Defining a variable in scala with two implicits

Question

I came across an interesting post on twitter by scalaLang. Where this code compiles and works

class A(implicit implicit val b: Int) 

val objA = new A()(42)

Can someone please explain me how is it working? I read the documentation of implicits but didn't found a case like this. Please explain me what's happening here.

Any help is appreciated !

Answer 1

After some digging I confirm what @Alexey Romanov said. Consider following example:

case class A(implicit implicit val a: Int)

def foo(x: Int)(implicit y: Int): Int = x * y

We could use it like this:

implicit val m: Int = 2
val myA = A()

And the following application:

val myAA = A()(2)
val n = myAA.a 
foo(3)

Now, foo(3) obviously yields 6 since it takes n implicitly. If we change the class to

case class A(implicit val a: Int)

it does not change the behavior of foo. Therefore, we arrive to the same conclusion that @Alexey - first implicit indicates that the constructor parameter can be passed implicitly; whereas the second one defines implicit value - even though in this case, they do the same thing.

Answer 2

You can have implicit before the last parameter list of a class or a method, and also before any member of a class or a trait. This simply combines both, which is probably legal just because forbidding it would make the language specification and the parser slightly more complex for no real benefit. I don't think there is any reason to ever use this or any difference from writing implicit once.