- 12.1 Functions as Values
- 12.2 Anonymous Functions
- 12.3 Parameters That Are Functions
- 12.4 Parameter Inference
- 12.5 Useful Higher-Order Functions
- 12.6 Closures
- 12.7 Interoperability with Lambda Expressions
- 12.8 Currying
- 12.9 Methods for Composing, Currying, and Tupling
- 12.10 Control Abstractions
- 12.11 Nonlocal Returns
- Exercises
This chapter is from the book
This chapter is from the book
This chapter is from the book
12.2 Anonymous Functions
In Scala, you don’t have to give a name to each function, just like you don’t have to give a name to each number. Here is an anonymous function:
(x: Double) => 3 * x
This function multiplies its argument by 3.
Of course, you can store this function in a variable:
val triple = (x: Double) => 3 * x
That’s just as if you had used a def:
def triple(x: Double) = 3 * x
But you don’t have to name the function. You can just pass it to another function:
Array(3.14, 1.42, 2.0).map((x: Double) => 3 * x) // Array(9.42, 4.26, 6.0)
Here, we tell the map method: “Multiply each element by 3.”
NOTE
If you prefer, you can enclose the function argument in braces instead of parentheses, for example:
Array(3.14, 1.42, 2.0).map{ (x: Double) => 3 * x }
This is more common when a method is used in infix notation (without the dot).
Array(3.14, 1.42, 2.0) map { (x: Double) => 3 * x }
NOTE
As noted in Chapter 2, some people consider anything that is declared with def to be a method. However, in this book, we use a conceptually cleaner mental model. Methods are invoked on objects. They are members of classes, traits, or objects. Top-level and block-level def statements declare functions.
In those cases, you cannot tell whether def declares anything other than a function. The expression triple is indistinguishable from the function (x: Double) => 3 * x. (This conversion is called eta expansion, using terminology from the lambda calculus.)
CAUTION
There is only one exception to eta expansion—when there are no parameters:
def heads() = scala.math.random() < 0.5
For arcane reasons, heads is a syntax error. The function is heads _, and it is invoked as heads().