Re: [Haskell-cafe] CBN, CBV, Lazy in the same final tagless framework

On Fri, Oct 09, 2009 at 01:27:57PM -0400, David Menendez wrote:

On Fri, Oct 9, 2009 at 11:12 AM, Felipe Lessa wrote:

...

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.

Hmmm, but my version also needs only one IORef, right? In fact I first wrote the same code as yours but I've frowned upon the need of having that 'undefined' and an extra 'writeIORef'. Thanks, -- Felipe.