As discussed on the libraries/haskell-cafe mailing lists
  http://www.haskell.org/pipermail/libraries/2010-April/013420.html

This is a replacement for block/unblock in the asychronous exceptions
API to fix a problem whereby a function could unblock asynchronous
exceptions even if called within a blocked context.

The new terminology is "mask" rather than "block" (to avoid confusion
due to overloaded meanings of the latter).

The following is the new API; the old API is deprecated but still
available for the time being.

Control.Exception
-----------------

mask  :: ((forall a. IO a -> IO a) -> IO b) -> IO b
mask_ :: IO a -> IO a

uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
uninterruptibleMask_ :: IO a -> IO 

getMaskingState :: IO MaskingState

data MaskingState
  = Unmasked
  | MaskedInterruptible 
  | MaskedUninterruptible


Control.Concurrent
------------------

forkIOUnmasked :: IO () -> IO ThreadId