Re: syntax match bug: 'keepend' not officially supported, but required in some cases

On Mon, Sep 19, 2016 at 9:05 AM, Bram Moolenaar <Bram@moolenaar.net> wrote:
>
> Brett Stahlman wrote:
>
>> The syntax documentation suggests that the 'keepend' argument applies
>> only to syntax regions, not syntax matches. But if I highlight a
>> buffer containing only the following line...
>>
>> ABCDE
>>
>> ...with the following syntax definitions...
>>
>> syn match A /A/
>> syn match Inside /[A-Z]\+/ transparent contained containedin=A
>> hi A guifg=blue
>>
>> ...the match group "A" extends (apparently incorrectly) from A through
>> E, though its pattern matches only the "A".
>
> This is correct, the inner match continues, causing the outer match not
> to end. :syn match does not have "keepend" but that doesn't mean it
> always keeps its end.

But syn-match *does* have 'keepend' - at least, it seems to be
respecting it (though as I mentioned, neither the docs nor the vim
syntax seem to think it should). Without 'keepend', group "A" extends
until "Inside" ends, at which point, "A" ends too. It's as though the
syntax engine knows A's pattern match has ended, but the presence of a
contained group is keeping the containing group active. I added
'keepend' because I wanted the end of "A" to determine the end of
"Inside", not the other way around, and the 'keepend' argument to
syn-match does indeed give the desired behavior. In fact, not having a
way to specify this behavior would be a real limitation if contained
groups can extend the containing syn-match group. Is :syn match's
current handling of 'keepend' not by design? I can fix the issue I was
having by using 'keepend', but don't want to do so if syn-match is
honoring 'keepend' only by accident...

Thanks,
Brett Stahlman

>
>> Stranger still, one
>> additional nested "Inside" group is recognized in each successive
>> letter. I.e., "Inside" is contained within "Inside", in spite of the
>> `containedin=A'. Thus, running synstack on successive characters, I
>> see...
>>
>> -- cursor on A --
>> A
>> Inside
>> -- cursor on B --
>> A
>> Inside
>> Inside
>> -- cursor on C --
>> A
>> Inside
>> Inside
>> Inside
>
> That looks like a bug.
>
>> I can fix the problem by adding the 'keepend' argument to the
>> definition for match group "A", but the "keepend" is highlighted as an
>> error by the vim syntax, and the help on :syn-match gives me no reason
>> to believe the argument should be supported.
>>
>> Here are the results of synstack() with the following, "fixed" definition for A:
>> syn match A /A/ keepend
>>
>> -- cursor on A --
>> A
>> Inside
>> -- cursor on B --
>> -- cursor on C --
>>
>> Is 'keepend' meant to be supported for syn-match? The purpose of
>> 'keepend' is to prevent a nested group from obscuring a match with a
>> containing group's "end pattern". Although a match group has no
>> explicit end pattern, the principle of checking for the end of the
>> match pattern certainly still applies, so I suppose it makes sense
>> that 'keepend' would be accepted for both syn-match and syn-region. If
>> that is the intent, then the documentation needs fixing.
>>
>> As for the infinitely nested "Inside" groups, I can't account for
>> that, and suspect it's a bug, but perhaps I'm misunderstanding
>> something about contained and containedin?
>
> Something seems wrong. Also try this:
>
> syn match A /A/
> syn match Inside /[A-Z]/ transparent contained containedin=A
> hi A guifg=green ctermfg=green
>
> Now every fourth character is highlighted. Hmm.
>
>
> --
> Facepalm statement #8: "Drive faster, the petrol is running out"
>
> /// Bram Moolenaar -- Bram@Moolenaar.net -- http://www.Moolenaar.net \\\
> /// sponsor Vim, vote for features -- http://www.Vim.org/sponsor/ \\\
> \\\ an exciting new programming language -- http://www.Zimbu.org ///
> \\\ help me help AIDS victims -- http://ICCF-Holland.org ///

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