在Scala中,为什么不可能从类型别名或从属类型推断TypeTag? [英] In Scala, why it is impossible to infer TypeTag from type alias or dependent type?
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问题描述
我有一个简单的scala程序来测试Scala推断类型类的能力:
I have a simple scala program to test the Capability of Scala to infer type classes:
import scala.reflect.ClassTag
object InferTypeTag {
import org.apache.spark.sql.catalyst.ScalaReflection.universe._
def infer(): Unit = {
type U = (Int, String)
val ttg1 = implicitly[TypeTag[(Int, String)]]
val ttg2 = implicitly[TypeTag[U]]
val ctg = implicitly[ClassTag[U]]
}
}
毫无疑问地推断出
ttg1
.
ttg2
触发了以下编译错误:
ttg2
triggered the following compilation error:
Error:(14, 26) No TypeTag available for U
val ttg2 = implicitly[TypeTag[U]]
问题1 :为什么它不起作用?类型U已经是最终类型,无法覆盖
Question 1: why it doesn't work? type U is already final and is impossible to override
问题2 :如果类型U不是最终的且与路径相关,为什么可以成功推断ctg?
Question 2: if type U is not final and path-dependent, why ctg can be inferred successfully?
推荐答案
也许有人会解释为什么本地类型看起来很抽象.也许只是一个错误.
Maybe someone will explain why a local type looks abstract. Maybe just a bug.
scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._
scala> { type X = Tuple2[Int, String] ; weakTypeTag[X] }
res0: reflect.runtime.universe.WeakTypeTag[X] = WeakTypeTag[X]
scala> { type X = Tuple2[Int, String] ; typeTag[X] }
^
error: No TypeTag available for X
scala> { type X = Tuple2[Int, String] ; weakTypeTag[X].tpe }
res2: reflect.runtime.universe.Type = X
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