Scala: type mismatch when calling a method from generic trait

Question

Let's look at some code first:

trait Foo[T] {
  def fooize(value : T) : Unit
}
object TestFoo {
  def testFooForType[T[_] <: Foo[_]](obj : T[Int]) {
    obj.fooize(5)
  }
}

Why doesn't it typecheck and how can I overcome this difficulty? What I want to do is to test any class that implements the given trait Foo for a particular type T (Int in the example).

I don't need T as such in the body of testFooForType, you might as well use simply def testFooForType(obj: Foo[Int])... — sjrd
– sjrd, Commented Oct 9, 2014 at 11:55

Shyamendra Solanki · Accepted Answer · 2014-10-09 11:17:51Z

2

T[_] <: Foo[_] means "T forSome type" extends "Foo forSome type".

It has to allow for cases where the types are different. (T[A] extends Foo[B])

Binding T with Int type has no effect on Foo's type, which is still unbounded.

You can either specify Foo's type:

def testFooForType[T[_] <: Foo[Int]](obj : T[_]) {
    obj.fooize(5)
}

or you can make the type relationship explicit:

def testFooForType[T[V] <: Foo[V]](obj : T[Int]) {
    obj.fooize(5)
}

answered Oct 9, 2014 at 11:17

Shyamendra Solanki

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1 Comment

matb Over a year ago

Thanks. I was so focused on "T will just be a class extending trait Foo" that I didn't think about that.

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