Close # 27.
Backtracking user state can be achieved via combination of ‘StateT’
monad transformer and ‘ParsecT’:
StateT StateType (ParsecT s m a)
This user state can be more flexible. This fact renders current built-in
user state redundant.
To help work with this new approach (combining monad transformers more
freely) we introduce ‘MonadParsec’ MTL-style type class. All tools that
come with Megaparsec library were modified to work smoothly with any
instance of ‘MonadParsec’, not only ‘ParsecT’.
Various languages may vary in how hexadecimal and octal literals should
be prefixed. Following the spirit of the new lexer we leave this to
programmer to decide.
Eliminated ‘Text.Megaparsec.Language’ module because at this point it is
clear that already existing definitions are of little use in
Megaparsec. I started writing “default” language definition in
‘Text.Megaparsec.Lexer’.
At this point it should be possible to parse languages where indentation
matters, although we will need to provide more helpers to make it
easier.
