[Haskell-cafe] Accepting and returning polyvariadic functions

[Will Jones]

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
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