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© 2014 Guillaume Laforge. All rights reserved. Do not distribute without permission. Guillaume Laforge 
 @glaforge Functional Groovy

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Guillaume Laforge Groovy project lead ! @glaforge http://www.google.com/+GuillaumeLaforge http://glaforge.appspot.com

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Pivotal related talks • Thursday – 2:15pm - 3:15pm •Advanced Web Development Techniques with Grails 2 — Jeff Brown •Making the Eclipse IDE fun again — Martin Lippert ! • Friday – 9:00am - 5:00pm •From the database into the web: End-to-end REST web services with Spring — Oliver Gierke 4

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Part 1
 a Groovy primer

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Simplify the life of (Java) developers

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Groovy as a Java superset

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It’s so easy to learn! Groovy as a Java superset

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Great for scripting

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Great for scripting Fit for Domain- Specific Languages

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Great for scripting Fit for Domain- Specific Languages Most seamless integration & interoperability wih java!

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class  MathSpec  extends  Specification  {          def  "maximum  of  two  numbers"()  {              expect:                  Math.max(a,  b)  ==  c   !            where:                  a  |  b  ||  c                  1  |  3  ||  3                  7  |  4  ||  4                  0  |  0  ||  0          }   }

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new  MarkupBuilder().html  {          head  {                  title  "The  Script  Bowl"          }   !        body  {                  div(class:  "banner")  {                          p  "Groovy  rocks!"                  }          }   }

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move  forward  at  3.km/h Expressive, Concise, Readable

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@RestController   class  App  {          @RequestMapping("/")          String  home()  {  "Hello  World!"  }   } Speaking of conciseness... A full Spring app in the span of a tweet!

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AssafeandfastasJava with statictypechecking & compilation

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AssafeandfastasJava with statictypechecking & compilation

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million 
 downloads
 in 2013 3.0

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Yup, we’re all using Groovy!

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Part 2
 Community

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A blossoming Ecosystem

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GVM

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Part 3 Functional

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What is a functional language?

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What is a functional language? Functions, closures, lambdas Immutability Side-effect free / pure Persistent collections Recursion Higher Order Functions Partial application Concurrency, parallelism Monads, functors, combinators... Composition Map / filter / reduce Lazy vs Eager Memoization Infinite streams

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Closures as first-class citizens of the language

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Closures everywhere in Groovy APIs • Closures are used in many situations: – iterators – callbacks – higher-order functions – custom control structures – dynamic method definition – resource allocation and handling – threads creation and launch – continuation-like coding – ... 26

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Closures — the basics 27 def  adder  =  {  a,  b  -­‐>  a  +  b  }   ! assert  adder(1,  2)  ==  3   assert  adder('a',  'b')  ==  'ab'

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Closures — the basics 27 def  adder  =  {  a,  b  -­‐>  a  +  b  }   ! assert  adder(1,  2)  ==  3   assert  adder('a',  'b')  ==  'ab' Closure parameters

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Closures — the basics 27 def  adder  =  {  a,  b  -­‐>  a  +  b  }   ! assert  adder(1,  2)  ==  3   assert  adder('a',  'b')  ==  'ab' Assign a function into a variable Closure parameters

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Closures — the basics 27 def  adder  =  {  a,  b  -­‐>  a  +  b  }   ! assert  adder(1,  2)  ==  3   assert  adder('a',  'b')  ==  'ab' Short form of: adder.call(‘a’, ‘b’) Assign a function into a variable Closure parameters

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Closures — the basics 27 def  adder  =  {  a,  b  -­‐>  a  +  b  }   ! assert  adder(1,  2)  ==  3   assert  adder('a',  'b')  ==  'ab' Short form of: adder.call(‘a’, ‘b’) Genericity with duck typing & operator overloading Assign a function into a variable Closure parameters

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Closures — explicit types 28 ! def  intAdder  =  {  int  a,  int  b  -­‐>  a  +  b  }  

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Closures — explicit types 28 ! def  intAdder  =  {  int  a,  int  b  -­‐>  a  +  b  }   Be explicit about the types

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Closures — implicit parameter 29 def  doubler  =  {  it  *  2  }   ! assert  doubler(3)  ==  6   assert  doubler('a')  ==  'aa'

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Closures — implicit parameter 29 def  doubler  =  {  it  *  2  }   ! assert  doubler(3)  ==  6   assert  doubler('a')  ==  'aa' Implicit parameter

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Closures — implicit parameter 29 def  doubler  =  {  it  *  2  }   ! assert  doubler(3)  ==  6   assert  doubler('a')  ==  'aa' Implicit parameter Multiply also defined on strings

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Closures — variable arguments 30 def  sum  =  {  ...  elements  -­‐>  
                                elements.sum()  }   ! assert  sum(1,  2)  ==  3   assert  sum('a',  'b',  'c')  ==  'abc'

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Closures — variable arguments 30 def  sum  =  {  ...  elements  -­‐>  
                                elements.sum()  }   ! assert  sum(1,  2)  ==  3   assert  sum('a',  'b',  'c')  ==  'abc' Variable number of arguments

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Closures — variable arguments 30 def  sum  =  {  ...  elements  -­‐>  
                                elements.sum()  }   ! assert  sum(1,  2)  ==  3   assert  sum('a',  'b',  'c')  ==  'abc' You can specify the type: int... Variable number of arguments

