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
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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