Multiple dummy bindings using underscore not allowed #7691

scabug · 2013-07-23T19:32:24Z

These show that the compiler knows that a wildcard is not a valid identifier in accordance with the spec:

scala> def _ = 3
<console>:1: error: identifier expected but '_' found.
       def _ = 3
           ^

scala> class _
<console>:1: error: identifier expected but '_' found.
       class _
             ^

scala> val _ = 3
<console>:5: error: identifier expected but '_' found.
  lazy val $result = _
                                          ^
<console>:10: error: identifier expected but '_' found.
 + "_: Int = " + scala.runtime.ScalaRunTime.replStringOf(_, 1000)
                                                                              ^

However, scalac complains about multiple bindings to _ :

scala> { val _ = 3; val _ = 5; 7 }
<console>:8: error: _ is already defined as value _
              { val _ = 3; val _ = 5; 7 }
                               ^

This was reported on a mailing list a while ago (http://www.scala-lang.org/node/7159), but never filed as an issue. One improvement since then:

scala> { val _ = 3; `_` }
<console>:1: error: wildcard invalid as backquoted identifier
       { val _ = 3; `_` }
                    ^

scabug · 2013-07-23T19:32:24Z

Imported From: https://issues.scala-lang.org/browse/SI-7691?orig=1
Reporter: @harrah
Affected Versions: 2.10.2, 2.11.0-M4

scabug · 2013-07-23T20:36:19Z

@paulp said:
Also from the world of poorly chosen ways to put _ to work is

object Test {
  def f[_](x: Int) = x      // compiles
  def g[_, _](x: Int) = x   // fails
  // ./a.scala:5: error: _ is already defined as type _
  //   def g[_, _](x: Int) = x
  //            ^
  // one error found
}

scabug · 2013-08-17T05:47:42Z

@Atry said (edited on Aug 17, 2013 5:51:33 AM UTC):
According to Scala Language Specification, {code} { val _ = "foo"; val _ = "bar" } {code} should be treated as valid , because _ is not a simple identifier but a pattern.

In Scala Language Specification 4.1 Value Declarations and Definitions

If p is some pattern other than a simple name or a name followed by a colon and a type,
then the value definition val p = e is expanded as follows:

If p has no bound variables:
e match { case p => ()}

In Scala Language Specification 1.1 Identifiers

The following names are reserved words instead of being members of the syntactic class id of lexical identifiers.
abstract case catch class def
do else extends false final
finally for forSome if implicit
import lazy match new null
object override package private protected
return sealed super this throw
trait try true type val
var while with yield
_ : = => <<:
<% >: # @

However, scalac should never accept { def _ = "foo"; def _ = "bar" }

This project now compiles with Scala 2.13 by default, while still cross-compiling to Scala 2.12. Issues addressed with this upgrade: ### Underscore (`_`) is no longer a valid identifer for vals Using underscore like this is no longer allowed... ``` implicit val _ = ... ``` ...we have to give the `val` a valid name: ``` implicit val v = ... ``` See also: * scala/bug#10384 * scala/bug#7691 * scala/scala#6218 ### `Map.mapValues()` now returns a `MapView` rather than a `Map` We usually want a `Map`, so we can just add in a `.toMap` to the end of the expression - it's harmless in Scala 2.12, and allows the code to compile in Scala 2.13. `Map.mapValues()` itself is deprecated in Scala 2.13, but using the new recommended alternative doesn't work in Scala 2.12. https://docs.scala-lang.org/overviews/core/collections-migration-213.html#what-are-the-breaking-changes See also: * scala/bug#10919 * scala/scala#7014

This project now compiles with Scala 2.13 by default, while still cross-compiling to Scala 2.12. Issues addressed with this upgrade: ### Underscore (`_`) is no longer a valid identifer for `val`s Using underscore like this is no longer allowed... ``` implicit val _ = ... ``` ...we have to give the `val` a valid name (even if it's just `v`): ``` implicit val v = ... ``` See also: * scala/bug#10384 * scala/bug#7691 * scala/scala#6218 ### `Map.mapValues()` now returns a `MapView` rather than a `Map` We usually want a `Map`, so we can just add in a `.toMap` to the end of the expression - it's harmless in Scala 2.12, and allows the code to compile in Scala 2.13. `Map.mapValues()` itself is deprecated in Scala 2.13, but using the new recommended alternative doesn't work in Scala 2.12. https://docs.scala-lang.org/overviews/core/collections-migration-213.html#what-are-the-breaking-changes See also: * scala/bug#10919 * scala/scala#7014 ### `.to` can no longer infer what resulting collection type you want This is all part of `CanBuildFrom` going away in Scala 2.13: https://www.scala-lang.org/blog/2018/06/13/scala-213-collections.html#life-without-canbuildfrom

