Getting FunctionalWhat is Functional Programming (FP)?Creating ListsNil and ::head, tail, and isEmptytake, drop, and splitAttoString and mkStringzip and unzipHigher-order functionsAbbreviationssortWithforall and existsforeachLeast upper boundClosuresmapflatMapfilterSome Map methodsAnother Map method“Houston, we have a problem.”The EndGetting Functional2What is Functional Programming (FP)?In FP,Functions are first-class objects. That is, they are value s, 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But we need random numbers, date and time, etc.Functions have referential transparency—any given function call could safely be replaced by the function’s result23Creating 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)34Nil 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)45head, 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!)56take, 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))67toString 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 = <: 1--2--3 :>scala> List(1, 2, 3) mkString("(", ", ", ")")res30: String = (1, 2, 3)78zip 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))89Higher-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 = 5910Abbreviations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Let me repeat that: Used oncescala> brag count (_ == 'a' || _ == 'e')<console>:7: error: wrong number of parameters; expected = 1 brag count (_ == 'a' || _ == 'e')1011sortWith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, !, , )1112forall 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 = false1213foreach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(_))Scalaisgreat!scala> brag foreach(println)Scalaisgreat!scala> List(3, 1, 4, 1, 6) foreach (x => if (x > 1) println(x))3461314Least upper boundThe following list contains only integers:scala> val list = List(1, 2, 3)list: