Vim folds out of sync with syntax/incorrectly inserted

Question

I have (something equivalent to) the following in a syntax file:

syn sync clear
syn sync fromstart
syn region fooBlock start="\<interface\>" end="\ze\<implementation\>" fold
syn region fooBlock start="\<implementation\>" end="\%$" fold

Syntax highlighting works just fine (as one can see by adding e.g. matchgroup=Keyword). Folding also basically works, but when I insert a new fold (by typing e.g. implementation, vim starts the fold one line later than it should. Example:

-  interface
|  l1
|  l2
   implementation
-  l3
|  l4
|  l5

When typing this, obviously I would expect the second fold to start one line earlier. Typing zx corrects that, as does saving and reopening the file or typing :syn sync fromstart.

What can I do to have vim correctly insert the folds as I type?

I already looked in :help fold-syntax, but as shown, :syn sync fromstart in the syntax file does not work.

Maybe there is a better way than using \ze (or the me=s-1 suffix, which also yields the same result) to highlight/fold multipart regions.

Answer 1

You're trying to express the fact that the interface section ends one line before an implementation line. Syntax regions (and apparently especially folding) work best when there's a definite region start and region end marker (like begin and end).

I've tried to make your example work by adding \n to the pattern, but that also didn't work. I guess to get reliable folding, you need to switch from syntax-folding to a 'foldexpr', like this:

:setlocal foldmethod=expr foldexpr=getline(v:lnum)=='interface'\|\|getline(v:lnum)=='implementation'?'>1':'='