如何确定类型参数是否是特征的子类型？

Question

Let's say I have the following types

class Foo
trait Bar

Is there a way to make a method which takes in a Type parameter, T, and determine if that T is a Bar? For example,

def isBar[T <: Foo: Manifest] = 
  classOf[Bar].isAssignableFrom(manifest[T].erasure)

Sadly, isBar[Foo with Bar] is false because erasure seems to erase mixins.

Also, manifest[Foo with Bar] <:< manifest[Bar] is false

Is this possible at all?

I looked at this question: How to tell if a Scala reified type extends a certain parent class?

but that answer doesn't work with mixed-in traits as they seem to be erased as evidenced above.

Answer 1

This can be achieved with TypeTags (at least 2.10M7):

scala> class Foo; trait Bar
defined class Foo
defined trait Bar

scala> import reflect.runtime.universe._
import reflect.runtime.universe._

scala> def isBar[A <: Foo : TypeTag] = typeOf[A].baseClasses.contains(typeOf[Bar].typeSymbol)
isBar: [A <: Foo](implicit evidence$1: reflect.runtime.universe.TypeTag[A])Boolean

scala> isBar[Foo]
res43: Boolean = false

scala> isBar[Foo with Bar]
res44: Boolean = true

TypeTags provide a 1:1 translation of Scala types because they represent the types the compiler knows. Therefore they are much more powerful than plain old Manifests:

scala> val fooBar = typeTag[Foo with Bar]
fooBar: reflect.runtime.universe.TypeTag[Foo with Bar] = TypeTag[Foo with Bar]

With the method tpe we get full access to Scalas new Reflection:

scala> val tpe = fooBar.tpe // equivalent to typeOf[Foo with Bar]
tpe: reflect.runtime.universe.Type = Foo with Bar

scala> val tpe.<tab><tab> // lot of nice methods here
=:=                 asInstanceOf        asSeenFrom          baseClasses         baseType            contains            declaration         
declarations        erasure             exists              find                foreach             isInstanceOf        kind                
map                 member              members             narrow              normalize           substituteSymbols   substituteTypes     
takesTypeArgs       termSymbol          toString            typeConstructor     typeSymbol          widen  

Answer 2

It is possible to do this pre-2.10, just not (as far as I know) with manifests:

def isBar[T <: Foo](implicit ev: T <:< Bar = null) = ev != null

It's a bit of a hack, but it works as desired.

scala> isBar[Foo with Bar]
res0: Boolean = true

scala> isBar[Foo]
res1: Boolean = false

Answer 3

You can resolve it without reflection by using typeclasses:

trait IsBar[T] {
  def apply():Boolean
}

trait LowerLevelImplicits {
  implicit def defaultIsBar[T] = new IsBar[T]{
    def apply() = false
  }
}

object Implicits extends LowerLevelImplicits {
  implicit def isBarTrue[T <: Bar] = new IsBar[T] {
    def apply() = true
  }
}

def isBar[T<:Foo]( t: T )( implicit ib: IsBar[T] ) = ib.apply()

scala> import Implicits._

scala> isBar( new Foo )
res6: Boolean = false

scala> isBar( new Foo with Bar )
res7: Boolean = true

Answer 4

Another typeclass usage(more generic):

  trait SubClassGauge[A, B] {
    def A_isSubclassOf_B: Boolean
  }

  implicit class IsSubclassOps[A](a: A) {
    def isSubclassOf[B](implicit ev: SubClassGauge[A, B]): Boolean = ev.A_isSubclassOf_B
  }

  trait LowerLevelImplicits {
    implicit def defaultSubClassGauge[A, B] = new SubClassGauge[A, B] {
      override def A_isSubclassOf_B: Boolean = false
    }
  }

  object Implicits extends LowerLevelImplicits {
    implicit def subClassGauge[A <: B, B]: SubClassGauge[A, B] = new SubClassGauge[A, B] {
      override def A_isSubclassOf_B: Boolean = true
    }
  }

  trait Prime
  class NotSuper
  class Super extends Prime
  class Sub extends Super
  class NotSub

Now, in REPL:

@ import Implicits._ 
import Implicits._
@ (new Sub).isSubclassOf[NotSuper] 
res29: Boolean = false
@ (new Sub).isSubclassOf[Super] 
res30: Boolean = true
@ (new Sub).isSubclassOf[Prime] 
res31: Boolean = true
@ (new Super).isSubclassOf[Prime] 
res32: Boolean = true
@ (new Super).isSubclassOf[Sub] 
res33: Boolean = false
@ (new NotSub).isSubclassOf[Super] 
res34: Boolean = false

TypeTag now belongs to scala reflect package. One needs to add extra dependency to use it.