Structural type in method type parameterization in Scala?

Question

Consider the following Scala code (e.g., in REPL)

object A{def foo:Unit = {}}
object B{def foo:Unit = {}}

def bar[T <: Any {def foo: Unit}](param: T*):Unit = param.foreach(x => x.foo)

bar(A, A)  // works fine
bar(B, B)  // works fine
bar(A, B)  // gives error

The first two work fine. The third ones gives an error:

error: inferred type arguments [ScalaObject] do not conform to method bar's type parameter bounds [T <: Any{def foo: Unit}]

Are there any ways to do what I want?

Answer 1

This is usually called structural typing, not duck typing. I edited your title. :)

I think that your problem is caused by defining the type parameter T and then using it in an invariant way. T can only refer to one concrete type, but you have parameters of different types A and B.

This works:

 def bar(param: {def foo: Unit}*) = param.foreach(x => x.foo)

Edit: Using a type alias also works:

 type T = {def foo: Unit}
 def bar(param: T*) = param.foreach(x => x.foo)

This works because the compiler will simply substitute the structural type in place of its alias, T. After the substitution, this example is exactly the same as the one above.