Support Pandoc fancy-lists

[SuibianP]

Description

Some aspects of Pandoc’s Markdown fancy lists are also accepted in org input, mimicking the option org-list-allow-alphabetical in Emacs. As in Org Mode, enabling this extension allows lowercase and uppercase alphabetical markers for ordered lists to be parsed in addition to arabic ones. Note that for Org, this does not include roman numerals or the # placeholder that are enabled by the extension in Pandoc’s Markdown.
The fancy_lists extension also allows ‘#’ to be used as an ordered list marker in place of a numeral:

#. one
#. two

Note: the ‘#’ ordered list marker doesn’t work with commonmark.

Benefits

This would allow using roman numerals and alphabets for ordered list bullets, without having to specify type or list-style-type, which is quite common for nested lists.

Solution Idea

Another extension to parse additional bullet types besides numbers.

[facelessuser]

While it is an interesting idea, it is not likely something I would use. Pydown Extensions is not meant to be a one-stop shop for all Python Markdown extensions, so I am often very selective about what I add. Generally, if I am to maintain an extension, it is usually one I am interested in personally using as well. In short, I need to be invested in the extension for me to take on supporting it.

Let me sit on this and think about it. It is possible already to wrap blocks in a div with a special class that can use CSS to target them and change the style, but I do get that you want a more convenient approach. I would just need to decide if this is something I'm willing to get behind and take on long term maintenance of.

@gir-bot add P: maybe

[facelessuser]

I'm exploring the possibility via prototype, but I am not sure if I'm all in yet.

One thing to note, this behaves very Pandoc-like currently in that:

• It supports lists of type #., 1., a., i., #), 1), a), i), (#), (1), (a), and (i). Also capitalized versions.
• Each list type is treated as its own list type, both uppercase and lowercase are treated unique. If starting a new list type, a new list is created and not appended to the previous list.
• The number (or symbol) indicating the start of the list is taken into account and the list will start at that value.
• pymdownx.saneheaders is required to be enabled for # style lists to work as otherwise they will be identified as headers.
• I'm not sure if I'm 100% sold on () style lists or # style lists, but implemented everything to understand how it would behave.
• Alpha lists are currently contained to a and do not allow aa.
• Obviously, there are rules to determine when some roman numerals are assumed roman numerals or alpha lists. Generally new lists are determined to be roman if they start with i or I or are wider than one character.
• Roman numerals are checked for validity and are ignored if they are not valid.
• NONE OF THIS IS STRICTLY TESTED CURRENTLY. I've made assertions above, but I have not extensively tested any of this.
• Lastly, all of this is subject to change if/when I decide to support this.

Here is the prototype: https://gist.github.com/facelessuser/a6613237425b78e843c32268ef464e2e

[facelessuser]

Linking another recent discussion: #2454.

[facelessuser]

Updated prototype to drop (marker) format. I don't feel that complicating patterns to try and support this when even CommonMark doesn't support it. CommonMark only supports 1. and 1). Python Markdown is not a CommonMark parser, but if they don't even support that, and there is little to no added benefit in doing so except yet another list format, so I'm not going to bother.

I also updated the prototype to follow the Pandoc rule of requiring uppercase, single letter list markers to need at least two spaces after the marker. I would rather keep this rule (which I am sure was added for some reason) and possibly relax it in the future if sufficient testing deemed it unnecessary.

[facelessuser]

Also fixed some bugs

[facelessuser]

I should note that #. and #) are also unique formats, but I may keep them if I move forward with this. I can see the usefulness of generic, numerical list markers.

[facelessuser]

This is what I'm thinking so far. By default, FancyList would extend list formats such that 1. and 1) would work. These formats would be added with functionality similar to Sane Lists but with the added behavior that swapping list types 1. vs 1) would force a new list.

All other formats are probably not supported syntactically in many editors. It's possible not everyone wants such behavior, and might actually control this automatically with CSS when lists are nested, but that's okay. I think additional list formats would be opt-in. You could enable one or all of them if you desire.

Someone using MkDocs might do this to enable all formats; decimal cannot be toggled and is always enabled.

markdown_extensions:
  - pymdownx.fancylists:
      addtional_ordered_styles:
      - alpha
      - roman
      - generic

[facelessuser]

Okay, I ended up leaving all the list types enabled by default. Instead, people can opt out of certain list formats.

I have PR with testing available here: #2455

[facelessuser]

Okay, I think I may be at a point where I am ready to stop fiddling. There are two inherient conflicts that exist when both alphabetically ordered and Roman numeral ordered lists are enabled.

Alphabetical lists cannot start/restart a list with I (or with the lowercase) and Roman numeral lists cannot start/restart a list with M, D, C, L, X, and V (or the lowercase).

This issue also exists in Pandoc, but mitigation is fairly straight forward. We relax the Roman numerals a little and the use can use things like VIIIII for X etc. Browsers won't render the list with VIIIII, but will render it more strictly as X. I think this is a reasonable work around.

viiiii. item 10
xi. item 11

<ol start="10" type="i">
<li>item 10</li>
<li>item 11</li>
</ol>

Alphabetical lists don't have such clean mitigation, but if you have the HTML Block extension enabled, you can work around the I case pretty easily. You could use this approach for the above Roman numeral case as well and use the more proper numerals.

/// html | ol[start="9"][type="a"]
i. item i
j. item j
///

<ol start="9" type="a">
<li>item i</li>
<li>item j</li>
</ol>

While maybe not as clean as the the Roman numeral case, it works fine and this case won't likely occur all that often. Pandoc has the same issue, but I don't know if they have a workaround.

Some may assume you can do the following while using md_in_html, but it doesn't work so well based on how md_in_html handles the elements in the HTML tree. So this doesn't work:

<ol start="9" type="i" markdown>
i. item i
j. item j
</ol>

You get something more like this, which is not desirable:

<ol start="9" type="i">
<ol type="i">
<li>item i</li>
</ol>
<ol start="10" type="a">
<li>item j</li>
</ol>
</ol>

[facelessuser]

I think I'm now open to people willing to test the PR and give feedback: #2455.

[facelessuser]

Apparently, I implemented # lists wrong. It appears they should take on the style of whatever list they are nested under, and if they are in an ambiguous list, they assume normal numbers. I will have to adjust that behavior.