Control.Concurrent.STM.Stats

hls-graph-2.9.0.0: Haskell Language Server internal graph API

Safe Haskell | None
Language | GHC2021

Control.Concurrent.STM.Stats

Synopsis

- atomicallyNamed :: String -> STM a -> IO a
- atomically :: STM a -> IO a
- getSTMStats :: IO (Map String (Int, Int))
- dumpSTMStats :: IO ()
- data TVar a
- data STM a
- retry :: STM a
- orElse :: STM a -> STM a -> STM a
- throwSTM :: Exception e => e -> STM a
- catchSTM :: Exception e => STM a -> (e -> STM a) -> STM a
- newTVar :: a -> STM (TVar a)
- newTVarIO :: a -> IO (TVar a)
- readTVarIO :: TVar a -> IO a
- readTVar :: TVar a -> STM a
- writeTVar :: TVar a -> a -> STM ()
- registerDelay :: Int -> IO (TVar Bool)
- newTChanIO :: IO (TChan a)
- newBroadcastTChanIO :: IO (TChan a)
- newTQueueIO :: IO (TQueue a)
- newTBQueueIO :: Natural -> IO (TBQueue a)
- newTMVarIO :: a -> IO (TMVar a)
- modifyTVar' :: TVar a -> (a -> a) -> STM ()
- data TArray i e
- data TBQueue a
- newTBQueue :: Natural -> STM (TBQueue a)
- writeTBQueue :: TBQueue a -> a -> STM ()
- readTBQueue :: TBQueue a -> STM a
- tryReadTBQueue :: TBQueue a -> STM (Maybe a)
- flushTBQueue :: TBQueue a -> STM [a]
- peekTBQueue :: TBQueue a -> STM a
- tryPeekTBQueue :: TBQueue a -> STM (Maybe a)
- unGetTBQueue :: TBQueue a -> a -> STM ()
- lengthTBQueue :: TBQueue a -> STM Natural
- isEmptyTBQueue :: TBQueue a -> STM Bool
- isFullTBQueue :: TBQueue a -> STM Bool
- capacityTBQueue :: TBQueue a -> Natural
- data TChan a
- newTChan :: STM (TChan a)
- newBroadcastTChan :: STM (TChan a)
- writeTChan :: TChan a -> a -> STM ()
- readTChan :: TChan a -> STM a
- tryReadTChan :: TChan a -> STM (Maybe a)
- peekTChan :: TChan a -> STM a
- tryPeekTChan :: TChan a -> STM (Maybe a)
- dupTChan :: TChan a -> STM (TChan a)
- unGetTChan :: TChan a -> a -> STM ()
- isEmptyTChan :: TChan a -> STM Bool
- cloneTChan :: TChan a -> STM (TChan a)
- data TMVar a
- newTMVar :: a -> STM (TMVar a)
- newEmptyTMVar :: STM (TMVar a)
- newEmptyTMVarIO :: IO (TMVar a)
- takeTMVar :: TMVar a -> STM a
- tryTakeTMVar :: TMVar a -> STM (Maybe a)
- putTMVar :: TMVar a -> a -> STM ()
- tryPutTMVar :: TMVar a -> a -> STM Bool
- readTMVar :: TMVar a -> STM a
- tryReadTMVar :: TMVar a -> STM (Maybe a)
- swapTMVar :: TMVar a -> a -> STM a
- writeTMVar :: TMVar a -> a -> STM ()
- isEmptyTMVar :: TMVar a -> STM Bool
- mkWeakTMVar :: TMVar a -> IO () -> IO (Weak (TMVar a))
- data TQueue a
- newTQueue :: STM (TQueue a)
- writeTQueue :: TQueue a -> a -> STM ()
- readTQueue :: TQueue a -> STM a
- tryReadTQueue :: TQueue a -> STM (Maybe a)
- flushTQueue :: TQueue a -> STM [a]
- peekTQueue :: TQueue a -> STM a
- tryPeekTQueue :: TQueue a -> STM (Maybe a)
- unGetTQueue :: TQueue a -> a -> STM ()
- isEmptyTQueue :: TQueue a -> STM Bool
- modifyTVar :: TVar a -> (a -> a) -> STM ()
- stateTVar :: TVar s -> (s -> (a, s)) -> STM a
- swapTVar :: TVar a -> a -> STM a
- mkWeakTVar :: TVar a -> IO () -> IO (Weak (TVar a))
- check :: Bool -> STM ()

# Documentation

atomicallyNamed :: String -> STM a -> IO a #

atomically :: STM a -> IO a #

getSTMStats :: IO (Map String (Int, Int)) #

dumpSTMStats :: IO () #

data TVar a #

Shared memory locations that support atomic memory transactions.

