AlternateLayoutRule

[John Meacham]

After some work, I have replaced jhcs front end with one using an
AlternateLayoutRule and it passes ghc's parsing regression tests. I
had to make some modifications so the code so it isn't as pretty as
I'd like, I'll see if I can distill the essence out into something
concrete.

Incidentally, I found a particularly troublesome case for the current
ghc rule out in the wild during my testing:

-- Lining up |'s and indenting new blocks 4 spaces. A fairly common practice.
bar x y = case x of
    Just z | y > z*2 -> case y of
        _ -> y
           | y > z -> y*10
    _  -> 1

bar (Just 2) 3  => 30

--add a constant true guard that should be a nop
bar x y = case x of
    Just z | y > z*2 -> case y of
        _ | True -> y              <=== added a no-op guard
           | y > z -> y*10
    _  -> 1

bar (Just 2) 3  => 1

The pattern match was fairly complicated making what happened pretty obscure.

One of the main benefits I have found is being able to process the
token stream between the lexer and parser, which drastically
simplified my parser making it almost shift/reduce conflict free. I
turn (+) and  case into single lexemes and turned reservedids into
varids for the keywords of unenabled extensions.

        John

-- John Meacham - http://notanumber.net/