Let's get Functional

Ever tried functional languages ? Ever been annoyed that every time you see the same Fibonacci examples or yet another maths problem ?

So what can functional languages really teach us on our day to day life as Java devs ?
This talk will take some concrete examples and show you how some functional approaches will make your life easier.
We will see how we can try and bring functional and immutability to our "beloved" Android Java 7 and what this brings us.

Video of the presentation at Droidcon Berlin 2015 available here:
https://www.youtube.com/watch?v=B4qbQ1wGpNM

Transcript

1. Let’s get Functional
   

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2. @Dorvaryn
   +BenjaminAugustin-Dorvaryn
   Dorvaryn
   Benjamin Augustin
   Android Software Craftsman
   

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3. What is Functional
   Programming ?
   

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4. “In computer science, functional programming is
   a programming paradigm that treats computation
   as the evaluation of mathematical functions and
   avoids changing-state and mutable data.”
   - Wikipedia contributors, 2015
   

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6. ● Less code
   ● Expressive code
   ● Small units
   What’s the point ?
   

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7. Focus on what not how
   

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8. Core concepts
   

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9. ● Not mutable, unchangeable, changeless
   ● Modification -> new value
   ● Safe to cache
   Immutability
   

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10. ● Same input -> Same output
    ● No side effect
    ● Referential transparency
    Pure functions
    

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12. ● Not a requirement but helps … a lot
    ● Building your domain
    ● Dealing with absence of data
    Strongly typed
    

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14. “I call it my billion-dollar mistake. It was
    the invention of the null reference in 1965.”
    - Sir Tony Hoare, 2009
    

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15. ● Null object
    ● Optional
    ● Mandatory
    Modeling absence
    

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17. You can use most common languages
    

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18. ● RxJava
    ● AutoValue
    ● Guava
    ● Or… Kotlin ?
    For java 7 and lower
    

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20. Kotlin Syntax
    fun aFunction(parameter: String): String {
    return parameter
    }
    fun highOrder(param: Int, func: (Int) -> Int): Int {
    return func(param)
    }
    val lambda = { x: Int, y: Int -> x + y }
    

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21. Let’s get Functional
    

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22. fun foo(words: Array): Map {
    val result = mapOf()
    for (word in words) {
    val value = result.get(word)
    if (value != null) {
    result.set(word, value + 1)
    } else {
    result.set(word, 1)
    }
    }
    return result
    }
    What does foo do ?
    

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24. fun foo(words: Array): List> {
    return words.groupBy {
    word -> word
    }.map {
    Pair(it.key, it.value.count())
    }
    }
    Can you tell now ?
    

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25. ● Code should be self explanatory
    ● Not just the method name
    ● Keep things concise
    Expressive code
    

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26. ● Get items occurring at least 3 times
    ● Sort by occurrences
    ● As a list of strings
    Extensible code
    

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27. fun mostFrequents(occurences: Array, threshold: Int):
    List {
    return countOccurences(occurences).filter {
    it.second > threshold
    }.sortedBy {
    it.second
    }.map {
    it.first
    }
    }
    Extended Code
    

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30. Focus on your data
    

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31. ● Article, Product, User...
    ● Value objects -> Immutable
    ● Lists of value objects
    What is your data ?
    

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32. ● Fetching a list of users
    ● Calculating the price of a basket
    ● Input List -> Output List
    What is your processing ?
    

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33. Think like shell
    #!/bin/bash
    for jpg; do
    png="${jpg%.jpg}.png"
    if convert "$jpg" jpg.to.png ;
    then
    mv jpg.to.png "$png"
    else
    echo 'error' >&2
    exit 1
    fi
    done
    echo all conversions successful
    exit 0
    du -h | grep M | sort -nr | head -15
    

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34. ● map, flatMap, filter, findAll, merge, zip…..
    ● first, last, find, fold..…
    ● Decompose your processing using those functions
    Functions upon lists
    

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35. ● Display the saved articles in a list
    ● Fetch the items in a basket requiring age check
    ● Any filtering logic -> list + filter
    Don’t trust me ?
    

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36. ● Get the total of a basket
    ● Find an item in a list
    ● Any list traversal logic -> list + fold
    Another one ?
    

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38. ● f(x,y) applied on each element in a list
    ● x element in list
    ● y accumulator
    Fold ?
    

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39. Fold ?
    

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40. Maximum using fold
    +
    +
    +
    ->
    ->
    ->
    

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42. Basket example
    

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43. data class Basket(val items: List)
    data class Item(val barcode: String, val price: Double)
    

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44. data class Basket(val items: List)
    data class Item(val barcode: String, val price: Double)
    fun calculateBasket(basket: Basket): Double {
    var total = 0.0
    for (item in basket.items) {
    total = total + item.price
    }
    return total
    }
    

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45. data class Basket(val items: List)
    data class Item(val barcode: String, val price: Double)
    fun calculateBasket(basket: Basket): Double {
    return basket.items
    .fold(0.0, { total, item -> total + item.price })
    }
    

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46. data class Basket(val items: List)
    data class Item(val barcode: String, val price: Double)
    fun calculateBasket(basket: Basket): Double {
    return basket.items
    .fold(0.0, sum())
    }
    private fun sum() = {
    total:Double, item: Item -> total + item.price
    }
    

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47. data class Basket(val items: List)
    data class Item(val barcode: String, val price: Double)
    fun calculateBasket(basket: Basket): Double {
    var total = 0.0
    for (item in basket.items) {
    total = sum(item, total)
    }
    return total
    }
    private fun sum(item: Item, total: Double) = {
    total + item.price
    }
    

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48. data class Basket(val items: List)
    data class Item(val barcode: String, val price: Double)
    fun calculateBasket(basket: Basket): Double {
    return basket.items
    .fold(0.0, sum())
    }
    private fun sum() = {
    total:Double, item: Item -> total + item.price
    }
    

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52. Make your data immutable
    Start small
    Aim for pure functions
    Validate your data
    Focus on what not how
    

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53. Learn a functional language
    Keep learning
    Spend some time on pet projects
    Have fun !
    

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56. ● Learn you a Haskell for great good by Miran Lipovača
    ● Functional Programming in Java by Venkat Subramaniam
    ● Category Theory for Programmers by Bartosz Milewski
    Good reads
    

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57. Questions ?
    

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58. @Dorvaryn
    +BenjaminAugustin-Dorvaryn
    Dorvaryn
    Benjamin Augustin
    Android Software Craftsman
    

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59. Click to add body
    Click to add code
    

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