Functions and Closures

1 Local Functions

Define functions insider other functions:

def processFile(filename: String, width: Int) = {
    def processLine(filename: String, width: Int, line: String) = {
        if (line.length > width)
            println(filename + ": " + line.trim)
    }

    val source = Source.fromFile(filename)
    for (line <- source.getLines())
        processLine(filename, width, line)
}

2 First-Class Functions

Scala has first-class functions. Not only can you define functions and call them, but you can write down functions as unnamed literals and then pass them around as values.

A function literal is compiled into a class that when instantiated at runtime is a function value. Thus the distinction between function literals and values is that function literals exist in the source code, whereas function values exist as objects at runtime.

(x: Int) => x + 1

The => designates that this function converts the thing on the left (any integer x) to the thing on the right (x + 1). Function values are objects, so you can store them in variables if you like.

var increase = (x: Int) => x + 1
increase(10) // res0: Int = 11

increase = (x: Int) => {
    Println("haha")
    x + 1
}

Many Scala libraries give you opportunities to use them:

val someNumbers = List(-11, -10, -5, 0, 5, 10)
someNumbers.foreach((x: Int) => println(x))
someNumbers.filter((x: Int) => x > 0)

3 Short Forms of Function Literals

Scala provides a number of ways to leave out redundant information and write function literals more bfiefly.

someNumbers.foreach((x） => x > 0)
someNumbers.filter(x => x > 0)

4 Placeholder Syntax

To make a function literal even more concise, you can use underscores as placehohlders for one or more parameters.

someNumbers.filter(_ > 0)

_ > 0 is very short notation for a function that checks whether a value is greater than zero.

5 Partially applied functions

You can also replace an entire parameter list with an underscore:

def sum(a: Int, b: Int, c: Int) = a + b + c
val a = sum _

someNumbers.foreach(println _)

6 Closures