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Crossmark and Creole have broadly similar goals, and striking similarities in syntax details. However, in their current drafts they are not compatible, and it is not practical to write an implementation that will natively support both syntaxes, at least without an egregious hack such as a mode declaration at the top of the document.
However, with slight modifications to both drafts, it is at least technically possible to specify a bastard hybrid of the two, that will mostly correctly render markup cut-and-pasted from either source. Most of the gratuitous syntax variations result in multiple ways of accomplishing the same effect, with a corresponding increased chance of collision. Aside from one or two required changes, this hybrid can be implemented with no significant changes to either source spec, through the magic of ExtensibleByOmission. However, in present form, the hybrid is unpleasantly warty, and it is my hope that many, if not most, of these warts can be removed by finding consensus on the differences.
Required changes#
As discussed in AlternateLinkSyntaxProposal and other places, the link syntax in Crossmark draft four is strongly incompatible with the existing link-first pipe syntax common in the majority of Wikis, and adopted in Creole 0.4.
Ivan Krstić has suggested the solution of changing the Crossmark delimiter from pipe to right arrow ->. This change still needs more discussion, but I think it is sound.
Source linebreaks not forcing linebreaks is a required non-change in the Creole 0.4 spec (and a required change from 0.3). I think that changing back to 0.3 would be a dealbreaker for this unification effort.
Link syntax#
It seems unlikely to me that Creole will abandon the prevailing pipe markup, so the hybrid clearly needs to support both. Ghestalt currently implements this option.
CornerCase: what if the link contains both pipe and right-arrow? Ghestalt gives the right arrow precedence. Thus, a link containing a pipe is possible as text -> link|containing|pipe.
CornerCase: what if the link contains more than one right-arrow? Ghestalt interprets the last one as the delimiter, thereby allowing right arrows in the link text, but not in the URL. Rationale: right angle bracket character is exceedingly rare in URL's, and can be escaped as %3e.
One vs two character inline markup#
Creole 0.4 requires two characters to begin inline markup, specifically // for italics and ** for bold. Crossmark uses a single character, but adds additional word-boundary constraints.
Ghestalt simply implements both. This is, however, one of the more unpleasant warts. Clearly, both are not needed, and the presence of single character markup greatly increases the collision risk for people using double character markup. Therefore, I would like to see it resolved one way or another. I don't have a strong feeling for which one to ditch, though - a case can be made for either.
Raw, nowiki, pre, and monospace#
Treatment of raw spans differs significantly between the two approaches. Creole uses a single inline markup to represent both raw spans (suppressing the interpretation of markup characters) and monospace font styling, where Crossmark uses two separate mechanisms. Thus, these two forms of markup:
Creole uses {{{{{{three curly braces}}}}}} for pre, while Crossmark uses {{{`backticks`}}}
for monospace, and the {{{<raw>}}} macro for raw spans.
Creole uses `{{{three curly braces}}}` for pre, while Crossmark uses `<raw `backticks`>`
for monospace, and the `<raw <raw>>` macro for raw spans.
Implementing both has a reasonably low collision risk, but is certainly warty. Crossmark's has more flexibility because it allows raw spans without monospace font styling, and vice versa.
It should also be noted that the Crossmark fourth draft spec contains backslash as an escape character, but I don't get the sense this is thoroughly thought through. My guess is that it can be simply dropped in favor of the raw macro. If left in, there would be a collision with the proposed \\ syntax for forced line breaks.
Since the hybrid has considerably greater collision risk than either source, I expect that the use of raw spans to avoid collisions will be fairly common. Therefore, I would recommend at the very least that the raw span markup be adopted in most Creole-only implementations. Of course, having Creole adopt Crossmark's raw span markup would remove this wart entirely and add a desirable feature.
CornerCases: yeah baby.
Headers#
Crossmark uses a row of equals or dashes to indicate first and second level headers, as in Markdown. Third and fourth level headers are already compatible.
Implementing both is not a serious problem, but an argument can be made that it is the Crossmark syntax that is warty, because of its lack of consistency. Going between second and third level headers is a much more major editing operation than in Creole. I weakly recommend that Crossmark drop its Markdown-style headers, but will tolerate both.
Numbered lists#
Crossmark supports both the # and 1. syntaxes, a la Markdown. The only real problem here is the collisions.
Macro syntax#
I'm not 100% thrilled with Crossmark's macro syntax. For one, the single angle brackets collide head-on into HTML/XML markup, and there are quite a number of wikis (MediaWiki in particular) who support this kind of hybrid.
For two, the preceding-colon syntax for block macros doesn't clearly telegraph that the block macro namespace is the same as the inline macro namespace. I'd much rather have a visual resemblance between the two.
Thus, I suggest double angle brackets as the macro delimiter for both inline and block usage. My suggestion for block syntax is:
<<math:
a < b
>>
I realize that having a block-close delimiter is not as Pythonic as the Crossmark draft, but I think my suggestion will be clearer for more people.
Indented blocks#
Crossmark uses indentation to delimit indented blocks. Only one level of indentation is mandated in the draft. It's not clear to me what's supposed to happen if there is more indentation.
Since Creole doesn't have indented block syntax, we can simply adopt Crossmark's. Ghestalt currently uses colons to introduce indentation, but this can be easily enough changed.
CornerCase: mixing indentation and lists.
Tables#
I don't get the strong sense that either camp really knows what they're doing with tables yet. Probably multiple syntaxes will evolve, perhaps one very simple and one richer. It will be necessary to implement both, and to make sure they don't collide.
Way forward#
I'd like to get a bit of a sense from both communities about whether this unification effort is worthwhile, or whether I'm just pissing into the wind. Also, for the items above that are relatively easily changed (example: dropping backslash escaping in Crossmark in favor of consistently requiring raw macro), it would be nice to get tentative agreement. Then, I think, the next step is to implement the bastard hybrid of the two so people can play with it.
If there is then a general sense that it's worth pursuing, I'd like to see focussed discussion on how to remove or at least mitigate the warts. I also wouldn't be surprised if tracking down all the CornerCases revealed other areas needing attention.
If, on the other hand, we decide that the goals are different enough that we really do need two incompatible standards, then I'll need to choose one or the other for the Ghilbert project -- more likely Crossmark than Creole, but hopefully I won't end up having to make that choice.