#### Instances

Instances details

Eq (TVar a) | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods (==) :: TVar a -> TVar a -> Bool # (/=) :: TVar a -> TVar a -> Bool #
PrimUnlifted (TVar a) |
Instance details Defined in Data.Primitive.Unlifted.Class Associated Types
type Unlifted (TVar a) |
Instance details Defined in Data.Primitive.Unlifted.Class type Unlifted (TVar a) = TVar# RealWorld a
Methods toUnlifted# :: TVar a -> Unlifted (TVar a) # fromUnlifted# :: Unlifted (TVar a) -> TVar a #
type Unlifted (TVar a) |
Instance details Defined in Data.Primitive.Unlifted.Class type Unlifted (TVar a) = TVar# RealWorld a

data STM a #

A monad supporting atomic memory transactions.

#### Instances

Instances details

MonadCatch STM |
Instance details Defined in Control.Monad.Catch Methods catch :: (HasCallStack, Exception e) => STM a -> (e -> STM a) -> STM a #
MonadThrow STM |
Instance details Defined in Control.Monad.Catch Methods throwM :: (HasCallStack, Exception e) => e -> STM a #
Alternative STM | Takes the first non-`retry`ing `STM` action. @since base-4.8.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods empty :: STM a # (<|>) :: STM a -> STM a -> STM a # some :: STM a -> STM [a] # many :: STM a -> STM [a] #
Applicative STM | @since base-4.8.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods pure :: a -> STM a # (<*>) :: STM (a -> b) -> STM a -> STM b # liftA2 :: (a -> b -> c) -> STM a -> STM b -> STM c # (*>) :: STM a -> STM b -> STM b # (<*) :: STM a -> STM b -> STM a #
Functor STM | @since base-4.3.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods fmap :: (a -> b) -> STM a -> STM b # (<$) :: a -> STM b -> STM a #
Monad STM | @since base-4.3.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods (>>=) :: STM a -> (a -> STM b) -> STM b # (>>) :: STM a -> STM b -> STM b # return :: a -> STM a #
MonadPlus STM | Takes the first non-`retry`ing `STM` action. @since base-4.3.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods mzero :: STM a # mplus :: STM a -> STM a -> STM a #
MonadBaseControl STM STM |
Instance details Defined in Control.Monad.Trans.Control Associated Types
type StM STM a |
Instance details Defined in Control.Monad.Trans.Control type StM STM a = a
Methods liftBaseWith :: (RunInBase STM STM -> STM a) -> STM a # restoreM :: StM STM a -> STM a #
MArray TArray e STM |
Instance details Defined in Control.Concurrent.STM.TArray Methods getBounds :: Ix i => TArray i e -> STM (i, i) # getNumElements :: Ix i => TArray i e -> STM Int # newArray :: Ix i => (i, i) -> e -> STM (TArray i e) # newArray_ :: Ix i => (i, i) -> STM (TArray i e) # unsafeNewArray_ :: Ix i => (i, i) -> STM (TArray i e) # unsafeRead :: Ix i => TArray i e -> Int -> STM e # unsafeWrite :: Ix i => TArray i e -> Int -> e -> STM () #
Monoid a => Monoid (STM a) | @since base-4.17.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods mempty :: STM a # mappend :: STM a -> STM a -> STM a # mconcat :: [STM a] -> STM a #
Semigroup a => Semigroup (STM a) | @since base-4.17.0.0
Instance details Defined in GHC.Internal.Conc.Sync Methods (<>) :: STM a -> STM a -> STM a # sconcat :: NonEmpty (STM a) -> STM a # stimes :: Integral b => b -> STM a -> STM a #
RandomGen g => FrozenGen (TGen g) STM | Since: random-1.2.1
Instance details Defined in System.Random.Stateful Associated Types
type MutableGen (TGen g) STM |
Instance details Defined in System.Random.Stateful type MutableGen (TGen g) STM = TGenM g
Methods freezeGen :: MutableGen (TGen g) STM -> STM (TGen g) # modifyGen :: MutableGen (TGen g) STM -> (TGen g -> (a, TGen g)) -> STM a # overwriteGen :: MutableGen (TGen g) STM -> TGen g -> STM () #
RandomGen g => StatefulGen (TGenM g) STM | Since: random-1.2.1
Instance details Defined in System.Random.Stateful Methods uniformWord32R :: Word32 -> TGenM g -> STM Word32 # uniformWord64R :: Word64 -> TGenM g -> STM Word64 # uniformWord8 :: TGenM g -> STM Word8 # uniformWord16 :: TGenM g -> STM Word16 # uniformWord32 :: TGenM g -> STM Word32 # uniformWord64 :: TGenM g -> STM Word64 # uniformByteArrayM :: Bool -> Int -> TGenM g -> STM ByteArray # uniformShortByteString :: Int -> TGenM g -> STM ShortByteString #
RandomGen g => ThawedGen (TGen g) STM |
Instance details Defined in System.Random.Stateful Methods thawGen :: TGen g -> STM (MutableGen (TGen g) STM) #
RandomGen r => RandomGenM (TGenM r) r STM |
Instance details Defined in System.Random.Stateful Methods applyRandomGenM :: (r -> (a, r)) -> TGenM r -> STM a #
type StM STM a |
Instance details Defined in Control.Monad.Trans.Control type StM STM a = a
type MutableGen (TGen g) STM |
Instance details Defined in System.Random.Stateful type MutableGen (TGen g) STM = TGenM g

