. Monad m => StateT s m s
get = forall (m :: * -> *) s a. Monad m => (s -> (a, s)) -> StateT s m a
state forall a b. (a -> b) -> a -> b
$ \ s
s -> (s
s, s
s)
{-# INLINE get #-}

-- | @'put' s@ sets the state within the monad to @s@.
put :: (Monad m) => s -> StateT s m ()
put :: forall (m :: * -> *) s. Monad m => s -> StateT s m ()
put s
s = forall (m :: * -> *) s a. Monad m => (s -> (a, s)) -> StateT s m a
state forall a b. (a -> b) -> a -> b
$ \ s
_ -> ((), s
s)
{-# INLINE put #-}

-- | @'modify' f@ is an action that updates the state to the result of
-- applying @f@ to the current state.
--
-- * @'modify' f = 'get' >>= ('put' . f)@
modify :: (Monad m) => (s -> s) -> StateT s m ()
modify :: forall (m :: * -> *) s. Monad m => (s -> s) -> StateT s m ()
modify s -> s
f = forall (m :: * -> *) s a. Monad m => (s -> (a, s)) -> StateT s m a
state forall a b. (a -> b) -> a -> b
$ \ s
s -> ((), s -> s
f s
s)
{-# INLINE modify #-}

-- | A variant of 'modify' in which the computation is strict in the
-- new state.
--
-- * @'modify'' f = 'get' >>= (('$!') 'put' . f)@
modify' :: (Monad m) => (s -> s) -> StateT s m ()
modify' :: forall (m :: * -> *) s. Monad m => (s -> s) -> StateT s m ()
modify' s -> s
f = do
    s
s <- forall (m :: * -> *) s. Monad m => StateT s m s
get
    forall (m :: * -> *) s. Monad m => s -> StateT s m ()
put forall a b. (a -> b) -> a -> b
$! s -> s
f s
s
{-# INLINE modify' #-}

-- | A variant of 'modify' in which the new state is generated by a
-- monadic action.
modifyM :: (Monad m) => (s -> m s) -> StateT s m ()
modifyM :: forall (m :: * -> *) s. Monad m => (s -> m s) -> StateT s m ()
modifyM s -> m s
f = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> do
    s
s' <- s -> m s
f s
s
    forall (m :: * -> *) a. Monad m => a -> m a
return ((), s
s')
{-# INLINE modifyM #-}

-- | Get a specific component of the state, using a projection function
-- supplied.
--
-- * @'gets' f = 'liftM' f 'get'@
gets :: (Monad m) => (s -> a) -> StateT s m a
gets :: forall (m :: * -> *) s a. Monad m => (s -> a) -> StateT s m a
gets s -> a
f = forall (m :: * -> *) s a. Monad m => (s -> (a, s)) -> StateT s m a
state forall a b. (a -> b) -> a -> b
$ \ s
s -> (s -> a
f s
s, s
s)
{-# INLINE gets #-}

-- | Uniform lifting of a @callCC@ operation to the new monad.
-- This version rolls back to the original state on entering the
-- continuation.
liftCallCC :: CallCC m (a,s) (b,s) -> CallCC (StateT s m) a b
liftCallCC :: forall (m :: * -> *) a s b.
CallCC m (a, s) (b, s) -> CallCC (StateT s m) a b
liftCallCC CallCC m (a, s) (b, s)
callCC (a -> StateT s m b) -> StateT s m a
f = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s ->
    CallCC m (a, s) (b, s)
callCC forall a b. (a -> b) -> a -> b
$ \ (a, s) -> m (b, s)
c ->
    forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT ((a -> StateT s m b) -> StateT s m a
f (\ a
a -> forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
_ -> (a, s) -> m (b, s)
c (a
a, s
s))) s
s
{-# INLINE liftCallCC #-}

-- | In-situ lifting of a @callCC@ operation to the new monad.
-- This version uses the current state on entering the continuation.
-- It does not satisfy the uniformity property (see "Control.Monad.Signatures").
liftCallCC' :: CallCC m (a,s) (b,s) -> CallCC (StateT s m) a b
liftCallCC' :: forall (m :: * -> *) a s b.
CallCC m (a, s) (b, s) -> CallCC (StateT s m) a b
liftCallCC' CallCC m (a, s) (b, s)
callCC (a -> StateT s m b) -> StateT s m a
f = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s ->
    CallCC m (a, s) (b, s)
callCC forall a b. (a -> b) -> a -> b
$ \ (a, s) -> m (b, s)
c ->
    forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT ((a -> StateT s m b) -> StateT s m a
f (\ a
a -> forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s' -> (a, s) -> m (b, s)
c (a
a, s
s'))) s
s
{-# INLINE liftCallCC' #-}

