Ordering of productions in visitor

[dhowe]

I'm sure this is an obvious question, but if I have a parser rule called expr which accepts zero or more of a or b or c or d, how can my visitor visit each production in the order it occurred in the input ?

grammar:

    expr
        : (a | b | c | d)+

parser:

    $.RULE("expr", () => {
      $.MANY(() => {
        $.OR([
          { ALT: () => $.SUBRULE($.a) },
          { ALT: () => $.SUBRULE($.b) },
          { ALT: () => $.SUBRULE($.c) },
          { ALT: () => $.SUBRULE($.d) }
        ])
      })
    })

visitor:

    expr(ctx) {
      // something like this, but the order is lost ...
      return Object.keys(ctx).map(r => ctx[r] && this.visit(r)).join('');
    }

thanks!

[bd82]

Option 1

Add logic to sort the CST children of exp by location/position information

Option 2

Wrap the OR in a sub-rule:

    $.RULE("expr", () => {
      $.MANY(() => {
        $.SUBRULE($.foo) // The visitor will only have `foo` sub-children elements **in-order**
      })
    })

    $.RULE("foo", () => {
      $.MANY(() => {
         $.OR([
          { ALT: () => $.SUBRULE($.a) },
          { ALT: () => $.SUBRULE($.b) },
          { ALT: () => $.SUBRULE($.c) },
          { ALT: () => $.SUBRULE($.d) }
        ])
      })
    })

[timneedham]

Great question @dhowe and reply @bd82 (this is exactly the advice I was after and you only wrote it 15 minutes ago ! 😳 )

[dhowe]

Thanks @bd82 (and @timneedham) The problem for me is that if there are many subrules (dozens or even hundreds) it gets painful to list them all, as in Option 2 (which I ended up using). So I would prefer the first option, but it was/is not quite clear to me how to implement this...

[bd82]

@dhowe maybe this feature will help:

• https://chevrotain.io/docs/guide/concrete_syntax_tree.html#cstnodes-location

Basically getting the location of each CstNode is complicated because you need to search "in-depth" to find its first and last tokens.

But with the CstNodeLocation enabled you will get said information directly on each CSTNode, e.g:

    {
        "name": "a",
        "children": [/* ... */ ]
	"location": {
		"startOffset": 0,
		"startLine": 1,
		"startColumn": 1
	}
    }

So you can easily sort by location.startOffset