Match an element only if alone on line

[dhowe]

I have a grammar where new-lines are generally ignored, except in the case where there is a specific element alone on a line.
How to specify that two different rules should fire, one if the element is alone on a line (/^ELEMENT$/), and the other if it is not?

[bd82]

Hello @dhowe

I would use a GATE to identify if the element is "alone on the line"

• https://chevrotain.io/docs/features/gates.html

Use the this.LA(x) to get the next / prev tokens and inspect their position information to identify how "alone" the element token is.

[dhowe]

Thanks. I did try that, though it was unclear to me how to specify the else part of the gate. If "alone", do subrule(X), else do subrule(Y)

I think I tried something like below:

$.RULE("ConditionalRule", () => {
  $.OR([
  {
    GATE: () => $.LA(1).tokenType === X,
    ALT: () => $.subrule('X')
  },
  {
    GATE: () => $.LA(1).tokenType === Y,
    ALT: () => $.subrule('Y')
  }
  ])
})

I guess one could also just allow define a separate rule for each of the two cases (?)

[bd82]

The example you gave is something the Parsing engine already does natively.
If you write:

$.RULE("ConditionalRule", () => {
  $.OR([
  {
    ALT: () => $.subrule('foo')
  },
  {
    ALT: () => $.subrule('bar')
  }
  ])
})

and foo starts with an X token and bar starts with a Y token.
Then Chevrotain will pick the correct alternative.

It is the additional logic for "alone on line" that you should be adding.
And I suspect you need to add it at the parser level, not the lexer level...

[dhowe]

Is there an example somewhere of a parser using lookahead/lookbehind to choose between rules ? thanks!

[bd82]

I did not find any good example, try expanding on something like this:

function isAloneOnLine() {
   const nextToken = $.LA(1)
   const nextNextToken = $.LA(2)
   const prevToken = $.LA(0)

   return prevToken.startLine < nextToken.startLine && nextToken.startLine < nextNextToken.startLine
}

$.RULE("ConditionalRule", () => {
  $.OR([
  {
    GATE: isAloneOnLine,
    ALT: () => $.subrule('singleLineElement')
  },
  {
    ALT: () => $.subrule('multiLineElement')
  }
  ])
})

Naturally this does not handle the edge cases of:

• first token in the stream (no prev token)
• last token in the stream (no nextNext token).

[dhowe]

thanks, will give it a try...