GettingFunctional

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,Getting Functional,What is Functional Programming (FP)?,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,But we need random numbers, date and time, etc.,Functions have,referential transparency,any given function call could safely be replaced by the functions result,2,Creating Lists,scala List(a, b, c),res0: ListChar = List(a, b, c),scala abc toList,res1: ListChar = List(a, b, c),scala Welcome to Scala split( ),res2: Arrayjava.lang.String = Array(Welcome, to, Scala),scala val scala = Scala toList,scala: ListChar = List(S, c, a, l, a),scala Hello : scala,res3: List,Any, = List(Hello, S, c, a, l, a),3,Nil,and,:,scala List(),res4: ListNothing = List(),scala Nil,res5: scala.collection.immutable.Nil.type = List(),scala List() = Nil,res6: Boolean = true,scala ListString(),res7: ListString = List(),scala xyz : Nil,res8: Listjava.lang.String = List(xyz),scala abc : xyz : Nil,res9: Listjava.lang.String = List(abc, xyz),4,head,tail, and,isEmpty,scala val penn = Pennsylvania toList,penn: ListChar = List(P, e, n, n, s, y, l, v, a, n, i, a),scala penn head,res10: Char = P,scala penn tail,res11: ListChar = 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,take,drop, and,splitAt,scala penn,res16: ListChar = List(P, e, n, n, s, y, l, v, a, n, i, a),scala penn take 4,res17: ListChar = List(P, e, n, n),scala penn drop 4,res18: ListChar = List(s, y, l, v, a, n, i, a),scala penn splitAt 4,res19: (ListChar, ListChar) = (List(P, e, n, n),List(s, y, l, v, a, n, i, a),scala penn.splitAt(4),res20: (ListChar, ListChar) = (List(P, e, n, n),List(s, y, l, v, a, n, i, a),6,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,zip,and,unzip,scala val words = one two three split ,words: Arrayjava.lang.String = Array(one, two, three),scala val numbers = List(1, 2, 3, 4, 5),numbers: ListInt = 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.Mapjava.lang.String,Int = Map(one,1), (two,2), (three,3),scala zz unzip,res32: (ListInt, Listjava.lang.String) = (List(1, 2, 3),List(one, two, three),8,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: ListChar = 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 aeiou contains e,res37: Boolean = true,scala Scala is great!.toList.count(ch: Char) = aeiou contains ch),res38: Int = 5,9,Abbreviations,In a literal function, you can usually omit the type (and, if theres only one parameter, the parentheses),scala brag,res40: ListChar = 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),:7: error: wrong number of parameters; expected = 1 brag count (_ = a | _ = e),10,sortWith,scala brag sortWith(x, y) = x brag sortWith (_ brag sortWith (_ _),res52: ListChar = List(t, s, r, l, i, g, e, c, a, a, a, S, !, , ),11,forall,and,exists,Whereas,count,returns an,Int,forall,and,exists,return a,Boolean,scala val n = List(3, 1, 4, 1, 6),n: ListInt = List(3, 1, 4, 1, 6),scala n forall(x = x n forall(x = x n exists(_ n exists(_ 8),res56: Boolean = false,12,foreach,foreach,returns the (uninteresting) Unit value,scala val brag = List(Scala, is, great!)brag: Listjava.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),346,13,Least upper bound,The following list contains only integers:,scala val list = List(1, 2, 3)list: ListInt = List(1, 2, 3),Now lets add a non-integer to it,scala hello : listres0: ListAny = 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,14,Closures,scala var c = 5,c: Int = 5,scala val mult = (x: Int) = c * x,mult: (Int) = Int = ,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 its value,15,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: ListInt = List(1, 2, 3, 4, 5),scala n map(x = x * x),res65: ListInt = List(1, 4, 9, 16, 25),scala n map (_ = 3),res66: ListBoolean = List(false, false, true, true, true),16,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: ListInt = List(1, 2, 3, 4, 5),scala n map (x = List(x, x*x),res72: ListListInt = 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: ListInt = List(1, 1, 2, 4, 3, 9, 4, 16, 5, 25),17,filter,filter,produces a new list consisting of the values that pass the given test,scala n,res67: ListInt = List(1, 2, 3, 4, 5),scala n filter (_ n filter (_ % 2 = 1),res69: ListInt = List(1, 3, 5),18,Some Map methods,These methods have little to do with “being functional,” so think of them as a bonus :-),Were 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.MapInt,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: 10.,.and many more lines of error message,scala,m.get(4,)res3: OptionInt = Some(16),scala,m.get(10,)res4: OptionInt = None,scala,m.get(4) match ,|,case Some(x) = x,|,case None = -999,|,res5: Int = 16,19,Another Map method,scala,m,res6: scala.collection.immutable.MapInt,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: ListAny = 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),20,“Houston, we have a problem.”,scala,val x = m.getOrElse(4, Hello),x: Any = 16,Thats pretty scary-but it gets worse:,scala,x + 1,:8: error: type mismatch;found : Int(1) required: String x + 1 ,21,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,