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Closures — default values 31 def  mult  =  {  int  a,  int  b  =  10  -­‐>  a  *  b  }   ! assert  mult(2,  3)  ==  6   assert  mult(5)  ==  50

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Closures — default values 31 def  mult  =  {  int  a,  int  b  =  10  -­‐>  a  *  b  }   ! assert  mult(2,  3)  ==  6   assert  mult(5)  ==  50 Default value

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Closures — default values 31 def  mult  =  {  int  a,  int  b  =  10  -­‐>  a  *  b  }   ! assert  mult(2,  3)  ==  6   assert  mult(5)  ==  50 Default value Provided value for b

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Closures — default values 31 def  mult  =  {  int  a,  int  b  =  10  -­‐>  a  *  b  }   ! assert  mult(2,  3)  ==  6   assert  mult(5)  ==  50 Default value Provided value for b Default value used for b

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Closures — methods as functions 32 def  logBase10  =  Math.&log10   def  printer  =  System.out.&println   ! assert  logBase10(10)  ==  1   printer  'abc'

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Closures — methods as functions 32 def  logBase10  =  Math.&log10   def  printer  =  System.out.&println   ! assert  logBase10(10)  ==  1   printer  'abc' Turn a method into a closure function

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Closures — iterations 33 def  fruits  =  ['apple',  
                            'orange',  
                            'banana']   ! fruits.each  {  println  it  }   fruits.each  printer

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Closures — iterations 33 def  fruits  =  ['apple',  
                            'orange',  
                            'banana']   ! fruits.each  {  println  it  }   fruits.each  printer Internal iteration

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Closures — iterations 33 def  fruits  =  ['apple',  
                            'orange',  
                            'banana']   ! fruits.each  {  println  it  }   fruits.each  printer Internal iteration Pass a method closure

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Closures — infinite streams 34 def  ints  =  1..1000   ! def  r  =  ints.dropWhile  {  it  <  10  }                          .take(  3  )   ! assert  r  ==  [10,  11,  12]

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Closures — infinite streams 34 def  ints  =  1..1000   ! def  r  =  ints.dropWhile  {  it  <  10  }                          .take(  3  )   ! assert  r  ==  [10,  11,  12] Could use an infinite iterator

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Closures — infinite streams 34 def  ints  =  1..1000   ! def  r  =  ints.dropWhile  {  it  <  10  }                          .take(  3  )   ! assert  r  ==  [10,  11,  12] Could use an infinite iterator take(), takeWhile(), drop(), dropWhile()

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Closures — infinite streams 34 def  ints  =  1..1000   ! def  r  =  ints.dropWhile  {  it  <  10  }                          .take(  3  )   ! assert  r  ==  [10,  11,  12] Could use an infinite iterator take(), takeWhile(), drop(), dropWhile() Returns an iterator

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Closures — infinite streams 34 def  ints  =  1..1000   ! def  r  =  ints.dropWhile  {  it  <  10  }                          .take(  3  )   ! assert  r  ==  [10,  11,  12] Could use an infinite iterator take(), takeWhile(), drop(), dropWhile() Returns an iterator Variants for maps too

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Closures — map / filter / reduce 35 3 } map filter reduce

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Closures — map / filter / reduce 36

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Closures — map / filter / reduce 36 @groovy.transform.Immutable
 class  Person  {
        String  name
        int  age
 }

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Closures — map / filter / reduce 36 @groovy.transform.Immutable
 class  Person  {
        String  name
        int  age
 }

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Closures — map / filter / reduce 36 @groovy.transform.Immutable
 class  Person  {
        String  name
        int  age
 } def  persons  =  [
        new  Person('Guillaume',  36),
        new  Person('Marion',  6),
        new  Person('Erine',  1)
 ]

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Closures — map / filter / reduce 36 @groovy.transform.Immutable
 class  Person  {
        String  name
        int  age
 } def  persons  =  [
        new  Person('Guillaume',  36),
        new  Person('Marion',  6),
        new  Person('Erine',  1)
 ]

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Closures — map / filter / reduce 36 @groovy.transform.Immutable
 class  Person  {
        String  name
        int  age
 } def  persons  =  [
        new  Person('Guillaume',  36),
        new  Person('Marion',  6),
        new  Person('Erine',  1)
 ] def  names  =
        persons.findAll  {  it.age  <  18  }
                      .collect  {  it.name.toUpperCase()  }
                      .sort()
                      .join(',  ')

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Closures — map / filter / reduce 36 @groovy.transform.Immutable
 class  Person  {
        String  name
        int  age
 } def  persons  =  [
        new  Person('Guillaume',  36),
        new  Person('Marion',  6),
        new  Person('Erine',  1)
 ] def  names  =
        persons.findAll  {  it.age  <  18  }
                      .collect  {  it.name.toUpperCase()  }
                      .sort()
                      .join(',  ')

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Closures — map / filter / reduce 36 @groovy.transform.Immutable
 class  Person  {
        String  name
        int  age
 } def  persons  =  [
        new  Person('Guillaume',  36),
        new  Person('Marion',  6),
        new  Person('Erine',  1)
 ] def  names  =
        persons.findAll  {  it.age  <  18  }
                      .collect  {  it.name.toUpperCase()  }
                      .sort()
                      .join(',  ') assert  names  ==  "ERINE,  MARION"

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Closures — map / filter / reduce 36 @groovy.transform.Immutable
 class  Person  {
        String  name
        int  age
 } def  persons  =  [
        new  Person('Guillaume',  36),
        new  Person('Marion',  6),
        new  Person('Erine',  1)
 ] def  names  =
        persons.findAll  {  it.age  <  18  }
                      .collect  {  it.name.toUpperCase()  }
                      .sort()
                      .join(',  ') assert  names  ==  "ERINE,  MARION" find/findAll, inject, collect, flatten, min/max, unique, reverse, collate, groupBy, any/ every, head/tail/last, count/ countBy, combinations/ permutations/subsequences/ transpose, withDefault/ withLazyDefault

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Closures — Closures vs Java 8 lambdas? 37

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Closures — Closures vs Java 8 lambdas? 37 IntStream.range(1,  100).forEach(s  -­‐>  System.out.println(s));   ! Files.lines(Paths.get('README.adoc'))            .map(it  -­‐>  it.toUpperCase())            .forEach(it  -­‐>  System.out.println  it);

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Closures — Closures vs Java 8 lambdas? 37 IntStream.range(1,  100).forEach  {  println  it  }   ! Files.lines(Paths.get('README.adoc'))            .map  {  it.toUpperCase()  }            .forEach  {  println  it  } IntStream.range(1,  100).forEach(s  -­‐>  System.out.println(s));   ! Files.lines(Paths.get('README.adoc'))            .map(it  -­‐>  it.toUpperCase())            .forEach(it  -­‐>  System.out.println  it);

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Closures — Closures vs Java 8 lambdas? 37 IntStream.range(1,  100).forEach  {  println  it  }   ! Files.lines(Paths.get('README.adoc'))            .map  {  it.toUpperCase()  }            .forEach  {  println  it  } IntStream.range(1,  100).forEach(s  -­‐>  System.out.println(s));   ! Files.lines(Paths.get('README.adoc'))            .map(it  -­‐>  it.toUpperCase())            .forEach(it  -­‐>  System.out.println  it); Use Groovy closures wherever you pass lambdas in Java 8

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Closures — map / filter / reduce & GPars 38

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Closures — map / filter / reduce & GPars 38 import  static  groovyx.gpars.GParsPool.withPool
 
 withPool  {

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Closures — map / filter / reduce & GPars 38 import  static  groovyx.gpars.GParsPool.withPool
 
 withPool  {        def  oneStarters  =  (1..30).parallel
                        .map        {  it  **  2  }
                        .filter  {  it  ==~  /1.*/  }

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Closures — map / filter / reduce & GPars 38 import  static  groovyx.gpars.GParsPool.withPool
 
 withPool  {        def  oneStarters  =  (1..30).parallel
                        .map        {  it  **  2  }
                        .filter  {  it  ==~  /1.*/  }

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Closures — map / filter / reduce & GPars 38 import  static  groovyx.gpars.GParsPool.withPool
 
 withPool  {        def  oneStarters  =  (1..30).parallel
                        .map        {  it  **  2  }
                        .filter  {  it  ==~  /1.*/  }        assert  oneStarters.collection  ==
                [1,  16,  100,  121,  144,  169,  196]

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Closures — map / filter / reduce & GPars 38 import  static  groovyx.gpars.GParsPool.withPool
 
 withPool  {        def  oneStarters  =  (1..30).parallel
                        .map        {  it  **  2  }
                        .filter  {  it  ==~  /1.*/  }        assert  oneStarters.collection  ==
                [1,  16,  100,  121,  144,  169,  196]

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Closures — map / filter / reduce & GPars 38 import  static  groovyx.gpars.GParsPool.withPool
 
 withPool  {        def  oneStarters  =  (1..30).parallel
                        .map        {  it  **  2  }
                        .filter  {  it  ==~  /1.*/  }        assert  oneStarters.collection  ==
                [1,  16,  100,  121,  144,  169,  196]        assert  oneStarters.max()  ==  196
        assert  oneStarters.reduce  {  a,  b  -­‐>  a+b  }  ==  747
        assert  oneStarters.sum()  ==  747
 }

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Closures — map / filter / reduce & GPars 38 import  static  groovyx.gpars.GParsPool.withPool
 
 withPool  {        def  oneStarters  =  (1..30).parallel
                        .map        {  it  **  2  }
                        .filter  {  it  ==~  /1.*/  }        assert  oneStarters.collection  ==
                [1,  16,  100,  121,  144,  169,  196]        assert  oneStarters.max()  ==  196
        assert  oneStarters.reduce  {  a,  b  -­‐>  a+b  }  ==  747
        assert  oneStarters.sum()  ==  747
 } in parallel!

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Closures — map / filter / reduce & GPars 38 import  static  groovyx.gpars.GParsPool.withPool
 
 withPool  {        def  oneStarters  =  (1..30).parallel
                        .map        {  it  **  2  }
                        .filter  {  it  ==~  /1.*/  }        assert  oneStarters.collection  ==
                [1,  16,  100,  121,  144,  169,  196]        assert  oneStarters.max()  ==  196
        assert  oneStarters.reduce  {  a,  b  -­‐>  a+b  }  ==  747
        assert  oneStarters.sum()  ==  747
 } in parallel! result still «parallel»

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GPars • Groovy library bundled with the distribution ! • Provides: – actors – safe agents – dataflow concurrency – data parallelism •fork / join, map / filter / reduce – asynchronous functions – parallel arrays – java.util.concurrent sugar 39

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GPars • Groovy library bundled with the distribution ! • Provides: – actors – safe agents – dataflow concurrency – data parallelism •fork / join, map / filter / reduce – asynchronous functions – parallel arrays – java.util.concurrent sugar 39 Concurrency & parallelism toolkit for Groovy

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Closures — resource handling 40 new  File('bible.txt').withReader  {  r  -­‐>          new  File('out.txt').withWriter  {  w  -­‐>                  r.eachLine  {  line  -­‐>                          if  (line.contains('Groovy'))                                  w  <<  line.toUpperCase()                  }          }   }

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Closures — resource handling 40 new  File('bible.txt').withReader  {  r  -­‐>          new  File('out.txt').withWriter  {  w  -­‐>                  r.eachLine  {  line  -­‐>                          if  (line.contains('Groovy'))                                  w  <<  line.toUpperCase()                  }          }   } Take care of properly opening / closing resources

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Closures — custom control structures 41 void  unless(boolean  cond,  Closure  c)  {          if  (!cond)  c()   }   ! unless  (10  <  9)  {          println  "less"   }

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Closures — custom control structures 41 void  unless(boolean  cond,  Closure  c)  {          if  (!cond)  c()   }   ! unless  (10  <  9)  {          println  "less"   } Closure as last argument

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Closures — custom control structures 41 void  unless(boolean  cond,  Closure  c)  {          if  (!cond)  c()   }   ! unless  (10  <  9)  {          println  "less"   } Closure as last argument Equivalent to: unless(10<9, {...})

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Closures — builders 42 new  MarkupBuilder().html  {          head  {                  title  "The  Script  Bowl"          }   !        body  {                  div(class:  "banner")  {                          p  "Groovy  rocks!"                  }          }   }

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Closures — builders 42 new  MarkupBuilder().html  {          head  {                  title  "The  Script  Bowl"          }   !        body  {                  div(class:  "banner")  {                          p  "Groovy  rocks!"                  }          }   } Closure

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Closures — higher order functions 43

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Closures — higher order functions 43 sell  100.shares  of  GOOG  at  1000.dollars

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Closures — higher order functions 43

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Closures — higher order functions 43 Number.metaClass.getShares  =  {  delegate  }
 Number.metaClass.getDollars  =  {  delegate  }


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Closures — higher order functions 43 Number.metaClass.getShares  =  {  delegate  }
 Number.metaClass.getDollars  =  {  delegate  }
 String  GOOG  =  "Google"

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Closures — higher order functions 43 Number.metaClass.getShares  =  {  delegate  }
 Number.metaClass.getDollars  =  {  delegate  }
 String  GOOG  =  "Google"

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Closures — higher order functions 43 Number.metaClass.getShares  =  {  delegate  }
 Number.metaClass.getDollars  =  {  delegate  }
 String  GOOG  =  "Google" def  sell(int  nShares)  {
        [of:  {  String  ticker  -­‐>
                [at:  {  int  price  -­‐>
                        println  "Sold  ${nShares}  ${ticker}  at  \$${price}"
                }]
        }]
 }

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Closures — higher order functions 43 Number.metaClass.getShares  =  {  delegate  }
 Number.metaClass.getDollars  =  {  delegate  }
 String  GOOG  =  "Google" def  sell(int  nShares)  {
        [of:  {  String  ticker  -­‐>
                [at:  {  int  price  -­‐>
                        println  "Sold  ${nShares}  ${ticker}  at  \$${price}"
                }]
        }]
 }

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Closures — higher order functions 43 Number.metaClass.getShares  =  {  delegate  }
 Number.metaClass.getDollars  =  {  delegate  }
 String  GOOG  =  "Google" def  sell(int  nShares)  {
        [of:  {  String  ticker  -­‐>
                [at:  {  int  price  -­‐>
                        println  "Sold  ${nShares}  ${ticker}  at  \$${price}"
                }]
        }]
 } sell  100.shares  of  GOOG  at  1000.dollars

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Closures — higher order functions 43 Number.metaClass.getShares  =  {  delegate  }
 Number.metaClass.getDollars  =  {  delegate  }
 String  GOOG  =  "Google" def  sell(int  nShares)  {
        [of:  {  String  ticker  -­‐>
                [at:  {  int  price  -­‐>
                        println  "Sold  ${nShares}  ${ticker}  at  \$${price}"
                }]
        }]
 } sell  100.shares  of  GOOG  at  1000.dollars Add properties to numbers

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Closures — higher order functions 43 Number.metaClass.getShares  =  {  delegate  }
 Number.metaClass.getDollars  =  {  delegate  }
 String  GOOG  =  "Google" def  sell(int  nShares)  {
        [of:  {  String  ticker  -­‐>
                [at:  {  int  price  -­‐>
                        println  "Sold  ${nShares}  ${ticker}  at  \$${price}"
                }]
        }]
 } sell  100.shares  of  GOOG  at  1000.dollars Add properties to numbers Nested maps and closures

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Closures — higher order functions 43 Number.metaClass.getShares  =  {  delegate  }
 Number.metaClass.getDollars  =  {  delegate  }
 String  GOOG  =  "Google" def  sell(int  nShares)  {
        [of:  {  String  ticker  -­‐>
                [at:  {  int  price  -­‐>
                        println  "Sold  ${nShares}  ${ticker}  at  \$${price}"
                }]
        }]
 } sell  100.shares  of  GOOG  at  1000.dollars Add properties to numbers Nested maps and closures Readable & expressive DSL

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Now what about some curry?

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Closures — partial application 45 def  mult  =  {  int  a,  int  b  -­‐>  a  *  b  }   def  multByTwo  =  mult.curry(2)   ! assert  multByTwo(3)  ==  6

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Closures — partial application 45 def  mult  =  {  int  a,  int  b  -­‐>  a  *  b  }   def  multByTwo  =  mult.curry(2)   ! assert  multByTwo(3)  ==  6 Param a is set to 2

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Closures — partial application 46 def  join  =  {  String  left,  String  sep,  String  right  -­‐>          left  +  sep  +  right   }   ! def  prefix  =  join.curry("Groovy")   assert  prefix("  is  ",  "cool")  ==  "Groovy  is  cool"   ! def  suffix  =  join.rcurry("cool")   assert  suffix("Functional",  "  is  ")  ==  "Functional  is  cool"   ! def  amperJoin  =  join.ncurry(1,  "  &  ")   assert  amperJoin("Marion",  "Érine")  ==  "Marion  &  Érine"

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Closures — partial application 46 def  join  =  {  String  left,  String  sep,  String  right  -­‐>          left  +  sep  +  right   }   ! def  prefix  =  join.curry("Groovy")   assert  prefix("  is  ",  "cool")  ==  "Groovy  is  cool"   ! def  suffix  =  join.rcurry("cool")   assert  suffix("Functional",  "  is  ")  ==  "Functional  is  cool"   ! def  amperJoin  =  join.ncurry(1,  "  &  ")   assert  amperJoin("Marion",  "Érine")  ==  "Marion  &  Érine" Left curry by default

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Closures — partial application 46 def  join  =  {  String  left,  String  sep,  String  right  -­‐>          left  +  sep  +  right   }   ! def  prefix  =  join.curry("Groovy")   assert  prefix("  is  ",  "cool")  ==  "Groovy  is  cool"   ! def  suffix  =  join.rcurry("cool")   assert  suffix("Functional",  "  is  ")  ==  "Functional  is  cool"   ! def  amperJoin  =  join.ncurry(1,  "  &  ")   assert  amperJoin("Marion",  "Érine")  ==  "Marion  &  Érine" Left curry by default Right curry

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Closures — partial application 46 def  join  =  {  String  left,  String  sep,  String  right  -­‐>          left  +  sep  +  right   }   ! def  prefix  =  join.curry("Groovy")   assert  prefix("  is  ",  "cool")  ==  "Groovy  is  cool"   ! def  suffix  =  join.rcurry("cool")   assert  suffix("Functional",  "  is  ")  ==  "Functional  is  cool"   ! def  amperJoin  =  join.ncurry(1,  "  &  ")   assert  amperJoin("Marion",  "Érine")  ==  "Marion  &  Érine" Left curry by default Right curry Curry n-th argument

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Composition

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Closures — composition 48

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Closures — composition 48 def  plus2    =  {  it  +  2  }
 def  times3  =  {  it  *  3  }

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Closures — composition 48 def  plus2    =  {  it  +  2  }
 def  times3  =  {  it  *  3  }

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Closures — composition 48 def  plus2    =  {  it  +  2  }
 def  times3  =  {  it  *  3  } def  times3plus2  =  plus2  <<  times3
 assert  times3plus2(3)  ==  11
 assert  times3plus2(4)  ==  plus2(times3(4))

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Closures — composition 48 def  plus2    =  {  it  +  2  }
 def  times3  =  {  it  *  3  } def  times3plus2  =  plus2  <<  times3
 assert  times3plus2(3)  ==  11
 assert  times3plus2(4)  ==  plus2(times3(4))

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Closures — composition 48 def  plus2    =  {  it  +  2  }
 def  times3  =  {  it  *  3  } def  times3plus2  =  plus2  <<  times3
 assert  times3plus2(3)  ==  11
 assert  times3plus2(4)  ==  plus2(times3(4)) def  plus2times3  =  times3  <<  plus2
 assert  plus2times3(3)  ==  15
 assert  plus2times3(5)  ==  times3(plus2(5))

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Closures — composition 48 def  plus2    =  {  it  +  2  }
 def  times3  =  {  it  *  3  } def  times3plus2  =  plus2  <<  times3
 assert  times3plus2(3)  ==  11
 assert  times3plus2(4)  ==  plus2(times3(4)) def  plus2times3  =  times3  <<  plus2
 assert  plus2times3(3)  ==  15
 assert  plus2times3(5)  ==  times3(plus2(5))

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Closures — composition 48 def  plus2    =  {  it  +  2  }
 def  times3  =  {  it  *  3  } def  times3plus2  =  plus2  <<  times3
 assert  times3plus2(3)  ==  11
 assert  times3plus2(4)  ==  plus2(times3(4)) def  plus2times3  =  times3  <<  plus2
 assert  plus2times3(3)  ==  15
 assert  plus2times3(5)  ==  times3(plus2(5)) assert  times3plus2(3)  ==  (times3  >>  plus2)(3)

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Closures — composition 48 def  plus2    =  {  it  +  2  }
 def  times3  =  {  it  *  3  } def  times3plus2  =  plus2  <<  times3
 assert  times3plus2(3)  ==  11
 assert  times3plus2(4)  ==  plus2(times3(4)) def  plus2times3  =  times3  <<  plus2
 assert  plus2times3(3)  ==  15
 assert  plus2times3(5)  ==  times3(plus2(5)) assert  times3plus2(3)  ==  (times3  >>  plus2)(3) Reverse composition

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Tail recursion!

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Closures — recursion 50 def  naiveFact  =  {  n  -­‐>          if  (n  <  2)  n          else  n  *  call(n  -­‐  1)   }   ! assert  naiveFact(10)  ==  3628800

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Closures — recursion 50 def  naiveFact  =  {  n  -­‐>          if  (n  <  2)  n          else  n  *  call(n  -­‐  1)   }   ! assert  naiveFact(10)  ==  3628800 What about naiveFact(1000)?

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Closures — recursion 50 def  naiveFact  =  {  n  -­‐>          if  (n  <  2)  n          else  n  *  call(n  -­‐  1)   }   ! assert  naiveFact(10)  ==  3628800 What about naiveFact(1000)?

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Trampoline!

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Closures — recursion with trampoline 52 def  fact  =  {  n,  accu  =  1G  -­‐>          if  (n  <  2)  accu          if  fact.trampoline(n  -­‐  1,  n  *  accu)   }.trampoline()   ! assert  fact(1)        ==  1
 assert  fact(3)        ==  1  *  2  *  3   assert  fact(1000)  ==  402387260...
 //  plus  another  2560  digits

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Closures — recursion with trampoline 52 def  fact  =  {  n,  accu  =  1G  -­‐>          if  (n  <  2)  accu          if  fact.trampoline(n  -­‐  1,  n  *  accu)   }.trampoline()   ! assert  fact(1)        ==  1
 assert  fact(3)        ==  1  *  2  *  3   assert  fact(1000)  ==  402387260...
 //  plus  another  2560  digits Not so elegant, needs rewriting

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Closures — @TailRecursive 53 @groovy.transform.TailRecursive   def  fact(n,  accu  =  1G)  {          if  (n  <  2)  accu          else  fact(n  -­‐  1,  n  *  accu)   }

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Closures — @TailRecursive 53 @groovy.transform.TailRecursive   def  fact(n,  accu  =  1G)  {          if  (n  <  2)  accu          else  fact(n  -­‐  1,  n  *  accu)   } Since Groovy 2.3

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Closures — @TailRecursive 53 @groovy.transform.TailRecursive   def  fact(n,  accu  =  1G)  {          if  (n  <  2)  accu          else  fact(n  -­‐  1,  n  *  accu)   } Since Groovy 2.3 Transformation applied to methods, not closures

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Closures — Memoization 54 def  plus  =  {  a,  b  -­‐>  
        sleep  1000
        return  a  +  b     }.memoize()   ! assert  plus(1,  2)  ==  3  //  after  1000ms   assert  plus(1,  2)  ==  3  //  return  immediately   assert  plus(2,  2)  ==  4  //  after  1000ms   assert  plus(2,  2)  ==  4  //  return  immediately

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Closures — Memoization 54 def  plus  =  {  a,  b  -­‐>  
        sleep  1000
        return  a  +  b     }.memoize()   ! assert  plus(1,  2)  ==  3  //  after  1000ms   assert  plus(1,  2)  ==  3  //  return  immediately   assert  plus(2,  2)  ==  4  //  after  1000ms   assert  plus(2,  2)  ==  4  //  return  immediately Also: memoizeAtLeast(10) memoizeAtMost(20) memoizeBetween(10, 20)

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Closures — Memoization • Cache the result of previous invocations of closures or methods with the same set 
 of argument values 55 def  fib  =  {  n  -­‐>          if  (n  <  2)  1          else  call(n  -­‐  1)  +  call(n  -­‐  2)   }.memoize()   ! assert  fib(10)  ==  89

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Compile-time meta-programming to the rescue!

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Memoization 57 import  groovy.transform.*   ! @Memoized   long  fib(long  n)  {          if  (n  <  2)  1          else  fib(n  -­‐  1)  +  fib(n  -­‐  2)   }   ! println  fib(40)

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Memoization 57 import  groovy.transform.*   ! @Memoized   long  fib(long  n)  {          if  (n  <  2)  1          else  fib(n  -­‐  1)  +  fib(n  -­‐  2)   }   ! println  fib(40) Best applied to side-effect free functions

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def  list  =  [1,  3,  5,  7]
                      .asImmutable()   ! list  <<  9

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Immutability is good! def  list  =  [1,  3,  5,  7]
                      .asImmutable()   ! list  <<  9

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Immutability is good! def  list  =  [1,  3,  5,  7]
                      .asImmutable()   ! list  <<  9 Throws UnsupportedOperationException

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Immutability is good! def  list  =  [1,  3,  5,  7]
                      .asImmutable()   ! list  <<  9 Throws UnsupportedOperationException Easier to reason about Only one state Can be shared in parallel Suitable for caching ....

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Immutability is good! def  list  =  [1,  3,  5,  7]
                      .asImmutable()   ! list  <<  9 Throws UnsupportedOperationException Easier to reason about Only one state Can be shared in parallel Suitable for caching .... What about immutable POJOs/POGOs?

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Immutability • Implement immutability 
 by the book ! – final class – tuple-style constructor – private final backing fields – defensive copying of collections – equals() and hashCode() methods – toString() method – ... 59

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Immutability • Implement immutability 
 by the book ! – final class – tuple-style constructor – private final backing fields – defensive copying of collections – equals() and hashCode() methods – toString() method – ... 59 Can be error-prone to write immutable classes oneself!

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Immutability • A Person class with – a String name – an int age 60 public final class Person {! private final String name;! private final int age;! ! public Person(String name, int age) {! this.name = name;! this.age = age;! }! ! public String getName() {! return name;! }! ! public int getAge() {! return age;! }! ! public int hashCode() {! return age + 31 * name.hashCode();! }! ! public boolean equals(Object other) {! if (other == null) {! return false;! }! if (this == other) {! return true;! }! if (Person.class != other.getClass()) {! return false;! }! Person otherPerson = (Person)other;! if (!name.equals(otherPerson.getName()) {! return false;! }! if (age != otherPerson.getAge()) {! return false;! }! return true;! }! ! public String toString() {! return "Person(" + name + ", " + age + ")";! }! }!

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Immutability • A Person class with – a String name – an int age 60 public final class Person {! private final String name;! private final int age;! ! public Person(String name, int age) {! this.name = name;! this.age = age;! }! ! public String getName() {! return name;! }! ! public int getAge() {! return age;! }! ! public int hashCode() {! return age + 31 * name.hashCode();! }! ! public boolean equals(Object other) {! if (other == null) {! return false;! }! if (this == other) {! return true;! }! if (Person.class != other.getClass()) {! return false;! }! Person otherPerson = (Person)other;! if (!name.equals(otherPerson.getName()) {! return false;! }! if (age != otherPerson.getAge()) {! return false;! }! return true;! }! ! public String toString() {! return "Person(" + name + ", " + age + ")";! }! }! Damn verbose Java!

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Immutability • A Person class with – a String name – an int age 60 public final class Person {! private final String name;! private final int age;! ! public Person(String name, int age) {! this.name = name;! this.age = age;! }! ! public String getName() {! return name;! }! ! public int getAge() {! return age;! }! ! public int hashCode() {! return age + 31 * name.hashCode();! }! ! public boolean equals(Object other) {! if (other == null) {! return false;! }! if (this == other) {! return true;! }! if (Person.class != other.getClass()) {! return false;! }! Person otherPerson = (Person)other;! if (!name.equals(otherPerson.getName()) {! return false;! }! if (age != otherPerson.getAge()) {! return false;! }! return true;! }! ! public String toString() {! return "Person(" + name + ", " + age + ")";! }! }! Damn verbose Java! Although it’s also a valid Groovy program!

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@Immutable 61 import  groovy.transform.*   ! @Immutable   class  Person  {          String  name          int  age   }

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Groovy allows you to be lazy

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Groovy allows you to be lazy The compiler will do the job for you

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Groovy allows you to be lazy The compiler will do the job for you More concise, more readable code

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Groovy allows you to be lazy The compiler will do the job for you More concise, more readable code Less stuff to maintain and worry about

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Speaking of laziness...

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@Lazy 64 class  CountryCodes  {          private  @Lazy  String[]  codes  =                                      ['FR',  'US',  'GB',  /*...*/]   }   ! class  Portfolio  {          @Lazy  List  positions  =  {                  //  fetch  positions  lazily  from  a  DB          }()   }

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@Lazy 64 class  CountryCodes  {          private  @Lazy  String[]  codes  =                                      ['FR',  'US',  'GB',  /*...*/]   }   ! class  Portfolio  {          @Lazy  List  positions  =  {                  //  fetch  positions  lazily  from  a  DB          }()   } Lazily allocate and create some collection

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@Lazy 64 class  CountryCodes  {          private  @Lazy  String[]  codes  =                                      ['FR',  'US',  'GB',  /*...*/]   }   ! class  Portfolio  {          @Lazy  List  positions  =  {                  //  fetch  positions  lazily  from  a  DB          }()   } Lazily allocate and create some collection A lazy computation for an expensive structure with closure logic

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Groovy-stream library 65 @Grab('com.bloidonia:groovy-­‐stream:0.5.2')   import  groovy.stream.Stream   ! def  integers  =  Stream.from  {  x++  }  using  x:  1   def  squares    =  Stream.from  integers  map  {  it  *  it  }   def  first5      =  squares.take(  5  ).collect()   ! assert  first5  ==  [  1,  4,  9,  16,  25  ]   ! def  s  =  Stream.from(  x:1..5,  y:1..3  )                              .filter  {  (x  +  y)  %  (x  +  2)  ==  0  }                              .map  {  x  +  y  }   ! assert  s.collect()  ==  [  3,  4,  5,  6,  7  ]

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Groovy-stream library 65 @Grab('com.bloidonia:groovy-­‐stream:0.5.2')   import  groovy.stream.Stream   ! def  integers  =  Stream.from  {  x++  }  using  x:  1   def  squares    =  Stream.from  integers  map  {  it  *  it  }   def  first5      =  squares.take(  5  ).collect()   ! assert  first5  ==  [  1,  4,  9,  16,  25  ]   ! def  s  =  Stream.from(  x:1..5,  y:1..3  )                              .filter  {  (x  +  y)  %  (x  +  2)  ==  0  }                              .map  {  x  +  y  }   ! assert  s.collect()  ==  [  3,  4,  5,  6,  7  ] Lazy streams and comprehensions

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A few other AST transformations • @Delegate – implement the delegate design pattern ! • @Canonical – like @Immutable but for mutable data – combines the @TupleConstructor, @EqualsAndHashcode, and @ToString transformations ! • @Singleton – to create singleton objects (potentially lazily) 66

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Persistent data structures • No built-in persistent collections — yet ! • But works well with existing – Functional Java – pcollections – clj-ds – totallylazy 67

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Persistent data structures 68

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Persistent data structures 68 @Grab("org.pcollections:pcollections:2.1.2")
 import  org.pcollections.*

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Persistent data structures 68 @Grab("org.pcollections:pcollections:2.1.2")
 import  org.pcollections.*

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Persistent data structures 68 @Grab("org.pcollections:pcollections:2.1.2")
 import  org.pcollections.* def  m0  =  HashTreePMap.from(a:  1,  b:  2)
 assert  m0.size()  ==  2

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Persistent data structures 68 @Grab("org.pcollections:pcollections:2.1.2")
 import  org.pcollections.* def  m0  =  HashTreePMap.from(a:  1,  b:  2)
 assert  m0.size()  ==  2

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Persistent data structures 68 @Grab("org.pcollections:pcollections:2.1.2")
 import  org.pcollections.* def  m0  =  HashTreePMap.from(a:  1,  b:  2)
 assert  m0.size()  ==  2 def  m1  =  m0  +  [c:  3]
 assert  m1.size()  ==  3  &&  m1.c  ==  3
 assert  m0.size()  ==  2

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Persistent data structures 68 @Grab("org.pcollections:pcollections:2.1.2")
 import  org.pcollections.* def  m0  =  HashTreePMap.from(a:  1,  b:  2)
 assert  m0.size()  ==  2 def  m1  =  m0  +  [c:  3]
 assert  m1.size()  ==  3  &&  m1.c  ==  3
 assert  m0.size()  ==  2

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Persistent data structures 68 @Grab("org.pcollections:pcollections:2.1.2")
 import  org.pcollections.* def  m0  =  HashTreePMap.from(a:  1,  b:  2)
 assert  m0.size()  ==  2 def  m1  =  m0  +  [c:  3]
 assert  m1.size()  ==  3  &&  m1.c  ==  3
 assert  m0.size()  ==  2 def  m2  =  m0  -­‐  'a'
 assert  m0.size()  ==  2
 assert  m1.size()  ==  3
 assert  m2.size()  ==  1

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That’s probably the part of the talk where you’d need a brain

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That’s probably the part of the talk where you’d need a brain But I don’t have one

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Monads, Functors, Option, Either… • Groovy doesn’t come built-in with monads, option, either, comprehensions, but third-party libraries bring that support ! – Monadologie – Fpiglet – Functional Groovy 70

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Monads, Functors, Option, Either… • Groovy doesn’t come built-in with monads, option, either, comprehensions, but third-party libraries bring that support ! – Monadologie – Fpiglet – Functional Groovy 70 References available in the «resources» part

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ForEach monad comprehension 71 import  static          hr.helix.monadologie.MonadComprehension.foreach   ! def  res  =  foreach  {          a  =  takeFrom  {  [1,  2,  3]  }          b  =  takeFrom  {  [4,  5]  }   !        yield  {  a  +  b  }   }   ! assert  res  ==  [5,  6,  6,  7,  7,  8]

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Part 4 Summary

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I’m functional!

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Part 5 Resources

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• Paul King – Concurrency with GPars • http://slideshare.net/paulk_asert/concurrency-gpars – Functional Groovy • http://slideshare.net/paulk_asert/functional-groovy • Andres Almiray – Functional Groovy • http://slideshare.net/aalmiray/functional-groovy-confess • Arturo Herrero – Functional programming with Groovy • http://slideshare.net/arturoherrero/functional-programming-with-groovy • Neal Ford – Functional thinking • http://www.youtube.com/watch?v=7aYS9PcAITQ&feature=youtu.be • Mario Fusco – If you think you can stay away from FP, you are wrong • http://slideshare.net/mariofusco/if-you-think-you-can-stay-away-from-functional-programming-you-are-wrong Resources — Presentations 75

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Resources — Interesting projects • GPars — Concurrency & parallelism library – http://gpars.codehaus.org/ • Fpiglet — Blog post experiments on combinators, monadic comprehensions – https://code.google.com/p/fpiglet/ – http://rpeszek.blogspot.ch/ • Monadologie — Monad comprehensions library – https://github.com/dsrkoc/monadologie • Groovy Stream — Lazy iterators and streams – http://timyates.github.io/groovy-stream/ • Functional Groovy — Functional Java wrapper – https://github.com/mperry/functionalgroovy 76

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Thank you! Q&A