Mario Gleichmann, profile picture
Uploaded byMario Gleichmann
PDF, PPTX2,685 views

Functional Scala II (in practice)

This document summarizes Mario Gleichmann's presentation on functional Scala to a Java user group in Frankfurt. It discusses functional programming concepts like functions as first-class values, higher-order functions, currying, partial application, and function composition using combinators. It provides examples of extracting prime numbers from a list and managing database resources functionally. The document concludes with an example problem of counting even words in a sentence.

Download as PDF, PPTX
1 / 112
2 / 112
3 / 112
4 / 112
5 / 112
6 / 112
7 / 112
8 / 112
9 / 112
10 / 112
11 / 112
12 / 112
13 / 112
14 / 112
15 / 112
16 / 112
17 / 112
18 / 112
19 / 112
20 / 112
21 / 112
22 / 112
23 / 112
24 / 112
25 / 112
26 / 112
27 / 112
28 / 112
29 / 112
30 / 112
31 / 112
32 / 112
33 / 112
34 / 112
35 / 112
36 / 112
37 / 112
38 / 112
39 / 112
40 / 112
41 / 112
42 / 112
43 / 112
44 / 112
45 / 112
46 / 112
47 / 112
48 / 112
49 / 112
50 / 112
51 / 112
52 / 112
53 / 112
54 / 112
55 / 112
56 / 112
57 / 112
58 / 112
59 / 112
60 / 112
61 / 112
62 / 112
63 / 112
64 / 112
65 / 112
66 / 112
67 / 112
68 / 112
69 / 112
70 / 112
71 / 112
72 / 112
73 / 112
74 / 112
75 / 112
76 / 112
77 / 112
78 / 112
79 / 112
80 / 112
81 / 112
82 / 112
83 / 112
84 / 112
85 / 112
86 / 112
87 / 112
88 / 112
89 / 112
90 / 112
91 / 112
92 / 112
93 / 112
94 / 112
95 / 112
96 / 112
97 / 112
98 / 112
99 / 112
100 / 112
101 / 112
102 / 112
103 / 112
104 / 112
105 / 112
106 / 112
107 / 112
108 / 112
109 / 112
110 / 112
111 / 112
112 / 112
Functional Scaλa … in Practice 20.10.2010 Java User Group Frankfurt / Main Mario Gleichmann Mario Gleichmann JUG Frankfurt / Main
Introduction Mario Gleichmann site: www.mg-informatik.de blog: 'brain driven development' gleichmann.wordpress.com mail: mario.gleichmann@mg-informatik.de Mario Gleichmann 2 JUG Frankfurt / Main
Summary Mario Gleichmann 3 JUG Frankfurt / Main
What is Functional Programming ? Functional Style val sum = fold( 1 to 10, add ) Expression based Computation method is function application Mario Gleichmann 4 fcn JUG Frankfurt / Main
What makes a Function ? Function Types val add = ( x :Int, y :Int ) => x + y Type of add : ( Int , Int ) => Int Mario Gleichmann 5 Obj JUG Frankfurt / Main
Closures 'closing over' var limit = 18 val isAdult = ( age :Int ) => age >= limit 'Closed term' Mario Gleichmann 6 appl JUG Frankfurt / Main
Algebraic Datatypes Example - Tree data Tree = Empty | Leaf Int | Node Int Tree Tree abstract case class Tree() case object Empty extends Tree case class Leaf( value: Int ) extends Tree case class Node( value: Int, left :Tree, right: Tree ) extends Tree Mario Gleichmann 7 insrt JUG Frankfurt / Main
Pattern Matching val depth :Tree => Int = _ match { case Empty => 0 case Leaf( _ ) => 1 case Node( _, left, right ) => 1 + max( depth( left ), depth( right ) ) } Mario Gleichmann 8 Prt isb JUG Frankfurt / Main
Partial Functions type =>?[-A, +B] = PartialFunction[A, B] val publisherRegionDE : Isbn =>? String = { case Isbn( 3, pubNr, _ ) => "D" + ( pubNr.toString charAt 0 ) } … publisherRegionDE.isDefinedAt( Isbn( 0, 677873, 8823 ) ) ) >> false Mario Gleichmann 9 chn JUG Frankfurt / Main
Comprehensions val factors = ( n :Int ) => for( x <- ( 1 to n ) if n % x == 0 ) yield x ... val prime = ( n :Int ) => factors( n ).toList == List( 1, n ) … val primes = (n :Int) => for( x <- (1 to n) if prime( x ) ) yield x Mario Gleichmann 10 Hgh rdr Fcn JUG Frankfurt / Main
Higher Order Functions val filter = ( predicate: Int => Boolean , xs :List[Int] ) => { for( x <- xs; if predicate( x ) ) yield x } Mario Gleichmann 11 Fcn Tp JUG Frankfurt / Main
Lambda Expressions filter( num => num % 2 == 0, List( 1, 2, 3, 4, 5, 6 ) ) … filter( _ > 0, List( -1, 5, 0, -3, 7 ) ) Mario Gleichmann 12 fst arg JUG Frankfurt / Main
Partial Application Mario Gleichmann 13 JUG Frankfurt / Main
Partial Application val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) Type of isPrime: Int => Boolean Mario Gleichmann 14 def prms wthn JUG Frankfurt / Main
Partial Application Delegation val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) val primesWithin = ( xs :List[Int] ) => filter( isPrime , xs ) Type of filter: ( Int => Boolean, List[Int] ) => List[Int] Mario Gleichmann 15 fxd var JUG Frankfurt / Main
Partial Application Delegation val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) val primesWithin = ( xs :List[Int] ) => filter( isPrime , xs ) fixed variable Mario Gleichmann 16 appl nappl JUG Frankfurt / Main
Partial Application New from Old ... val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) val primesWithin = filter( isPrime, _ :List[Int] ) applied unapplied Mario Gleichmann 17 Fcn Tp JUG Frankfurt / Main
Partial Application New from Old ... val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) val primesWithin = filter( isPrime, _ :List[Int] ) Type of primesWithin: List[Int] => List[Int] Mario Gleichmann 18 Crrng JUG Frankfurt / Main
Currying Mario Gleichmann 19 JUG Frankfurt / Main
Currying val filter = ( pred: Int => Boolean , xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean , List[Int] ) => List[Int] Mario Gleichmann 20 Crrd fcn JUG Frankfurt / Main
Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Mario Gleichmann 21 Crrd fcn Tp JUG Frankfurt / Main
Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => ( List[Int] ) => List[Int] Mario Gleichmann 22 sngl arg JUG Frankfurt / Main
Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => ( List[Int] ) => List[Int] Curried Function Curried Function Mario Gleichmann 23 sngl xpln arg JUG Frankfurt / Main
Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => ( List[Int] ) => List[Int] ... accepting one ... resulting in another A function (Function) Arg function Mario Gleichmann 24 lmbd xpr JUG Frankfurt / Main
Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => List[Int] => List[Int] ... is defined as a Lambda expession Mario Gleichmann 25 cls ovr JUG Frankfurt / Main
Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => List[Int] => List[Int] ... closes over to Arguments (Scope) of surrounding Function 'filter' Mario Gleichmann 26 Smp prms wthn JUG Frankfurt / Main
Example 1 – Extract primes val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } val primesWithin = filter( isPrime ) Mario Gleichmann 27 Fcn Tp JUG Frankfurt / Main
Example 1 – Extract primes val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } val primesWithin = filter( isPrime ) Type of primesWithin: List[Int] => List[Int] Mario Gleichmann 28 appl JUG Frankfurt / Main
Example 1 – Extract primes val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } val primesWithin = filter( isPrime ) ... primesWithin( List( 3, 4, 5, 6, 7 ) ) >> List( 3, 5, 7 ) Mario Gleichmann 29 Smp rc mgmt JUG Frankfurt / Main
Example 2 – Ressource Management val initQuery = (dataSource :DataSource) => (query :String) => ( extractor :ResultSet => Unit ) => { try{ val conn = dataSource.getConnection val stmt = conn.createStatement val resultSet = stmt.executeQuery( query ) extractor( resultSet ) } finally{ try{ if( conn != null ) conn.close } finally{ if( stmt != null ) stmt.close } } } Mario Gleichmann 30 appl JUG Frankfurt / Main
Example 2 – Ressource Management val dataSource :DataSource = ...; ... val query = initQuery( dataSource ) … query( "select * from User where age > 18" ) { result :ResultSet => while( result.next ) { .... } } … query( "select * from Account where balance < 1000" ) { result :ResultSet => while( result.next ) { .... } } Mario Gleichmann 31 smmy JUG Frankfurt / Main
Combinators Mario Gleichmann 32 JUG Frankfurt / Main
Combinators val power : Int => Int => Int = ( base :Int ) => ( exp :Int ) => if( exp == 1 ) base else base * power( base )( exp - 1 ) ... val square = power( _ :Int )( 2 ) Mario Gleichmann 33 JUG Frankfurt / Main
Combinators val power : Int => Int => Int = ( base :Int ) => ( exp :Int ) => if( exp == 1 ) base else base * power( base )( exp - 1 ) ... val square = power( _ :Int )( 2 ) Type of square : Int => Int Mario Gleichmann 34 JUG Frankfurt / Main
Combinators val power : Int => Int => Int = ( base :Int ) => ( exp :Int ) => if( exp == 1 ) base else base * power( base )( exp - 1 ) ... val square = power( _ :Int )( 2 ) Partial Application 'in the middle' Mario Gleichmann 35 JUG Frankfurt / Main
Combinators def flipArgs [A, B, C] ( f: A => B => C ) : B => A => C = ( x: B ) => ( y: A ) => f (y) (x) Transforms to another Function, expecting arguments in reversed order Mario Gleichmann 36 JUG Frankfurt / Main
Combinators def flipArgs [A, B, C] ( f: A => B => C ) : B => A => C = ( x: B ) => ( y: A ) => f (y) (x) … val square = flipArgs( power )( 2 ) … now just exploit Currying Mario Gleichmann 37 JUG Frankfurt / Main
Combinators Function Composition f :: B => C g :: A => B x ∈ A ( f o g )( x ) = f ( g ( x ) ) ( f o g ) :: A => C Mario Gleichmann 38 JUG Frankfurt / Main
Combinators : Function Composition def o [A,B,C] ( f: B => C ) ( g: A => B ) : A => C = ( x :A ) => f( g( x ) ) ... val mult = ( x :Int ) => ( y: Int ) => x * y val add = ( x :Int ) => ( y: Int ) => x + y ... val doubleSucc = o ( mult(2) ) ( add(1) ) Type of doubleSucc : Int => Int Mario Gleichmann 39 JUG Frankfurt / Main
