This is a literate haskell version of Control.Concurrent.MSem for increased clarity.
This semaphore gracefully handles threads which die while blocked waiting. The fairness guarantee is that blocked threads are servied in a FIFO order.
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.
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? ]
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.
wait will take one unit of value from the sempahore, but will block if the quantity available
is not positive.
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
returns without interruption then it is known that each earlier waiter has definitely either been
interrupted or has retured without interruption (the FIFO guarantee).
peekAvail skips the queue of any blocked
wait threads, but may momentarily block on
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
Note that Control.Concurrent.MSemN offers a more powerful API for making decisions based on the available amount.