On Fri, Oct 9, 2009 at 11:12 AM, Felipe Lessa
On Thu, Oct 08, 2009 at 12:54:14AM -0700, oleg@okmij.org wrote:
Actually it is possible to implement all three evaluation orders within the same final tagless framework, using the same interpretation of types and reusing most of the code save the semantics of lam. That is where the three orders differ, by their own definition.
That's really nice, Oleg, thanks! I just wanted to comment that I'd prefer to write
share :: IO a -> IO (IO a) share m = mdo r <- newIORef (do x <- m writeIORef r (return x) return x) return (readIORef r >>= id)
which unfortunately needs {-# LANGUAGE RecursiveDo #-} or some ugliness from mfix
share :: IO a -> IO (IO a) share m = do r <- mfix $ \r -> newIORef (do x <- m writeIORef r (return x) return x) return (readIORef r >>= id)
Alternatively,
share m = do
r <- newIORef undefined
writeIORef r $ do
x <- m
writeIORef r (return x)
return x
return $ readIORef r >>= id
Which is basically the same as your version, but only needs one IORef.
--
Dave Menendez