Hello Will, 2010/8/11 Will Jones <w...@sacharissa.co.uk>: > 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: > > vtuple f :: IO (Int -> (Int, ())) > vtuple g :: IO (Int -> Int -> (Int, (Int, ())))
If we drop the IO (as pointed out by Ryan Ingram), vtuple seems weird - the only sensible function of the type "Int -> Int -> (Int, (Int, ()))" is a function that collects its arguments and returns them in a tuple, so it doesn't touch the input function g at all, it only cares about g's arity. Here's the solution: > vtuple f = eat (arity f) `mcomp` hListToTuple > class HListToTuple l r | l -> r where > hListToTuple :: l -> r > > instance HListToTuple HNil () where > hListToTuple _ = () > > instance HListToTuple xs ys => HListToTuple (HCons x xs) (x,ys) where > hListToTuple (HCons x xs) = (x,hListToTuple xs) Rest of the code (functions eat, arity and mcomp) is presented here: http://paczesiowa.blogspot.com/2010/03/generalized-zipwithn.html Regards, Bartek Ćwikłowski _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
