Nested labels / fat arrow support

[Mike Austin]

I've used PEG.js for fun building a JavaScript dialect, and now I'm coming back to it after a year. I love that it's simple and easy to use.

I was messing around and learned it has support for nested labels and blocks. I didn't see any examples, but it does make sense now.

I never really liked the buildList() with array indices. It feels fragile and uses magic numbers:

Factor

  = "(" _ head:Expression _ tail:("," _ Expression)* ")" {

      return buildList(head, tail, 2)

    }

I recently learned you can do this, instead. I like that it doesn't need a helper function or indices, but it's not the prettiest with the inline block.

Factor

  = "(" _ head:Expression _ tail:("," _ e:Expression { return e })* ")" {

      return [head, ...tail]
    }

If fat arrow syntax is added, this would look fairly clean and look like modern JavaScript:

Factor

  = "(" _ head:Expression _ tail:("," _ expr:Expression => expr)* ")" {

      return [head, ...tail]
    }

Just some thoughts.

Maybe I'll take a look at the PEG source and see how difficult it would be to add.

Mike