我对scala还是比较陌生，并且在使用scala编译器时遇到了最初完全没有信息的NullPointerException，现在我可以缩小问题范围并消除其原因，但我仍然不完全了解编译器的行为。
环境:
IntelliJ IDEA 2020.2.3，JDK 1.8.0_271，Scala 2.13.3，SBT 1.3.13，Breeze 1.1
编辑:在没有IntelliJ的情况下使用SBT 1.4.2从命令行使用sbt compile进行了验证，编译器行为完全相同
我目前正在观察的内容:
MWE:
import breeze.linalg._
object MWE {
def test(v: DenseVector[Double], m: DenseMatrix[Double]): Unit = {
var value = 0.5
value += v
value += 0.5 * v.t * m * v
}
}
+=和DenseVector value上错误地使用运算符v value += v
尝试编译它，我得到了有用的信息
value += is not a member of Double
Expression does not convert to assignment because:
overloaded method + with alternatives:
(x: Double)Double <and>
(x: Float)Double <and>
(x: Long)Double <and>
(x: Int)Double <and>
(x: Char)Double <and>
(x: Short)Double <and>
(x: Byte)Double
cannot be applied to (breeze.linalg.DenseVector[Double])
expansion: value = value.<$plus: error>(v)
value += v
-=，IDE 不会突出显示为红色。尝试编译它，我得到了一个不同但仍有用的信息:
value -= is not a member of Double
Expression does not convert to assignment because:
type mismatch;
found : breeze.linalg.DenseVector[Double]
required: Double
expansion: value = value.<$minus: error>(v)
value -= v
我对scala中的这些运算符进行了简短的处理(请参阅例如Where is Scala's += defined in the context of Int?)，并在查看了scala文档(尤其是https://www.scala-lang.org/api/2.13.3/scala/Double.html)之后，看不出
+=和-=有区别的任何理由吗？回到我们的原始代码，行为上的差异以另一种方式体现出来:
在行中使用
+=value += 0.5 * v.t * m * v
但是，与更简单的情况相比，使用
-=会显示出完全不同的编译器错误:scalac: Error while emitting MWE.scala
assertion failed:
Bad superClass for class Double: <none>
while compiling: C:\Users\bt306644\typeBugMWE\src\main\scala\MWE.scala
during phase: jvm
library version: version 2.13.3
compiler version: version 2.13.3
reconstructed args: -classpath C:\Program Files\Java\jdk1.8.0_271\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_271\jre\lib\rt.jar;C:\Users\bt306644\typeBugMWE\target\scala-2.13\classes;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\com\chuusai\shapeless_2.13\2.3.3\shapeless_2.13-2.3.3.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\com\github\fommil\netlib\core\1.1.2\core-1.1.2.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\com\github\wendykierp\JTransforms\3.1\JTransforms-3.1.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\net\sf\opencsv\opencsv\2.3\opencsv-2.3.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\net\sourceforge\f2j\arpack_combined_all\0.1\arpack_combined_all-0.1.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\apache\commons\commons-math3\3.5\commons-math3-3.5.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\scala-lang\modules\scala-collection-compat_2.13\2.1.1\scala-collection-compat_2.13-2.1.1.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\scala-lang\scala-library\2.13.3\scala-library-2.13.3.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\scala-lang\scala-reflect\2.13.3\scala-reflect-2.13.3.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\scalanlp\breeze-macros_2.13\1.1\breeze-macros_2.13-1.1.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\scalanlp\breeze_2.13\1.1\breeze_2.13-1.1.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\slf4j\slf4j-api\1.7.5\slf4j-api-1.7.5.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\typelevel\algebra_2.13\2.0.0-M2\algebra_2.13-2.0.0-M2.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\typelevel\cats-kernel_2.13\2.0.0-M4\cats-kernel_2.13-2.0.0-M4.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\typelevel\machinist_2.13\0.6.8\machinist_2.13-0.6.8.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\typelevel\spire-macros_2.13\0.17.0-M1\spire-macros_2.13-0.17.0-M1.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\typelevel\spire-platform_2.13\0.17.0-M1\spire-platform_2.13-0.17.0-M1.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\typelevel\spire-util_2.13\0.17.0-M1\spire-util_2.13-0.17.0-M1.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\org\typelevel\spire_2.13\0.17.0-M1\spire_2.13-0.17.0-M1.jar;C:\Users\bt306644\AppData\Local\Coursier\cache\v1\https\repo1.maven.org\maven2\pl\edu\icm\JLargeArrays\1.5\JLargeArrays-1.5.jar
