problem with objects with unspecified type parameters that have an upper bound

[clemens]

Hi everyone,

I am struggling to figure out why I am getting a compile error from the following code:

trait Vertex
trait Material[TPos <: Vertex]


object Main {
  def makeTuple(x: Material[_]) = (0, x)
}

$ scalac test.scala

test.scala:5: error: type arguments [_$1] do not conform to trait Material's type parameter bounds [TPos <: Vertex]

  def makeTuple(x: Material[_]) = (0, x)

$ scalac -version

Scala compiler version 2.11.4 -- Copyright 2002-2013, LAMP/EPFL

The problem seems to be related to the inferred type parameter in the constructor:

def makeList(x: Material[_]) = List(x)

gives the same error, while the following works (presumably because this defaults to List[Any] which avoids the issue):

def makeList(x: Material[_]) = List(0, x)

In these examples, it is possible to get makeTuple to work by adding explicit type parameters.

However, my problem is that I am working with a collection of Material objects with different type parameters (i.e. something like a List[Material[_]]. maybe that is not the correct way to express this?).

So I only have elements of type Material[_] to work with.

I would love to understand what is going on here, any insights would be much appreciated.

Thanks,

Clemens

[Stephen Compall]

On Thu, 2015-01-22 at 12:18 -0800, clemens wrote:

$ scalac test.scala

test.scala:5: error: type arguments [_$1] do not conform to trait Material's type parameter bounds [TPos <: Vertex]

  def makeTuple(x: Material[_]) = (0, x)

Yes; you need to say Material[_ <: Vertex] here. See SI-1786. For an explanation of the Java behavior that might have led you to write your existential this way, see SI-6169 analysis starting here and the 5 comments that follow.

However, my problem is that I am working with a collection of Material objects with different type parameters (i.e. something like a List[Material[_]]. maybe that is not the correct way to express this?).

Right. You are actually dealing with a List[Material[_ <: Vertex]].

-- Stephen Compall "^aCollection allSatisfy: [:each|aCondition]": less is better