[Haskell-cafe] Accepting and returning polyvariadic functions
Ryan Ingram
ryani.spam at gmail.com
Wed Aug 11 14:08:00 EDT 2010
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
>
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