retry :: STM a #

Retry execution of the current memory transaction because it has seen values in `TVar`s which mean that it should not continue (e.g. the `TVar`s represent a shared buffer that is now empty). The implementation may block the thread until one of the `TVar`s that it has read from has been updated. (GHC only)

orElse :: STM a -> STM a -> STM a #

Compose two alternative STM actions (GHC only).

If the first action completes without retrying then it forms the result of the `orElse`. Otherwise, if the first action retries, then the second action is tried in its place. If both actions retry then the `orElse` as a whole retries.

throwSTM :: Exception e => e -> STM a #

A variant of `throw` that can only be used within the `STM` monad.

Throwing an exception in `STM` aborts the transaction and propagates the exception. If the exception is caught via `catchSTM`, only the changes enclosed by the catch are rolled back; changes made outside of `catchSTM` persist.

If the exception is not caught inside of the `STM`, it is re-thrown by `atomically`, and the entire `STM` is rolled back.

Although `throwSTM` has a type that is an instance of the type of `throw`, the two functions are subtly different:

    throw e    `seq` x  ===> throw e
    throwSTM e `seq` x  ===> x

The first example will cause the exception `e` to be raised, whereas the second one won't. In fact, `throwSTM` will only cause an exception to be raised when it is used within the `STM` monad. The `throwSTM` variant should be used in preference to `throw` to raise an exception within the `STM` monad because it guarantees ordering with respect to other `STM` operations, whereas `throw` does not.

catchSTM :: Exception e => STM a -> (e -> STM a) -> STM a #

Exception handling within STM actions.

``catchSTM` m f` catches any exception thrown by `m` using `throwSTM`, using the function `f` to handle the exception. If an exception is thrown, any changes made by `m` are rolled back, but changes prior to `m` persist.

newTVar :: a -> STM (TVar a) #

Create a new `TVar` holding a value supplied

newTVarIO :: a -> IO (TVar a) #

`IO` version of `newTVar`. This is useful for creating top-level `TVar`s using `unsafePerformIO`, because using `atomically` inside `unsafePerformIO` isn't possible.

readTVarIO :: TVar a -> IO a #

Return the current value stored in a `TVar`. This is equivalent to

     readTVarIO = atomically . readTVar

but works much faster, because it doesn't perform a complete transaction, it just reads the current value of the `TVar`.

readTVar :: TVar a -> STM a #

Return the current value stored in a `TVar`.

writeTVar :: TVar a -> a -> STM () #

Write the supplied value into a `TVar`.

registerDelay :: Int -> IO (TVar Bool) #

Switch the value of returned `TVar` from initial value `False` to `True` after a given number of microseconds. The caveats associated with `threadDelay` also apply.

