SIP-27: Trailing Commas, revised document

sips/completed/_posts/2016-06-25-trailing-commas.md

@@ -3,31 +3,33 @@ layout: sip
 disqus: true
 title: SIP-27 - Trailing Commas
 
-vote-status: accepted 
+vote-status: under review
 vote-text: The following proposal needs to be updated, since only the specialized case version (with new lines) has been accepted. For more information, check the <a href="http://docs.scala-lang.org/sips/minutes/sip-20th-september-minutes.html">minutes</a>.
 ---
 
+// TODO: Move from sips/completed to sips/pending
+
 **By: Dale Wijnand**
 
 ## History
 
-| Date           | Version                                         |
-| ---------------|-------------------------------------------------|
-| Jun 25th 2016  | Initial Draft                                   |
-| Jun 27th 2016  | Added drawback of changing existing tools       |
-| Jun 27th 2016  | Added motivation that it simplifies codegen     |
-| Jun 28th 2016  | Fixed a typo                                    |
-| Aug 10th 2016  | Renamed from SIP-NN to SIP-27                   |
-| Aug 10th 2016  | Changed scala-commas URL (repo was moved)       |
-| Aug 10th 2016  | Dialed back some of the language from review    |
-| Sep 04th 2016  | Split the motivation into sections              |
-| Sep 04th 2016  | Add VCS authorship attribution to motivation    |
-| Sep 04th 2016  | Add Cross building hinderance to drawbacks      |
-| Sep 12th 2016  | Remove Cross building hinderance from drawbacks |
+| Date           | Version                                                    |
+| ---------------|------------------------------------------------------------|
+| Jun 25th 2016  | Initial Draft ([#533][])                                   |
+| Jun 27th 2016  | New drawback: changing existing tools ([#533][])           |
+| Jun 27th 2016  | New motivation: simplifies codegen ([#533][])              |
+| Aug 10th 2016  | SIP numbered: Renamed to SIP-27 ([#533][])                 |
+| Aug 10th 2016  | Changed scala-commas URL (repo was moved) ([#533][])       |
+| Aug 10th 2016  | Dialed back some of the language ([#533][])                |
+| Sep 04th 2016  | Split the motivation into sections ([#533][])              |
+| Sep 04th 2016  | New motivation: VCS authorship attribution ([#533][])      |
+| Sep 04th 2016  | New drawback: Cross building hinderance ([#533][])         |
+| Sep 12th 2016  | Remove cross building hinderance from drawbacks ([#533][]) |
+| Nov 12th 2016  | Major rework: multi-line, 2 variants & spec ([#625][])     |
 
 ## Motivation
 
-### Easy to modify lists
+### Ease of modification
 
 When using a comma-separated sequence of elements on multiple lines, such as:
 
@@ -39,33 +41,33 @@ Seq(
 )
 {% endhighlight %}
 
-It is quite inconvenient to remove or comment out any element because one has to think about the fact that the last element mustn't have a trailing comma:
+It is inconvenient to remove or comment out elements because the last element mustn't have a trailing comma:
 
 {% highlight scala %}
-Map(
+Seq(
   foo,
-  bar //,
+  bar,
 //  baz
-)
+)       // error: illegal start of simple expression
 {% endhighlight %}
 
-Secondly, it is quite inconvenient to re-order the sequence, for instance if you wanted `baz` before `bar` you need to micromanage which is followed by a comma and which isn't:
+It is also inconvenient to reorder because every element but the last one must be followed by a comma:
 
 {% highlight scala %}
 val xs = Seq(
   foo,
-  baz   // This isn't going to work
+  baz
   bar,
-)
+)       // error: illegal start of simple expression
 {% endhighlight %}
 
-### Reduce diff noise
+### Diff noise reduction
 
-Allowing trailing commas also reduces a lot of noise in diffs, such as:
+Adding and removing commas also introduces unnecessary noise in diffs:
 
 {% highlight diff %}
 @@ -4,7 +4,8 @@
- Map(
+ Seq(
    foo,
    bar,
 -  baz
@@ -76,15 +78,15 @@ Allowing trailing commas also reduces a lot of noise in diffs, such as:
 
 ### VCS authorship attribution
 
-Using the example above, the authorship of the `baz` line would be preserved, instead of becoming that of the author of the `quux` line.
+Using the example above, adding a comma after `baz` also unnecessarily changed the authorship of the line.
 
