Make source positions a parser option?

[jgm]

Instead of handling it with typeclasses.
This would allow simpler typeclasses: Html, Pandoc.
It might also allow us to improve performance by avoiding the work of storing and computing ranges.

[jgm]

One idea would be to add an Options data structure

data Options m =
  Options{
    optIncludeSourceRanges :: Bool,
    optWarn :: Warning -> m (),
    optAddToSourceMap :: Text -> SourceRange -> m ()
  }

optWarn would allow us to issue warnings for things like duplicate references in a flexible way (#48).
optAddToSourceMap would allow us to construct source maps in a flexible way (#33).
Putting these in options may be more flexible than using typeclasses.
Defaults could be False, \_ -> return (), and \_ _ -> return ().

[jgm]

We could use a reader monad transformer to make these available in block and inline parsers.

[alerque]

I don't really care how they get there as long as the option is there! We're really looking forward to having a source maps available for vim-pandoc! We have a working prototype right now of a new syntax system using an external source map provided by pulldown-cmark which reports start and stop byte ranges. This is working great but we really need the full range of Pandoc extensions, not just CommonMark.

[jgm]

The option is there already (as well as the ability to create a source map). This was just an idea about an alternative way to put it there.

[jgm]

@alerque for a demo of the source map functionality, you can do commonmark --highlight -xall README.md. This does syntax highlighting with all the extensions enabled. To see how this is done see the code for commonmark-cli.