On Apr 26, 2008, at 7:18 PM, Conal Elliott wrote:
Here's another angle on part of Jake's question:
Can we implement a type 'TIVal a' (preferably without unsafePerformIO) with the following interface:
newIVal :: STM (TIVal a, a -> STM ()) -- or IO (...) force :: TIVal a -> STM a
instance Functor IVal instance Applicative IVal instance Monad IVal
where
* 'newIVal' makes something like an IVar that can be written/defined (just once) with the returned a->STM(). * 'force' gets the value, retrying if not yet defined; once force is able to succeed, it always yields the same value. * 'fmap f tiv' becomes defined (force yields a value instead of retrying) when tiv does. Similarly for (<*>) and join. * Forcing 'fmap f tiv' more than once results in f being called only once, i.e., the result is cached and reused, as in pure values. Similarly for (<*>) and join.
Well, I think I may have done it! This is only code that I typed up really quick. I haven't even made sure it compiles. Regardless, I think the gist is pretty clear... data TIVal a = TIVal (STM a) (TMVar a) newTIVal = do uc <- newEmptyTMVar c <- newEmptyTMVar return (TIVal (takeTMVar uc) c, putTMVar uc) force (TIVal uc c) = readTMVar c `orElse` cache where cache = do x <- uc putTMVar c x return x unsafeNewEmptyTMVar = unsafePerformIO newEmptyTMVarIO -- insert NOINLINE and/or other magical pragmas here instance Functor TIVal where f `fmap` x = TIVal (return . f =<< force x) unsafeNewEmptyTMVar -- Applicative, Monad, and Monoid omitted I did have to resort to unsafePerformIO, but I think the reason is innocent enough to still feel good about. This implementation, if it works, seems to be embarrassingly simple.