 ### Simplify code generation
 
-Such a feature would also simplify generating Scala source code.
+Allowing trailing commas would also simplify generating Scala source code.
 
 ### Long standing ticket
 
-There is an open ticket ([SI-4986][]) where this feature was requested, referencing the fact that it facilitates code generation by tools and allows for easier sorting of the values, initially in the context of import selectors but later also for other constructs in the syntax.
+([SI-4986][]) was opened in 2011 requesting support for trailing commas, referencing that it facilitates code generation by tools and allows easier sorting of values. It was initially in the context of import selectors but later also for other constructs in the syntax.
 
 ### Real-world use-cases
 
@@ -95,45 +97,120 @@ Some real-world use-cases where elements of a sequence are typically added, remo
 
 ## Design Decisions
 
-There are a number of different elements of the Scala syntax that are comma separated, but instead of changing them all a subset of the more useful ones was chosen:
+### Multi-line
+
+It is not the intent of introducing trailing commas to promote a code style such as:
+
+{% highlight scala %}
+val xs = Seq(foo, baz, bar, )
+{% endhighlight %}
+
+for a number of reasons:
+
+1. Subjectively, it's an ugly style.
+2. Some people utilise commas as a mechanism for counting, so introducing an optional trailing commas interferes with this technique; when elements are one by line, then line-counting can be used.
+3. Adding or removing elements is less cumbersome on one line.
+4. Commenting out elements isn't any less cumbersome with an optional trailing comma.
+
+Trailing comma support is therefore restricted to only comma-separated elements that are on separate lines:
+
+{% highlight scala %}
+val xs = Seq(
+  foo,
+  baz,
+  bar,
+)
+{% endhighlight %}
+
+### What parts of the Scala grammar to change
+
+There are a number of different parts of the Scala grammar that are comma-separated and, therefore, could support trailing commas. Specifically:
+
+* `ArgumentExprs`
+* `Params` and `ClassParams`
+* `SimpleExpr1`
+* `TypeArgs`, `TypeParamClause` and `FunTypeParamClause`
+* `SimpleType` and `FunctionArgTypes`
+* `SimplePattern`
+* `ImportSelectors`
+* `Import`
+* `Bindings`
+* `ids`, `ValDcl`, `VarDcl`, `VarDef` and `PatDef`
 
-* tuples
-* argument and parameter groups, including for implicits, for functions, methods and constructors
-* import selectors
+With this proposal I would like to present 2 variants:
 
-From the spec these are:
+1. The first variant adds trailing comma support to only multi-line `ArgumentExprs`, `Params` and `ClassParams`, which I consider to be the parts of the grammar that would most benefit from trailing commas.
 
-* SimpleExpr1, ArgumentExprs via Exprs
-* ParamClause, ParamClauses via Params
-* ClassParamClause, ClassParamClauses via ClassParams
-* ImportSelector
+2. The second variant adds trailing comma support to the whole grammar (again, only for multi-line), which means more consistency, but also supporting trailing commas in places that in practice don't really require it as much, such as `ids`.
 
-The elements that have not changed are:
+In this proposal, only the first variant is considered: trailing comma support for `ArgumentExprs`, `Params` and `ParamClasses` for the sake of simplicity.
 
