Support for links like [this]?

[rudymatela]

Consider the following markdown file links.md:

### Reference-style links

This is a link to the [Python](https://python.org) website.

This is a link to the [Python] website.

This is a link to the [Python][] website.

[Python]: https://python.org

I can compile it with markdown2 as follows:

markdown2 links.md > links.html

When I open the HTML, this is what I get:

1. The first link to python.org renders as "Python";
2. The second link to python.org literally renders as [Python];
3. The third link renders as "Python"

Is writing links as just [Python] supported? Or are only links with [Python][] supported?

Some other markdown engines (e.g.: GFM) support [Python]. Could this be added to markdown2 as an extra to be activated with -x?

I actually initially thought that [Python] was standard markdown. Upon looking at the syntax specification it seems only [Python][] would be standard. It nevertheless seems to be that the former is quite common, so I suggest adding it as an extension. (If you think this is a good idea of course.)

Debug info

$ python --version
Python 3.12.4

$ markdown2 --version
markdown2 2.5.0

I also tried with:

markdown2 -x footnotes links.md > links.html

[nicholasserra]

Interesting, I wouldn't be opposed to it. Assuming there's a good name for the extra lol.

[rudymatela]

@nicholasserra GFM calls [this] a shortcut reference link. I know that some other engines support this, but I don't know what they call it. "Shortcut reference link" seems appropriate enough.

[this]: https://example.com

In terms of the "extra" name on --help, you could have it called:

1. shortcut-reference-link
2. short-links
3. link-shortcuts
4. link-short
5. link-shortref

Of the above, I personally would prefer 4 (link-short) as it would appear beside the related link-patterns in markdown2 --help. It is also brief. One suggestion for the help message would be "link-short: allow shortcut reference links, not followed by [] or a link label"

$ markdown2 --help
usage: markdown2 [PATHS...]

...
...
...
* html-classes: ...
* link-patterns: Auto-link given regex patterns in text (e.g. bug number
  references, revision number references).
* link-short: allow shortcut reference links, not followed by [] or a link label.
* markdown-in-html: ...
...
...
...

[nicholasserra]

It seems like we support the full reference links right now, without an extra. Wondering if we should just support the short without an extra too. This is just from a quick glance at the _do_links method. I'll have to do some testing.

[Crozzers]

I would agree with that approach.

I took a look at implementing this as a new style extra class and it's tricky to nail down. I settled on detecting snippets like [Python] and just adding the [] after them so that _do_links can process it normally. However, this didn't work particularly well and there were loads of edge cases.
Unless we spun the link processing out into a seperate link processing class (like the ItalicAndBoldProcessor) I don't think this approach is feasible.

I think making this part of _do_links is probably the most straightforward approach, and I don't mind if we gate it behind an extra or if we make it the default behaviour