Partibles for composing monads

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Having praised monads to the hilt, let me level one criticism. Monads tend to be an all-or-nothing proposition. If you discover that you need interaction deep within your program, you must rewrite that segment to use a monad. If you discover that you need two sorts of interaction, you tend to make a single monad support both sorts. It seems to me that instead we should be able to move smoothly from no monads (no interactions) to one monad (a single form of interaction) to many monads (several independent forms of interactions). How to achieve this remains a challenge for the future.

  • How to Declare an Imperative, Philip Wadler.

Some initial definitions:

        class Partible a where
            part  :: a -> (a, a)
            parts :: a -> [a]   

             -- Minimal complete definition: part or parts
            part u  = case parts u of u1:u2:_ -> (u1, u2) 
            parts u = case part u of (u1, u2) -> u1 : parts u2

        instance Partible a => Monad ((->) a) where
            return x = \ u -> part u `seq` x
            m >>= k  = \ u -> case part u of (u1, u2) -> (k $! m u1) u2
            m >> w   = \ u -> case part u of (u1, u2) -> m u1 `seq` w u2
            fail s   = \ u -> part u `seq` error s

        data OI  -- abstract
        primPartOI :: OI -> (OI, OI)  -- primitive
         -- type IO a = OI -> a

        instance Partible OI where part = primPartOI

         -- more primitives
        primGetChar :: OI -> Char
        primPutChar :: Char -> OI -> ()

         -- copy 'n' paste from Wadler's paper
        type Dialogue = [Response] -> [Request]
        data Request  = Getq | Putq Char
        data Response = Getp Char | Putp

        respond :: Request -> OI -> Response
        respond Getq     = primGetChar >>= return . Getp
        respond (Putq c) = primPutChar c >> return Putp 

        runDialogue :: Dialogue -> OI -> ()
        runDialogue d =
            \u -> let rspl = zipWith respond (d rspl) (parts u) in
                  foldr (\ _ r -> r) () rspl

        instance Partible a => MonadFix ((->) a) where
            mfix m = \ u -> fix (\ x -> m x u)          -- fix f = f (fix f)

         -- to be made into an ADT...
        data Fresh a = Fresh (OI -> a) OI

        afresh :: (OI -> a) -> OI -> Fresh a
        afresh g u = Fresh g u

        instance Partible (Fresh a) where
            part (Fresh g u) = case part u of (u1, u2) -> (Fresh g u1, Fresh g u2)

        fresh :: Fresh a -> [a]
        fresh u = [ g v | Fresh g v <- parts u ]

        instance Functor Fresh where
            fmap f (Fresh g u) = Fresh (f . g) u

         -- one more primitive
        primGensym :: OI -> Int

        supplyInts :: OI -> Fresh Int
        supplyInts = \ u -> afresh primGensym u

        instance (Partible a, Partible b) => Partible (a, b) where
            part (u, v) = case (part u, part v) of
                           ((u1, u2), (v1, v2)) -> ((u1, v1), (u2, v2))

        instance (Partible a, Partible b) => Partible (Either a b) where
            part (Left u)  = case part u of (u1, u2) -> (Left u1, Left u2)
            part (Right v) = case part v of (v1, v2) -> (Right v1, Right v2)

        type M1 a = (Fresh Int, OI) -> a
        type M2 a = Either (Fresh a) OI -> a
         -- ...whatever suits the purpose

        class (Monad m1, Monad m2) => MonadCommute m1 m2 where
            mcommute :: m1 (m2 a) -> m2 (m1 a)

        instance (Partible a, Partible b) => MonadCommute ((->) a) ((->) b) where
            mcommute m = \ u2 u1 -> m u1 u2

So what qualifies as being partible?

A partible value can be used only once to generate new values that can be used for the same purpose. Think of a very large sheet of paper - new sheets can be made from it, other sheets can be made from those, etc, with the original sheet no longer in existence. Unlike paper sheets, partible values are intended to have no limits e.g. the result of applying supplyInts.

If its violation causes a runtime error, the use-once property of partible values aids in maintaining referential transparency in the effectful segments of a program; using another example from Wadler's paper minimally rewritten in Haskell syntax using OI values:

                  x = (primPutChar 'h' u `seq` primPutChar 'a' u)
                in x `seq` x

would trigger the error; the working version being:

                  x = (\ v -> case part v of
                                (v1, v2) -> primPutChar 'h' v1 `seq` primPutChar 'a' v2)
                  case part u of
                    (u1, u2) -> x u1 `seq` x u2

...rather tedious, if it weren't for Haskell's standard monadic methods:

                  x = primPutChar 'h' >> primPutChar 'a'
                in x >> x

Higher-order functions allows the manipulation of control e.g. Prelude.until in Haskell. As the definition of runDialogue shows, monadic types with visible definitions based on types of partible values may also allow the manipulation of control in ways beyond what the standard monadic methods provide.

Other references and articles:

  • An alternative approach to I/O, Maarten Fokkinga and Jan Kuper.
  • Functional Pearl: On generating unique names, Lennart Augustsson, Mikael Rittri and Dan Synek.
  • Can functional programming be liberated from the von Neumann paradigm?, Conal Elliott.
  • Non-Imperative Functional Programming, Nobuo Yamashita.
  • MTL style for free, Tom Ellis.
  • Functional I/O Using System Tokens, Lennart Augustsson.

Thank you to those who commented on early drafts of this document.

Atravers (talk) 04:31, 10 April 2018 (UTC)