Combinators : Function Composition def o [A,B,C] ( f: B => C ) ( g: A => B ) : A => C = ( x :A ) => f( g( x ) ) ... val mult = ( x :Int ) => ( y: Int ) => x * y val add = ( x :Int ) => ( y: Int ) => x + y ... val doubleSucc = o ( mult(2) ) ( add(1) ) doubleSucc( 7 ) >> 16 Mario Gleichmann 40 JUG Frankfurt / Main
Combinators : Function Composition def o [A,B,C] ( f: B => C ) ( g: A => B ) : A => C = ( x :A ) => f( g( x ) ) ... val mult = ( x :Int ) => ( y: Int ) => x * y val add = ( x :Int ) => ( y: Int ) => x + y ... val doubleSucc = o ( mult(2) ) ( add(1) ) doubleSucc( 7 ) Postfix ... >> 16 Mario Gleichmann 41 JUG Frankfurt / Main
Combinators : Function Composition class RichFunc [B, C] ( f: B => C ) { def o[A] ( g: A => B ) = (x :A) => f( g( x ) ) } implicit def toRichFunc[B,C]( f: B => C ) = new RichFunc( f ) ... val doubleSucc = mult(2) o add(1) doubleSucc( 7 ) Infix !!! >> 16 Mario Gleichmann 42 JUG Frankfurt / Main
Combinators : Function Composition class RichFunc [B, C] ( f: B => C ) { def o[A] ( g: A => B ) = (x :A) => f( g( x ) ) } implicit def toRichFunc[B,C]( f: B => C ) = new RichFunc( f ) ... val doubleSucc = mult(2) o add(1) doubleSucc( 7 ) 'Point free style' of function definition >> 16 Mario Gleichmann 43 JUG Frankfurt / Main
Example: Word Counter Count all even words within a sentence: “Hello World“ → 0 “The fox jumps over the lazy dog“ → 2 "This sentence contains five even words" → 5 Mario Gleichmann 44 JUG Frankfurt / Main
Example: Word Counter Count all even words within a sentence: “Hello World“ → 0 “The fox jumps over the lazy dog“ → 2 "This sentence contains five even words" → 5 Basic Functions: parse Words → Word to length → filter even → count Mario Gleichmann 45 JUG Frankfurt / Main
Example: Word Counter Basic Functions val words = ( sentence : String ) => sentence.split( " " ).toList val length = ( ws :List[String] ) => for( w <- ws ) yield w.length val filter = ( predicate: Int => Boolean ) => ( xs :List[Int] ) => ... val even = ( i :Int ) => i % 2 == 0 val size = ( xs :List[ _ ] ) => xs.size Mario Gleichmann 46 JUG Frankfurt / Main
Example: Word Counter val words = ( sentence : String ) => sentence.split( " " ).toList val length = ( ws :List[String] ) => for( w <- ws ) yield w.length val filter = ( predicate: Int => Boolean ) => ( xs :List[Int] ) => ... val even = ( i :Int ) => i % 2 == 0 val size = ( xs :List[ _ ] ) => xs.size val evenWordCount = size o filter( even ) o length o words Type of evenWordCount : String => Int Mario Gleichmann 47 JUG Frankfurt / Main
Example: Word Counter val words = ( sentence : String ) => sentence.split( " " ).toList val length = ( ws :List[String] ) => for( w <- ws ) yield w.length val filter = ( predicate: Int => Boolean ) => ( xs :List[Int] ) => ... val even = ( i :Int ) => i % 2 == 0 val size = ( xs :List[ _ ] ) => xs.size val evenWordCount = size o filter( even ) o length o words vs. val evenWordCount = (sentence :String ) => size ( filter( even ) ( length( words( sentence ) ) ) ) Mario Gleichmann 48 JUG Frankfurt / Main
Example: Word Counter val words = ( sentence : String ) => sentence.split( " " ).toList val length = ( ws :List[String] ) => for( w <- ws ) yield w.length val filter = ( predicate: Int => Boolean ) => ( xs :List[Int] ) => ... val even = ( i :Int ) => i % 2 == 0 val size = ( xs :List[ _ ] ) => xs.size val evenWordCount = size o filter( even ) o length o words evenWordCount( "this sentence contains five even words" ) >> 5 Mario Gleichmann 49 JUG Frankfurt / Main
Fundamental Functions Mario Gleichmann 50 JUG Frankfurt / Main
Fundamental Functions def sum ( list : List[Int] ) : Int = list match { case Nil => 0 case x :: xs => x + sum( xs ) } Mario Gleichmann 51 JUG Frankfurt / Main
Fundamental Functions def product ( list : List[Int] ) : Int = list match { case Nil => 1 case x :: xs => x * product( xs ) } Mario Gleichmann 52 JUG Frankfurt / Main
Fundamental Functions def allTrue ( ys : List[Boolean] ) : Boolean = ys match { case Nil => true case x :: xs => x and allTrue( xs ) } Mario Gleichmann 53 JUG Frankfurt / Main
Fundamental Functions : reduce def reduce ( list : List[ T ] ) : R = list match { case Nil => ? case x :: xs => x ʘ reduce( xs ) } Mario Gleichmann 54 JUG Frankfurt / Main
Fundamental Functions : reduce def reduce ( list : List[ T ] ) : R = list match { case Nil => ? 'unit' case x :: xs => x ʘ reduce( xs ) } 'subsuming operation' Mario Gleichmann 55 JUG Frankfurt / Main
Fundamental Functions : reduce def reduce[T,R] ( fun : T => R => R ) ( unit : R ) ( list : List[T] ) : R = list match { case Nil => unit case x :: xs => fun ( x ) ( reduce( fun )( unit )( xs ) ) } Mario Gleichmann 56 JUG Frankfurt / Main
Fundamental Functions : reduce def reduce[T,R] ( fun : T => R => R ) ( unit : R ) ( list : List[T] ) : R = list match { case Nil => unit case x :: xs => fun ( x ) ( reduce( fun )( unit )( xs ) ) } reduce ʘ unit t1 :: t2 :: t3 :: t4 :: Nil => t1 ʘ ( t2 ʘ ( t3 ʘ ( t4 ʘ unit ) ) ) Mario Gleichmann 57 JUG Frankfurt / Main
Fundamental Functions : reduce val add = (x : Int) => (y : Int) => x + y val mult = (x : Int) => (y : Int) => x * y ... val sum = reduce ( add ) ( 0 ) _ val product = reduce ( mult ) ( 1 ) _ ... sum( List( 1, 2, 3, 4 ) ) >> 10 product( List( 1, 2, 3, 4 ) ) >> 24 Mario Gleichmann 58 JUG Frankfurt / Main
Fundamental Functions : reduce val or = ( x :Boolean ) => ( y :Boolean ) => x || y val and = ( x :Boolean ) => ( y :Boolean ) => x && y val not = ( x :Boolean ) => !x ... val existOneTrue = reduce ( or ) ( false ) _ val allFalse = not o reduce ( or ) ( false ) _ val allTrue = reduce ( and ) ( true ) _ ... allTrue( List( 1 < 2, 3 == 3, 1 + 1 == 2 ) ) >> true Mario Gleichmann 59 JUG Frankfurt / Main
Example: List concatenation def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = reduce ( append ) _ listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) >> List( 8, 9, 1, 2, 3, 4 ) listConcat ( List( true, true, false ) ) ( List( true, false ) ) ) >> List( true, false, true, true, false ) Mario Gleichmann 60 JUG Frankfurt / Main
Example: List concatenation def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = reduce ( append ) _ listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) >> List( 8, 9, 1, 2, 3, 4 ) 'unit' listConcat ( List( true, true, false ) ) ( List( true, false ) ) ) >> List( true, false, true, true, false ) Mario Gleichmann 61 JUG Frankfurt / Main
Example: List concatenation def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = flipArgs( reduce ( append ) _ ) listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) >> List( 1, 2, 3, 4, 8, 9 ) listConcat ( List( true, true, false ) ) ( List( true, false ) ) ) >> List( true, true, false, true, false ) Mario Gleichmann 62 JUG Frankfurt / Main
Example: List concatenation def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = flipArgs( reduce ( append ) _ ) listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) >> List( 1, 2, 3, 4, 8, 9 ) 'unit' listConcat ( List( true, true, false ) ) ( List( true, false ) ) ) >> List( true, true, false, true, false ) Mario Gleichmann 63 JUG Frankfurt / Main
Example: List concatenation ! def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = flipArgs( reduce ( append ) _ ) listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) Type of result: List[Any] Mario Gleichmann 64 JUG Frankfurt / Main
Example: List concatenation def append[T] (x:T) (xs:List[ T ]) = x :: xs def listConcat[A] ( a: List[A]) ( b: List[A] ) : List[A] = reduce[ A, List[A] ] ( ( append[A] ) _ ) (b) (a) listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) Type of result: List[Int] Mario Gleichmann 65 JUG Frankfurt / Main
More Fundamental Functions Mario Gleichmann 66 JUG Frankfurt / Main
Fundamental Functions val doubleAll : List[Int] => List[Int] = _ match { case Nil => Nil case y :: ys => y * 2 :: doubleAll( ys ) } doubleAll( List( 1, 2, 3, 4 ) ) >> List( 2, 4, 6, 8 ) Mario Gleichmann 67 JUG Frankfurt / Main
Fundamental Functions val toLength : List[String] => List[Int] = _ match { case Nil => Nil case y :: ys => y.length :: toLength( ys ) } toLength( List( "a", "ab", "abc", "abcde" ) ) ) >> List( 1, 2, 3, 5 ) Mario Gleichmann 68 JUG Frankfurt / Main
Fundamental Functions : map val map : List[T] => List[R] = _ match { case Nil => Nil case y :: ys => f?( y ) :: map( ys ) } Mario Gleichmann 69 JUG Frankfurt / Main
Fundamental Functions : map def map[T, R] ( f: T => R ) : List[T] => List[R] = ( xs : List[T] ) => xs match { case Nil => Nil case y :: ys => f( y ) :: map( f )( ys ) } Mario Gleichmann 70 JUG Frankfurt / Main
Fundamental Functions : map def map[T, R] ( f: T => R ) : List[T] => List[R] = ( xs : List[T] ) => xs match { case Nil => Nil case y :: ys => f( y ) :: map( f )( ys ) } Accepting a function to Resulting in another function, apply on every element accepting the List of elements Mario Gleichmann 71 JUG Frankfurt / Main
Fundamental Functions : map def map[T, R] ( f: T => R ) : List[T] => List[R] = ... ... val mult = (x : Int) => (y : Int) => x * y val doubleAll = map( mult( 2 ) ) ... val toLength = map( ( s :String ) => s.length ) Mario Gleichmann 72 JUG Frankfurt / Main
