Re: [Haskell-cafe] Is it usual to read a Maybe (IORef a) ?

3 Sep 2008


      On Wednesday 03 September 2008 12:09:38 minh thu wrote:
...
Hi,
I'd like to write a data structure to be used inside the IO monad.
The structure has some handles of type Maybe (IORef a),
i.e. IORef are pointers and the Maybe is like null pointers.
So I came up with the following functions :
readHandle :: Maybe (IORef a) -> IO (Maybe a)
readField :: (a -> b) -> Maybe (IORef a) -> IO (Maybe b)
readHandle Nothing  = do
  return Nothing
readHandle (Just r) = do
  v <- readIORef r
  return $ Just v
readField f h = do
  m <- readHandle h
  return $ fmap f m
Is it something usual ?
Are there any related functions in the standard libraries ?
A value of type Maybe (IORef a) is an optional pointer that must point to an 
object. If you want a pointer that points to either Nothing (aka null) or to a 
value, you should use IORef (Maybe a).

Then 

readHandle :: IORef (Maybe a) -> IO (Maybe a)
readHandle = readIORef

readfield :: (a -> b) -> IORef (Maybe a) -> IO (Maybe b)
readfield f p = (fmap . fmap) f (readIORef p)

Re: [Haskell-cafe] Is it usual to read a Maybe (IORef a) ?

hs