Introducing Pattern Matching in Scala

1. Introducing Pattern Matching
Ayush Kumar Mishra
Sr. Software Consultant
Knoldus

2. What is Pattern Matching
Pattern Matching lets you match a value against several
cases, sort of like a switch statement in Java. So in Java,
one might write something like this:
public boolean checkPrime(int number) {
// checks if a number between 1 and 10 is prime
switch (number) {
case 1: return true;
case 2: return true;
case 3: return true;
case 5: return true;
case 7: return true;
default: return false;
}
}

3. ●
One of the major limitations of switch/case in Java
(and really any C derivative language) is that it can
only be used on primitives.
●
You were unable to use strings as entries in switch-
case statements.
●
As the name suggests, pattern matching enables the
checking of a sequence of tokens for the presence
of the same pattern.
●
The pattern matching is not something new. It is
something that appears already in some other
languages like OCaml, Haskell.

4. ●
A pattern match includes a sequence of alternatives
●
An alternative starts with the keyword case.
General syntax:
case pattern => result
●
Each alternative includes a pattern and one or more
expressions, which will be evaluated if the pattern
matches.
●
An arrow symbol => separates the pattern from the
expressions.
●
Symbol _ is used to formulate the default case.

5. REPL
Traditional approach:
object Test {
def knolX(x: Int): String = x match {
case 1 => "one"
case 2 => "two"
case _ => "many"
}
}

6. REPL
defined module Test
scala> Test.knolX(2)
res2: String = two
scala> Test.knolX(3)
res3: String = many
scala>

7. Constant Pattern
●
ConstantMatch is a function that takes a parameter of
type Any.
●
Any is the Scala pendant to java.lang.Object.
●
This value is now matched against several constants.
object Test {
def knolX(x: Any): Any = x match {
case 1 => "one"
case "two" => 2
case y: Int => "scala.Int"
case _ => "many"
}
}

8. REPL
defined module Test
scala> Test.knolX(1)
res4: Any = one
scala> Test.knolX("two")
res5: Any = 2
scala>

9. Matching Using case Classes:
●
The case classes are special classes that are used
in pattern matching with case expressions.
●
Syntactically, these are standard classes with a
special modifier: case.
●
Case classes are classes with part of their
behavior predefined in order to make easier their
construction and their use in a pattern.

10. REPL
object Test {
val alice = new KnolX("Alice", 25)
val bob = new KnolX("Bob", 32)
val charlie = new KnolX("Charlie", 32)
for (knolX <- List(alice, bob, charlie)) {
knolX match {
case KnolX("Alice", 25) => println("Hi Alice!")
case KnolX("Bob", 32) => println("Hi Bob!")
}
}
// case class, empty one.
case class KnolX(name: String, age: Int)
}
scala> Test
Hi Alice!
Hi Bob!

11. Use OR in Case
def toYesOrNo(choice: Int): String = choice match {
case 1 | 2 | 3 => "yes"
case 0 => "no"
case _ => "error"
}
scala> toYesOrNo(1)
res7: String = yes
scala> toYesOrNo(2)
res8: String = yes
scala>

12. Functional approach to pattern matching
It provides an alternative way to design functions. For example, consider the
factorial function. If you choose the recursive version, usually, you would
define it like this:
def fact(n:Int):Int = if (n==0) 1 else n * factorial(n-1)
But you can use Scala’s pattern matching in this case:
def fact(n: Int): Int = n match {
case 0 => 1
case n => n * fact(n - 1)
}
scala> fact(5)
res10: Int = 120

13. Pattern matching and collection
Pattern matching may be applied to the collections.
Below is a function that computes the length of a list without
pattern matching:
def length[A](list : List[A]) : Int = {
if (list.isEmpty) 0
else 1 + length(list.tail)
}

14. Here is the same function with pattern matching:In this function, there are
two cases.
The last one checks if the list is empty with the Nil value. The first one
checks if there is at least one element is the list.
The notation _ :: tail should be understood “a list with whatever head
followed by a tail”. Here the tail can be Nil (ie. empty list) or a non-empty
list.
def length[A](list : List[A]) : Int = list match {
case _ :: tail => 1 + length(tail)
case Nil => 0
}

15. length: [A](list: List[A])Int
scala> length(List(1,2,3))
res8: Int = 3
scala> length(List())
res10: Int = 0

16. Typed pattern
Type patterns allow to calculate the result based on the type of the value.
● Use a type annotation to match certain types only:
def whatIsIt(any: Any) = any match {
case x: String => "A String: " + x
case _: Int => "An Int value"
case _ => "Something unknown"
}
scala> whatIsIt("12:01")
res01: java.lang.String = A String: 12:01
scala> whatIsIt(1)
res1: java.lang.String = An Int value
The typed pattern is always combined with the wildcard or variable pattern

17. Tuple pattern
• Use tuple syntax to match and decompose tuples:
def whatIsIt(any: Any) = any match {
case ("12:00", "12:01") => "12:00..12:01"
case ("12:00", x) => "High noon and " + x
case _ => "Something else"
}
•
scala> whatIsIt("12:00" -> "midnight")
res0: java.lang.String = High noon and midnight
• The tuple pattern is combined with other patterns, e.g. with the
constant or variable pattern

18. Constructor pattern
● Use constructor syntax to match and decompose case classes:
def whatIsIt(any: Any) = any match {
case Time(12, 00) => "High noon"
case Time(12, minutes) => "12:%02d" format minutes
}
scala> whatIsIt(Time(12, 01))
res0: java.lang.String = 12:01
● The constructor pattern is combined with other pattern, e.g. with the
constant or variable pattern or with deeply nested constructor
patterns

19. For-expressions
with Scala

20. ●
For-expressions are for iteration, but they aren’t loops,
they yield a collection.
●
General syntax:
for (seq) yield expr
●
seq contains generators, definitions and filters
●
expr creates an element of the resulting collection

21. Generators
●
Generators drive the iteration
x <- coll
●
coll is the collection to be iterated .
●
x is a variable bound to the current element of the iteration.
●
The (first) generator determines the type of the result:
●
scala> for (i <- List(1, 2, 3)) yield i + 1
res0: List[Int] = List(2, 3, 4)
●
scala> for (i <- Set(1, 2, 3)) yield i + 1
res1: ...Set[Int] = Set(2, 3, 4)

22. Multiple generators
●
Either separate multiple generators by semicolon
●
Or better use curly braces and new lines:
●
scala> for {
| i <- 1 to 3
| j <- 1 to i
| } yield i * j
res0: ...IndexedSeq[Int] = Vector(1, 2, 4, 3, 6, 9)

23. Filters
●
Filters control the iteration
if expr
●
expr must evaluate to a Boolean
●
Filters can follow generators without semicolon or new line:
●
scala> for {
| i <- 1 to 3 if i % 2 == 1
| j <- 1 to i
| } yield i * j
res0: ...IndexedSeq[Int] = Vector(1, 3, 6, 9)

24. Definitions
●
Definitions are like local val definitions
x = expr
●
Definitions can also be directly followed by a filter:
●
scala> for {
| time <- times
| hours = time.hours if hours > 12
| } yield (hours - 12) + "pm"
res0: List[String] = List(1pm, 2pm)