[Haskell-cafe] Accepting and returning polyvariadic functions

[Will Jones]

Hi Tillmann,

That's worked a treat -- thanks ever so much :)

Will

On Wed, Aug 11, 2010 at 7:50 PM, Tillmann Rendel <
rendel at mathematik.uni-marburg.de> wrote:

> Will Jones wrote:
>
>>  > 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; [...]
>>
>>
>> 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.
>>
>
> So you need to use recursion to handle the arbitrary deeply nested
> arrows in the type of vtuple's argument. I tried it with type families,
> but I don't see a reason why functional dependencies should not work.
>
>    {-# LANGUAGE FlexibleInstances, TypeFamilies #-}
>    module VTupleWithTypeFamilies where
>
> We use two type families to handle the two places where the result type
> of vtuple changes for different argument types.
>
>    type family F a
>    type family G a r
>
> So the intention is that the type of vtuple is as follows.
>
>    class VTuple a where
>      vtuple :: a -> IO (G a (F a))
>
> The base case:
>
>    type instance F (IO ())   = ()
>    type instance G (IO ()) r = r
>
>    instance VTuple (IO ()) where
>      vtuple = undefined
>
> And the step case:
>
>    type instance F (a -> b)   = (a, F b)
>    type instance G (a -> b) r = a -> G b r
>
>    instance VTuple b => VTuple (a -> b) where
>      vtuple = undefined
>
> A test case:
>
>    f :: Int -> Bool -> Char -> Double -> IO ()
>    f = undefined
>
>    test = do
>      vt <- vtuple f
>      return (vt 5 True 'x' 1.3)
>
> Testing it with ghci yields the following type for test, which looks
> good to me.
>
>    test :: IO (Int, (Bool, (Char, (Double, ()))))
>
> HTH, Tillmann
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20100812/05a65dfd/attachment.html