Fundamental Functions : map val add = (x : Int) => (y : Int) => x + y val sum = reduce ( add ) ( 0 ) _ val sumMatrix = sum o map( sum ) Type of sum : List[Int] => Int Type of map( sum ) : List[List[Int]] => List[Int] Type of sumMatrix : List[List[Int]] => Int Mario Gleichmann 73 JUG Frankfurt / Main
Fundamental Functions : map val add = (x : Int) => (y : Int) => x + y val sum = reduce ( add ) ( 0 ) _ val sumMatrix = sum o map( sum ) ... val matrix = List( List( 1, 2, 3 ), List( 4, 5, 6 ), List( 7, 8, 9 ) ) sumMatrix( matrix ) >> 45 Mario Gleichmann 74 JUG Frankfurt / Main
map via reduce def applyAndPrepend [A, B] ( f: A => B ) = ( a :A ) => ( bs :List[B] ) => f( a ) :: bs def map [A, B] ( f: A => B ) : List[A] => List[B] = reduce ( applyAndPrepend( f ) ) ( Nil ) _ Mario Gleichmann 75 JUG Frankfurt / Main
map via reduce def applyAndPrepend [A, B] ( f: A => B ) = ( a :A ) => ( bs :List[B] ) => f( a ) :: bs def map [A, B] ( f: A => B ) : List[A] => List[B] = reduce ( applyAndPrepend( f ) ) ( Nil ) _ f :: f :: f :: f :: Nil X1 :: X2 :: X3 :: X4 :: Nil Mario Gleichmann 76 JUG Frankfurt / Main
filter via reduce def filter[T]( pred :T => Boolean ) : List[T] => List[T] = reduce ( ( x :T ) => ( acc :List[T] ) => if( pred( x ) ) x :: acc else acc ) ( Nil ) Mario Gleichmann 77 JUG Frankfurt / Main
Type Classes Mario Gleichmann 78 JUG Frankfurt / Main
Type Classes def sum( xs : List[Int] ) : Int = xs match { case Nil => 0 case head :: tail => head + sum( tail ) } Mario Gleichmann 79 JUG Frankfurt / Main
Type Classes def sum( xs : List[String] ) : String = xs match { case Nil => ““ case head :: tail => head + sum( tail ) } Concatenation ! Mario Gleichmann 80 JUG Frankfurt / Main
Type Classes def sum[A]( xs : List[A] ) : A = xs match { case Nil => ? case head :: tail => head ? sum( tail ) } Mario Gleichmann 81 JUG Frankfurt / Main
Type Classes def sum[A]( xs : List[A] ) : A = xs match { case Nil => ? 'neutral' element case head :: tail => head ? sum( tail ) } element composition Mario Gleichmann 82 JUG Frankfurt / Main
Type Classes trait SemiGroup[S] { def add( x : S, y : S ) : S element composition } trait Monoid[M] extends SemiGroup[M] { def unit : M 'neutral' element } Mario Gleichmann 83 JUG Frankfurt / Main
Type Classes def sum [A] ( xs : List[A], m: Monoid[A] ) : A = xs match { case Nil => m.unit case head :: tail => m.add( head, sum( tail, m) ) } Mario Gleichmann 84 JUG Frankfurt / Main
Type Classes def sum [A] ( xs : List[A], m: Monoid[A] ) : A = 'type constraint' xs match { case Nil => m.unit case head :: tail => m.add( head, sum( tail, m) ) } Mario Gleichmann 85 JUG Frankfurt / Main
Type Classes object IntMonoid extends Monoid[Int]{ def add(x : Int, y : Int): Int = x + y def unit : Int = 0 } … sum( List( 1, 2, 3, 4 ), IntMonoid ) Mario Gleichmann 86 JUG Frankfurt / Main
Type Classes def sum [A] ( xs : List[A] ) ( implicit m: Monoid[A] ) : A = ... implicit object IntMonoid extends Monoid[Int]{ … } … import typeclasses.monoid._ ... sum( List( 1, 2, 3, 4 ) ) Mario Gleichmann 87 JUG Frankfurt / Main
Example: Clock-Monoid sealed abstract case class ClockHour( hour: Int ) case object HOUR_0 extends ClockHour( 0 ) case object HOUR_1 extends ClockHour( 1 ) ... case object HOUR_11 extends ClockHour( 11 ) … implicit object ClockHourMonoid extends Monoid[ClockHour]{ val hours = Array( HOUR_0, HOUR_1, HOUR_2, ..., HOUR_11 ) def add(x : ClockHour, y : ClockHour) = hours( ( x.hour + y.hour ) % 12 ) def unit : ClockHour = HOUR_0 } Mario Gleichmann 88 JUG Frankfurt / Main
Example: Clock-Monoid import typeclasses.monoid.ClockHourMonoid ... sum( List( HOUR_5, HOUR_3, HOUR_8 ) ) >> HOUR_4 Mario Gleichmann 89 JUG Frankfurt / Main
Example: Contains element ? trait Eq[E]{ def equals( e1 :E, e2 :E ) : Boolean } def contains [T] ( x :T, xs : List[T] ) ( implicit eq :Eq[T] ) : Boolean = reduce ( ( y :T ) => ( acc :Boolean ) => if( eq.equals( x, y ) ) true else acc ) ( false ) 1st Arg: 'reducing' function ( xs ) 2nd Arg: 'unit' 3rd Arg: List to reduce Mario Gleichmann 90 JUG Frankfurt / Main
Example: Contains element ? case class Person( name :String, age :Int ) implicit object PersonEq extends Eq[Person]{ def equals( p1 :Person, p2 :Person ) : Boolean = (p1.name == p2.name) && (p1.age == p2.age) } ... val persons = List( Person("Hans", 22), Person( "Helga", 32 ), Person( "Hugo", 47 ) ) contains( Person( "Olga", 22 ), persons ) >> false contains( Person( "Helga", 32 ), persons ) >> true Mario Gleichmann 91 JUG Frankfurt / Main
Monads Mario Gleichmann 92 JUG Frankfurt / Main
Monads A simple Monad: Option << abstract >> Option[+A] Some[+A] None presence absence Handling the or of something ...as the result of a computation (computational environment) Mario Gleichmann JUG Frankfurt / Main
Monads A simple Monad: Option class CustomerDAO{ def findCustomer( custId: Long ) : Option<Customer> = { ... if( found( customer ) ) Some( customer ) else None } } Mario Gleichmann JUG Frankfurt / Main
Monads A simple Monad: Option class CustomerDAO{ def findCustomer( custId: Long ) : Option<Customer> = { ... if( found( customer ) ) Some( customer ) else None } } Explicit Notion, that there may be 'none' result Mario Gleichmann JUG Frankfurt / Main
Monads A simple Monad: Option val customerHit = customerDAO.findCustomer( 123 ); ... customerHit match { case Some( customer ) => println( customer.name ) case None => println( “not found“ ) } Mario Gleichmann JUG Frankfurt / Main
Monads A simple Monad: Option val customerHit = customerDAO.findCustomer( 123 ); ... customerHit match { case Some( customer ) => println( customer.name ) case None => println( “not found“ ) } Explicit Handling the absensce of a result Forces 'Awareness' Mario Gleichmann JUG Frankfurt / Main
Monads A simple Monad: Option val customerHit = customerDAO.findCustomer( 123 ); ... customerHit match { case Some( customer ) => println( customer.name ) case None => println( “not found“ ) } ... beside from that ... what's the deal ??? Mario Gleichmann JUG Frankfurt / Main
Monads A simple Monad: Option val customerHit = customerDAO.findCustomer( 123 ); ... customerHit match { case Some( customer ) => println( customer.name ) case None => println( “not found“ ) } 'Protected' Function Composition ... Mario Gleichmann JUG Frankfurt / Main
Monads val projects = Map( "Jan" -> "IKT", "Joe" -> "TensE", "Luca" -> "InTA" ) val customers = Map( "IKT" -> "Hanso GmbH", "InTA" -> "RAIA Duo" ) val cities = Map( "Hanso GmbH" -> "Stuttgart", "Mogno" -> "Mailand" ) Mario Gleichmann JUG Frankfurt / Main
Monads val projects = Map( "Jan" -> "IKT", "Joe" -> "TensE", "Luca" -> "InTA" ) val customers = Map( "IKT" -> "Hanso GmbH", "InTA" -> "RAIA Duo" ) val cities = Map( "Hanso GmbH" -> "Stuttgart", "Mogno" -> "Mailand" ) Where is Jan ? Jan -> IKT -> Hanso GmbH -> Stuttgart Mario Gleichmann JUG Frankfurt / Main
Monads Jav public String whereIs( String name ){ a String project = projects.get( name ); if( project != null ){ String customer = customers.get( project ); if( customer != null ){ String city = cities.get( customer ) if( city != null ) return city; else return “unknown“; } else return ''unknown''; } else return ''unknown''; } Mario Gleichmann JUG Frankfurt / Main
Monads Scal A simple Monad: Option a def whereIs( name: String ) = { projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Mario Gleichmann JUG Frankfurt / Main
Monads Scal A simple Monad: Option a def whereIs( name: String ) = { Results in Option[String] projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Mario Gleichmann JUG Frankfurt / Main
Monads Scal A simple Monad: Option a def whereIs( name: String ) = { Results in Option[String] projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Option[A].map( A => B ) >> Option[B] Mario Gleichmann JUG Frankfurt / Main
Monads Scal A simple Monad: Option a def whereIs( name: String ) = { Results in Option[String] projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Option[A].map( A => B ) >> Option[B] Option[A].map( A => Option[B] ) >> Option[Option[B]] Mario Gleichmann JUG Frankfurt / Main
Monads Scal A simple Monad: Option a def whereIs( name: String ) = { Results in Option[String] projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Option[A].map( A => B ) >> Option[B] Option[A].map( A => Option[B] ) >> Option[Option[B]] Option[A].flatMap( A => Option[B] ) >> Option[B] Mario Gleichmann JUG Frankfurt / Main
Monads Protected Composition map( A => B ) map( B => C ) map( C => D ) None None None ... Some[A] Some[B] Some[C] Some[D] Mario Gleichmann JUG Frankfurt / Main
Monads A simple Monad: Option def whereIs( name: String ) = ( for( project <- projects get name; customer <- customers get project; city <- cities get customer ) yield city ).getOrElse( "unknown!" ) Mario Gleichmann JUG Frankfurt / Main
Summary Scala allows for ... ● Currying ● Combinators ● Fundamental Functions ● Type classes ● Monads Mario Gleichmann JUG Frankfurt / Main
Thank you ! Mario Gleichmann 111 JUG Frankfurt / Main
References Scala Home www.scala-lang.org Dr. Erik Meijer C9 Lectures – Functional Programming Fundamentals http://channel9.msdn.com Graham Hutton Programming in Haskell Cambridge Odersky, Spoon, Venners Programming in Scala artima Mario Gleichmann 112 JUG Frankfurt / Main

