Differences in "prose" and "dot notation" when invoking a method with (explicit) implicit parameters

Question

I'm not a Scala expert, and I'm confused about the syntax of calling a method with an implicit parameter.

Here's my situation:

I have a Spark RDD like this:

val myData: RDD[Array[String]] = ...

and have defined an Ordering for it:

object MyOrdering extends Ordering[Array[String]] = ...

I want to filter this RDD and take the top n entries in the RDD according to my Ordering. A Spark RDD has a method for taking the top n entries with this signature:

def top(num: Int)(implicit ord: Ordering[T]): Array[T] 

Initially I tried this code

myData filter { D =>
  D(9) == "yes"
} top(50)(MyOrdering)

Which fails with this error:

error: Int(50) does not take parameters
    } top(50)(MyOrdering)

However, this code works:

myData.filter(D => D(9) == "yes").top(50)(MyOrdering)

To my beginner's eye, the failing code sample and the working code sample look to be specifying equivalent logic. Am I wrong here? Am I actually doing something different in the two code samples? Or is this an issue with how the Scala compiler parses the code?

Answer 1

You can only replace the dot . with a space for method invocation when the method is arity-1 (1 parameter list) or arity-0 (no parameter lists). For example, this fails to compile:

class Foo {
    def baz(x: Int)(y: Int) = this
    def qux(x: Int) = this
}

(new Foo) baz(1)(1) //Does not compile
(new Foo).baz(1)(1) //Allowed
(new Foo) qux(1) //Allowed, since qux is arity-1
(new Foo).qux(1) //Also allowed, of course

When the the last parameter list is implicit the compiler can treat a method with n parameter lists as if it has arity n-1 if the method is being invoked with an implicit argument:

class Foo {
    def baz(x: Int)(implicit y: Int) = this
}

implicit val y: Int = 1
(new Foo) baz(1) //Allowed
(new Foo).baz(1) //Also fine
(new Foo) baz(1)(2) //Still not allowed
(new Foo).baz(1)(2) //Still allowed

Read more about the rules for method invocation in the style guide.