Implement SIP-27: Trailing Commas

[smarter]

Accepted SIP: http://docs.scala-lang.org/sips/completed/trailing-commas.html
Scala 2 implementation: scala/scala#5245

[CucumisSativus]

Hey @smarter I'd like to work on this.

[smarter]

Go for it! 😄

[CucumisSativus]

One thing i'm not sure @smarter , where should I place tests for this?

[smarter]

Tests that should compile go in ./tests/pos/, tests that shouldn't compile go in ./tests/neg, tests that should compile and run go in ./tests/run/. You should be able to port the tests from scala/scala#5245, the only difference is that we do not use check files for neg tests, instead we put // error in the source files on each line containing a compiler error.

[smarter]

Here's something that may be useful during development: you can pass the -Xprint:parser flag to the compiler to pretty-print the output of the parser, and If you add -Yplain-printer you'll see raw trees instead of pretty-printed output.

[CucumisSativus]

Thanks!

[rsoeldner]

Can someone help with the tests/neg cases. Starting with the this simple case (from here):

package foo

trait ArgumentExprs1 { f(23, "bar", )(Ev0, Ev1) } // error

//// Test utilities
object `package` {
  sealed trait Ev0; implicit object Ev0 extends Ev0
  sealed trait Ev1; implicit object Ev1 extends Ev1
}

and > dotty-compiler/testOnly dotty.tools.dotc.CompilationTests
I receive:

...
Wrong number of errors encountered when compiling ../out/optimised/testOptimised/neg/trailing-comma, expected: 1, actual: 2

[smarter]

If it's normal to have two errors on this line, you can put the error marker twice on the same line: // error // error.

[rsoeldner]

@smarter Thank you.

Currently i struggle with a few tests. Maybe the syntax/ gramma has changed and now the tests are ok ?

pos tests

  // test ': _*' syntax in patterns
List(1, 2, 3) match {
   case List(1, 2, _ @ _*,
   ) => 1
}

  // test varargs in patterns
val List(x, y, _*,
) = 42 :: 17 :: Nil

which fail with:

- [E030] Syntax Error: bla.scala:4:23 -----------------------------------------
4 |   case List(1, 2, _ @ _*,
  |                       ^
  |                       `_*' can be used only for last argument

longer explanation available when compiling with `-explain`
-- Error: bla.scala:4:19 -------------------------------------------------------
4 |   case List(1, 2, _ @ _*,
  |                   ^
  |          The syntax `x @ _*' is no longer supported; use `x : _*' instead
-- [E031] Syntax Error: bla.scala:5:3 ------------------------------------------
5 |   ) => 1
  |   ^
  |   illegal start of simple pattern

longer explanation available when compiling with `-explain`
-- [E030] Syntax Error: bla.scala:9:17 -----------------------------------------
9 |  val List(x, y, _*,
  |                 ^
  |                 `_*' can be used only for last argument

longer explanation available when compiling with `-explain`
-- [E031] Syntax Error: bla.scala:10:2 -----------------------------------------
10 |  ) = 42 :: 17 :: Nil
   |  ^
   |  illegal start of simple pattern

longer explanation available when compiling with `-explain`

5 errors found

And one neg test that doesn't fail:

package foo

trait Import { import foo.Ev0, foo.Ev1, }

object `package` {
  sealed trait Ev0; implicit object Ev0 extends Ev0
  sealed trait Ev1; implicit object Ev1 extends Ev1
}