Control.Concurrent.MSem

This is a literate haskell version of Control.Concurrent.MSem for increased clarity.

A semaphore in which operations may wait for or signal single units of value. This modules is intended to improve on Control.Concurrent.QSem.

This semaphore gracefully handles threads which die while blocked waiting. The fairness guarantee is that blocked threads are servied in a FIFO order.

If with is used to guard a critical section then no quantity of the semaphore will be lost if the activity throws an exception or if this thread is killed by the rest of the program.

new can initialize the semaphore to negative, zero, or positive quantity. wait always leaves the MSem with non-negative quantity. signal alawys adds one to the quantity.

The functions below are generic in (Integral i) with specialization to Int, Word, and Integer.

Overflow warning: These operations do not check for overflow errors. If the Integral type is too small to accept the new total then the behavior of signal is undefined. Using (MSem Integer) prevents the possibility of an overflow error. [ A version of signal that checks the upper bound could be added, but how would it report failure and how would you use this sanely? ]

Documentation

data MSem i

A MSem is a semaphore in which the available quantity can be added and removed in single units, and which can start with positive, zero, or negative value.

Instances

Typeable1 MSem

Eq (MSem i)

new :: Integral i => i -> IO (MSem i)

new allows positive, zero, and negative initial values. The initial value is forced here to better localize errors.

The only way to achieve a negative value with MSem is to start negative with new. Once a negative quantity becomes non-negative by use of signal it will never later be negative.

with :: Integral i => MSem i -> IO a -> IO a

with takes a unit of value from the semaphore to hold while performing the provided operation. with ensures the quantity of the sempahore cannot be lost if there are exceptions or if killThread is used.

with uses bracket_ to ensure wait and signal get called correctly.

wait :: Integral i => MSem i -> IO ()

wait will take one unit of value from the sempahore, but will block if the quantity available is not positive.

If wait returns normally (not interrupted) then it left the MSem with a remaining quantity that was greater than or equal to zero. If wait is interrupted then no quantity is lost. If wait returns without interruption then it is known that each earlier waiter has definitely either been interrupted or has retured without interruption (the FIFO guarantee).

signal :: Integral i => MSem i -> IO ()

signal adds one unit to the sempahore. Overflow is not checked.

signal may block, but it cannot be interrupted, which allows it to dependably restore value to the MSem. All signal, peekAvail, and the head waiter may momentarily block in a fair FIFO manner.

peekAvail :: Integral i => MSem i -> IO i

peekAvail skips the queue of any blocked wait threads, but may momentarily block on signal, other peekAvail, and the head waiter. This returns the amount of value available to be taken. Using this value without producing unwanted race conditions is left up to the programmer.

Note that Control.Concurrent.MSemN offers a more powerful API for making decisions based on the available amount.