c# - 有没有办法在 Scala 中使用命名字段的元组，类似于 C# 中的匿名类？

Question

参见:Can I specify a meaningful name for an anonymous class in C#?

在 C# 中你可以这样写:

var e = new { ID = 5, Name= "Prashant" };
assertEquals( 5, e.ID )

但在 Scala 中我最终写成:

var e = (5, "Prashant")
assertEquals( 5, e._1 )

Scala 通过使用泛型来维护类型安全(C# 也是如此)，但失去了每个字段名称的可读性，例如我使用“_1”而不是“ID”。

Scala 中有这样的东西吗？

Answer 1

object T {
  def main(args: Array[String]) {  
    val e = new { var id = 5; var name = "Prashant" }
    assert(e.id == 5)
  }
}

好吧，让我们把事情说清楚。这确实在 Scala 2.7 和 Scala 2.8 上使用了反射，因为在这种情况下，e 的类型是 结构类型，Scala 通过反射处理它。这是清理时生成的代码 (scalac -Xprint:cleanup):

package <empty> {
  final class T extends java.lang.Object with ScalaObject {
    private <synthetic> <static> var reflMethod$Cache1: java.lang.reflect.Method = null;
    private <synthetic> <static> var reflClass$Cache1: java.lang.Class = null;
    <synthetic> <static> def reflMethod$Method1(x$1: java.lang.Class): java.lang.reflect.Method = {
      if (T.this.reflMethod$Cache1.eq(null).||(T.this.reflClass$Cache1.ne(x$1)))
        {
          T.this.reflMethod$Cache1 = x$1.getMethod("id", Array[java.lang.Class]{});
          T.this.reflClass$Cache1 = x$1;
          ()
        };
      T.this.reflMethod$Cache1
    };
    @remote def $tag(): Int = scala.ScalaObject$class.$tag(T.this);
    def main(args: Array[java.lang.String]): Unit = {
      val e: java.lang.Object = {
        new T$$anon$1()
      };
      scala.this.Predef.assert(scala.Int.unbox({
        var exceptionResult1: java.lang.Object = _;
        try {
          exceptionResult1 = T.reflMethod$Method1(e.getClass()).invoke(e, Array[java.lang.Object]{})
        } catch {
          case ($1$ @ (_: java.lang.reflect.InvocationTargetException)) => {
            exceptionResult1 = throw $1$.getCause()
          }
        };
        exceptionResult1
      }.$asInstanceOf[java.lang.Integer]()).==(5))
    };
    def this(): object T = {
      T.super.this();
      ()
    }
  };
  final class T$$anon$1 extends java.lang.Object {
    private[this] var id: Int = _;
    <accessor> def id(): Int = T$$anon$1.this.id;
    <accessor> def id_=(x$1: Int): Unit = T$$anon$1.this.id = x$1;
    private[this] var name: java.lang.String = _;
    <accessor> def name(): java.lang.String = T$$anon$1.this.name;
    <accessor> def name_=(x$1: java.lang.String): Unit = T$$anon$1.this.name = x$1;
    def this(): T$$anon$1 = {
      T$$anon$1.this.id = 5;
      T$$anon$1.this.name = "Prashant";
      T$$anon$1.super.this();
      ()
    }
  }
}

有一些缓存正在进行，但如果我在 id 和 name 之间交替，它已经使缓存无效。 Scala 2.8 也有反射和缓存，但它使用了更高效的缓存技术，应该能提供更好的整体性能。作为引用，这里是 Scala 2.8 的清理:

package <empty> {
  final class T extends java.lang.Object with ScalaObject {
    final private <synthetic> <static> var reflParams$Cache1: Array[java.lang.Class] = Array[java.lang.Class]{};
    @volatile
    private <synthetic> <static> var reflPoly$Cache1: scala.runtime.MethodCache = new scala.runtime.EmptyMethodCache();
    <synthetic> <static> def reflMethod$Method1(x$1: java.lang.Class): java.lang.reflect.Method = {
      var method1: java.lang.reflect.Method = T.reflPoly$Cache1.find(x$1);
      if (method1.ne(null))
        return method1
      else
        {
          method1 = x$1.getMethod("id", T.reflParams$Cache1);
          T.reflPoly$Cache1 = T.reflPoly$Cache1.add(x$1, method1);
          return method1
        }
    };
    def main(args: Array[java.lang.String]): Unit = {
      val e: java.lang.Object = {
        new T$$anon$1()
      };
      scala.this.Predef.assert(scala.Int.unbox({
        val qual1: java.lang.Object = e;
        {
          var exceptionResult1: java.lang.Object = _;
          try {
            exceptionResult1 = T.reflMethod$Method1(qual1.getClass()).invoke(qual1, Array[java.lang.Object]{})
          } catch {
            case ($1$ @ (_: java.lang.reflect.InvocationTargetException)) => {
              exceptionResult1 = throw $1$.getCause()
            }
          };
          exceptionResult1
        }.$asInstanceOf[java.lang.Integer]()
      }).==(5))
    };
    def this(): object T = {
      T.reflParams$Cache1 = Array[java.lang.Class]{};
      T.reflPoly$Cache1 = new scala.runtime.EmptyMethodCache();
      T.super.this();
      ()
    }
  };
  final class T$$anon$1 extends java.lang.Object {
    private[this] var id: Int = _;
    <accessor> def id(): Int = T$$anon$1.this.id;
    <accessor> def id_=(x$1: Int): Unit = T$$anon$1.this.id = x$1;
    private[this] var name: java.lang.String = _;
    <accessor> def name(): java.lang.String = T$$anon$1.this.name;
    <accessor> def name_=(x$1: java.lang.String): Unit = T$$anon$1.this.name = x$1;
    def this(): T$$anon$1 = {
      T$$anon$1.super.this();
      T$$anon$1.this.id = 5;
      T$$anon$1.this.name = "Prashant";
      ()
    }
  }
}