once-0.4: memoization for IO actions and functions

Package provides single polymorphic function once, that allows you to memoize IO actions and functions, evaluating them at most once.

>>> let mkStamp = (putStrLn "stamping" >> writeFile "/tmp/stamp" "") :: IO ()
>>> -- onceStamp :: IO ()
>>> onceStamp <- once mkStamp
>>> -- onceStamp actually evaluates mkStamp it wraps first time.
>>> onceStamp
stamping
>>> -- but second time result `()' is memoized, no action is performed.
>>> onceStamp
>>> -- we can memoize functions too
>>> foo <- once $ \x -> print "foo" >> print (x :: Int)
>>> -- action will be performed once for every distinct argument
>>> foo 10
foo
10
>>> foo 10
10
>>> foo 4
foo
4