More Related Content

PDF
Functional Scala I
byMario Gleichmann
 
PPT
Java cơ bản java co ban
byifis
 
PDF
CP Handout#3
bytrupti1976
 
PDF
CP Handout#5
bytrupti1976
 
PPTX
Scala 2.10.0 (english version)
byDaniel Sobral
 
PDF
Scala Workshop
byClueda AG
 
PDF
Scala for Java Developers (Silicon Valley Code Camp 13)
byRamnivas Laddad
 
KEY
Scala For Java Programmers
byEnno Runne
 
Functional Scala I
byMario Gleichmann
 
Java cơ bản java co ban
byifis
 
CP Handout#3
bytrupti1976
 
CP Handout#5
bytrupti1976
 
Scala 2.10.0 (english version)
byDaniel Sobral
 
Scala Workshop
byClueda AG
 
Scala for Java Developers (Silicon Valley Code Camp 13)
byRamnivas Laddad
 
Scala For Java Programmers
byEnno Runne
 

Similar to Functional Scala II (in practice)

PDF
Hitchhiker's Guide to Functional Programming
bySergey Shishkin
 
PDF
Scala Functional Patterns
byleague
 
PPTX
Scala
bysuraj_atreya
 
PDF
02. haskell motivation
bySebastian Rettig
 
PDF
Introduction à Scala - Michel Schinz - January 2010
byJUG Lausanne
 
ODP
Scala as a Declarative Language
byvsssuresh
 
PDF
High Wizardry in the Land of Scala
bydjspiewak
 
PDF
ScalaFlavor4J
byKazuhiro Sera
 
DOC
White box-sol
byNguyen Duc Nhieu
 
KEY
Haskellで学ぶ関数型言語
byikdysfm
 
KEY
An Introduction to Functional Programming using Haskell
byMichel Rijnders
 
PDF
Scala at GenevaJUG by Iulian Dragos
byGenevaJUG
 
PPTX
Softshake 2013: 10 reasons why java developers are jealous of Scala developers
byMatthew Farwell
 
PDF
Lecture 5: Functional Programming
byEelco Visser
 
PDF
Why Haskell Matters
byromanandreg
 
PDF
FSharp Talk
byHaiBin Chang
 
PDF
(How) can we benefit from adopting scala?
byTomasz Wrobel
 
PPTX
Kotlin
byBoKaiRuan
 
PDF
A taste of Functional Programming
byJordan Open Source Association
 
PPTX
Scala, Play 2.0 & Cloud Foundry
byPray Desai
 
Hitchhiker's Guide to Functional Programming
bySergey Shishkin
 
Scala Functional Patterns
byleague
 
Scala
bysuraj_atreya
 
02. haskell motivation
bySebastian Rettig
 
Introduction à Scala - Michel Schinz - January 2010
byJUG Lausanne
 
Scala as a Declarative Language
byvsssuresh
 
High Wizardry in the Land of Scala
bydjspiewak
 
ScalaFlavor4J
byKazuhiro Sera
 
White box-sol
byNguyen Duc Nhieu
 
Haskellで学ぶ関数型言語
byikdysfm
 
An Introduction to Functional Programming using Haskell
byMichel Rijnders
 
Scala at GenevaJUG by Iulian Dragos
byGenevaJUG
 
Softshake 2013: 10 reasons why java developers are jealous of Scala developers
byMatthew Farwell
 
Lecture 5: Functional Programming
byEelco Visser
 
Why Haskell Matters
byromanandreg
 
FSharp Talk
byHaiBin Chang
 
(How) can we benefit from adopting scala?
byTomasz Wrobel
 
Kotlin
byBoKaiRuan
 
A taste of Functional Programming
byJordan Open Source Association
 
Scala, Play 2.0 & Cloud Foundry
byPray Desai
 

Recently uploaded

PDF
Atlantix is an AI-first venture studio, building companies with AI at the core
byadmin625551
 
PDF
Session 1 - Agentic Automation Building the Enterprise Agent of Tomorrow
byDianaGray10
 
PDF
Fairness and Bias in AI Ethics and Explainability
byShiwani Gupta
 
PPTX
Cloud Metrics: The Cost-Value Equation (All Things Open 2025)
byEric D. Schabell
 
