如何从 TypeClass 解析中排除特定类型？

Question

我想为大多数类型提供 ToCondition 操作，但不为字符串提供。（这是在移植 Javascript 代码时，非空和非零测试经常进行，重写所有此类测试以进行适当比较是乏味的）。这样做很容易，所以我收到错误运行时：

trait ToCondition[T] {
  def apply(x: T): Boolean
}
implicit object ToConditionInt extends ToCondition[Int] {
  override def apply(x: Int) = x != 0
}
implicit object ToConditionString extends ToCondition[String] {
  override def apply(x: String) = throw new UnsupportedOperationException("Cannot use String as a condition")
}
implicit object ToConditionAnyRef extends ToCondition[AnyRef] {
  override def apply(x: AnyRef) = x != null
}

def toCondition[T: ToCondition](a: T): Boolean = implicitly[ToCondition[T]].apply(a)

toCondition(1) // true

toCondition(null:AnyRef) // false

toCondition("") // throws runtime

能否以某种方式表达此约束，以便我在编译时得到错误？

Answer 1

如果你被允许使用 shapeless，那么使用类型不等式 =:!=。这是 String:

引发编译错误的版本

import shapeless._

trait ToCondition[T] {
  def apply(x: T): Boolean
}
implicit object ToConditionInt extends ToCondition[Int] {
  override def apply(x: Int) = x != 0
}
implicit object ToConditionString extends ToCondition[String] {
  override def apply(x: String) = throw new UnsupportedOperationException("Cannot use String as a condition")
}
implicit object ToConditionAnyRef extends ToCondition[AnyRef] {
  override def apply(x: AnyRef) = x != null
}

//Note the implicit value here
def toCondition[T: ToCondition](a: T)(implicit ev: T =:!= String) : Boolean = implicitly[ToCondition[T]].apply(a)

def main(args : Array[String]) {
  toCondition(1) // true

  toCondition(null:AnyRef) // false

  toCondition("") // compile error
}

Answer 2

您可以将 广义类型约束 与条件一起使用，这永远不会为真，例如 String =:= Nothing:

implicit def ToConditionString(implicit ev: String =:= Nothing) = new ToCondition[String] {
  override def apply(x: String) = throw new UnsupportedOperationException("Cannot use String as a condition")
}

然后

toCondition("")

编译时失败：

could not find implicit value for evidence parameter of type ToCondition[String]

如何从 TypeClass 解析中排除特定类型？

How can I exclude a specific type from a TypeClass resolution?

scala

typeclass

type-constraints