Parser error prettyprinting

[coclique]

As title, it is good if we can format the Expectations into Strings. Ideally, Trifecta's output style (with line numbers and offending lines) will be very nice.

[johnynek]

Yes. See the LocationMap class that can compute the line and column.

It would be nice to have some utility that allows you to easily print.

I have some in another project but I use typelevel/paiges to produce the result and I didn't want to add that dependency here.

But we could do something with String or StringBuilder perhaps.

[coclique]

I wrote a simple prettyprinting utility: gist link.

I tried to test with the JSON parser in README. It gave these errors:

(repl):2:7: error: expected one of 'false', 'null', 'true'
2 | {"x": except}
  |       ^
(repl):2:7: error: expected '"'
2 | {"x": except}
  |       ^
(repl):2:7: error: expected '0' ~ '9'
2 | {"x": except}
  |       ^
(repl):2:7: error: expected '['
2 | {"x": except}
  |       ^
(repl):2:7: error: expected '{'
2 | {"x": except}
  |       ^

Should these errors (all at one position) be combined in some way?

[coclique]

Also, LocationMap does not support getting the EOF position (offset == length) which is inconvenient.

[johnynek]

Yeah, that is an issue.

If you get None for the line/col, it might be EOL. It would have been nicer if I had noticed that earlier and I could have made an ADT return type:

sealed abstract class RowColResult {
  def toValid: Option[(Int, Int)]
}

object RowColResult {
  case object OutOfBounds extends RowColResult {
    def toValid = None
  }
  case class Valid(row: Int, col: Int) extends RowColResult {
    def toValid = Some((row, col))
  }
  case object EOF extends RowColResult {
    def toValid = None
  }
}

something like that...

We could probably add a method like that, and then have the current method just call toValid on that.

Want to make a PR?

[johnynek]

About this: #239 (comment)

yes, I would group by location, and then aggregate all the expecations:
expected false, true, null, ", [, or {

something like that.

[coclique]

This comment:

I was thinking about also giving the EOF a position, which is one column after the last character. This way the offending line can be at the right position.

[johnynek]

I think that's right. The EOF should be on the same row as the last real character, unless that character is an end of line itself.

Put another way, EOF should be at the position that would be the same if we did: str + "e" and asked for the position of that final "e".

[johnynek]

I made some pretty printing in bosatsu here:

johnynek/bosatsu#808

Maybe something general could make it in here, but people could also copy that code and tweak to suit them.

[MasseGuillaume]

@re-xyr I implemented a sketch for a Show[Error] at

#410

you can see it in action in the test suite:
https://github.com/typelevel/cats-parse/blob/03cc03cdfe9fe7e326ee640fcfdff9eb5674b0f5/core/shared/src/test/scala/cats/parse/ErrorShowTest.scala