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
|
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE PatternSynonyms #-}
-- | A state monad which is strict in its state.
module GHC.Utils.Monad.State.Strict
( -- * The State monad
State(State)
, state
, evalState
, execState
, runState
-- * Operations
, get
, gets
, put
, modify
) where
import GHC.Prelude
import GHC.Exts (oneShot)
-- | A state monad which is strict in the state.
newtype State s a = State' { runState' :: s -> (# a, s #) }
deriving (Functor)
pattern State :: (s -> (# a, s #))
-> State s a
-- This pattern synonym makes the monad eta-expand,
-- which as a very beneficial effect on compiler performance
-- See #18202.
-- See Note [The one-shot state monad trick] in GHC.Utils.Monad
pattern State m <- State' m
where
State m = State' (oneShot $ \s -> m s)
instance Applicative (State s) where
pure x = State $ \(!s) -> (# x, s #)
m <*> n = State $ \s -> case runState' m s of
(# f, !s' #) -> case runState' n s' of
(# x, !s'' #) -> (# f x, s'' #)
instance Monad (State s) where
m >>= n = State $ \s -> case runState' m s of
(# r, !s' #) -> runState' (n r) s'
state :: (s -> (a, s)) -> State s a
state f = State $ \s -> case f s of
(r, !s') -> (# r, s' #)
get :: State s s
get = State $ \(!s) -> (# s, s #)
gets :: (s -> a) -> State s a
gets f = State $ \(!s) -> (# f s, s #)
put :: s -> State s ()
put !s' = State $ \_ -> (# (), s' #)
modify :: (s -> s) -> State s ()
modify f = State $ \s -> let !s' = f s in (# (), s' #)
evalState :: State s a -> s -> a
evalState s i = case runState' s i of
(# a, _ #) -> a
execState :: State s a -> s -> s
execState s i = case runState' s i of
(# _, s' #) -> s'
runState :: State s a -> s -> (a, s)
runState s i = case runState' s i of
(# a, !s' #) -> (a, s')
|
