On 2020-10-26 at 07:21 -07, Nicolas Goaziou <[email protected]> wrote...
> Ken Mankoff <[email protected]> writes:
>
>> I question if this is possible because EBNF is for context-free
>> grammars, but I *think* Org syntax is context-free.
>
> It's not as explained in a footnote in the Org syntax document.
Yes, I meant to write that I think Org syntax is maybe *not* context-free, and
therefore EBNF can't capture all of it. But it could still be very helpful and
capture most of it.
But the more I think about it, the more I think Org may be context-free.
For the footnotes, I'm not sure that "(1) In particular, the parser requires
stars at column 0 to be quoted by a comma when they do not define a headline"
violates context. An "*" in the first column defines a header. It can be
escaped by anything else too (" *" works too). If ",*" has a special meaning,
that can be captured elsewhere in the syntax.
I'm also not sure (2) violates context-freeness, at least in the EBNF sense
where a context can include a newline. See for example:
section ::= "*"+ string (tag+) newline (planning newline)? (property_drawer
newline)?
planning ::= ("SCHEDULED:" "<" date_or_time ">")? ("DEADLINE:" "<" date_or_time
">")?
property_drawer ::= ":PROPERTIES:" newline drawer_contents newline ":END:"
drawer_contents ::= ":" property ":" whitespace string
Where the first line, "section" is represented graphically as the attached
image.
I guess I'm not 100% clear what "context-free" means. EBNF can represent a
language where a for loop has an opening and closing brace. The closing brace
is context-dependent, just as the planning or property drawers are.
I recently used EBNF to represent a CSV file with header, and I was unable to
capture the requirement that the header column must have the same number of
fields or commas as the data section. I think that is context-free.
-k.