PDF
Ethical Initiatives in AI and accountability.pdf
byShiwani Gupta
 
PDF
The invisible key: Securing the new attack vector of OAuth tokens
byGianluca Varisco
 
PDF
Digital Sovereignty has to start with Open Source.pdf
byMindtrek
 
PPTX
Open Source business is of strategic importance to the EUpptx
byMindtrek
 
PDF
Dream Companion Reviews, Pricing, & Alternatives
byDream Companion
 
PPTX
⚡Level Up Your Slack Game: Workflows & Conditional Branching🎉
bySanjeetMishra29
 
PPTX
This Could Have Been a Slack Message: Diagnosing and Treating Bad Meetings
byMike Clement
 
PPTX
TechVerse Kent (Combining Tech, AI, Innovation and Creativity)
byKritarthDevli
 
PDF
Google Agentspace: Enterprise AI search platform
byKAVAIYAYASHKUMARAMRU
 
PPTX
WUG-DMV: Workfront Wizards: Tips & Tricks Exchange (Sept 2025)
byWUG-DC MARYLAND VIRGINIA
 
PDF
Supercharge your JVM performance with Project Leyden and Spring Boot
byAmmbra
 
PPTX
Building Europe with Open LLMs - Manu Setälä.pptx
byMindtrek
 
PPTX
UiPath Automation Suite Installation (Hands-On) [2/3]
bysuhanisingh58689
 
PDF
Phishing for Answers: A Tech Filler Quiz.pdf
byConquiztadors- the Quiz Society of Sri Venkateswara College
 
PPTX
Momentum Developer Con 2025 - How To Better Developer Estimates
byBrian McKeiver
 
PDF
UiPath DevConnect 2025: Introduction to Agentic Automation
byDianaGray10
 
Atlantix is an AI-first venture studio, building companies with AI at the core
byadmin625551
 
Session 1 - Agentic Automation Building the Enterprise Agent of Tomorrow
byDianaGray10
 
Fairness and Bias in AI Ethics and Explainability
byShiwani Gupta
 
Cloud Metrics: The Cost-Value Equation (All Things Open 2025)
byEric D. Schabell
 
Ethical Initiatives in AI and accountability.pdf
byShiwani Gupta
 
The invisible key: Securing the new attack vector of OAuth tokens
byGianluca Varisco
 
Digital Sovereignty has to start with Open Source.pdf
byMindtrek
 
Open Source business is of strategic importance to the EUpptx
byMindtrek
 
Dream Companion Reviews, Pricing, & Alternatives
byDream Companion
 
⚡Level Up Your Slack Game: Workflows & Conditional Branching🎉
bySanjeetMishra29
 
This Could Have Been a Slack Message: Diagnosing and Treating Bad Meetings
byMike Clement
 
TechVerse Kent (Combining Tech, AI, Innovation and Creativity)
byKritarthDevli
 
Google Agentspace: Enterprise AI search platform
byKAVAIYAYASHKUMARAMRU
 
WUG-DMV: Workfront Wizards: Tips & Tricks Exchange (Sept 2025)
byWUG-DC MARYLAND VIRGINIA
 
Supercharge your JVM performance with Project Leyden and Spring Boot
byAmmbra
 
Building Europe with Open LLMs - Manu Setälä.pptx
byMindtrek
 
UiPath Automation Suite Installation (Hands-On) [2/3]
bysuhanisingh58689
 
Phishing for Answers: A Tech Filler Quiz.pdf
byConquiztadors- the Quiz Society of Sri Venkateswara College
 
Momentum Developer Con 2025 - How To Better Developer Estimates
byBrian McKeiver
 
UiPath DevConnect 2025: Introduction to Agentic Automation
byDianaGray10
 

Functional Scala II (in practice)

