Reflection API - how to get actual types of a constructor of a generic object?

[Piotr Kolaczkowski]

scala> class Generic[T](arg: T)

defined class Generic

scala> typeOf[Generic[Int]]

res37: reflect.runtime.universe.Type = Generic[Int]   // fine, so a full type info is here

scala> typeOf[Generic[Int]].declaration(nme.CONSTRUCTOR).asMethod.paramss(0).map(_.asTerm.typeSignature)

res38: List[reflect.runtime.universe.Type] = List(T)

T? Huh. Looks like we get a formal type here, not the actual. So how to get the actual type Int here? 

I know I can grab the actual type args for the whole class type by using match { case TypeRef(_, _, args) => args }.

Then possibly use the "substituteTypes" method, but I  didn't figure it out how to make it all fit together.

Can you give me a hint how to get actual unerased types of arguments to any method/constructor obtained by reflection?

Thanks,

Piotr

[Oliver Ruebenacker]

     Hello,

On Sun, May 25, 2014 at 7:23 AM, Piotr Kolaczkowski <pkol...@datastax.com> wrote:

scala> class Generic[T](arg: T)

defined class Generic

scala> typeOf[Generic[Int]]

res37: reflect.runtime.universe.Type = Generic[Int]   // fine, so a full type info is here

  Full type info is there at compile time, not at run-time.

     Best,

     Oliver

 

scala> typeOf[Generic[Int]].declaration(nme.CONSTRUCTOR).asMethod.paramss(0).map(_.asTerm.typeSignature)

res38: List[reflect.runtime.universe.Type] = List(T)

T? Huh. Looks like we get a formal type here, not the actual. So how to get the actual type Int here? 

I know I can grab the actual type args for the whole class type by using match { case TypeRef(_, _, args) => args }.

Then possibly use the "substituteTypes" method, but I  didn't figure it out how to make it all fit together.

Can you give me a hint how to get actual unerased types of arguments to any method/constructor obtained by reflection?

Thanks,

Piotr

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Oliver Ruebenacker

Founder at Relomics Consulting

Be always grateful, but never satisfied.

[Piotr Kolaczkowski]

Wrong, for sure it is there at runtime. 

Actually I managet to do it, but it is limited to a constructors or methods without generic arguments, and it is slightly convoluted solution:

  private val tpe = implicitly[TypeTag[T]].tpe

  private val paramTypes: Array[Type] = {

    val formalTypeArgs = tpe.typeSymbol.asClass.typeParams

    val actualTypeArgs = tpe match {

      case TypeRef(_, _, args) => args

      case _ => List.empty[Type]

    }

    val scalaConstructor: MethodSymbol = tpe.declaration(nme.CONSTRUCTOR).asMethod

    def symbolToType(s: Symbol) = s.asTerm.typeSignature.substituteTypes(formalTypeArgs, actualTypeArgs)

    scalaConstructor.paramss(0).map(symbolToType).toArray

  }

I'd appreciate if someone has a better, more-elegant solution.

Thanks,

Piotr

[Eugene Burmako]

Use typeSignatureIn instead of typeSignature - that should help.

[Oliver Ruebenacker]

     Hello,

  Yes, you can capture type information at compile time and store it in an extra variable.

  I thought you were attempting to query an actual object for the types of its declared parameters.

     Best,

     Oliver

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[Jason Zaugg]

On Sun, May 25, 2014 at 1:57 PM, Eugene Burmako <xen...@gmail.com> wrote:

Use typeSignatureIn instead of typeSignature - that should help.

Yep, that's the one. 

scala> val tp = typeOf[Generic[Int]]
tp: reflect.runtime.universe.Type = Generic[Int]

scala> val constructor = tp.declaration(termNames.CONSTRUCTOR).asMethod
constructor: reflect.runtime.universe.MethodSymbol = constructor Generic

scala> val typeSig = constructor.typeSignatureIn(tp)
typeSig: reflect.runtime.universe.Type = (arg: Int)Generic[Int]

​

Type#declaration just gives you back a Symbol representing a method. You would get the same symbol back from typeOf[Generic[_].decl(...). 

​

-jason

[Piotr Kolaczkowski]

Nice! That works! Thanks.