variadic.hs

-- http://stackoverflow.com/questions/7828072/how-does-haskell-printf-work 

{-# LANGUAGE FlexibleInstances #-} 

class FooType a where 
    bar :: IO () -> a 

instance FooType (IO ()) where 
    bar = id 

instance (Show x, FooType r) => FooType (x -> r) where 
    bar s x = bar (s >> print x) 

foo :: FooType a => a 
foo = bar (return ()) 

main :: IO ()
main = foo 3 "hello" True