Tuesday, April 25, 2006

Some code functions+devices

Here is some Haskell code to illustrate my previous post (I cheat a bit with tuples in unary cases):

-- AND join in workflow terminology.
binaryJoin :: (STM a, STM b) -> STM (a, b)
binaryJoin (sa, sb) = do
a <- sa
b <- sb
return (a, b)

-- Very simple code function.
inc :: Num a => STM a -> STM a
inc sa = do
a <- sa
return (a+1)

-- Code that delegates to other code.
add :: Num a => (STM a, STM a) -> STM a
add inputs = do
(a, b) <- binaryJoin inputs
return (a+b)

This code covers so called instances in terminology of Mark's thesis.
What about devices? For output the answer looks simple: code for output devices has type:
(STM i1, ..., STM im) -> IO (), but such object would probably require a separate thread to run (can we say that's because, unlike STM, IO is not composable?). Thus, I suggest splitting (code for) output devices into a transactional matcher part (if needed at all) of type (STM i1, ..., STM im) -> STM a and a reactive handler part of type a -> IO (). This gives the framework runtime a chance to run matchers transactionally, and only when one of them matches, run the handler.
So, we have (pseudo-Haskell):
data OutputDevice = forall a . OutputDevice ((STM i1, ..., STM im) -> STM a) (a -> IO ())

What about input devices? The first idea is to type them:
(a -> STM ()) -> IO ()
with the contract that the device performs some input, then applies the (parameterised) transaction to it, then runs it.
Why not just IO a? Deal, I can minimize input device's responsibility by saying the framework will "apply and run", while the device will just read and return, so the type is:
IO a

Well, by its nature (blocking input) each input device will be run on a separate thread, injecting its values into the transactional thread via TChan'nels. I don't like this (especially when we dealt with multithreading issues of output devices), but currently see no other alternative. I have to explore the Haskell standard library to see, whether the input is mostly blocking.
If nothing better can be done with input devices, then maybe it makes sense to run output device on separate threads as well, and let STM runtime to handle blocking (so output devices will be typed (STM i1, ..., STM im) -> IO (), or even a -> IO ())...

No comments: