[Haskell-cafe] Continuable and serializable parsers.

[Felipe Lessa]

On Sun, Dec 27, 2009 at 08:20:53AM -0500, Edward Kmett wrote:
> On Fri, Dec 25, 2009 at 8:31 PM, Felipe Lessa <felipe.lessa at gmail.com>wrote:
> > However you're right in a sense, you can't use this scheme to
> > serialize any functions taking functions, like
> >
> >    something :: (a -> Parser a) -> a -> Parser a
> >
> > because
> >
> >    data MyParser = FunSomething (a -> MyParser) a
> >
> > wouldn't be serializable.  Well, maybe if your parsers were
> > arrows... :)
> >
>
> You can't use full arrows because you need to be able to lift an arbitrary
> function into an arrow type, which precludes meeting the serialization
> criterion.

Hmmm... but, assuming a preprocessor, you probably would be able
to transform this code:


  number :: MyArrow String Int
  number = ...

  showMe :: MyArrow Int String
  showMe = ...

  doSomething :: Int -> Maybe Int
  doSomething = ...

  f = showMe . arr doSomething . number


...into this code:


  data Action a b where
    Number          :: Action String Int
    ShowMe          :: Action Int String
    ArrDoSomething  :: Action Int (Maybe Int)
    (:*:)           :: Action b c -> Action a b -> Action a c

  -- repeat definitions of number, showMe, doSomething

  f = ShowMe :*: ArrDoSomething :*: Number

  interpret :: Action a b -> MyArrow a b
  interpret Number         = number
  interpret ShowMe         = showMe
  interpret ArrDoSomething = arr doSomething
  interpret (f :*: g)      = f . g

  instance Binary (Action a b) where
    put Number         = putWord8 1
    put ShowMe         = putWord8 2
    put ArrDoSomething = putWord8 3
    put (f :*: g)      = putWord8 4 >> put f >> put g
    get = do i <- getWord8
             case i of
               1 -> return Number
               2 -> return ShowMe
               3 -> return ArrDoSomething
               4 -> (:*:) <$> get <*> get

The only part missing here is being able to run only a small part
of the arrow's computation and then return another Action.

Is my example contrived?  Am I missing something? :)

Thanks!

--
Felipe.