Hello Will,
2010/8/11 Will Jones
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