[Haskell] Implicit parameters

[Per Larsson]

When using implicit parameters I have noticed (at least for me) a rather 
puzzling behaviour with GHC and Hugs.

Given the declarations

data Env = Env {numlines :: Int, numcols  :: Int}

initEnv = Env {numlines = 0, numcols = 1}

withEnv :: ((?env :: Env) => IO a) -> IO a
withEnv io = let ?env = initEnv in io

I can write code like:

main = withEnv (do
  let lines = numlines ?env
  putStrLn ("Initial number of lines is " ++ (show lines)))

which works as expected, but the version below

main = withEnv $ do
  let lines = numlines ?env
  putStrLn ("Initial number of lines is " ++ (show lines))

is not accepted by either GHC or Hugs! Is this a bug or have stumbled into a 
context where it actually matters if you use '$' or explicit grouping with 
parentheses?

Per Larsson