我对TypeTag所了解的只是它们以某种方式替代了Manifests。互联网上的信息稀缺,无法使我对这个主题有很好的了解。
因此,如果有人共享指向TypeTag上一些有用 Material 的链接(包括示例和流行的用例),我将感到非常高兴。也欢迎提供详细的答案和解释。
最佳答案
TypeTag
解决了Scala类型在运行时被擦除的问题(类型擦除)。如果我们想做
class Foo
class Bar extends Foo
def meth[A](xs: List[A]) = xs match {
case _: List[String] => "list of strings"
case _: List[Foo] => "list of foos"
}
我们将收到警告:
<console>:23: warning: non-variable type argument String in type pattern List[String]↩
is unchecked since it is eliminated by erasure
case _: List[String] => "list of strings"
^
<console>:24: warning: non-variable type argument Foo in type pattern List[Foo]↩
is unchecked since it is eliminated by erasure
case _: List[Foo] => "list of foos"
^
为了解决这个问题,将Manifests引入了Scala。但是它们有一个问题,就是不能表示很多有用的类型,例如路径依赖类型:
scala> class Foo{class Bar}
defined class Foo
scala> def m(f: Foo)(b: f.Bar)(implicit ev: Manifest[f.Bar]) = ev
warning: there were 2 deprecation warnings; re-run with -deprecation for details
m: (f: Foo)(b: f.Bar)(implicit ev: Manifest[f.Bar])Manifest[f.Bar]
scala> val f1 = new Foo;val b1 = new f1.Bar
f1: Foo = Foo@681e731c
b1: f1.Bar = Foo$Bar@271768ab
scala> val f2 = new Foo;val b2 = new f2.Bar
f2: Foo = Foo@3e50039c
b2: f2.Bar = Foo$Bar@771d16b9
scala> val ev1 = m(f1)(b1)
warning: there were 2 deprecation warnings; re-run with -deprecation for details
ev1: Manifest[f1.Bar] = [email protected]#Foo$Bar
scala> val ev2 = m(f2)(b2)
warning: there were 2 deprecation warnings; re-run with -deprecation for details
ev2: Manifest[f2.Bar] = [email protected]#Foo$Bar
scala> ev1 == ev2 // they should be different, thus the result is wrong
res28: Boolean = true
因此,它们被TypeTags代替,它不仅易于使用,而且可以很好地集成到新的Reflection API中。有了它们,我们可以优雅地解决上述有关路径依赖类型的问题:
scala> def m(f: Foo)(b: f.Bar)(implicit ev: TypeTag[f.Bar]) = ev
m: (f: Foo)(b: f.Bar)(implicit ev: reflect.runtime.universe.TypeTag[f.Bar])↩
reflect.runtime.universe.TypeTag[f.Bar]
scala> val ev1 = m(f1)(b1)
ev1: reflect.runtime.universe.TypeTag[f1.Bar] = TypeTag[f1.Bar]
scala> val ev2 = m(f2)(b2)
ev2: reflect.runtime.universe.TypeTag[f2.Bar] = TypeTag[f2.Bar]
scala> ev1 == ev2 // the result is correct, the type tags are different
res30: Boolean = false
scala> ev1.tpe =:= ev2.tpe // this result is correct, too
res31: Boolean = false
它们也很容易用于检查类型参数:
import scala.reflect.runtime.universe._
def meth[A : TypeTag](xs: List[A]) = typeOf[A] match {
case t if t =:= typeOf[String] => "list of strings"
case t if t <:< typeOf[Foo] => "list of foos"
}
scala> meth(List("string"))
res67: String = list of strings
scala> meth(List(new Bar))
res68: String = list of foos
在这一点上,理解使用
=:=
(类型相等)和<:<
(子类型关系)进行相等性检查非常重要。切勿使用==
或!=
,除非您完全知道自己要做什么:scala> typeOf[List[java.lang.String]] =:= typeOf[List[Predef.String]]
res71: Boolean = true
scala> typeOf[List[java.lang.String]] == typeOf[List[Predef.String]]
res72: Boolean = false
后者检查结构是否相等,通常不应该这样做,因为它不关心诸如前缀之类的东西(如示例中所示)。
TypeTag
