From the report(s):
An ordinary comment begins with a sequence of two or more consecutive dashes
(e.g. --) and extends to the following newline. The sequence of dashes must
not form part of a legal lexeme. For example, “-->” or “|--” do not begin a
comment, because both of these are legal lexemes; however “--foo” does start
a comment.
Haskell98: http://goo.gl/CucLL
Haskell2010: http://goo.gl/1OYzN