Scala 3 泛型方法类型参数不起作用

Question

作品：

trait Transformer[A, B] {
  def transform(a: A): B
}

class TransformerImpl extends Transformer[Int, String] {
  override def transform(value: Int): String = {
    value.toString
  }
}

这就是我想要的，因为只有一个参数的特征更简单，而且B的具体值由A决定。但是，它不起作用。

trait Transformer2[A] {
  def transform[B](a: A): B
}

class Transformer2Impl extends Transformer2[Int] {
  override def transform[String](value: Int): String = {
    value.toString
  }
}

错误：

-- [E007] Type Mismatch Error: --------
3 |    value.toString
  |    ^^^^^^^^^^^^^^
  |    Found:    String
  |    Required: String

这也用不同的方法签名，不起作用：

class Transformer2IntToIntImpl extends Transformer2[Int] {
  override def transform(value: Int): Int = {
    value
  }
}

错误：

-- Error: ----------------------------------------------------------------------
1 |class Transformer2IntToIntImpl extends Transformer2[Int] {
  |      ^
  |class Transformer2IntToIntImpl needs to be abstract, since def transform[B](a: A): B in trait Transformer2 is not defined 
  |(Note that
  | parameter A in def transform[B](a: A): B in trait Transformer2 does not match
  | parameter Int in override def transform(value: Int): Int in class Transformer2IntToIntImpl
  | )
-- [E038] Declaration Error: ---------------------------------------------------
2 |  override def transform(value: Int): Int = {
  |               ^
  |method transform has a different signature than the overridden declaration
  |-----------------------------------------------------------------------------

2 errors found

根据错误，我尝试了几种替代方案，但没有成功。

Answer 1

这对我有用：

trait Transformer2[A] {
  type B
  def transform(a: A): B
}

class Transformer2Impl extends Transformer2[Int] {
  type B = String
  override def transform(value: Int): String = {
    value.toString
  }
}

class Transformer2IntToIntImpl extends Transformer2[Int] {
  type B = Int
  override def transform(value: Int): Int = {
    value
  }
}

val ans1 = new Transformer2Impl().transform(5)
val ans2 = new Transformer2IntToIntImpl().transform(5)

我还是想知道为什么原来的代码失败了。