[SOLVED] Scala syntax question in Rocket-chip config.scala

Scala syntax question in Rocket-chip config.scala

I just learned about the scalar to study rocket chips.
I see some strange codes in the Config.scala of Rocket-chip

abstract class Field[T] private (val default: Option[T])
{
   def this() // 1st-this
              = this(None) // 2nd-this
   def this(default: T) // 3rd-this
              = this(Some(default)) // 4th-this
}

The above code has 4 of this. I think 2nd/4th-this are identical.
But I'm not sure 2nd/4th-this are represent Field class self-type or not. If they are self-type, 1st/3rd-this are to be what?? I'm frustrated since I can't tell the definition of the above four this. Could you explain this?

Solution

These are called auxiliary constructors (see https://docs.scala-lang.org/scala3/book/domain-modeling-tools.html#classes).

The "main constructor" is the one defined by the class declaration:

class Field[T] private (val default: Option[T])

With this you can create instances of Field only by passing a Option[T]. Like Field(None) or Field(Some(...)).

Then, you have 2 additional auxiliary constructors. They are defined as regular methods but they need to be called this.

The following adds a constructor that accepts no parameter so that you can create instances with Field() and it will be the same as Field(None). The 2nd this refers to the main constructor.

def this() = this(None)

Same principle for the other auxiliary constructors which allows to call Field(x) instead of Field(Some(x)).

Note that you could achieve the same with apply methods in a companion object.