This pull request implements most of currently missing types of expressions,
with the exception of:
* Do notation
* List comprehensions (list comprehensions are considered a kind of do
notation in GHC)
* Arrow notation
* Template Haskell
Most expressions are pretty printed in an uncontroversial manner, preferably
similar to existing syntax.
It was decided that we're going to make the project compatible with just one
GHC version at a time. Right now this version is going to be 8.6.4.
A small refactoring included, plus support for the “deriving via” feature.
‘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.
