Accessing compiler-generated apply$default$1 from inside the companion object

[Daniel Armak]

Hi,

I asked this on SO but there were no answers, so I’ll try here.

When a method (in this example a constructor, apply) has a parameter with a default value, scala generates a synthetic method on the companion object named [method]$default$[index] where index is the parameter’s position.

I can access this method from outside the companion object, but not from inside it. This works:

case class C(i: Int = 1)
object C 
def x: Int = C.apply$default$1

But this doesn’t:

case class C(i: Int = 1)
object C {
  def x: Int = C.apply$default$1
}

scalac complains that value apply$default$1 is not a member of object C. Presumably the method is generated later than my own def x is processed.

I need to access this method from inside the companion object because I’m generating implicit typeclasses from a macro. Is there a way to do this?

Right now I’m generating code that uses runtime reflection to call the method, but that is inefficient and unsatisfying.

Thanks!

Daniel Armak

​

[Som Snytt]

I offered a partial answer on SO. Now, off to my day job.

There's an example for entering default params in a macro:

https://github.com/scala/scala/blob/v2.11.5/test/files/run/macro-default-params/Macros_1.scala

Probably the macro folks want to make it easier, over to time, to interact with the pipeline in this way.

It would be nice if a macro could say, Let me view this module symbol after the default-args micro-phase.

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[Daniel Armak]

Thanks Som, but I don’t understand what this macro (in Macros_1) has to do with the problem at hand. The macro creates a class C with a method f with a parameter a with a default value. So what?

I’ve been trying to follow up on your hint on SO. Unfortunately, I don’t understand Scala compiler internals either! You suggest that I “substitute your own namer in the attachment to enter your additional methods at the same time”. At the same time as what? Adding what methods?

I got my macro to find the ConstructorDefaultsAttachment and its Namer, and I should be able to use it to add methods to the module.

Thanks you for your help, really. I’m sorry I need more instructions, but it would probably take me way too long to figure out by myself everything that’s going on here.

​

Daniel Armak