This project now compiles with Scala 2.13 by default, while still cross-compiling to Scala 2.12 & 2.11. Issues addressed with this upgrade: ### Underscore (`_`) is no longer a valid identifer for `val`s Using underscore like this is no longer allowed... ``` implicit val _ = ... ``` ...we have to give the `val` a valid name (even if it's just `v`): ``` implicit val v = ... ``` See also: * scala/bug#10384 * scala/bug#7691 * scala/scala#6218 ### `Map.mapValues()` now returns a `MapView` rather than a `Map` We usually want a `Map`, so we can just add in a `.toMap` to the end of the expression - it's harmless in Scala 2.12, and allows the code to compile in Scala 2.13. `Map.mapValues()` itself is deprecated in Scala 2.13, but using the new recommended alternative doesn't work in Scala 2.12. https://docs.scala-lang.org/overviews/core/collections-migration-213.html#what-are-the-breaking-changes See also: * scala/bug#10919 * scala/scala#7014 ### `.to` can no longer infer what resulting collection type you want This is all part of `CanBuildFrom` going away in Scala 2.13: https://www.scala-lang.org/blog/2018/06/13/scala-213-collections.html#life-without-canbuildfrom

This project now compiles with Scala 2.13 by default, while still cross-compiling to Scala 2.12 & 2.11. Although Scala 2.13.1 has been released, this change sticks with 2.13.0, as `scoverage` has not yet released an update compatible with 2.13.1: scoverage/sbt-scoverage#295 scoverage/scalac-scoverage-plugin#283 scoverage/sbt-scoverage#299 Migration issues addressed with the update to Scala 2.13: ### Underscore (`_`) is no longer a valid identifer for `val`s Using underscore like this is no longer allowed... ``` implicit val _ = ... ``` ...we have to give the `val` a valid name (even if it's just `v`): ``` implicit val v = ... ``` See also: * scala/bug#10384 * scala/bug#7691 * scala/scala#6218 ### `Map.mapValues()` now returns a `MapView` rather than a `Map` We usually want a `Map`, so we can just add in a `.toMap` to the end of the expression - it's harmless in Scala 2.12, and allows the code to compile in Scala 2.13. `Map.mapValues()` itself is deprecated in Scala 2.13, but using the new recommended alternative doesn't work in Scala 2.12. https://docs.scala-lang.org/overviews/core/collections-migration-213.html#what-are-the-breaking-changes See also: * scala/bug#10919 * scala/scala#7014 ### `.to` can no longer infer what resulting collection type you want This is all part of `CanBuildFrom` going away in Scala 2.13: https://www.scala-lang.org/blog/2018/06/13/scala-213-collections.html#life-without-canbuildfrom

scabug mentioned this issue Apr 7, 2017

val _ should not create an identifier named "_" #7761

Closed

This was referenced Dec 7, 2017

Disallow _ as an identifier #10384

Closed

Import selector respects backquotes [ci: last-only] scala/scala#6218

Merged

som-snytt added the has PR label Dec 9, 2017

adriaanm closed this as completed in scala/scala#6218 Nov 27, 2018

SethTisue assigned som-snytt Nov 27, 2018

SethTisue added this to the 2.13.0-RC1 milestone Nov 27, 2018

szeiger mentioned this issue Jan 21, 2019

latest 2.13.x compiler crash with "val _ = 42" #11374

Closed

SethTisue mentioned this issue Jul 8, 2019

Scala 2.13 regression in for comprehension with wildcard syntax which mixes Unit and Nothing #11604

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rtyley mentioned this issue Oct 21, 2019

Add Scala 2.13 support DanielaSfregola/twitter4s#275

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Multiple dummy bindings using underscore not allowed #7691

Multiple dummy bindings using underscore not allowed #7691

scabug commented Jul 23, 2013

scabug commented Jul 23, 2013

scabug commented Jul 23, 2013

scabug commented Aug 17, 2013 •

edited by retronym

Multiple dummy bindings using underscore not allowed #7691

Multiple dummy bindings using underscore not allowed #7691

Comments

scabug commented Jul 23, 2013

scabug commented Jul 23, 2013

scabug commented Jul 23, 2013

scabug commented Aug 17, 2013 • edited by retronym

scabug commented Aug 17, 2013 •

edited by retronym