Although Markdown is good enough for 📓 Zettelkasten, wikis and such (cf. Emanote), for writing outline content Org Mode in Emacs shines the best.
For parsing org files in Haskell, checkout org-parser (also provides Heist templating) by lucasvreis … as well as the Ema app abacateiro by the same author.
The development version (not 0.4) supports Pandoc for Markdown. What this means is that it is possible to support other markup formats like reST, Org Mode, Asciidoc, etc. All they must have in common is a syntax (like the markdown autolinks) that will produce the appropriate Pandoc AST node for neuron to process.
It is worth emphasizing that I designed Ema to facilitate a new model of application development – one where the data is managed and edited outside of the application itself (such as in text editors, or even a database), but the view of the data is provided by the application. Neuron is a great example of that (and it might well be eventually rewritten on top of Ema; EDIT: indeed it is). Another example is writing a diary / journal view on top of Org Mode daily notes, which is what I’m exploring in the orgself project that also uses Ema. A third example is to take a OPML file and present a daily-digest view of new content from the RSS feeds; the application would provide only a “view” into your data, which is a XML file that you may edit however you see fit. I like this model of apps, because the data and the data format remains under my control, all the while yet I’m not limited when it comes to presentation of that data, and I think the web is a great platform for the later.