Question

scala> class A(implicit a: Int);
defined class A

scala> class B()(implicit a: Int);
defined class B

scala> new A()(1)
res1: A = A@159d450

scala> new B()(1)
res2: B = B@171f735

scala> new A(1)
<console>:7: error: too many arguments for constructor A: ()(implicit a: Int)A
       new A(1)

Why does Scalac insert an empty parameter list before the implicit parameter list provided in the class declaration?

This seems to be a feature, not a bug, judging by the commentary in the scalac sources:

// convert (implicit … ) to
()(implicit … ) if its the only
parameter section

I’m curious to know why this is done. I find it rather surprising.

Answer 1

The way I see it is that implicit parameter list does not replace the regular one(s). Since for constructor definitions at least one parameter list is needed, if nothing is indicated explicitly ‘()’ is generated.

While this might be indeed puzzling, it’s in line with generating an empty constructor when no parameter lists at all are present.