PrettyPrinter(minimizeEmpty = true) injects a new line into elements with attributes

[hosamaly]

Using scala-xml v1.1.0, pretty-printing a leaf node with attributes when minimizeEmpty is set to true injects a new line into the element:

scala> new scala.xml.PrettyPrinter(width = 4, step = 2, minimizeEmpty = true).format(<a b="c"/>)
res0: String =
"<a
b="c">
</a>
"

scala> scala.xml.XML.loadString(res0).child.map(_.getClass.getName)
res2: scala.collection.immutable.Seq[String] = List(scala.xml.Text)

This behaviour renders the XML invalid if the schema dictates a non-mixed element.

[ashawley]

Hosam,

Thanks for reporting this with an example.

Indeed, it looks like the method PrettyPrinter.traverse will prioritize the width limit regardless of the empty element setting, and break up the element to try to fit the width.

Perhaps, there could be a check of minimizeMode in the conditional on line 160 of PrettyPrinter.scala?

[hosamaly]

Thanks, Aaron. I think that the conditional on line 160 may cause a different side effect. Specifically, I'm worried that an element with a long label and short attributes wouldn't be broken into multiple lines.

To avoid this, I made a different change in PR #233 that should keep the behaviour of breaking the line in these cases.

[ashawley]

Ok, fair enough. Indeed, enforcing the width on attributes is a requirement.

Did you see the nearbytodo comment? Seems it is trying to do some logic related to breaking spaces? Maybe it could handle attributes? I fear it is probably naive, and not useful to us, though.

Thanks for the PR and including some minimal tests.

[hosamaly]

Yes, I saw the todo. It's been there for ~14 years. It isn't clear to me why it's (still) commented out, so I ignored it.

[ashawley]

Why reviewing the change, it was suggested that PrettyPrinter.format should preserve the XML structure. While that is true for this defect, related to empty elements getting both a closing element and a text newline inserted, but I predict this rule will not scale generally for all outputted XML, like when there are a lot of nested tags.

[hosamaly]

By definition, pretty-printing means injecting whitespaces around XML tags, changing the structure of the document.

However, empty tags (i.e. ones without child nodes) are a special case. They should remain empty because the XML schema may not allow them to contain text. As such, the safest way to pretty-print them is to always make them self-closing. They can still be formatted, and the closing slash (/>) can be on a line by its own, meeting both requirements.