‘ghc-exactprint’ (or perhaps lexer of GHC itself) does a fairly poor job at
associating comments with elements of AST. In many cases the result is not
what you'd expect. We ran into insuperable problems with that to the effect
that correct comment placement were impossible.
The new approach is to exploit the raw position information provided by the
GHC lexer, that is, spans attached to AST elements and comments. This
allowed us to place comments in output in a very precise and satisfactory
fashion.
