Higher order functions allow functions to be passed as parameters to other functions and returned from functions. This document discusses higher order functions in Scala including examples of functions that take functions as parameters and return functions. It also covers call-by-value vs call-by-name parameter passing and provides an example of using call-by-name to avoid evaluating expensive expressions multiple times. Finally, it demonstrates using a using method to ensure resources are closed in a finally block to avoid resource leaks.

1. Higher order functions

2. Higher order functions
Functions which take functions as parameters
and/or return functions

3. Higher order functions
Short summary of first class functions:
val even: (Int => Boolean) = (i: Int) => i % 2 == 0
Same type definition:
def test(numbers: List[Int], f: Int => Boolean) = ...
Call:
test(List(1, 2, 3), (i: Int) => i % 2 == 0)

4. Higher order functions
def test(numbers: List[Int], f: Int => Boolean) =
numbers.map(tall => f(tall))
// List[Boolean]

5. Higher order functions
Functions with several parameters must list them
in parenthesis:
def test(l: List[String], f: (Int, String) => Boolean)

6. call-by-value vs. call-by-name
by-value: expressions are evaluated before being
passed to the function
by-name: expressions evaluated inside function
nice when computationally expensive
possible to create nice APIs

7. call-by-value vs. call-by-name
Example: Logging
def thisTakesTime = {
println(“Slow computation”)
“result”
}
logger.debug(thisTakesTime())

8. call-by-value
def debug(s: String) {
println(“debug”)
if (logLevel <= DEBUG) println(s)
}
// Slow computation
// debug
// result

9. call-by-name
def debug(s: => String) {
println(“debug”)
if (logLevel <= DEBUG) println(s)
}
// debug
// Slow computation
// result

10. BufferedReader reader = null;
try {
reader = new BufferedReader(new FileReader("f.txt"));
System.out.println(reader.readLine());
} finally {
if (reader != null) {
try {
reader.close();
} catch (IOException e) {
// Exception on close, ignore
}
}
}
using(new BufferedReader(new FileReader("f.txt"))) {
reader => println(reader.readLine())
}

11. def using[T <: { def close() }, A]
(closeable: T)
(f: T => A) = {
try {
f(closeable)
} finally {
if (closeable != null) {
try {
closeable.close()
}
catch {
case e: Exception => // Do something clever!?
}
}
}
}

12. Tasks (30 min)
Open 'higher-order-functions' project
Tests in scalaexamples.higherorderfunctions
Add @Test to one and one method
Implement missing functions in PersonFilter and
so on.
Follow instructions in code
Make tests pass