Two sets of constructor parameters in a scala class

Question

What does this code do? Why is there two sets of constructor parameters?

class A(val x: Int)(val y: Int)

I can initialize an object and use both fields:

val a = new A(5)(7)
println(a.x + ", " + a.y)

If I make it a case class, I can match only by the first set of parameters.

case class A(x: Int)(y: Int)
val a = A(5)(7)
a match {
  A(x) => println(x)
}

It's not possible to create 3 sets of parameters. It doesn't compile. So what is the meaning of the two sets of constructor parameters?

It's not something I made up. I found this construction in the scala swing library: lampsvn.epfl.ch/trac/scala/browser/scala/trunk/src/swing/scala/… — PeWu
– PeWu, Commented Jul 28, 2010 at 10:16

Nicolas · Accepted Answer · 2010-07-28 09:48:12Z

7

According to the scala specification (see section 5.3), the second set of parameters is dedicated to implicit parameters. Dividing the parameters in two sets allow you to define only non-implicit paameter and let the other be contextually defined.

It is quite strange actually that the compiler accpet non-implicit parameters in the second set.

answered Jul 28, 2010 at 9:48

Nicolas

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3 Comments

PeWu Over a year ago

Thanks. If I add the implicit keyword into the second set, it starts to be implicit, but it's not implicitly implicit without the implicit keyword. Anyway, the scala specification does not allow non-implicit parameters in the second set.

Nicolas Over a year ago

Well... this section of the spec seems very strange: the example given right below the grammar does not seem to correspond to the gramar. And "class Test (a:Int)(b:Int)(c:Int)" is a valid class definition.

I82Much Over a year ago

@PeWu +1 for having the phrase "it's not implicitly implicit without the implicit keyword" :)

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