-- | Lift a @catchE@ operation to the new monad.
liftCatch :: Catch e m (a,s) -> Catch e (StateT s m) a
liftCatch :: forall e (m :: * -> *) a s.
Catch e m (a, s) -> Catch e (StateT s m) a
liftCatch Catch e m (a, s)
catchE StateT s m a
m e -> StateT s m a
h =
    forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m a
m s
s Catch e m (a, s)
`catchE` \ e
e -> forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT (e -> StateT s m a
h e
e) s
s
{-# INLINE liftCatch #-}

-- | Lift a @listen@ operation to the new monad.
liftListen :: (Monad m) => Listen w m (a,s) -> Listen w (StateT s m) a
liftListen :: forall (m :: * -> *) w a s.
Monad m =>
Listen w m (a, s) -> Listen w (StateT s m) a
liftListen Listen w m (a, s)
listen StateT s m a
m = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> do
    ~((a
a, s
s'), w
w) <- Listen w m (a, s)
listen (forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m a
m s
s)
    forall (m :: * -> *) a. Monad m => a -> m a
return ((a
a, w
w), s
s')
{-# INLINE liftListen #-}

-- | Lift a @pass@ operation to the new monad.
liftPass :: (Monad m) => Pass w m (a,s) -> Pass w (StateT s m) a
liftPass :: forall (m :: * -> *) w a s.
Monad m =>
Pass w m (a, s) -> Pass w (StateT s m) a
liftPass Pass w m (a, s)
pass StateT s m (a, w -> w)
m = forall s (m :: * -> *) a. (s -> m (a, s)) -> StateT s m a
StateT forall a b. (a -> b) -> a -> b
$ \ s
s -> Pass w m (a, s)
pass forall a b. (a -> b) -> a -> b
$ do
    ~((a
a, w -> w
f), s
s') <- forall s (m :: * -> *) a. StateT s m a -> s -> m (a, s)
runStateT StateT s m (a, w -> w)
m s
s
    forall (m :: * -> *) a. Monad m => a -> m a
return ((a
a, s
s'), w -> w
f)
{-# INLINE liftPass #-}

{- $examples

Parser from ParseLib with Hugs:

> type Parser a = StateT String [] a
>    ==> StateT (String -> [(a,String)])

For example, item can be written as:

> item = do (x:xs) <- get
>        put xs
>        return x
>
> type BoringState s a = StateT s Identity a
>      ==> StateT (s -> Identity (a,s))
>
> type StateWithIO s a = StateT s IO a
>      ==> StateT (s -> IO (a,s))
>
> type StateWithErr s a = StateT s Maybe a
>      ==> StateT (s -> Maybe (a,s))

-}

{- $counting

A function to increment a counter.
Taken from the paper \"Generalising Monads to Arrows\",
John Hughes (<http://www.cse.chalmers.se/~rjmh/>), November 1998:

> tick :: State Int Int
> tick = do n <- get
>           put (n+1)
>           return n

Add one to the given number using the state monad:

> plusOne :: Int -> Int
> plusOne n = execState tick n

A contrived addition example. Works only with positive numbers:

> plus :: Int -> Int -> Int
> plus n x = execState (sequence $ replicate n tick) x

-}

{- $labelling

An example from /The Craft of Functional Programming/, Simon
Thompson (<http://www.cs.kent.ac.uk/people/staff/sjt/>),
Addison-Wesley 1999: \"Given an arbitrary tree, transform it to a
tree of integers in which the original elements are replaced by
natural numbers, starting from 0.  The same element has to be
replaced by the same number at every occurrence, and when we meet
an as-yet-unvisited element we have to find a \'new\' number to match
it with:\"

> data Tree a = Nil | Node a (Tree a) (Tree a) deriving (Show, Eq)
> type Table a = [a]

> numberTree :: Eq a => Tree a -> State (Table a) (Tree Int)
> numberTree Nil = return Nil
> numberTree (Node x t1 t2) = do
>     num <- numberNode x
>     nt1 <- numberTree t1
>     nt2 <- numberTree t2
>     return (Node num nt1 nt2)
>   where
>     numberNode :: Eq a => a -> State (Table a) Int
>     numberNode x = do
>         table <- get
>         case elemIndex x table of
>             Nothing -> do
>                 put (table ++ [x])
>                 return (length table)
>             Just i -> return i

numTree applies numberTree with an initial state:

> numTree :: (Eq a) => Tree a -> Tree Int
> numTree t = evalState (numberTree t) []

> testTree = Node "Zero" (Node "One" (Node "Two" Nil Nil) (Node "One" (Node "Zero" Nil Nil) Nil)) Nil
> numTree testTree => Node 0 (Node 1 (Node 2 Nil Nil) (Node 1 (Node 0 Nil Nil) Nil)) Nil

-}