[pmwiki-users] On the order of Markup

[Patrick R. Michaud]

On Thu, Mar 23, 2017 at 11:21:06AM -0400, Peter Kay wrote:
> textvar:       <split           B>><
> nl0              <split           B>><
> Sidebar          <split           B>><
> input+sp         <split           B>><
> nl1              >nl0             B>><>
> [...]
> I am wondering:
> 
> Is there any particular benefit to this non-intuitive approach to
> ordering markup?  A tree structure strikes me as the more intuitive
> way to order things, with each node having "before" and "after"
> branches.  Then MarkupToHTML can traverse the tree and "nl1" will
> happen immediately after "nl0".

Technically speaking, PmWiki *is* using a tree structure for ordering --
it's just not a binary tree.  The <'s and >'s are specifying ordering, 
and it's always possible to insert nodes before or after an existing one.

The problem with a strictly binary tree structure (as described with
"each node having 'before' and 'after' branches") is that it's not
precisely clear what to do when you need to insert something "before" a node
that already has a "before" branch.  

For example, if we have a node 'B' that already has 'A' before and 
'C' after, and we have a rule that needs to go before 'B', it's not
entirely correct to simply assume that it should go after 'A'... 
especially if "A" was supplied by a recipe that also assumes it
will be happening immediately before "B".  And PmWiki can't know
the necessary ordering in advance, since these are coming from
recipes, and I didn't want the order in which recipes are included
to be the driving factor of markup rule order.

So, the approach PmWiki takes is to enable recipes to specify
whatever level of detail is necessary for ordering.  If you specify 
"<split", you're guaranteed that your rule will occur before the split 
rule, along with all of the other rules that specified "<split".  
If the ordering of two of those rules ends up being important 
(as happened in your case), then the way to resolve it is to specify 
the ordering of one of the rules in terms of the other... which is 
exactly the solution you came up with.  

Pm