On Wed, 11 Feb 2015, Simon Peyton Jones wrote:
($) has its own *typing rule*; it does not have a special type. It's very ad hoc, but ($) is used so much to decrease parens that (e1 $ e2) is almost special syntax!
I already wondered why runST $ do foo bar worked in some versions of GHC and not in others. I guess it is better to tell the programmer the truth, i.e. remove the special treatment of ($). If you save a programmer from wondering about why "runST $ do ... " does not work with the special typing rule for ($), the programmer will later wonder why (.) does not work or why his privately defined function application operator (e.g. for reversed order of argument and function) will not work. I think that I stay with "runST (do ...)" in order to be compatible with several GHC versions.