Hi Tillmann, That's worked a treat -- thanks ever so much :) Will On Wed, Aug 11, 2010 at 7:50 PM, Tillmann Rendel < rendel@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