3. IOExts

This library provides the following extensions to the IO monad:

• The operations fixIO, unsafePerformIO and unsafeInterleaveIO described in [ImperativeFP]
• References (aka mutable variables) and mutable arrays (but no form of mutable byte arrays)
• performGC triggers an immediate garbage collection
• When called, trace prints the string in its first argument, and then returns the second argument as its result. The trace function is not referentially transparent, and should only be used for debugging, or for monitoring execution.
• unsafePtrEq compares two values for pointer equality without evaluating them. The results are not referentially transparent and may vary significantly from one compiler to another or in the face of semantics-preserving program changes. However, pointer equality is useful in creating a number of referentially transparent constructs such as this simplified memoisation function:

  > cache :: (a -> b) -> (a -> b)
  > cache f = \x -> unsafePerformIO (check x)
  >  where
  >   ref = unsafePerformIO (newIORef (error "cache", error "cache"))
  >   check x = readIORef ref >>= \ (x',a) ->
  >	       if x `unsafePtrEq` x' then
  >		 return a
  >	       else
  >		 let a = f x in
  >		 writeIORef ref (x, a) >>
  >		 return a
  

module IOExts where

fixIO               :: (a -> IO a) -> IO a
unsafePerformIO     :: IO a -> a
unsafeInterleaveIO  :: IO a -> IO a
		    
data IORef a        -- mutable variables containing values of type a
newIORef     	    :: a -> IO (IORef a)
readIORef    	    :: IORef a -> IO a
writeIORef   	    :: IORef a -> a -> IO ()
instance Eq (IORef a)

data IOArray ix elt -- mutable arrays indexed by values of type ix
                    -- containing values of type a.
newIOArray          :: Ix ix => (ix,ix) -> elt -> IO (IOArray ix elt)
boundsIOArray       :: Ix ix => IOArray ix elt -> (ix, ix)
readIOArray         :: Ix ix => IOArray ix elt -> ix -> IO elt
writeIOArray        :: Ix ix => IOArray ix elt -> ix -> elt -> IO ()
freezeIOArray       :: Ix ix => IOArray ix elt -> IO (Array ix elt)
instance Eq (IOArray ix elt)

performGC           :: IO ()
trace               :: String -> a -> a
unsafePtrEq         :: a -> a -> Bool