  • 1.
    Functional Scaλa … in Practice 20.10.2010 Java User Group Frankfurt / Main Mario Gleichmann Mario Gleichmann JUG Frankfurt / Main
  • 2.
    Introduction Mario Gleichmann site: www.mg-informatik.de blog: 'brain driven development' gleichmann.wordpress.com mail: mario.gleichmann@mg-informatik.de Mario Gleichmann 2 JUG Frankfurt / Main
  • 3.
    Summary Mario Gleichmann 3 JUG Frankfurt / Main
  • 4.
    What is FunctionalProgramming ? Functional Style val sum = fold( 1 to 10, add ) Expression based Computation method is function application Mario Gleichmann 4 fcn JUG Frankfurt / Main
  • 5.
    What makes aFunction ? Function Types val add = ( x :Int, y :Int ) => x + y Type of add : ( Int , Int ) => Int Mario Gleichmann 5 Obj JUG Frankfurt / Main
  • 6.
    Closures 'closing over' var limit = 18 val isAdult = ( age :Int ) => age >= limit 'Closed term' Mario Gleichmann 6 appl JUG Frankfurt / Main
  • 7.
    Algebraic Datatypes Example - Tree data Tree = Empty | Leaf Int | Node Int Tree Tree abstract case class Tree() case object Empty extends Tree case class Leaf( value: Int ) extends Tree case class Node( value: Int, left :Tree, right: Tree ) extends Tree Mario Gleichmann 7 insrt JUG Frankfurt / Main
  • 8.
    Pattern Matching val depth :Tree => Int = _ match { case Empty => 0 case Leaf( _ ) => 1 case Node( _, left, right ) => 1 + max( depth( left ), depth( right ) ) } Mario Gleichmann 8 Prt isb JUG Frankfurt / Main
  • 9.
    Partial Functions type =>?[-A, +B] = PartialFunction[A, B] val publisherRegionDE : Isbn =>? String = { case Isbn( 3, pubNr, _ ) => "D" + ( pubNr.toString charAt 0 ) } … publisherRegionDE.isDefinedAt( Isbn( 0, 677873, 8823 ) ) ) >> false Mario Gleichmann 9 chn JUG Frankfurt / Main
  • 10.
    Comprehensions val factors = ( n :Int ) => for( x <- ( 1 to n ) if n % x == 0 ) yield x ... val prime = ( n :Int ) => factors( n ).toList == List( 1, n ) … val primes = (n :Int) => for( x <- (1 to n) if prime( x ) ) yield x Mario Gleichmann 10 Hgh rdr Fcn JUG Frankfurt / Main
  • 11.
    Higher Order Functions val filter = ( predicate: Int => Boolean , xs :List[Int] ) => { for( x <- xs; if predicate( x ) ) yield x } Mario Gleichmann 11 Fcn Tp JUG Frankfurt / Main
  • 12.
    Lambda Expressions filter( num => num % 2 == 0, List( 1, 2, 3, 4, 5, 6 ) ) … filter( _ > 0, List( -1, 5, 0, -3, 7 ) ) Mario Gleichmann 12 fst arg JUG Frankfurt / Main
  • 13.
    Partial Application Mario Gleichmann 13 JUG Frankfurt / Main
  • 14.
    Partial Application val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) Type of isPrime: Int => Boolean Mario Gleichmann 14 def prms wthn JUG Frankfurt / Main
  • 15.
    Partial Application Delegation val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) val primesWithin = ( xs :List[Int] ) => filter( isPrime , xs ) Type of filter: ( Int => Boolean, List[Int] ) => List[Int] Mario Gleichmann 15 fxd var JUG Frankfurt / Main
  • 16.
    Partial Application Delegation val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) val primesWithin = ( xs :List[Int] ) => filter( isPrime , xs ) fixed variable Mario Gleichmann 16 appl nappl JUG Frankfurt / Main
  • 17.
    Partial Application New from Old ... val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) val primesWithin = filter( isPrime, _ :List[Int] ) applied unapplied Mario Gleichmann 17 Fcn Tp JUG Frankfurt / Main
  • 18.
    Partial Application New from Old ... val isPrime = (x :Int) => ( 2 to x/2 ).forall( x % _ != 0 ) val primesWithin = filter( isPrime, _ :List[Int] ) Type of primesWithin: List[Int] => List[Int] Mario Gleichmann 18 Crrng JUG Frankfurt / Main
  • 19.
    Currying Mario Gleichmann 19 JUG Frankfurt / Main
  • 20.
    Currying val filter = ( pred: Int => Boolean , xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean , List[Int] ) => List[Int] Mario Gleichmann 20 Crrd fcn JUG Frankfurt / Main
  • 21.
    Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Mario Gleichmann 21 Crrd fcn Tp JUG Frankfurt / Main
  • 22.
    Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => ( List[Int] ) => List[Int] Mario Gleichmann 22 sngl arg JUG Frankfurt / Main
  • 23.
    Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => ( List[Int] ) => List[Int] Curried Function Curried Function Mario Gleichmann 23 sngl xpln arg JUG Frankfurt / Main
  • 24.
    Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => ( List[Int] ) => List[Int] ... accepting one ... resulting in another A function (Function) Arg function Mario Gleichmann 24 lmbd xpr JUG Frankfurt / Main
  • 25.
    Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => List[Int] => List[Int] ... is defined as a Lambda expession Mario Gleichmann 25 cls ovr JUG Frankfurt / Main
  • 26.
    Currying 'Higher Order Lambda Closures' ... val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } Type of filter: ( Int => Boolean ) => List[Int] => List[Int] ... closes over to Arguments (Scope) of surrounding Function 'filter' Mario Gleichmann 26 Smp prms wthn JUG Frankfurt / Main
  • 27.
    Example 1 –Extract primes val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } val primesWithin = filter( isPrime ) Mario Gleichmann 27 Fcn Tp JUG Frankfurt / Main
  • 28.
    Example 1 –Extract primes val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } val primesWithin = filter( isPrime ) Type of primesWithin: List[Int] => List[Int] Mario Gleichmann 28 appl JUG Frankfurt / Main
  • 29.
    Example 1 –Extract primes val filter = ( pred: Int => Boolean ) => ( xs :List[Int] ) => { for( x <- xs; if pred( x ) ) yield x } val primesWithin = filter( isPrime ) ... primesWithin( List( 3, 4, 5, 6, 7 ) ) >> List( 3, 5, 7 ) Mario Gleichmann 29 Smp rc mgmt JUG Frankfurt / Main
  • 30.
    Example 2 –Ressource Management val initQuery = (dataSource :DataSource) => (query :String) => ( extractor :ResultSet => Unit ) => { try{ val conn = dataSource.getConnection val stmt = conn.createStatement val resultSet = stmt.executeQuery( query ) extractor( resultSet ) } finally{ try{ if( conn != null ) conn.close } finally{ if( stmt != null ) stmt.close } } } Mario Gleichmann 30 appl JUG Frankfurt / Main
  • 31.
    Example 2 –Ressource Management val dataSource :DataSource = ...; ... val query = initQuery( dataSource ) … query( "select * from User where age > 18" ) { result :ResultSet => while( result.next ) { .... } } … query( "select * from Account where balance < 1000" ) { result :ResultSet => while( result.next ) { .... } } Mario Gleichmann 31 smmy JUG Frankfurt / Main
  • 32.
    Combinators Mario Gleichmann 32 JUG Frankfurt / Main
  • 33.
    Combinators val power : Int => Int => Int = ( base :Int ) => ( exp :Int ) => if( exp == 1 ) base else base * power( base )( exp - 1 ) ... val square = power( _ :Int )( 2 ) Mario Gleichmann 33 JUG Frankfurt / Main
  • 34.
    Combinators val power : Int => Int => Int = ( base :Int ) => ( exp :Int ) => if( exp == 1 ) base else base * power( base )( exp - 1 ) ... val square = power( _ :Int )( 2 ) Type of square : Int => Int Mario Gleichmann 34 JUG Frankfurt / Main
  • 35.
    Combinators val power : Int => Int => Int = ( base :Int ) => ( exp :Int ) => if( exp == 1 ) base else base * power( base )( exp - 1 ) ... val square = power( _ :Int )( 2 ) Partial Application 'in the middle' Mario Gleichmann 35 JUG Frankfurt / Main
  • 36.
    Combinators def flipArgs [A, B, C] ( f: A => B => C ) : B => A => C = ( x: B ) => ( y: A ) => f (y) (x) Transforms to another Function, expecting arguments in reversed order Mario Gleichmann 36 JUG Frankfurt / Main
  • 37.
    Combinators def flipArgs [A, B, C] ( f: A => B => C ) : B => A => C = ( x: B ) => ( y: A ) => f (y) (x) … val square = flipArgs( power )( 2 ) … now just exploit Currying Mario Gleichmann 37 JUG Frankfurt / Main
  • 38.
    Combinators Function Composition f :: B => C g :: A => B x ∈ A ( f o g )( x ) = f ( g ( x ) ) ( f o g ) :: A => C Mario Gleichmann 38 JUG Frankfurt / Main
  • 39.
    Combinators : FunctionComposition def o [A,B,C] ( f: B => C ) ( g: A => B ) : A => C = ( x :A ) => f( g( x ) ) ... val mult = ( x :Int ) => ( y: Int ) => x * y val add = ( x :Int ) => ( y: Int ) => x + y ... val doubleSucc = o ( mult(2) ) ( add(1) ) Type of doubleSucc : Int => Int Mario Gleichmann 39 JUG Frankfurt / Main
  • 40.
    Combinators : FunctionComposition def o [A,B,C] ( f: B => C ) ( g: A => B ) : A => C = ( x :A ) => f( g( x ) ) ... val mult = ( x :Int ) => ( y: Int ) => x * y val add = ( x :Int ) => ( y: Int ) => x + y ... val doubleSucc = o ( mult(2) ) ( add(1) ) doubleSucc( 7 ) >> 16 Mario Gleichmann 40 JUG Frankfurt / Main
  • 41.
    Combinators : FunctionComposition def o [A,B,C] ( f: B => C ) ( g: A => B ) : A => C = ( x :A ) => f( g( x ) ) ... val mult = ( x :Int ) => ( y: Int ) => x * y val add = ( x :Int ) => ( y: Int ) => x + y ... val doubleSucc = o ( mult(2) ) ( add(1) ) doubleSucc( 7 ) Postfix ... >> 16 Mario Gleichmann 41 JUG Frankfurt / Main
  • 42.
    Combinators : FunctionComposition class RichFunc [B, C] ( f: B => C ) { def o[A] ( g: A => B ) = (x :A) => f( g( x ) ) } implicit def toRichFunc[B,C]( f: B => C ) = new RichFunc( f ) ... val doubleSucc = mult(2) o add(1) doubleSucc( 7 ) Infix !!! >> 16 Mario Gleichmann 42 JUG Frankfurt / Main