Be careful not to exceed `maxBound :: Int`, which on 32-bit machines is only 2147483647 μs, less than 36 minutes.

newTChanIO :: IO (TChan a) #

`IO` version of `newTChan`. This is useful for creating top-level `TChan`s using `unsafePerformIO`, because using `atomically` inside `unsafePerformIO` isn't possible.

newBroadcastTChanIO :: IO (TChan a) #

`IO` version of `newBroadcastTChan`.

Since: stm-2.4

newTQueueIO :: IO (TQueue a) #

`IO` version of `newTQueue`. This is useful for creating top-level `TQueue`s using `unsafePerformIO`, because using `atomically` inside `unsafePerformIO` isn't possible.

newTBQueueIO :: Natural -> IO (TBQueue a) #

`IO` version of `newTBQueue`. This is useful for creating top-level `TBQueue`s using `unsafePerformIO`, because using `atomically` inside `unsafePerformIO` isn't possible.

newTMVarIO :: a -> IO (TMVar a) #

`IO` version of `newTMVar`. This is useful for creating top-level `TMVar`s using `unsafePerformIO`, because using `atomically` inside `unsafePerformIO` isn't possible.

modifyTVar' :: TVar a -> (a -> a) -> STM () #

Strict version of `modifyTVar`.

Since: stm-2.3

data TArray i e #

`TArray` is a transactional array, supporting the usual `MArray` interface for mutable arrays.

It is conceptually implemented as `Array i (TVar e)`.

#### Instances

Instances details

MArray TArray e STM |
Instance details Defined in Control.Concurrent.STM.TArray Methods getBounds :: Ix i => TArray i e -> STM (i, i) # getNumElements :: Ix i => TArray i e -> STM Int # newArray :: Ix i => (i, i) -> e -> STM (TArray i e) # newArray_ :: Ix i => (i, i) -> STM (TArray i e) # unsafeNewArray_ :: Ix i => (i, i) -> STM (TArray i e) # unsafeRead :: Ix i => TArray i e -> Int -> STM e # unsafeWrite :: Ix i => TArray i e -> Int -> e -> STM () #
MArray TArray e IO | Writes are slow in `IO`.
Instance details Defined in Control.Concurrent.STM.TArray Methods getBounds :: Ix i => TArray i e -> IO (i, i) # getNumElements :: Ix i => TArray i e -> IO Int # newArray :: Ix i => (i, i) -> e -> IO (TArray i e) # newArray_ :: Ix i => (i, i) -> IO (TArray i e) # unsafeNewArray_ :: Ix i => (i, i) -> IO (TArray i e) # unsafeRead :: Ix i => TArray i e -> Int -> IO e # unsafeWrite :: Ix i => TArray i e -> Int -> e -> IO () #
(Eq i, Eq e) => Eq (TArray i e) |
Instance details Defined in Control.Concurrent.STM.TArray Methods (==) :: TArray i e -> TArray i e -> Bool # (/=) :: TArray i e -> TArray i e -> Bool #

data TBQueue a #

`TBQueue` is an abstract type representing a bounded FIFO channel.

Since: stm-2.4

#### Instances

Instances details

Eq (TBQueue a) |
Instance details Defined in Control.Concurrent.STM.TBQueue Methods (==) :: TBQueue a -> TBQueue a -> Bool # (/=) :: TBQueue a -> TBQueue a -> Bool #

newTBQueue #

Arguments

:: Natural | maximum number of elements the queue can hold
-> STM (TBQueue a) |

Builds and returns a new instance of `TBQueue`.

writeTBQueue :: TBQueue a -> a -> STM () #

Write a value to a `TBQueue`; blocks if the queue is full.

readTBQueue :: TBQueue a -> STM a #

Read the next value from the `TBQueue`.

tryReadTBQueue :: TBQueue a -> STM (Maybe a) #

A version of `readTBQueue` which does not retry. Instead it returns `Nothing` if no value is available.

flushTBQueue :: TBQueue a -> STM [a] #

Efficiently read the entire contents of a `TBQueue` into a list. This function never retries.

Since: stm-2.4.5

peekTBQueue :: TBQueue a -> STM a #

Get the next value from the `TBQueue` without removing it, retrying if the channel is empty.

tryPeekTBQueue :: TBQueue a -> STM (Maybe a) #

A version of `peekTBQueue` which does not retry. Instead it returns `Nothing` if no value is available.

unGetTBQueue :: TBQueue a -> a -> STM () #

Put a data item back onto a channel, where it will be the next item read. Blocks if the queue is full.

lengthTBQueue :: TBQueue a -> STM Natural #

Return the length of a `TBQueue`.

Since: stm-2.5.0.0

isEmptyTBQueue :: TBQueue a -> STM Bool #

Returns `True` if the supplied `TBQueue` is empty.

isFullTBQueue :: TBQueue a -> STM Bool #

Returns `True` if the supplied `TBQueue` is full.

Since: stm-2.4.3

capacityTBQueue :: TBQueue a -> Natural #

The maximum number of elements the queue can hold.

Since: stm-2.5.2.0

data TChan a #

`TChan` is an abstract type representing an unbounded FIFO channel.

#### Instances

Instances details

Eq (TChan a) |
Instance details Defined in Control.Concurrent.STM.TChan Methods (==) :: TChan a -> TChan a -> Bool # (/=) :: TChan a -> TChan a -> Bool #

newTChan :: STM (TChan a) #

Build and return a new instance of `TChan`

newBroadcastTChan :: STM (TChan a) #

Create a write-only `TChan`. More precisely, `readTChan` will `retry` even after items have been written to the channel. The only way to read a broadcast channel is to duplicate it with `dupTChan`.

Consider a server that broadcasts messages to clients:

    serve :: TChan Message -> Client -> IO loop
    serve broadcastChan client = do
        myChan <- dupTChan broadcastChan
        forever $ do
            message <- readTChan myChan
            send client message

The problem with using `newTChan` to create the broadcast channel is that if it is only written to and never read, items will pile up in memory. By using `newBroadcastTChan` to create the broadcast channel, items can be garbage collected after clients have seen them.

Since: stm-2.4

writeTChan :: TChan a -> a -> STM () #

Write a value to a `TChan`.

readTChan :: TChan a -> STM a #

Read the next value from the `TChan`.

tryReadTChan :: TChan a -> STM (Maybe a) #

A version of `readTChan` which does not retry. Instead it returns `Nothing` if no value is available.

Since: stm-2.3

peekTChan :: TChan a -> STM a #

Get the next value from the `TChan` without removing it, retrying if the channel is empty.

Since: stm-2.3

tryPeekTChan :: TChan a -> STM (Maybe a) #

A version of `peekTChan` which does not retry. Instead it returns `Nothing` if no value is available.

Since: stm-2.3

dupTChan :: TChan a -> STM (TChan a) #

Duplicate a `TChan`: the duplicate channel begins empty, but data written to either channel from then on will be available from both. Hence this creates a kind of broadcast channel, where data written by anyone is seen by everyone else.

unGetTChan :: TChan a -> a -> STM () #

Put a data item back onto a channel, where it will be the next item read.

isEmptyTChan :: TChan a -> STM Bool #

Returns `True` if the supplied `TChan` is empty.

cloneTChan :: TChan a -> STM (TChan a) #

Clone a `TChan`: similar to dupTChan, but the cloned channel starts with the same content available as the original channel.

Since: stm-2.4

data TMVar a #

A `TMVar` is a synchronising variable, used for communication between concurrent threads. It can be thought of as a box, which may be empty or full.

#### Instances

Instances details

Eq (TMVar a) |
Instance details Defined in Control.Concurrent.STM.TMVar Methods (==) :: TMVar a -> TMVar a -> Bool # (/=) :: TMVar a -> TMVar a -> Bool #

newTMVar :: a -> STM (TMVar a) #

Create a `TMVar` which contains the supplied value.

newEmptyTMVar :: STM (TMVar a) #

Create a `TMVar` which is initially empty.

newEmptyTMVarIO :: IO (TMVar a) #

`IO` version of `newEmptyTMVar`. This is useful for creating top-level `TMVar`s using `unsafePerformIO`, because using `atomically` inside `unsafePerformIO` isn't possible.

