What is the ellipsis (empty string) used for in a Treetop(PEG) grammar?
Question
The Treetop website gives the following explanation that I don't understand
Ellipsis An empty string matches at any position and consumes no input. It's useful when you wish to treat a single symbol as part of a sequence, for example when an alternate rule will be processed using shared code.
rule alts ( foo bar / baz '' ) { def value elements.map{|e| e.text_value } end } end
when is useful to treat a symbol as a part of sequence? Can anybody provide a meaningful example of that?
Solution
I am not familiar with Treetop. From the example it would seem that ( foo bar / baz '' )
would either produce ['foo', 'bar']
or ['baz', '']
.
If you remove the ellipsis, you would get either ['foo', 'bar']
or just 'baz'
(no sequence/list/array).