1. Getting Functional

2. Functional Programming (FP) in Scala
In FP,
Functions are first-class objects. That is, they are values, just like other objects are values, and can be treated as such
Functions can be assigned to variables, passed as parameters to higher-order functions, returned as results of functions
There is some way to write function literals
Functions should only transform their inputs into their outputs
A function should have no side effects
It should not do any input/output
It should not change any state (any external data)
Given the same inputs, a function should produce the same outputs, every time--it is deterministic
If a function is side-effect free and deterministic, it has referential transparency—all calls to the function could be replaced in the program text by the result of the function
But we need random numbers, date and time, etc. 2

3. Creating Lists scala> List('a', 'b', 'c')
res0: List[Char] = List(a, b, c)
scala> "abc" toList
res1: List[Char] = List(a, b, c)
scala> "Welcome to Scala" split(" ")
res2: Array[java.lang.String] = Array(Welcome, to, Scala)
scala> val scala = "Scala" toList
scala: List[Char] = List(S, c, a, l, a)
scala> "Hello" :: scala
res3: List[Any] = List(Hello, S, c, a, l, a) 3

4. Nil and :: scala> List() res4: List[Nothing] = List() scala> Nil
res5: scala.collection.immutable.Nil.type = List()
scala> List() == Nil
res6: Boolean = true
scala> List[String]()
res7: List[String] = List()
scala> "xyz" :: Nil
res8: List[java.lang.String] = List(xyz)
scala> "abc" :: "xyz" :: Nil
res9: List[java.lang.String] = List(abc, xyz) 4

5. head, tail, and isEmpty scala> val penn = "Pennsylvania" toList
penn: List[Char] = List(P, e, n, n, s, y, l, v, a, n, i, a)
scala> penn head
res10: Char = P
scala> penn tail
res11: List[Char] = List(e, n, n, s, y, l, v, a, n, i, a)
scala> penn isEmpty
res12: Boolean = false
scala> Nil isEmpty
res13: Boolean = true
scala> Nil head
java.util.NoSuchElementException: head of empty list (plus many more lines!) 5

6. take, drop, and splitAt scala> penn
res16: List[Char] = List(P, e, n, n, s, y, l, v, a, n, i, a)
scala> penn take 4
res17: List[Char] = List(P, e, n, n)
scala> penn drop 4
res18: List[Char] = List(s, y, l, v, a, n, i, a)
scala> penn splitAt 4
res19: (List[Char], List[Char]) = (List(P, e, n, n),List(s, y, l, v, a, n, i, a))
scala> penn.splitAt(4)
res20: (List[Char], List[Char]) = (List(P, e, n, n),List(s, y, l, v, a, n, i, a)) 6

7. toString and mkString scala> List(1, 2, 3).toString
res25: String = List(1, 2, 3)
scala> List(1, 2, 3).toString == "List(1, 2, 3)"
res26: Boolean = true
scala> List(1, 2, 3) mkString(" is less than ")
res27: String = 1 is less than 2 is less than 3
scala> List(1, 2, 3) mkString("*")
res28: String = 1*2*3
scala> List(1, 2, 3) mkString("<: ", "--", " :>")
res29: String = <: :>
scala> List(1, 2, 3) mkString("(", ", ", ")")
res30: String = (1, 2, 3) 7

8. zip and unzip scala> val words = "one two three" split " "
words: Array[java.lang.String] = Array(one, two, three)
scala> val numbers = List(1, 2, 3, 4, 5)
numbers: List[Int] = List(1, 2, 3, 4, 5)
scala> val z = words zip numbers
z: Array[(java.lang.String, Int)] = Array((one,1), (two,2), (three,3))
scala> val zz = numbers zip words
zz: List[(Int, java.lang.String)] = List((1,one), (2,two), (3,three))
scala> z toMap
res31: scala.collection.immutable.Map[java.lang.String,Int] = Map((one,1), (two,2), (three,3))
scala> zz unzip
res32: (List[Int], List[java.lang.String]) = (List(1, 2, 3),List(one, two, three)) 8

9. Higher-order functions
The basic syntax of a function literal is parameter_list => function_body
A higher-order function is one that takes a function as a parameter, or returns a function as a result
scala> val brag = "Scala is great!" toList
brag: List[Char] = List(S, c, a, l, a, , i, s, , g, r, e, a, t, !)
scala> brag count((ch: Char) => ch == 'a')
res34: Int = 3
scala> brag count((ch: Char) => !(ch isLetter))
res35: Int = 3
scala> "Scala is great!".toList.count((ch: Char) => ch < 'f')
res36: Int = 9
scala> "aeiou" contains 'e'
res37: Boolean = true
scala> "Scala is great!".toList.count((ch: Char) => "aeiou" contains ch)
res38: Int = 5 9

10. Abbreviations
In a literal function, you can usually omit the type (and, if there’s only one parameter, the parentheses)
scala> brag res40: List[Char] = List(S, c, a, l, a, , i, s, , g, r, e, a, t, !)
scala> brag count((ch: Char) => ch == 'a') res41: Int = 3
scala> brag count (ch => ch == 'a') res42: Int = 3
In fact, if there is only one parameter, used once, you can omit the parameter and the => and just use _ to stand in for the parameter
scala> brag count (_ == 'a') res44: Int = 3
You can use underscores to stand for multiple parameters, provided each is used once, and they are used in order
scala> (1 to 10).foldLeft(0)(_ + _) res27: Int = 55 10