takeTMVar :: TMVar a -> STM a #

Return the contents of the `TMVar`. If the `TMVar` is currently empty, the transaction will `retry`. After a `takeTMVar`, the `TMVar` is left empty.

tryTakeTMVar :: TMVar a -> STM (Maybe a) #

A version of `takeTMVar` that does not `retry`. The `tryTakeTMVar` function returns `Nothing` if the `TMVar` was empty, or ``Just` a` if the `TMVar` was full with contents `a`. After `tryTakeTMVar`, the `TMVar` is left empty.

putTMVar :: TMVar a -> a -> STM () #

Put a value into a `TMVar`. If the `TMVar` is currently full, `putTMVar` will `retry`.

tryPutTMVar :: TMVar a -> a -> STM Bool #

A version of `putTMVar` that does not `retry`. The `tryPutTMVar` function attempts to put the value `a` into the `TMVar`, returning `True` if it was successful, or `False` otherwise.

readTMVar :: TMVar a -> STM a #

This is a combination of `takeTMVar` and `putTMVar`; ie. it takes the value from the `TMVar`, puts it back, and also returns it.

tryReadTMVar :: TMVar a -> STM (Maybe a) #

A version of `readTMVar` which does not retry. Instead it returns `Nothing` if no value is available.

Since: stm-2.3

swapTMVar :: TMVar a -> a -> STM a #

Swap the contents of a `TMVar` for a new value.

writeTMVar :: TMVar a -> a -> STM () #

Non-blocking write of a new value to a `TMVar` Puts if empty. Replaces if populated.

Since: stm-2.5.1

isEmptyTMVar :: TMVar a -> STM Bool #

Check whether a given `TMVar` is empty.

mkWeakTMVar :: TMVar a -> IO () -> IO (Weak (TMVar a)) #

Make a `Weak` pointer to a `TMVar`, using the second argument as a finalizer to run when the `TMVar` is garbage-collected.

Since: stm-2.4.4

data TQueue a #

`TQueue` is an abstract type representing an unbounded FIFO channel.

Since: stm-2.4

#### Instances

Instances details

Eq (TQueue a) |
Instance details Defined in Control.Concurrent.STM.TQueue Methods (==) :: TQueue a -> TQueue a -> Bool # (/=) :: TQueue a -> TQueue a -> Bool #

newTQueue :: STM (TQueue a) #

Build and returns a new instance of `TQueue`

writeTQueue :: TQueue a -> a -> STM () #

Write a value to a `TQueue`.

readTQueue :: TQueue a -> STM a #

Read the next value from the `TQueue`.

tryReadTQueue :: TQueue a -> STM (Maybe a) #

A version of `readTQueue` which does not retry. Instead it returns `Nothing` if no value is available.

flushTQueue :: TQueue a -> STM [a] #

Efficiently read the entire contents of a `TQueue` into a list. This function never retries.

Since: stm-2.4.5

peekTQueue :: TQueue a -> STM a #

Get the next value from the `TQueue` without removing it, retrying if the channel is empty.

tryPeekTQueue :: TQueue a -> STM (Maybe a) #

A version of `peekTQueue` which does not retry. Instead it returns `Nothing` if no value is available.

unGetTQueue :: TQueue a -> a -> STM () #

Put a data item back onto a channel, where it will be the next item read.

isEmptyTQueue :: TQueue a -> STM Bool #

Returns `True` if the supplied `TQueue` is empty.

modifyTVar :: TVar a -> (a -> a) -> STM () #

Mutate the contents of a `TVar`. N.B., this version is non-strict.

Since: stm-2.3

stateTVar :: TVar s -> (s -> (a, s)) -> STM a #

Like `modifyTVar'` but the function is a simple state transition that can return a side value which is passed on as the result of the `STM`.

Since: stm-2.5.0

swapTVar :: TVar a -> a -> STM a #

Swap the contents of a `TVar` for a new value.

Since: stm-2.3

mkWeakTVar :: TVar a -> IO () -> IO (Weak (TVar a)) #

Make a `Weak` pointer to a `TVar`, using the second argument as a finalizer to run when `TVar` is garbage-collected

Since: stm-2.4.3

check :: Bool -> STM () #

Check that the boolean condition is true and, if not, `retry`.

In other words, `check b = unless b retry`.

Since: stm-2.1.1