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Anonymous Scala function syntax

Asked 2009-10-11T14:31:08.517
15

I'm learning more about Scala, and I'm having a little trouble understanding the example of anonymous functions in http://www.scala-lang.org/node/135. I've copied the entire code block below:

object CurryTest extends Application {
    def filter(xs: List[Int], p: Int => Boolean): List[Int] =
        if (xs.isEmpty) xs
        else if (p(xs.head)) xs.head :: filter(xs.tail, p)
        else filter(xs.tail, p)

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

    val nums = List(1, 2, 3, 4, 5, 6, 7, 8)
    println(filter(nums, modN(2)))
    println(filter(nums, modN(3)))
}

I'm confused with the application of the modN function

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

In the example, it's called with one argument

modN(2) and modN(3)

What does the syntax of modN(n: Int)(x: Int) mean?

Since it's called with one argument, I'm assuming they're not both arguments, but I can't really figure out how the values from nums get used by the mod function.

syntax scala anonymous-function
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2

In that example, modN returns a function that mods by the particular N. It saves you from having to do this:

def mod2(x:Int): Boolean = (x%2) == 0
def mod3(x:Int): Boolean = (x%3) == 0

The two pairs of parens delimit where you can stop passing arguments to the method. Of course, you can also just use a placeholder to achieve the same thing even when the method only has a single argument list.

def modN(n: Int, x: Int): Boolean = (x % n) == 0

val nums = List(1, 2, 3, 4, 5)
println(nums.filter(modN(2, _)))
println(nums.filter(modN(3, _)))
answered 2009-10-11T14:53:04.060

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