-* ValDcl, VarDcl, VarDef via ids
-* Type via FunctionArgTypes
-* SimpleType, TypeArgs via Types
-* Expr, ResultExpr via Bindings
-* SimplePattern via Patterns
-* TypeParamClause, FunTypeParamClause
-* ImportExp
-* PatDef
+See below for more details on what that would mean.
+
+#### Changing `ArgumentExprs`
+
+**Spec change**
+
+{% highlight diff %}
+         Exprs ::= Expr {‘,’ Expr}
+-ArgumentExprs ::= ‘(’ [Exprs] ‘)’
++ArgumentExprs ::= ‘(’ [Exprs] [‘,’] ‘)'
+{% endhighlight %}
+
+**Example**
+{% highlight scala %}
+Seq(
+  foo,
+  bar,
+  baz,
+)
+{% endhighlight %}
+
+## `Params` and `ClassParams`
+
+**Spec change**
+{% highlight diff %}
+       Params ::=  Param {‘,’ Param}
+- ParamClause ::=  [nl] ‘(’ [Params] ‘)’
+-ParamClauses ::=  {ParamClause} [[nl] ‘(’ ‘implicit’ Params ‘)’]
++ ParamClause ::=  [nl] ‘(’ [Params] [‘,’] ‘)’
++ParamClauses ::=  {ParamClause} [[nl] ‘(’ ‘implicit’ Params [‘,’] ‘)’]
+
+       ClassParams ::=  ClassParam {‘,’ ClassParam}
+- ClassParamClause ::=  [nl] ‘(’ [ClassParams] ‘)’
+-ClassParamClauses ::=  {ClassParamClause} [[nl] ‘(’ ‘implicit’ ClassParams ‘)’]
++ ClassParamClause ::=  [nl] ‘(’ [ClassParams] [‘,’] ‘)’
++ClassParamClauses ::=  {ClassParamClause} [[nl] ‘(’ ‘implicit’ ClassParams [‘,’] ‘)’]
+{% endhighlight %}
+
+**examples**
+{% highlight scala %}
+def bippy(
+  foo: Int,
+  bar: String,
+  baz: Boolean,
+)
+
+class Bippy(
+  foo: Int,
+  bar: String,
+  baz: Boolean,
+)
+{% endhighlight %}
 
 ## Implementation
 
-The implementation is a simple change to the parser, allowing for a trailing comma, for the groups detailed above, and has been proposed in [scala/scala#5245][].
+The implementation of trailing commas is is limited to changing Parsers.scala in the Scala compiler. An implementation of this proposal can be found at [scala/scala#5245][].
 
 ## Drawbacks/Trade-offs
 
-The drawback, or trade-off, to this change is that it adds another way in which it is possible to do something in Scala. But it is the opinion of this SIP that the pragmatic advantage of being able to have trailing commas is worth this drawback.
+One drawback, or trade-off, to this change is that it adds an alternative way in which it is possible to do something in Scala. But I believe that the pragmatic advantage of being able to have trailing commas is worth this drawback.
 
-Given that this is a change in syntax, another drawback is that it requires changing the existing tools, such as those that parse Scala: intellij-scala, scalariform, scala.meta and scalaparse.
+Another drawback, given this is a change in syntax, is that it requires changing the existing tools, such as those that parse Scala: intellij-scala, scalariform, scala.meta and scalaparse.
 
 ## Alternatives
 
-As an alternative, trailing commas support could be added universally to all the comma-separated elements of the syntax. This would mean changing more (but still only in the parser), but it would make it consistent.
-
-As an alternative to changing the language, there already exists today a compiler plugin called [scala-commas][] that provides this feature. It also provides some evidence that people would even use unsupported compiler apis and reflection to add this functionality, even when such a plugin won't compose with other plugins well, though arguably only weak evidence as it's a young and obscure plugin.
+As an alternative to changing the language, there already exists today a compiler plugin called [scala-commas][] that provides a variant of this feature. It also provides some evidence that people would even use unsupported compiler apis and reflection to add this functionality, even when such a plugin won't compose with other plugins well, though arguably only weak evidence as it's a young and obscure plugin.
 
 ## References
 
@@ -144,3 +221,5 @@ As an alternative to changing the language, there already exists today a compile
 [SI-4986]: https://issues.scala-lang.org/browse/SI-4986
 [scala/scala#5245]: https://github.com/scala/scala/pull/524://github.com/scala/scala/pull/5245
 [scala-commas]: https://github.com/47deg/scala-commas
+[#533]: https://github.com/scala/scala.github.com/pull/533
+[#625]: https://github.com/scala/scala.github.com/pull/625