在以下示例中,使用=:=
运算符时,我想替换标准的隐式未找到消息:
sealed trait TBoolean
sealed trait TTrue extends TBoolean
sealed trait TFalse extends TBoolean
class Builder[X <: TBoolean, Y <: TBoolean, Z <: TBoolean] private(x: Int, y: Int, z: List[List[Int]]) {
protected def this() = this(-1, -1, List.empty)
def withX(xx: Int)(implicit ev: X =:= TFalse) = new Builder[TTrue, Y, Z](xx, this.y, this.z)
def withY(yy: Int)(implicit ev: Y =:= TFalse) = new Builder[X, TTrue, Z](this.x, yy, this.z)
def withZ(zz: List[List[Int]])(implicit ev: Z =:= TFalse) = new Builder[X, Y, TTrue](this.x, this.y, zz)
def build()(implicit ev1: Y =:= TTrue, ev2: X =:= TTrue, ev3: Z =:= TTrue) = println(s"${x} ${y} ${z}")
}
object Builder {
def apply() = new Builder[TFalse, TFalse, TFalse]
}
如果我尝试构建这样的非法对象:
Builder().withX(1).withY(1).withZ(List.empty).withY(1).build // i.e. two withY calls
我照常:
error: Cannot prove that TTrue =:= TFalse.
我想自定义此消息,以便提供更多信息。我尝试使用@scala.annotation.implicitNotFound("custom message: blah")
,但似乎不起作用。
有没有办法做到这一点?
根据implicitNotFound
注释Scaladoc(https://www.scala-lang.org/api/2.12.5/scala/annotation/implicitNotFound.html):
...找不到,用@implicitNotFound注释the class C。
因此,如果使用带有此注释的=:=
类,则无法覆盖它,除非您将使用自己的错误消息实现自己的类型相等证明类型类:
@implicitNotFound(msg = "Custom type equals prove message ${From} =::= ${To}.")
sealed abstract class =::=[From, To]
private final val singleton_=::= = new =::=[Any,Any] {}
object =::= {
implicit def tpEquals[A]: A =::= A = singleton_=::=.asInstanceOf[A =::= A]
}
并使用它:
def withZ(zz: List[List[Int]])(implicit ev: Z =::= TFalse)
所以结果错误消息将是:
Custom type equals prove message TTrue =::= TFalse.
希望这会有所帮助!