scala-vs-java

Grouped Parameters

Java

In Java, there is no way to group parameters of a method. We will see later that grouping parameters will enable us to do currying and later on we will see how difficult and severely verbose the Java code can become in an attempt to support currying. Lacking parameter grouping is the main contributor to this problem.

Passing multiple varargs is also impossible in Java but with the help of parameter grouping we can easily define a function supporting multiple varargs.

Scala

GroupedParameters.scala

def divisibleBy(n: Int)(x: Int) = (x % n) == 0

divisibleBy(5)(10)

val divisibleByFive = divisibleBy(5)(_)

divisibleByFive(10)

def multiSum(as: Int*)(bs: Int*)(cs: Int*) = as.sum * bs.sum * cs.sum

multiSum(1, 2, 3)(4, 5, 6, 7, 9)(10, 11)

As you see in the example above you can group function parameters into several groups. Each group can have multiple parameters all defined in the same enclosing round brackets.

There are several reasons to group parameters. We briefly mention some of them here:

1. Implicit Parameters need to be grouped. We see them shortly in the next section.
2. Currying
   • A powerful technique that we will see shortly. If you want to know a bit more about it right now just look at the example above. The divisibleBy(5)(_) is an example of currying. As you see we’re passing only the first group of arguments and we leave the second group (x:Int ).
3. Type inference
   • Sometimes in order to improve type inference we have to group parameters. Don’t worry if this does not make total sense, we will discuss it in details later. For now just keep in mind that in some scenarios, in order to help compiler infer types we can group parameters. Compiler will infer types per group in the same order that they are defined.
4. Multiple Varargs
   • In the example above multiSum is an example of defining multiple varargs params. This is not possible in Java.