Scala / Dotty -将特征混合到现有对象中

Question

在Dotty或Scala中，有没有办法将特征混合到现有的对象中？

class SomeClass
trait SomeTrait

// This works, but it's not what I'm looking for:
new SomeClass with SomeTrait

// This is what I'm looking for, but it breaks:
val someClass = new SomeClass
someClass with SomeTrait

This answer提供了一个宏观解决方案，但它已经有7年的历史了，我希望如此(祈祷吧！)为了更简单的东西。

Answer 1

那么使用类型类来代替呢？

从您在对您的问题的评论中提供的示例：

trait Organism
trait Winged[O <: Organism]
trait Legged[O <: Organism]

class Dog extends Organism
object Dog {
   implicit val legged: Legged[Dog] = new Legged[Dog] { ... }
}

class Fly extends Organism
object Fly {
   implicit val winged: Winged[Fly] = new Winged[Fly] { ... }
   implicit val legged: Legged[Fly] = new Legged[Fly] { ... }
}

这是一种非常灵活的方法，允许您在设计特定有机体时定义Legged和Winged属性，或者稍后通过在相应的伴生对象外部隐式添加它们。您可以通过在伴生对象中提供隐式设置来强制有机体始终具有腿/翅膀，或者将其留给代码的用户。

然后，您可以定义

// Only Winged organisms (ie. `O` for which `Winged[O]` is available implicitly
def makeItFly[O <: Organism : Winged](o: O) 

Answer 2

看一下看似废弃但相当新的库zio-delegate

import zio.delegate._

class SomeClass

trait SomeTrait {
  def test() = println("It just works!")
}

val someClass = new SomeClass

val result: SomeClass with SomeTrait =
  Mix[SomeClass, SomeTrait].mix(someClass, new SomeTrait {})

result.test()

它仍然是基于宏的，而且在Scala中使用那样程度的混合是不常见的。Zio完全改变了另一种模式，IIUC。

Answer 3

如果你想要类，你仍然需要一个宏

class SomeClass1 extends SomeClass with SomeTrait

将自动生成。

我检查了一下，宏仍然有效(只做了很小的修改)

def toPersisted[T](instance: T, id: Long): T with Persisted = macro impl[T]

def impl[T: c.WeakTypeTag](c: blackbox.Context)(instance: c.Tree, id: c.Tree): c.Tree = {
  import c.universe._

  val typ = weakTypeOf[T]
  val symbol = typ.typeSymbol
  if (!symbol.asClass.isCaseClass)
    c.abort(c.enclosingPosition, s"toPersisted only accepts case classes, you provided $typ")

  val accessors = typ.members.sorted.collect { case x: TermSymbol if x.isCaseAccessor && x.isMethod => x }
  val fieldNames = accessors map (_.name)

  val instanceParam = q"val instance: $typ"
  val idParam = q"${Modifiers(Flag.PARAMACCESSOR)} val id: Long"
  val superArgs = fieldNames map (fieldName => q"instance.$fieldName")
  val ctor =
    q"""def ${termNames.CONSTRUCTOR}($instanceParam, $idParam) = {
      super.${termNames.CONSTRUCTOR}(..$superArgs)
      ()
    }"""
  val idVal = idParam.duplicate
  val tmpl = Template(List(tq"$typ", tq"Persisted"), noSelfType, List(idVal, ctor))
  val cname = TypeName(c.freshName(symbol.name.toString + "$Persisted"))
  val cdef = ClassDef(NoMods, cname, Nil, tmpl)

  q"""
     $cdef
     new $cname($instance, $id)
    """
}

case class MyClass(i: Int, s: String)

val x = MyClass(1, "a")

val y = toPersisted(x, 2L)

y.i // 1
y.s // a
y.id // 2