Scalafmt doesn't handle 2.12.2 syntax for commas at the end of a multi-line list

[lespea]

This template is a guideline, not a strict requirement.

• Version: 0.6.8
• Integration: IntelliJ / console tool
• Configuration: any

Steps

Having a comma in the last item of a list (spanning multiple lines) is now valid syntax in scala 2.12.2. Scalafmt doesn't seem to support this and throws this exception: illegal start of simple expression.

# cat t.scala
object Test {
  val a = Seq(
    1,
    2,
    3,
  )
}
# scalac -version
Scala compiler version 2.12.2 -- Copyright 2002-2017, LAMP/EPFL and Lightbend, Inc.
# scalac t.scala
# scalafmt
[warn] Error in .../t.scala: <input>:6: error: illegal start of simple expression
)
    ^

Expectations

Scalafmt handles this syntax

Wishlist

It would also be cool if there was a config option to force all lists to include a comma on the last item.

[olafurpg]

Thank you for reporting! This should actually be easy to accommodate, I added support for parsing trailing commas in scala.meta (scalameta/scalameta#660) as soon as they got merged into scala/scala. I am very excited to use them myself :)

I agree that it would be nice to have a flag to insert them where appropriate (e.g. in config style only). That should totally be doable!

[mushtaq]

+1

[lespea]

Any update on this?

[olafurpg]

I am currently swamped with tasks in scalameta+scalafix, so I haven't had much bandwidth to work on scalafmt lately. All the pieces for this issue are already in place, the only thing missing is to add support to use a scala.meta.Dialect that supports trailing commas. The current supported dialects are here

scalafmt/scalafmt-core/shared/src/main/scala/org/scalafmt/config/ScalafmtConfDecoders.scala

Lines 27 to 28 in d5c3b0a

	implicit lazy val dialectReader: ConfDecoder[Dialect] =
	ReaderUtil.oneOf[Dialect](Scala211, Sbt0137, Dotty, Paradise211)

I believe we need to add another case for Scala212 where

val Scala212 = scala.meta.dialects.Scala212.copy(allowTrailingCommas = true) // we forgot to change in last 1.7 release

[olafurpg]

I don't have the luxury to work with 2.12-only projects (only cross 2.11/2.12 even 2.10) so trailing commas haven't hit my day to day workflow yet 😄

[olafurpg]

BTW, I implemented a while ago a rewrite to insert trailing commas where they might make sense olafurpg/scalafix@21e35a3#diff-601bbf131241a9bce3e6683e6143eb83R17 Here's a diff from running it olafurpg/scala-repos#19 We could probably add that as a rewrite.rules = [UseTrailingCommas] to force usage of trailing commas in for example config style

[lespea]

Cool. It obviously isn't a showstopper or anything I was just curious. Thanks for the update :)

[olafurpg]

This got fixed in #979

[mjaghouri]

This still fails if there a comment on the line after the trailing comma.

object Test {
  val a = Seq(
    1,
    2,
    3,  // this comment makes formatting fail
  )
}

[gabro]

@mjaghouri just stumbled upon this ticket and the error you were seeing was a Scalameta parser bug, that recently got fixed (scalameta/scalameta#1532). The upcoming Scalafmt 1.6 will handle that case too.

[olafurpg]

It can already be tested in 1.6.0-RC1