Efficiency of using field labels vs pattern matching

[Brian Hulley]

Bulat Ziganshin wrote:
> btw, if you want beter efficiency, you may use unboxed
> references (http://haskell.org/haskellwiki/Library/ArrayRef)

Thanks for the pointer to your ArrayRef library. I've downloaded it and it 
will be very useful - its extremely neat that the fact that something is 
stored as unboxed can be hidden from the rest of the program.

One thing I wondered was if the functional dependency in Data.Ref.Universal 
from the result type to the monad is actually necessary, since this FD 
prevents me adding an instance for MonadIO ie the following instance is not 
valid:

    instance MonadIO m => URef m IOURef where
        -- m -> r is fine
        -- r -> m restricts m too much

Of course this isn't a big problem because I can simply define lifted 
versions separately ie:

    import Data.Ref hiding(newURef, readURef, writeURef)
    import GHC.Unboxed
    import Control.Monad.Trans

    -- instance MonadIO m => URef m IOURef where

    newURef :: (Unboxed a, MonadIO m) => a -> m (IOURef a)
    newURef v = liftIO $ newIOURef v

    readURef :: (Unboxed a, MonadIO m) => IOURef a -> m a
    readURef ref = liftIO $ readIOURef ref

    writeURef :: (Unboxed a, MonadIO m) => IOURef a -> a -> m ()
    writeURef ref v = liftIO $ writeIOURef ref v

    -- test monad
    newtype SomeIO a = SomeIO {runSomeIO :: (IO a)} deriving (Monad, 
MonadIO)

    foo :: SomeIO Int
    foo = do
        xRef <- newURef (57::Int)
        readURef xRef

    main = do
        x <- runSomeIO foo
        print x
        _ <- getChar
        return ()

Anyway thanks for sharing your library. I'm going to put the URef functions 
above into a module so I can use the same names for URef  functions (ie 
URef.T, new, read, write) as I'm already using for boxed refs.

Best regards,
Brian.
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