Question

I need to restrict a Scala method parameter so that it implements both a trait and a structural subtype. How can I do this?

trait Foo
// ...
def someMethod[A <: Foo xxx { def close() }](resource: A)(block: A => Unit) {
  // ...
}

What do I put in place of xxx? I tried both extends and with, but got syntax errors.

Can it be done using a type definition for the structural subtype?

Answer 1

Yes, you can use type for this:

type CanBeClosed = {def close()}

def someMethod[A <: Foo with CanBeClosed](resource: A)(block: A => Unit) {
  // ...
}

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Recently I also wrote post about similar topic:

http://hacking-scala.posterous.com/composing-your-types-on-fly