What does {% id %} do?

I have read the README.md here, and the Nearley web page. Then I was reading how-to-grammar-good.md and I’ve read the page about Earley. I understand what regular expression, Backus Naur Form, context free grammar, terminal, nonterminal, and ambiguous grammar mean, but when it said

You should know nearley syntax well enough to understand what something like

int -> [0-9]        {% id %}
    | int [0-9]     {% function(d) {return d[0] + d[1]} %}

means

I thought: no, not fully. Where can I learn more?

I understand the declaration syntax, and I know that JavaScript snippets can be included between {% and %}, but what does the {% id %} do? (I’m guessing the first argument of the function is the array of elements of the word, so the first 2 elements are returned.)

[kach]

Also, @pepa65, we do mention it in the README! https://github.com/Hardmath123/nearley#postprocessors

data is an array whose elements match the nonterminals in order. The postprocessor id returns the first token in the match (literally function(data) {return data[0];}).

I guess the problem is that it’s hidden away and hard to find — and I guess the solution is to restructure the documentation to be easier to skim. Currently, we have some sort of hybrid between an in-depth tutorial and legit documentation; maybe that needs to change…

[deltaidea]

Yep, you got it right! See, it is pretty straight-forward. 😄

In the generated JS you can find this: function id(x) {return x[0]; } - this operation is so often-used that they include a helper for it. Since {% ... %} can contain any JS, {% id %} simply refers to this id function defined in the same file.

I agree, docs are lacking sometimes. At least the syntax is intuitive.