11. sortWith scala> brag sortWith((x, y) => x < y)
res49: List[Char] = List( , , !, S, a, a, a, c, e, g, i, l, r, s, t)
Since there are two parameters, we can use two underscores
scala> brag sortWith (_ < _)
res50: List[Char] = List( , , !, S, a, a, a, c, e, g, i, l, r, s, t)
Order matters!
scala> brag sortWith (_ > _)
res52: List[Char] = List(t, s, r, l, i, g, e, c, a, a, a, S, !, , ) 11

12. forall and exists
Whereas count returns an Int, forall and exists return a Boolean
scala> val n = List(3, 1, 4, 1, 6)
n: List[Int] = List(3, 1, 4, 1, 6)
scala> n forall(x => x < 8)
res53: Boolean = true
scala> n forall(x => x < 5)
res54: Boolean = false
scala> n exists(_ < 5)
res55: Boolean = true
scala> n exists(_ > 8)
res56: Boolean = false 12

13. foreach foreach returns the (uninteresting) Unit value
scala> val brag = List("Scala", "is", "great!") brag: List[java.lang.String] = List(Scala, is, great!)
scala> brag foreach (println(_))
Scala is great!
scala> brag foreach(println)
scala> List(3, 1, 4, 1, 6) foreach (x => if (x > 1) println(x))
3 4 6 13

14. Least upper bound The following list contains only integers:
scala> val list = List(1, 2, 3) list: List[Int] = List(1, 2, 3)
Now let’s add a non-integer to it
scala> "hello" :: list res0: List[Any] = List(hello, 1, 2, 3)
The type of this new list is the lowest class in the hierarchy that is a superclass of all the elements of the list

15. Closures scala> var c = 5 c: Int = 5
scala> val mult = (x: Int) => c * x
mult: (Int) => Int = <function1>
scala> mult(3)
res63: Int = 15
scala> c = 7
c: Int = 7
scala> mult(10)
res64: Int = 70
What will happen if we pass the mult function into another function in another context?
It continues to “link to” (enclose?) the original variable c, not it’s value 15

16. map
map produces a new list by applying the given function to each element of the given list
scala> val n = List(1, 2, 3, 4, 5)
n: List[Int] = List(1, 2, 3, 4, 5)
scala> n map(x => x * x)
res65: List[Int] = List(1, 4, 9, 16, 25)
scala> n map (_ >= 3)
res66: List[Boolean] = List(false, false, true, true, true) 16

17. flatMap
flatMap produces a new list by applying the given function to each element of the given list, and “flattening” the result
scala> n
res70: List[Int] = List(1, 2, 3, 4, 5)
scala> n map (x => List(x, x*x))
res72: List[List[Int]] = List(List(1, 1), List(2, 4), List(3, 9), List(4, 16), List(5, 25))
scala> n flatMap(x => List(x, x*x))
res73: List[Int] = List(1, 1, 2, 4, 3, 9, 4, 16, 5, 25) 17

18. filter
filter produces a new list consisting of the values that pass the given test
scala> n
res67: List[Int] = List(1, 2, 3, 4, 5)
scala> n filter (_ < 4)
res68: List[Int] = List(1, 2, 3)
scala> n filter (_ % 2 == 1)
res69: List[Int] = List(1, 3, 5) 18

19. Some Map methods
These methods have little to do with “being functional,” so think of them as a bonus :-)
We’re going to use a map that takes small numbers (1..5) to their squares:
scala> val m = Map(1 -> 1, 2 -> 4, 3 -> 9, 4 -> 16, 5 -> 25) m: scala.collection.immutable.Map[Int,Int] = Map((5,25), (1,1), (2,4), (3,9), (4,16))
scala> m(4) res1: Int = 16
scala> m(10) java.util.NoSuchElementException: key not found: and many more lines of error message
scala> m.get(4) res3: Option[Int] = Some(16)
scala> m.get(10) res4: Option[Int] = None
scala> m.get(4) match { | case Some(x) => x | case None => | } res5: Int = 16 19

20. Another Map method Now consider the following carefully: Given that
scala> m res6: scala.collection.immutable.Map[Int,Int] = Map((5,25), (1,1), (2,4), (3,9), (4,16))
scala> m.getOrElse(4, -999) res7: Int = 16
scala> m.getOrElse(10, -999) res8: Int = -999
Now consider the following carefully: Given that
scala> "hello" :: List(1, 2, 3) res9: List[Any] = List(hello, 1, 2, 3)
What do you think the type and value of the following will be?
scala> val x = m.getOrElse(4, "Hello")

21. “Houston, we have a problem.”
scala> val x = m.getOrElse(4, "Hello") x: Any = 16
That’s pretty scary--but it gets worse:
scala> x + 1 <console>:8: error: type mismatch; found : Int(1) required: String x ^

22. The End
If I were to pick a language to use today other than Java, it would be Scala James Gosling, creator of Java 22