  • 43.
    Combinators : FunctionComposition class RichFunc [B, C] ( f: B => C ) { def o[A] ( g: A => B ) = (x :A) => f( g( x ) ) } implicit def toRichFunc[B,C]( f: B => C ) = new RichFunc( f ) ... val doubleSucc = mult(2) o add(1) doubleSucc( 7 ) 'Point free style' of function definition >> 16 Mario Gleichmann 43 JUG Frankfurt / Main
  • 44.
    Example: Word Counter Count all even words within a sentence: “Hello World“ → 0 “The fox jumps over the lazy dog“ → 2 "This sentence contains five even words" → 5 Mario Gleichmann 44 JUG Frankfurt / Main
  • 45.
    Example: Word Counter Count all even words within a sentence: “Hello World“ → 0 “The fox jumps over the lazy dog“ → 2 "This sentence contains five even words" → 5 Basic Functions: parse Words → Word to length → filter even → count Mario Gleichmann 45 JUG Frankfurt / Main
  • 46.
    Example: Word Counter Basic Functions val words = ( sentence : String ) => sentence.split( " " ).toList val length = ( ws :List[String] ) => for( w <- ws ) yield w.length val filter = ( predicate: Int => Boolean ) => ( xs :List[Int] ) => ... val even = ( i :Int ) => i % 2 == 0 val size = ( xs :List[ _ ] ) => xs.size Mario Gleichmann 46 JUG Frankfurt / Main
  • 47.
    Example: Word Counter val words = ( sentence : String ) => sentence.split( " " ).toList val length = ( ws :List[String] ) => for( w <- ws ) yield w.length val filter = ( predicate: Int => Boolean ) => ( xs :List[Int] ) => ... val even = ( i :Int ) => i % 2 == 0 val size = ( xs :List[ _ ] ) => xs.size val evenWordCount = size o filter( even ) o length o words Type of evenWordCount : String => Int Mario Gleichmann 47 JUG Frankfurt / Main
  • 48.
    Example: Word Counter val words = ( sentence : String ) => sentence.split( " " ).toList val length = ( ws :List[String] ) => for( w <- ws ) yield w.length val filter = ( predicate: Int => Boolean ) => ( xs :List[Int] ) => ... val even = ( i :Int ) => i % 2 == 0 val size = ( xs :List[ _ ] ) => xs.size val evenWordCount = size o filter( even ) o length o words vs. val evenWordCount = (sentence :String ) => size ( filter( even ) ( length( words( sentence ) ) ) ) Mario Gleichmann 48 JUG Frankfurt / Main
  • 49.
    Example: Word Counter val words = ( sentence : String ) => sentence.split( " " ).toList val length = ( ws :List[String] ) => for( w <- ws ) yield w.length val filter = ( predicate: Int => Boolean ) => ( xs :List[Int] ) => ... val even = ( i :Int ) => i % 2 == 0 val size = ( xs :List[ _ ] ) => xs.size val evenWordCount = size o filter( even ) o length o words evenWordCount( "this sentence contains five even words" ) >> 5 Mario Gleichmann 49 JUG Frankfurt / Main
  • 50.
  • 51.
    Fundamental Functions def sum ( list : List[Int] ) : Int = list match { case Nil => 0 case x :: xs => x + sum( xs ) } Mario Gleichmann 51 JUG Frankfurt / Main
  • 52.
    Fundamental Functions def product ( list : List[Int] ) : Int = list match { case Nil => 1 case x :: xs => x * product( xs ) } Mario Gleichmann 52 JUG Frankfurt / Main
  • 53.
    Fundamental Functions def allTrue ( ys : List[Boolean] ) : Boolean = ys match { case Nil => true case x :: xs => x and allTrue( xs ) } Mario Gleichmann 53 JUG Frankfurt / Main
  • 54.
    Fundamental Functions :reduce def reduce ( list : List[ T ] ) : R = list match { case Nil => ? case x :: xs => x ʘ reduce( xs ) } Mario Gleichmann 54 JUG Frankfurt / Main
  • 55.
    Fundamental Functions :reduce def reduce ( list : List[ T ] ) : R = list match { case Nil => ? 'unit' case x :: xs => x ʘ reduce( xs ) } 'subsuming operation' Mario Gleichmann 55 JUG Frankfurt / Main
  • 56.
    Fundamental Functions :reduce def reduce[T,R] ( fun : T => R => R ) ( unit : R ) ( list : List[T] ) : R = list match { case Nil => unit case x :: xs => fun ( x ) ( reduce( fun )( unit )( xs ) ) } Mario Gleichmann 56 JUG Frankfurt / Main
  • 57.
    Fundamental Functions :reduce def reduce[T,R] ( fun : T => R => R ) ( unit : R ) ( list : List[T] ) : R = list match { case Nil => unit case x :: xs => fun ( x ) ( reduce( fun )( unit )( xs ) ) } reduce ʘ unit t1 :: t2 :: t3 :: t4 :: Nil => t1 ʘ ( t2 ʘ ( t3 ʘ ( t4 ʘ unit ) ) ) Mario Gleichmann 57 JUG Frankfurt / Main
  • 58.
    Fundamental Functions :reduce val add = (x : Int) => (y : Int) => x + y val mult = (x : Int) => (y : Int) => x * y ... val sum = reduce ( add ) ( 0 ) _ val product = reduce ( mult ) ( 1 ) _ ... sum( List( 1, 2, 3, 4 ) ) >> 10 product( List( 1, 2, 3, 4 ) ) >> 24 Mario Gleichmann 58 JUG Frankfurt / Main
  • 59.
    Fundamental Functions :reduce val or = ( x :Boolean ) => ( y :Boolean ) => x || y val and = ( x :Boolean ) => ( y :Boolean ) => x && y val not = ( x :Boolean ) => !x ... val existOneTrue = reduce ( or ) ( false ) _ val allFalse = not o reduce ( or ) ( false ) _ val allTrue = reduce ( and ) ( true ) _ ... allTrue( List( 1 < 2, 3 == 3, 1 + 1 == 2 ) ) >> true Mario Gleichmann 59 JUG Frankfurt / Main
  • 60.
    Example: List concatenation def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = reduce ( append ) _ listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) >> List( 8, 9, 1, 2, 3, 4 ) listConcat ( List( true, true, false ) ) ( List( true, false ) ) ) >> List( true, false, true, true, false ) Mario Gleichmann 60 JUG Frankfurt / Main
  • 61.
    Example: List concatenation def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = reduce ( append ) _ listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) >> List( 8, 9, 1, 2, 3, 4 ) 'unit' listConcat ( List( true, true, false ) ) ( List( true, false ) ) ) >> List( true, false, true, true, false ) Mario Gleichmann 61 JUG Frankfurt / Main
  • 62.
    Example: List concatenation def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = flipArgs( reduce ( append ) _ ) listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) >> List( 1, 2, 3, 4, 8, 9 ) listConcat ( List( true, true, false ) ) ( List( true, false ) ) ) >> List( true, true, false, true, false ) Mario Gleichmann 62 JUG Frankfurt / Main
  • 63.
    Example: List concatenation def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = flipArgs( reduce ( append ) _ ) listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) >> List( 1, 2, 3, 4, 8, 9 ) 'unit' listConcat ( List( true, true, false ) ) ( List( true, false ) ) ) >> List( true, true, false, true, false ) Mario Gleichmann 63 JUG Frankfurt / Main
  • 64.
    Example: List concatenation ! def append[T] (x:T) (xs:List[ _ ]) = x :: xs val listConcat = flipArgs( reduce ( append ) _ ) listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) Type of result: List[Any] Mario Gleichmann 64 JUG Frankfurt / Main
  • 65.
    Example: List concatenation def append[T] (x:T) (xs:List[ T ]) = x :: xs def listConcat[A] ( a: List[A]) ( b: List[A] ) : List[A] = reduce[ A, List[A] ] ( ( append[A] ) _ ) (b) (a) listConcat ( List( 1, 2, 3, 4 ) ) ( List( 8, 9 ) ) ) Type of result: List[Int] Mario Gleichmann 65 JUG Frankfurt / Main
  • 66.
    More Fundamental Functions Mario Gleichmann 66 JUG Frankfurt / Main
  • 67.
    Fundamental Functions val doubleAll : List[Int] => List[Int] = _ match { case Nil => Nil case y :: ys => y * 2 :: doubleAll( ys ) } doubleAll( List( 1, 2, 3, 4 ) ) >> List( 2, 4, 6, 8 ) Mario Gleichmann 67 JUG Frankfurt / Main
  • 68.
    Fundamental Functions val toLength : List[String] => List[Int] = _ match { case Nil => Nil case y :: ys => y.length :: toLength( ys ) } toLength( List( "a", "ab", "abc", "abcde" ) ) ) >> List( 1, 2, 3, 5 ) Mario Gleichmann 68 JUG Frankfurt / Main
  • 69.
    Fundamental Functions :map val map : List[T] => List[R] = _ match { case Nil => Nil case y :: ys => f?( y ) :: map( ys ) } Mario Gleichmann 69 JUG Frankfurt / Main
  • 70.
    Fundamental Functions :map def map[T, R] ( f: T => R ) : List[T] => List[R] = ( xs : List[T] ) => xs match { case Nil => Nil case y :: ys => f( y ) :: map( f )( ys ) } Mario Gleichmann 70 JUG Frankfurt / Main
  • 71.
    Fundamental Functions :map def map[T, R] ( f: T => R ) : List[T] => List[R] = ( xs : List[T] ) => xs match { case Nil => Nil case y :: ys => f( y ) :: map( f )( ys ) } Accepting a function to Resulting in another function, apply on every element accepting the List of elements Mario Gleichmann 71 JUG Frankfurt / Main
  • 72.
    Fundamental Functions :map def map[T, R] ( f: T => R ) : List[T] => List[R] = ... ... val mult = (x : Int) => (y : Int) => x * y val doubleAll = map( mult( 2 ) ) ... val toLength = map( ( s :String ) => s.length ) Mario Gleichmann 72 JUG Frankfurt / Main
  • 73.
    Fundamental Functions :map val add = (x : Int) => (y : Int) => x + y val sum = reduce ( add ) ( 0 ) _ val sumMatrix = sum o map( sum ) Type of sum : List[Int] => Int Type of map( sum ) : List[List[Int]] => List[Int] Type of sumMatrix : List[List[Int]] => Int Mario Gleichmann 73 JUG Frankfurt / Main
  • 74.
    Fundamental Functions :map val add = (x : Int) => (y : Int) => x + y val sum = reduce ( add ) ( 0 ) _ val sumMatrix = sum o map( sum ) ... val matrix = List( List( 1, 2, 3 ), List( 4, 5, 6 ), List( 7, 8, 9 ) ) sumMatrix( matrix ) >> 45 Mario Gleichmann 74 JUG Frankfurt / Main
  • 75.
    map via reduce def applyAndPrepend [A, B] ( f: A => B ) = ( a :A ) => ( bs :List[B] ) => f( a ) :: bs def map [A, B] ( f: A => B ) : List[A] => List[B] = reduce ( applyAndPrepend( f ) ) ( Nil ) _ Mario Gleichmann 75 JUG Frankfurt / Main
  • 76.
    map via reduce def applyAndPrepend [A, B] ( f: A => B ) = ( a :A ) => ( bs :List[B] ) => f( a ) :: bs def map [A, B] ( f: A => B ) : List[A] => List[B] = reduce ( applyAndPrepend( f ) ) ( Nil ) _ f :: f :: f :: f :: Nil X1 :: X2 :: X3 :: X4 :: Nil Mario Gleichmann 76 JUG Frankfurt / Main
  • 77.
    filter via reduce def filter[T]( pred :T => Boolean ) : List[T] => List[T] = reduce ( ( x :T ) => ( acc :List[T] ) => if( pred( x ) ) x :: acc else acc ) ( Nil ) Mario Gleichmann 77 JUG Frankfurt / Main
  • 78.
    Type Classes Mario Gleichmann 78 JUG Frankfurt / Main
  • 79.
    Type Classes def sum( xs : List[Int] ) : Int = xs match { case Nil => 0 case head :: tail => head + sum( tail ) } Mario Gleichmann 79 JUG Frankfurt / Main
  • 80.
    Type Classes def sum( xs : List[String] ) : String = xs match { case Nil => ““ case head :: tail => head + sum( tail ) } Concatenation ! Mario Gleichmann 80 JUG Frankfurt / Main
  • 81.
    Type Classes def sum[A]( xs : List[A] ) : A = xs match { case Nil => ? case head :: tail => head ? sum( tail ) } Mario Gleichmann 81 JUG Frankfurt / Main
  • 82.
    Type Classes def sum[A]( xs : List[A] ) : A = xs match { case Nil => ? 'neutral' element case head :: tail => head ? sum( tail ) } element composition Mario Gleichmann 82 JUG Frankfurt / Main
  • 83.
    Type Classes trait SemiGroup[S] { def add( x : S, y : S ) : S element composition } trait Monoid[M] extends SemiGroup[M] { def unit : M 'neutral' element } Mario Gleichmann 83 JUG Frankfurt / Main
  • 84.
    Type Classes def sum [A] ( xs : List[A], m: Monoid[A] ) : A = xs match { case Nil => m.unit case head :: tail => m.add( head, sum( tail, m) ) } Mario Gleichmann 84 JUG Frankfurt / Main
  • 85.
    Type Classes def sum [A] ( xs : List[A], m: Monoid[A] ) : A = 'type constraint' xs match { case Nil => m.unit case head :: tail => m.add( head, sum( tail, m) ) } Mario Gleichmann 85 JUG Frankfurt / Main
  • 86.
    Type Classes object IntMonoid extends Monoid[Int]{ def add(x : Int, y : Int): Int = x + y def unit : Int = 0 } … sum( List( 1, 2, 3, 4 ), IntMonoid ) Mario Gleichmann 86 JUG Frankfurt / Main
  • 87.
    Type Classes def sum [A] ( xs : List[A] ) ( implicit m: Monoid[A] ) : A = ... implicit object IntMonoid extends Monoid[Int]{ … } … import typeclasses.monoid._ ... sum( List( 1, 2, 3, 4 ) ) Mario Gleichmann 87 JUG Frankfurt / Main
  • 88.
    Example: Clock-Monoid sealed abstract case class ClockHour( hour: Int ) case object HOUR_0 extends ClockHour( 0 ) case object HOUR_1 extends ClockHour( 1 ) ... case object HOUR_11 extends ClockHour( 11 ) … implicit object ClockHourMonoid extends Monoid[ClockHour]{ val hours = Array( HOUR_0, HOUR_1, HOUR_2, ..., HOUR_11 ) def add(x : ClockHour, y : ClockHour) = hours( ( x.hour + y.hour ) % 12 ) def unit : ClockHour = HOUR_0 } Mario Gleichmann 88 JUG Frankfurt / Main
  • 89.
    Example: Clock-Monoid import typeclasses.monoid.ClockHourMonoid ... sum( List( HOUR_5, HOUR_3, HOUR_8 ) ) >> HOUR_4 Mario Gleichmann 89 JUG Frankfurt / Main
  • 90.
    Example: Contains element? trait Eq[E]{ def equals( e1 :E, e2 :E ) : Boolean } def contains [T] ( x :T, xs : List[T] ) ( implicit eq :Eq[T] ) : Boolean = reduce ( ( y :T ) => ( acc :Boolean ) => if( eq.equals( x, y ) ) true else acc ) ( false ) 1st Arg: 'reducing' function ( xs ) 2nd Arg: 'unit' 3rd Arg: List to reduce Mario Gleichmann 90 JUG Frankfurt / Main
  • 91.
    Example: Contains element? case class Person( name :String, age :Int ) implicit object PersonEq extends Eq[Person]{ def equals( p1 :Person, p2 :Person ) : Boolean = (p1.name == p2.name) && (p1.age == p2.age) } ... val persons = List( Person("Hans", 22), Person( "Helga", 32 ), Person( "Hugo", 47 ) ) contains( Person( "Olga", 22 ), persons ) >> false contains( Person( "Helga", 32 ), persons ) >> true Mario Gleichmann 91 JUG Frankfurt / Main
  • 92.
    Monads Mario Gleichmann 92 JUG Frankfurt / Main
  • 93.
    Monads A simple Monad: Option << abstract >> Option[+A] Some[+A] None presence absence Handling the or of something ...as the result of a computation (computational environment) Mario Gleichmann JUG Frankfurt / Main
  • 94.
    Monads A simple Monad: Option class CustomerDAO{ def findCustomer( custId: Long ) : Option<Customer> = { ... if( found( customer ) ) Some( customer ) else None } } Mario Gleichmann JUG Frankfurt / Main
  • 95.
    Monads A simple Monad: Option class CustomerDAO{ def findCustomer( custId: Long ) : Option<Customer> = { ... if( found( customer ) ) Some( customer ) else None } } Explicit Notion, that there may be 'none' result Mario Gleichmann JUG Frankfurt / Main
  • 96.
    Monads A simple Monad: Option val customerHit = customerDAO.findCustomer( 123 ); ... customerHit match { case Some( customer ) => println( customer.name ) case None => println( “not found“ ) } Mario Gleichmann JUG Frankfurt / Main
  • 97.
    Monads A simple Monad: Option val customerHit = customerDAO.findCustomer( 123 ); ... customerHit match { case Some( customer ) => println( customer.name ) case None => println( “not found“ ) } Explicit Handling the absensce of a result Forces 'Awareness' Mario Gleichmann JUG Frankfurt / Main
  • 98.
    Monads A simple Monad: Option val customerHit = customerDAO.findCustomer( 123 ); ... customerHit match { case Some( customer ) => println( customer.name ) case None => println( “not found“ ) } ... beside from that ... what's the deal ??? Mario Gleichmann JUG Frankfurt / Main
  • 99.
    Monads A simple Monad: Option val customerHit = customerDAO.findCustomer( 123 ); ... customerHit match { case Some( customer ) => println( customer.name ) case None => println( “not found“ ) } 'Protected' Function Composition ... Mario Gleichmann JUG Frankfurt / Main
  • 100.
    Monads val projects = Map( "Jan" -> "IKT", "Joe" -> "TensE", "Luca" -> "InTA" ) val customers = Map( "IKT" -> "Hanso GmbH", "InTA" -> "RAIA Duo" ) val cities = Map( "Hanso GmbH" -> "Stuttgart", "Mogno" -> "Mailand" ) Mario Gleichmann JUG Frankfurt / Main
  • 101.
    Monads val projects = Map( "Jan" -> "IKT", "Joe" -> "TensE", "Luca" -> "InTA" ) val customers = Map( "IKT" -> "Hanso GmbH", "InTA" -> "RAIA Duo" ) val cities = Map( "Hanso GmbH" -> "Stuttgart", "Mogno" -> "Mailand" ) Where is Jan ? Jan -> IKT -> Hanso GmbH -> Stuttgart Mario Gleichmann JUG Frankfurt / Main
  • 102.
    Monads Jav public String whereIs( String name ){ a String project = projects.get( name ); if( project != null ){ String customer = customers.get( project ); if( customer != null ){ String city = cities.get( customer ) if( city != null ) return city; else return “unknown“; } else return ''unknown''; } else return ''unknown''; } Mario Gleichmann JUG Frankfurt / Main
  • 103.
    Monads Scal A simple Monad: Option a def whereIs( name: String ) = { projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Mario Gleichmann JUG Frankfurt / Main
  • 104.
    Monads Scal A simple Monad: Option a def whereIs( name: String ) = { Results in Option[String] projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Mario Gleichmann JUG Frankfurt / Main
  • 105.
    Monads Scal A simple Monad: Option a def whereIs( name: String ) = { Results in Option[String] projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Option[A].map( A => B ) >> Option[B] Mario Gleichmann JUG Frankfurt / Main
  • 106.
    Monads Scal A simple Monad: Option a def whereIs( name: String ) = { Results in Option[String] projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Option[A].map( A => B ) >> Option[B] Option[A].map( A => Option[B] ) >> Option[Option[B]] Mario Gleichmann JUG Frankfurt / Main
  • 107.
    Monads Scal A simple Monad: Option a def whereIs( name: String ) = { Results in Option[String] projects.get( name ) .flatMap( project => customers get project ) .flatMap( customer => cities get customer ) .getOrElse( "unknown!" ) } Option[A].map( A => B ) >> Option[B] Option[A].map( A => Option[B] ) >> Option[Option[B]] Option[A].flatMap( A => Option[B] ) >> Option[B] Mario Gleichmann JUG Frankfurt / Main
  • 108.
    Monads Protected Composition map( A => B ) map( B => C ) map( C => D ) None None None ... Some[A] Some[B] Some[C] Some[D] Mario Gleichmann JUG Frankfurt / Main
  • 109.
    Monads A simple Monad: Option def whereIs( name: String ) = ( for( project <- projects get name; customer <- customers get project; city <- cities get customer ) yield city ).getOrElse( "unknown!" ) Mario Gleichmann JUG Frankfurt / Main
  • 110.
    Summary Scala allows for ... ● Currying ● Combinators ● Fundamental Functions ● Type classes ● Monads Mario Gleichmann JUG Frankfurt / Main
  • 111.
    Thank you ! MarioGleichmann 111 JUG Frankfurt / Main
  • 112.
    References Scala Home www.scala-lang.org Dr. Erik Meijer C9 Lectures – Functional Programming Fundamentals http://channel9.msdn.com Graham Hutton Programming in Haskell Cambridge Odersky, Spoon, Venners Programming in Scala artima Mario Gleichmann 112 JUG Frankfurt / Main