last tree to typer: Literal(Constant(0.5))
tree position: line 6 of C:\Users\bt306644\typeBugMWE\src\main\scala\MWE.scala
tree tpe: Double(0.5)
symbol: null
call site: constructor MWE in object MWE in package <empty>
== Source file context for tree position ==
3 def test(v: DenseVector[Double], m: DenseMatrix[Double]): Unit = {
4 var value = 0.5
5 // value += v
6 value -= 0.5 * v.t * m * v
7 }
8 }
9
除了这些主要问题外，还有一些上下文信息和注意事项:
回到我遇到此问题的原始代码和设置的另一步，我们实际上使用的是Scala 2.11.11而不是2.13.3(因此使用Breeze 0.13.2而不是Breeze 1.1)。
这导致编译器在第二种情况下根本不显示任何有用的信息，而是在编译器中产生了NullPointerException:
scalac: Error: org.jetbrains.jps.incremental.scala.remote.ServerException
java.lang.NullPointerException
at scala.tools.nsc.transform.Mixin$MixinTransformer.scala$tools$nsc$transform$Mixin$MixinTransformer$$postTransform(Mixin.scala:1154)
at scala.tools.nsc.transform.Mixin$MixinTransformer$$anonfun$transform$1.apply(Mixin.scala:1261)
at scala.tools.nsc.transform.Mixin$MixinTransformer$$anonfun$transform$1.apply(Mixin.scala:1261)
at scala.reflect.internal.SymbolTable.enteringPhase(SymbolTable.scala:235)
at scala.reflect.internal.SymbolTable.exitingPhase(SymbolTable.scala:256)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:1261)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:468)
at scala.reflect.internal.Trees$class.itransform(Trees.scala:1347)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:1258)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:468)
at scala.reflect.internal.Trees$class.itransform(Trees.scala:1386)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:1258)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:468)
at scala.reflect.api.Trees$Transformer$$anonfun$transformStats$1.apply(Trees.scala:2589)
at scala.reflect.api.Trees$Transformer$$anonfun$transformStats$1.apply(Trees.scala:2587)
at scala.collection.immutable.List.loop$1(List.scala:176)
at scala.collection.immutable.List.mapConserve(List.scala:200)
at scala.reflect.api.Trees$Transformer.transformStats(Trees.scala:2587)
at scala.reflect.internal.Trees$class.itransform(Trees.scala:1366)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:1258)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:468)
at scala.reflect.internal.Trees$$anonfun$itransform$2.apply(Trees.scala:1363)
at scala.reflect.internal.Trees$$anonfun$itransform$2.apply(Trees.scala:1361)
at scala.reflect.api.Trees$Transformer.atOwner(Trees.scala:2600)
at scala.reflect.internal.Trees$class.itransform(Trees.scala:1360)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:1258)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:468)
at scala.reflect.api.Trees$Transformer$$anonfun$transformStats$1.apply(Trees.scala:2589)
at scala.reflect.api.Trees$Transformer$$anonfun$transformStats$1.apply(Trees.scala:2587)
at scala.collection.immutable.List.loop$1(List.scala:176)
at scala.collection.immutable.List.mapConserve(List.scala:200)
at scala.reflect.api.Trees$Transformer.transformStats(Trees.scala:2587)
at scala.reflect.internal.Trees$class.itransform(Trees.scala:1404)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:1258)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:468)
at scala.reflect.api.Trees$Transformer.transformTemplate(Trees.scala:2563)
at scala.reflect.internal.Trees$$anonfun$itransform$4.apply(Trees.scala:1408)
at scala.reflect.internal.Trees$$anonfun$itransform$4.apply(Trees.scala:1407)
at scala.reflect.api.Trees$Transformer.atOwner(Trees.scala:2600)
at scala.reflect.internal.Trees$class.itransform(Trees.scala:1406)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:1258)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:468)
at scala.reflect.api.Trees$Transformer$$anonfun$transformStats$1.apply(Trees.scala:2589)
at scala.reflect.api.Trees$Transformer$$anonfun$transformStats$1.apply(Trees.scala:2587)
at scala.collection.immutable.List.loop$1(List.scala:176)
at scala.collection.immutable.List.mapConserve(List.scala:200)
at scala.reflect.api.Trees$Transformer.transformStats(Trees.scala:2587)
at scala.reflect.internal.Trees$$anonfun$itransform$7.apply(Trees.scala:1426)
at scala.reflect.internal.Trees$$anonfun$itransform$7.apply(Trees.scala:1426)
at scala.reflect.api.Trees$Transformer.atOwner(Trees.scala:2600)
at scala.reflect.internal.Trees$class.itransform(Trees.scala:1425)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:1258)
at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:468)
at scala.tools.nsc.ast.Trees$Transformer.transformUnit(Trees.scala:147)
at scala.tools.nsc.transform.Transform$Phase.apply(Transform.scala:30)
at scala.tools.nsc.Global$GlobalPhase$$anonfun$applyPhase$1.apply$mcV$sp(Global.scala:467)
at scala.tools.nsc.Global$GlobalPhase.withCurrentUnit(Global.scala:458)
at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:467)
at scala.tools.nsc.Global$GlobalPhase$$anonfun$run$1.apply(Global.scala:425)
at scala.tools.nsc.Global$GlobalPhase$$anonfun$run$1.apply(Global.scala:425)
at scala.collection.Iterator$class.foreach(Iterator.scala:891)
at scala.collection.AbstractIterator.foreach(Iterator.scala:1334)
at scala.tools.nsc.Global$GlobalPhase.run(Global.scala:425)
at scala.tools.nsc.Global$Run.compileUnitsInternal(Global.scala:1528)
at scala.tools.nsc.Global$Run.compileUnits(Global.scala:1513)
at scala.tools.nsc.Global$Run.compileSources(Global.scala:1508)
at scala.tools.nsc.Global$Run.compile(Global.scala:1609)
at xsbt.CachedCompiler0.run(CompilerInterface.scala:130)
at xsbt.CachedCompiler0.run(CompilerInterface.scala:105)
at xsbt.CompilerInterface.run(CompilerInterface.scala:31)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at sbt.internal.inc.AnalyzingCompiler.call(AnalyzingCompiler.scala:237)
at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:111)
at sbt.internal.inc.AnalyzingCompiler.compile(AnalyzingCompiler.scala:90)
at org.jetbrains.jps.incremental.scala.local.IdeaIncrementalCompiler.compile(IdeaIncrementalCompiler.scala:42)
at org.jetbrains.jps.incremental.scala.local.LocalServer.compile(LocalServer.scala:43)
at org.jetbrains.jps.incremental.scala.remote.Main$.compileLogic(Main.scala:145)
at org.jetbrains.jps.incremental.scala.remote.Main$.$anonfun$handleCommand$1(Main.scala:131)
at org.jetbrains.jps.incremental.scala.remote.Main$.decorated$1(Main.scala:121)
at org.jetbrains.jps.incremental.scala.remote.Main$.handleCommand(Main.scala:128)
at org.jetbrains.jps.incremental.scala.remote.Main$.serverLogic(Main.scala:105)
at org.jetbrains.jps.incremental.scala.remote.Main$.nailMain(Main.scala:63)
at org.jetbrains.jps.incremental.scala.remote.Main.nailMain(Main.scala)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.martiansoftware.nailgun.NGSession.run(NGSession.java:319)
实际导致这整个东西出现的原因还有些奇怪(不，我没想到能够从Double xD中减去DenseVectors)。
初始代码中的操作稍微复杂一些，但实际上可以归结为MWE中的这一行:
value -= 0.5 * v.t * m * v
撇开0.5，我们将转置的 vector ，方矩阵和 vector 相乘，它们都具有相同的尺寸。此操作导致标量，而不是 vector !
但是0.5至少会在代码中造成困惑(至少在类型方面如此)-将标量乘以转置的DenseVector(
0.5 * v.t)不会得到预期的转置的DenseVector，结果是DenseMatrix类型的。然后，这导致整行导致DenseVector的类型错误，而不是Double。附加问题:代码为什么显示有关类型的这种行为？任何人都知道这是否是一个合理的选择，或者甚至可能是Breeze中的错误？
因此事实证明，通过添加括号可以很容易地修复我们的原始代码:
value -= 0.5 * (v.t * m * v)
但是问题的原因仍然让我感到困惑，我期待收到一些有用的信息:)
最佳答案
第一个问题:这是有根据的猜测。 Scala对+和+=的处理有些怪异，因为Scala可以让您执行(x: Any) + (y: String)，而且我已经看到了由此引起的不同运算符的错误怪异。
第二个问题:这是一个编译器错误，应该受到支持。您可以尝试在scalac中使用-no-specialization标志运行，看看是否遇到相同的错误。特化在Breeze中使用很多，多年来，我偶然发现了许多与之相关的编译器错误。
附带问题:这是Breeze中的限制/错误。 Scala将.5 * v.t * m * v解析为0.5.*(v.t).*(m).*(v)(即从左到右)。 v.t是Transpose[DenseVector[Double]](即行 vector )，说实话，Transpose的东西并没有我想要的完全烘焙。从历史上讲，我们将行 vector 表示为矩阵，但我怀疑它会回落到那，因此它看到矩阵*矩阵* vector 当然是 vector 。
关于scala - 对不兼容的类型(Double和DenseVector)使用-=会导致奇怪的Scala编译器错误，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/64753094/