What is a higher-order function?

Explanation

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Higher-order Functions
Scala allows the definition of higher-order functions. These are functions that take other functions as parameters, or whose result is a function. Here is a function apply which takes another function f and a value v and applies function f to v:
```
def apply(f: Int => String, v: Int) = f(v)
```
Note: methods are automatically coerced to functions if the context requires this.
Read more: Scala documentation
Higher-order Functions
Here is another example of usage of higher-order functions:
```
class Decorator(left: String, right: String) {
  def layout[A](x: A) = left + x.toString() + right
}
object QuizfulTest extends App {
  def apply(f: Int => String, v: Int) = f(v)
  val decorator = new Decorator("[", "]")
  println(apply(decorator.layout, 2015))
}
```
Execution yields the output:
```
[2015]
```
In this example, the method decorator.layout is coerced automatically to a value of type Int => String as required by method apply. Please note that method decorator.layout is a polymorphic method (i.e. it abstracts over some of its signature types) and the Scala compiler has to instantiate its method type first appropriately.
Read more: Scala documentation