Why this structural type bound does not work as expected?

Question

I'm trying to write a simple helper method that receives something that can be closed and some function which receives the former and ensures the "closeable" is closed after executing the function.

For example, I want to use it like this:

  closing(new FileOutputStream("/asda"))(_.write("asas"))

My impl is

object Helpers {

  def closing[T <: { def close }](closeable: T)(action: T => Unit): Unit =
    try action apply closeable finally closeable close

}

But when trying to compile this simple test:

object Test {

  import Helpers._

  closing(new FileOutputStream("/asda"))(_.write("asas"))

}

The compiler complains with:

inferred type arguments [java.io.FileOutputStream] do not conform to method closing's type parameter bounds [T <: AnyRef{def close: Unit}]

Any ideas why?

Answer 1

You need to write

def closing[T <: { def close() }]

there is a difference in Scala between methods with empty parentheses and methods without parentheses at all.

Answer 2

Type bounds are tricky. In particular, Scala keeps track of the number of parameter lists in addition to the parameters themselves. Try these out!

class A { def f = 5 }
class B { def f() = 5 }
class C { def f()() = 5 }
def useA[X <: { def f: Int }](x: X) = x.f
def useB[X <: { def f(): Int }](x: X) = x.f
def useC[X <: { def f()(): Int}](x: X) = x.f

useA(new A)  // This works, but what other combinations do?

In your case, you want

def closing[T <: { def close() }] ...

P.S. If you really plan on using this a lot, you probably ought also play with

class D extends B { override def f = 6 }
class E extends A { override def f() = 6 }

and see which use you need to use in each case.