Control.Concurrent.WVar

WVar is waitable IORef. It is similar to MVar but different at some points.

• The latest (cached) value can be read while someone is updating the value.
• Put operation can overwrite the value if the value is fresh and cannot be blocked for waiting empty.
• WVar is strict. It means that the new value storing into the WVar will be evaluated (WHNF) before actual storing.

There are two states in the user viewpoint.

Fresh

The WVar is not being updated. This state corresponds to to full state of MVar.

Updating

The WVar is being updated by someone. This state corresponds to to empty state of MVar. However, cached previous value can be read while Updating.

WVar

data WVar a

"a" is the type of data in the WVar.

Instances

Eq (WVar a)

newWVar :: a -> IO (WVar a)

Create a fresh WVar that contains the supplied value.

takeWVar :: WVar a -> IO a

Take the value of a WVar like takeMVar. It blocks when the WVar is being updated.

tryTakeWVar :: WVar a -> IO (Bool, a)

Non-blocking version of takeWVar.

putWVar :: WVar a -> a -> IO ()

Put the supplied value into a WVar. It performs simple "write" when the WVar is Fresh. When the supplied value is already evaluated, it never blocks.

readWVar :: WVar a -> IO a

Read the cached value of the WVar. It never blocks.

readFreshWVar :: WVar a -> IO a

Read the fresh value of the WVar. It blocks and waits for a fresh value when the WVar is being updated by someone.

tryReadFreshWVar :: WVar a -> IO (Bool, a)

Non-blocking version of readFreshWVar

WCached

Low level types and functions of WVar.

data WCached a

WCached consists of WVar and its cached ticket.

Constructors

WCached

Fields

cachedVar :: !(WVar a)

cachedTicket :: WTicket a

Instances

Eq (WCached a)

Show a => Show (WCached a)

type WTicket a = Ticket (WContent a)

cacheWVar :: WVar a -> IO (WCached a)

Cache the current value of the WVar and create WCached.

recacheWCached :: WCached a -> IO (WCached a)

Recache the WCached.

recacheWCached = cacheWVar . cachedVar

readWTicket :: WTicket a -> a

Read the value of the WTicket

takeWCached :: WCached a -> IO (WTicket a)

Take the value of the WCached like takeMVar. It blocks when the WCached is being updated.

tryTakeWCached :: WCached a -> IO (Bool, WTicket a)

Non-blocking version of takeWCached.

putWCached :: WCached a -> a -> IO (WTicket a)

Put the value to the WCached. It performs simple "write" when the WVar is Fresh. When the supplied value is already evaluated, it never blocks.

tryPutWCached :: WCached a -> a -> IO (Bool, WTicket a)

Put the value to a WCached. It performs simple "write" when the WVar is Fresh. It fails when the cache is obsoleted. When the supplied value is already evaluated, it never blocks.

readWCached :: WCached a -> a

Read the cached value of the WCached. It never blocks.

readFreshWCached :: WCached a -> IO (WTicket a)

Read the Fresh value of the WCached. It blocks and waits for a Fresh value when the WCached is being updated by someone.

tryReadFreshWCached :: WCached a -> IO (Bool, WTicket a)

Non-blocking version of readFreshWCached