4

有没有办法将特征混合到 Dotty 或 Scala 中的现有对象中？

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

这个答案提供了一个宏观解决方案，但它已经 7 岁了，我希望（手指交叉！）更简单的东西。

4

3 回答 3

3

看看看似废弃但最近的图书馆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，在 Scala 中使用 mixins 到那种程度是不常见的。Zio 完全改变了另一种模式，IIUC。

于 2020-04-10T10:14:19.990 回答
3

如果你想上课，你仍然需要一个宏

class SomeClass1 extends SomeClass with SomeTrait

会自动生成。

我检查了，宏仍然有效（稍作修改）

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
于 2020-04-10T11:05:34.810 回答
3

那么使用类型类呢？

从您在评论中提供的示例到您的问题：

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和属性，或者稍后通过各自伴随对象之外的隐式添加它们。Winged您可以通过在伴随对象中提供隐式来强制有机体始终拥有腿/翅膀，或者将其留给代码的用户。

然后你可以定义

// Only Winged organisms (ie. `O` for which `Winged[O]` is available implicitly
def makeItFly[O <: Organism : Winged](o: O)
于 2020-04-10T14:47:49.313 回答