1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
|
{-# LANGUAGE CPP #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.StateStack
-- Copyright : (c) 2011 Brent Yorgey
-- License : BSD-style (see LICENSE)
-- Maintainer : byorgey@cis.upenn.edu
--
-- A state monad which allows the state to be saved and restored on a
-- stack.
--
-- [Computation type:] Computations with implicit access to a
-- read/write state, with additional operations for pushing the
-- current state on a stack and later restoring the state from the top
-- of the stack.
--
-- [Binding strategy:] Same as for the usual state monad; the state
-- and accompanying stack of saved states are threaded through
-- computations.
--
-- [Useful for:] Remembering state while emitting commands for some
-- system which itself has saveable/restorable state, such as OpenGL
-- or Cairo.
--
-- Simple example:
--
-- > ghci> let p = get >>= liftIO . print
-- > ghci> evalStateStackT (put 2 >> p >> save >> put 3 >> p >> restore >> p) 0
-- > 2
-- > 3
-- > 2
--
-----------------------------------------------------------------------------
module Control.Monad.StateStack
(
-- * The @MonadStateStack@ class
MonadStateStack(..)
-- * The @StateStackT@ transformer
, StateStackT(..), StateStack
-- * Running @StateStackT@ and @StateStack@ computations
, runStateStackT, evalStateStackT, execStateStackT
, runStateStack, evalStateStack, execStateStack
, liftState
) where
import Control.Arrow (second)
import Control.Arrow (first, (&&&))
import qualified Control.Monad.State as St
import Control.Monad
import Control.Monad.Identity
import Control.Monad.Trans
import Control.Monad.Trans.Cont
import Control.Monad.Trans.Except
import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Reader (ReaderT)
import Control.Monad.Trans.State.Lazy as Lazy
import Control.Monad.Trans.State.Strict as Strict
import Control.Monad.Trans.Writer.Lazy as Lazy
import Control.Monad.Trans.Writer.Strict as Strict
import qualified Control.Monad.Cont.Class as CC
import qualified Control.Monad.IO.Class as IC
------------------------------------------------------------
-- Implementation
------------------------------------------------------------
-- | A monad transformer which adds a save/restorable state to an
-- existing monad.
newtype StateStackT s m a = StateStackT { unStateStackT :: St.StateT (s,[s]) m a }
deriving (Functor, Applicative, Monad, MonadTrans, IC.MonadIO)
-- | Class of monads which support a state along with a stack for
-- saving and restoring states.
class St.MonadState s m => MonadStateStack s m where
save :: m () -- ^ Save the current state on the stack
restore :: m () -- ^ Restore the top state from the stack
instance Monad m => St.MonadState s (StateStackT s m) where
get = StateStackT $ St.gets fst
put s = StateStackT $ (St.modify . first) (const s)
instance Monad m => MonadStateStack s (StateStackT s m) where
save = StateStackT $ St.modify (fst &&& uncurry (:))
restore = StateStackT . St.modify $ \(cur,hist) ->
case hist of
[] -> (cur,hist)
(r:hist') -> (r,hist')
-- | Run a @StateStackT@ computation from an initial state, resulting
-- in a computation of the underlying monad which yields the return
-- value and final state.
runStateStackT :: Monad m => StateStackT s m a -> s -> m (a, s)
runStateStackT m s = (liftM . second) fst . flip St.runStateT (s,[]) . unStateStackT $ m
-- | Like 'runStateStackT', but discard the final state.
evalStateStackT :: Monad m => StateStackT s m a -> s -> m a
evalStateStackT m s = liftM fst $ runStateStackT m s
-- | Like 'runStateStackT', but discard the return value and yield
-- only the final state.
execStateStackT :: Monad m => StateStackT s m a -> s -> m s
execStateStackT m s = liftM snd $ runStateStackT m s
type StateStack s a = StateStackT s Identity a
-- | Run a @StateStack@ computation from an initial state, resulting
-- in a pair of the final return value and final state.
runStateStack :: StateStack s a -> s -> (a,s)
runStateStack m s = runIdentity $ runStateStackT m s
-- | Like 'runStateStack', but discard the final state.
evalStateStack :: StateStack s a -> s -> a
evalStateStack m s = runIdentity $ evalStateStackT m s
-- | Like 'runStateStack', but discard the return value and yield
-- only the final state.
execStateStack :: StateStack s a -> s -> s
execStateStack m s = runIdentity $ execStateStackT m s
-- | @StateT@ computations can always be lifted to @StateStackT@
-- computations which do not manipulate the state stack.
liftState :: Monad m => St.StateT s m a -> StateStackT s m a
liftState st = StateStackT . St.StateT $ \(s,ss) -> (liftM . second) (flip (,) ss) (St.runStateT st s)
------------------------------------------------------------
-- Applying monad transformers to MonadStateStack monads
------------------------------------------------------------
instance MonadStateStack s m => MonadStateStack s (ContT r m) where
save = lift save
restore = lift restore
instance MonadStateStack s m => MonadStateStack s (ExceptT e m) where
save = lift save
restore = lift restore
instance MonadStateStack s m => MonadStateStack s (IdentityT m) where
save = lift save
restore = lift restore
instance MonadStateStack s m => MonadStateStack s (MaybeT m) where
save = lift save
restore = lift restore
instance MonadStateStack s m => MonadStateStack s (ReaderT r m) where
save = lift save
restore = lift restore
instance MonadStateStack s m => MonadStateStack s (Lazy.StateT s m) where
save = lift save
restore = lift restore
instance MonadStateStack s m => MonadStateStack s (Strict.StateT s m) where
save = lift save
restore = lift restore
instance (Monoid w, MonadStateStack s m) => MonadStateStack s (Lazy.WriterT w m) where
save = lift save
restore = lift restore
instance (Monoid w, MonadStateStack s m) => MonadStateStack s (Strict.WriterT w m) where
save = lift save
restore = lift restore
------------------------------------------------------------
-- Applying StateStackT to other monads
------------------------------------------------------------
instance CC.MonadCont m => CC.MonadCont (StateStackT s m) where
callCC c = StateStackT $ CC.callCC (unStateStackT . (\k -> c (StateStackT . k)))
{- -- These require UndecidableInstances =(
instance EC.MonadError e m => EC.MonadError e (StateStackT s m) where
throwError = lift . EC.throwError
catchError m h = StateStackT $ EC.catchError (unStateStackT m) (unStateStackT . h)
instance RC.MonadReader r m => RC.MonadReader r (StateStackT s m) where
ask = lift RC.ask
local f = StateStackT . RC.local f . unStateStackT
-}
|
