Implementation of skip variables - a special case of skip channels, the difference being that a value stored in a skip variable can be read at most once, while a skip channel can have multiple readers (i.e. multiple readers can read the same value).

Writing into a skip variable doesn't block - if there's a value stored, it's overwritten. Reading, on the other hand, blocks, if there isn't a new value since the last read, or if the skip variable is empty.

Some examples:

s <- newEmptySVar :: IO (SVar Int)
print =<< readSVar s -- blocks

s <- newSVar 42 :: IO (SVar Int)
print =<< readSVar s -- prints 42
print =<< readSVar s -- blocks

The next few lines will print some of the numbers between 0 and 10000. Note that at least one number will be printed, but many will be skipped, because printing is slow.

import Control.Concurrent (forkIO, killThread)
import Control.Monad (forever)

s <- newEmptySVar :: IO (SVar Int)
tid <- forkIO $ forever $ print =<< readSVar s
mapM_ (putSVar s) [0..10000]
killThread tid