Change the language grammar to allow a trailing comma in "import" selectors

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Change the grammar for ImportSelectors to allow a trailing comma. These will facilitate generation by tools and allow easier sorting of the values.

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Imported From: https://issues.scala-lang.org/browse/SI-4986?orig=1
Reporter: G. Ralph Kuntz, MD (grkuntzmd)
Affected Versions: 2.9.1

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@paulp said:
Boy, I'd like trailing commas to work everywhere. So much suffering could be alleviated.

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Benoit Sigoure (tsuna) said:
I signed up on JIRA just so I could +1 this issue. After a bit of Googling, it seems that Scala actually used to support this, but it was removed from the language.

I'm also in the code generation camp, having to deal with "output the comma except if it's for the last element" is really annoying. Granted it's simple to work around, but it's just wasted time.

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Mikhail Trishchenkov (kriomant) said:
I vote for this too, not for code generation, but because of settings list in SBT configuration:

settings = Seq(
  name := 'bla',
  version := 2,
  something <<= path \{ _ / "foo" \}
)

Let's comment last line:

  settings = Seq(
  name := 'bla',
  version := 2,
  //something <<= path \{ _ / "foo" \}
}

Ooops, syntax error, fix:

settings = Seq(
  name := 'bla',
  version := 2 /*,
  something <<= path \{ _ / "foo" \} */
}

• ugly and annoying

Ok, add new line:

settings = Seq(
  name := 'bla',
  version := 2,
  something <<= path \{ _ / "foo" \}
  another := ""
}

Damn, I always forget to add comma to previous line!

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Erik Allik (eallik) said:

I signed up on JIRA just so I could +1 this issue.

same here.

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@gkossakowski said:
Unassigning and rescheduling to M7 as previous deadline was missed.

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Erik Allik (eallik) said (edited on Nov 18, 2013 6:48:09 PM UTC):
Does the "... in import selectors" part of the title still hold? People, including myself, seem to be wanting trailing commas to be accepted universally.

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G. Ralph Kuntz, MD (grkuntzmd) said (edited on Nov 18, 2013 6:56:09 PM UTC):
The more commas, the better :-). Anywhere in the grammar that treats comma as a separator should probably allow a trailing comma. It makes machine-generation of code easier.

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Chris Martin (chris-martin) said:
Trailing commas everywhere make version control diffs much happier.

.
 foo(
     one,
     two,
+    three,
 )

versus

.
 foo(
     one,
-    two
+    two,
+    three
 )

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Gabriel Jones (AEthyrSurfer) said:
Commit where trailing commas were deprecated:
scala/scala@e4bc488

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Gabriel Jones (AEthyrSurfer) said:
I think this is where trailing commas were removed altogether:
scala/scala@b3ce4c4

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Gabriel Jones (AEthyrSurfer) said:
Maybe we could add an allowTrailing parameter to tokenSeparated(...)?

https://github.com/scala/scala/blob/b8deaa11cba964f686e33fe416758989df0f31eb/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala#L757-L768

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Gabriel Jones (AEthyrSurfer) said:
Not as simple as I thought. tokenSeparated would need to know the closing token... Maybe not so hard to pass that in from the various calls to commaSeparated?

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Eran Medan (eranation) said:
Why was it ever removed? :) The one feature that I actually liked in Java...
Is there any pull request on this? should I try fixing it? or is someone else working on it?

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@dwijnand said:
scala/scala#5245