-- Inspired by blaze2007@FuncProgram.smth.org

data Id a = Id { unboxId :: a } deriving Show

instance Monad Id where
  return = Id
  (Id x) >>= k = k x

mfix f =
    (mfix f) `bind` f
-- eager bind forces its first argument to match the pattern (Id x), thereby enforcing sequence
        where bind = (>>=)
-- lazy bind delays the unboxing computation until needed
--        where bind x k = k $ unboxId x
-- or a lazy pattern match
--        where bind ~(Id x) k = k x

mrec f = return (\i ->
  if i == 0
    then 1
    else i * f (i - 1)
  )

mfac :: Id (Int -> Int)
mfac = mfix mrec

-- compute factorial
main = print $ unboxId mfac 3