是完全由编译器生成的,这意味着编译器会在调用需要此类TypeTag
的方法时创建并填充TypeTag
。标签存在三种不同形式:ClassTag
替代ClassManifest
,而TypeTag
或多或少是Manifest
的替代品。前者可以完全使用通用数组:
scala> import scala.reflect._
import scala.reflect._
scala> def createArr[A](seq: A*) = Array[A](seq: _*)
<console>:22: error: No ClassTag available for A
def createArr[A](seq: A*) = Array[A](seq: _*)
^
scala> def createArr[A : ClassTag](seq: A*) = Array[A](seq: _*)
createArr: [A](seq: A*)(implicit evidence$1: scala.reflect.ClassTag[A])Array[A]
scala> createArr(1,2,3)
res78: Array[Int] = Array(1, 2, 3)
scala> createArr("a","b","c")
res79: Array[String] = Array(a, b, c)
ClassTag
仅提供在运行时创建类型所需的信息(已擦除类型):scala> classTag[Int]
res99: scala.reflect.ClassTag[Int] = ClassTag[int]
scala> classTag[Int].runtimeClass
res100: Class[_] = int
scala> classTag[Int].newArray(3)
res101: Array[Int] = Array(0, 0, 0)
scala> classTag[List[Int]]
res104: scala.reflect.ClassTag[List[Int]] =↩
ClassTag[class scala.collection.immutable.List]
从上面可以看到,他们并不关心类型擦除,因此如果要“完整”类型,则应使用
TypeTag
:scala> typeTag[List[Int]]
res105: reflect.runtime.universe.TypeTag[List[Int]] = TypeTag[scala.List[Int]]
scala> typeTag[List[Int]].tpe
res107: reflect.runtime.universe.Type = scala.List[Int]
scala> typeOf[List[Int]]
res108: reflect.runtime.universe.Type = scala.List[Int]
scala> res107 =:= res108
res109: Boolean = true
可以看到,
tpe
的TypeTag
方法产生完整的Type
,这与调用typeOf
时得到的相同。当然,可以同时使用ClassTag
和TypeTag
:scala> def m[A : ClassTag : TypeTag] = (classTag[A], typeTag[A])
m: [A](implicit evidence$1: scala.reflect.ClassTag[A],↩
implicit evidence$2: reflect.runtime.universe.TypeTag[A])↩
(scala.reflect.ClassTag[A], reflect.runtime.universe.TypeTag[A])
scala> m[List[Int]]
res36: (scala.reflect.ClassTag[List[Int]],↩
reflect.runtime.universe.TypeTag[List[Int]]) =↩
(scala.collection.immutable.List,TypeTag[scala.List[Int]])
现在剩下的问题是
WeakTypeTag
的含义是什么?简而言之,TypeTag
代表具体类型(这意味着它仅允许完全实例化的类型),而WeakTypeTag
仅允许任何类型。大多数时候,人们并不在乎哪个是什么(这意味着应该使用TypeTag
),但是例如,当使用的宏应该与泛型类型一起使用时,就需要它们:object Macro {
import language.experimental.macros
import scala.reflect.macros.Context
def anymacro[A](expr: A): String = macro __anymacro[A]
def __anymacro[A : c.WeakTypeTag](c: Context)(expr: c.Expr[A]): c.Expr[A] = {
// to get a Type for A the c.WeakTypeTag context bound must be added
val aType = implicitly[c.WeakTypeTag[A]].tpe
???
}
}
如果将
WeakTypeTag
替换为TypeTag
,则会引发错误:<console>:17: error: macro implementation has wrong shape:
required: (c: scala.reflect.macros.Context)(expr: c.Expr[A]): c.Expr[String]
found : (c: scala.reflect.macros.Context)(expr: c.Expr[A])(implicit evidence$1: c.TypeTag[A]): c.Expr[A]
macro implementations cannot have implicit parameters other than WeakTypeTag evidences
def anymacro[A](expr: A): String = macro __anymacro[A]
^
有关
TypeTag
和WeakTypeTag
之间的区别的更详细说明,请参见以下问题:Scala Macros: “cannot create TypeTag from a type T having unresolved type parameters”Scala的官方文档站点还包含guide for Reflection。