11 Jan 2002 17:10:16 -0500, Albert Lai <[EMAIL PROTECTED]> pisze: > Now my grief is that I cannot write a subprogram with state > variables and have it reused in ST and IO. Fortunately I can write > a subprogram with mutable arrays and have it reused in ST and IO, > so I can write "sort a given array"; but I cannot write "increment > a given integer variable".
You can if you write your own class describing the common interface. Here is mine. The same code can work with MVars too, as long as the order of operations is consistent with the empty/full state. The empty/full state is real for MVars and imagined for IORef/STRef. module UnifiedRef (Ref(..), STRef, IORef, MVar) where import ST import IOExts import Concurrent class Monad m => Ref m c | c -> m where -- Minimal definition: -- 'newRef' or 'newEmptyRef', -- 'getRef' or 'cutRef', -- 'setRef' or 'insertRef'. newRef :: a -> m (c a) newEmptyRef :: m (c a) copyRef :: c a -> m (c a) getRef :: c a -> m a setRef :: c a -> a -> m () cutRef :: c a -> m a insertRef :: c a -> a -> m () modifyRef :: c a -> (a -> m a) -> m () withRef :: c a -> (a -> m b) -> m b changeRef :: c a -> (a -> m (b, a)) -> m b newRef a = do c <- newEmptyRef; insertRef c a; return c newEmptyRef = newRef (error "Empty reference") copyRef c = newRef =<< getRef c getRef c = do a <- cutRef c; insertRef c a; return a setRef c a = do cutRef c; insertRef c a cutRef = getRef insertRef = setRef modifyRef c f = insertRef c =<< f =<< cutRef c withRef c f = do a <- cutRef c; b <- f a; insertRef c a; return b changeRef c f = do a <- cutRef c; (b, a') <- f a; insertRef c a'; return b instance Ref (ST s) (STRef s) where newRef = newSTRef getRef = readSTRef setRef = writeSTRef instance Ref IO IORef where newRef = newIORef getRef = readIORef setRef = writeIORef instance Ref IO MVar where newRef = newMVar newEmptyRef = newEmptyMVar getRef = readMVar cutRef = takeMVar insertRef = putMVar modifyRef = modifyMVar_ withRef = withMVar changeRef r f = modifyMVar r (\a -> do (b, a') <- f a; return (a', b)) -- __("< Marcin Kowalczyk * [EMAIL PROTECTED] http://qrczak.ids.net.pl/ \__/ ^^ QRCZAK _______________________________________________ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users