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.

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) -> (\ x -> x `seq` k x u2) (m u1)
            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 -> foldr seq () (yet (\ l -> zipWith respond (d l) (parts u)))


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


         -- to be made into an abstract data type...
        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)

        data Some a = Only a | More a (Some a)

        instance Partible a => Partible (Some a) where
            part (Only u)    = case part u of
                                 (u1, u2) -> (Only u1, Only u2)
            part (More u us) = case part u of
                                 (u1, u2) ->
                                   case part us of
                                     (us1, us2) -> (More u1 us1, More u2 us2)

        type M1 a = (Fresh Int, OI) -> a
        type M2 a = Either (Fresh a) OI -> a
        type M3 a = Some (Either (Fresh Char) (Fresh Int)) -> 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 = \ v u -> m u v


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 can help to maintain referential transparency in the effectful segments of a program; using another example from Wadler's paper minimally rewritten in Haskell syntax using OI values:

         \ u -> let
                  x = (primPutChar 'h' u `seq` primPutChar 'a' u)
                in x `seq` x


would trigger the error; the working version being:

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


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

                let
                  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.

The patches for an initial implementation in GHC are available:

Other references and articles:

  • Functional Pearl: On generating unique names, Lennart Augustsson, Mikael Rittri and Dan Synek.
  • Non-Imperative Functional Programming, Nobuo Yamashita.
  • Functional I/O Using System Tokens, Lennart Augustsson.
  • I/O Trees and Interactive Lazy Functional Programming, Samuel A. Rebelsky.
  • Arborescent data structures and lazy evaluation: A new approach to numerical problems, Manuel Carcenac.

See also:


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

Atravers 04:31, 10 April 2018 (UTC)