[Haskell-cafe] Accepting and returning polyvariadic functions

[Will Jones]

Hi Ryan,

Thanks for the reply. The specification I've given is just to illustrate the
kind of relationship I'm trying to establish between the types of the
argument and the result. In reality the type of the argument function is
something a little more usable; you could generalise it with type families
vis:

class HasDual t where
  type Dual t

class VTuple ia ir a r | r -> a where
  vtuple :: (ia -> ir) -> IO (a -> r)

-- m is some monad.
instance (HasDual t, Dual t ~ dual) => VTuple dual (m a) t (t, ())

etc.

I hope that clears things up; to be honest I'm not sure it's relevant -- the
more I look at it the more I'm stumped.

Cheers,
Will

On Wed, Aug 11, 2010 at 7:08 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:

> There's no (safe) way to go from
>
> a -> IO b
>
> to
>
> IO (a -> b)
>
> which is part of what vtuple does.
>
> Consider
>
> foo :: Int -> IO String
> foo 0 = return "zero"
> foo _ = launchMissles >> return "fired!"
>
> How would you implement foo2 :: IO (Int -> String) with the same behavior?
>
> You can't; you would somehow need to know the argument the function
> was called at, and when it was going to be called, to implement foo2.
>
> So I think you need a better specification!
>
>  -- ryan
>
> On Wed, Aug 11, 2010 at 8:50 AM, Will Jones <will at sacharissa.co.uk> wrote:
> > Hi all,
> >
> > I'm trying to write a function (I'll call it `vtuple' for lack of a
> better
> > name)
> > that returns a function that itself returns multiple arguments in the
> form
> > of a
> > tuple. For example:
> >
> >> {-# LANGUAGE FlexibleInstances #-}
> >> {-# LANGUAGE FunctionalDependencies #-}
> >> {-# LANGUAGE MultiParamTypeClasses #-}
> >
> >> f :: Int -> IO ()
> >> f = undefined
> >
> >> g :: Int -> Int -> IO ()
> >> g = undefined
> >
> >> h :: Int -> Int -> Int -> IO ()
> >> h = undefined
> >
> > vtuple f :: IO (Int -> (Int, ()))
> > vtuple g :: IO (Int -> Int -> (Int, (Int, ())))
> >
> > I've tried to type vtuple using a type class; my current effort is
> something
> > like:
> >
> >> class VTuple ia ir a r | r -> a, a -> ia where
> >>   vtuple :: (ia -> ir) -> IO (a -> r)
> >
> >> instance VTuple Int (IO ()) Int (Int, ()) where
> >> --vtuple :: (Int -> IO ()) -> IO (Int -> (Int, ()))
> >>   vtuple = undefined
> >
> >> instance VTuple ia ir a r
> >>       => VTuple Int (ia -> ir) Int (a -> (Int, r)) where
> >
> >> --vtuple :: (Int -> ia -> ir) -> IO (Int -> a -> (Int, r))
> >>   vtuple = undefined
> >
> > But this is problematic, since arrows creep in:
> >
> > For one argument (fine):
> >   vtuple :: (Int -> IO ()) -> IO (Int -> (Int, ()))
> >
> >> vf :: IO (Int -> (Int, ()))
> >> vf = vtuple f
> >
> > For two arguments (also fine):
> >   vtuple  :: (Int -> Int -> IO ())
> >           -> IO (Int -> Int -> (Int, (Int, ())))
> >
> >> vg :: IO (Int -> Int -> (Int, (Int, ())))
> >> vg = vtuple g
> >
> > For three (noooo!):
> >   vtuple  :: (Int -> Int -> IO ())
> >           -> IO (Int -> Int -> (Int, (Int -> (Int32, (Int32, ())))))
> >
> > And so on. I've thought about it and it seems impossible to solve this
> > problem
> > -- you keep needing to ``split'' the function type one arrow further on.
> Is
> > this a job for Template Haskell or is there a solution I'm missing here?
> > Note
> > that I'd also like to use types other than Int, but I don't think this is
> > the
> > primary complication here (touch wood).
> >
> > Any help much appreciated, thanks,
> > Will
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > Haskell-Cafe at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell-cafe
> >
> >
>
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