Johan Tibell wrote:

   insertWithKey' :: Ord k => (k -> a -> a -> a) -> k -> a -> Map k a ->
Map k a
   insertWithKey' f kx x t0 = kx `seq` go t0
     where
       go t = case t of
         Tip -> singleton kx $! x
         Bin sy ky y l r
             -> case compare kx ky of
                  LT -> balance ky y (go l) r
                  GT -> balance ky y l (go r)
                  EQ -> let x' = f kx x y in seq x' (Bin sy kx x' l r)

As a style issue, I'd suggest using a pattern instead of the first case:

   go Tip = singleton kx $! x
   go (Bin sy ky y l r) =

       case compare kx ky of
       LT -> balance ky y (go l) r
       GT -> balance ky y l (go r)
       EQ -> let x' = f kx x y in seq x' (Bin sy kx x' l r)