Thanks to all for your replies!

I asked the question because I often make this kind of transformations (please don't mind the non-sensical example):

test :: Bool -> IO ()
test foo = do
   bar <- case foo of
      True ->  return "Foo"
      False -> return "Bar"
   return ()
into

test :: Bool -> IO ()
test foo = do
   let bar = case foo of
        True ->  "Foo"
        False -> "Bar"
   return ()
And was wondering why can't I maintain the initial (and nicer) indentation.

But since let allows for several bindings, it make sense...

Best,
Corentin