Coding in Kotlin with Arrow NIDC 2018

training@instil.co
June 2018
© Instil Software 2018
Coding in Kotlin with Arrow
NIDC 2018
https://gitlab.com/instil-training/nidc-arrow-2018

About Me
Experienced trainer
• 20 years in the trenches
• Over 1000 deliveries
The day job
• Head of Learning at Instil
• Coaching, mentoring etc…
The night job(s)
• Husband and father
• Krav Maga Instructor

A quick Kotlin refresher
• Kotlin compared to Java
Introducing Arrow
• What is it and why does it matter?
Arrow Pt1: Data Types
• Option, Try, Either and Validated
Arrow Pt2: Enhanced FP
• Partial Invocation, Currying and Composition
Arrow Pt3: Esoteric Stuff
• Lenses, IO etc…
Agenda For This Talk

• A Quick Language Overview
Kotlin Refresher

Enter some numbers or three 'X' to finish
10
20
30
40
50
XXX
Total of numbers is: 150
wibble
Ignoring wibble
12
13
14
XXX
XXX

public class Program {
public static void main(String[] args) {
@SuppressWarnings("resource")
Scanner scanner = new Scanner(System.in);
List<Integer> numbers = new ArrayList<>();
System.out.println("Enter some numbers or three 'X' to finish");
Pattern endOfInput = Pattern.compile("X{3}");
while(scanner.hasNextLine()) {
if (scanner.hasNext(endOfInput)) {
break;
} else if (scanner.hasNextInt()) {
numbers.add(scanner.nextInt());
} else {
String mysteryText = scanner.nextLine();
System.out.printf("Ignoring %sn", mysteryText);
}
}
int total = numbers.stream().reduce((a,b) -> a + b).orElse(0);
System.out.printf("Total of numbers is: %sn",total);
}
}
A Sample Java Program

fun main(args: Array<String>) {
val numbers = mutableListOf<Int>()
val scanner = Scanner(System.`in`)
val endOfInput = Regex("X{3}")
println("Enter some numbers or three 'X' to finish")
while (scanner.hasNextLine()) {
if (scanner.hasNext(endOfInput.toPattern())) {
break
} else if (scanner.hasNextInt()) {
numbers += scanner.nextInt()
} else {
val mysteryText = scanner.nextLine()
println("Ignoring $mysteryText")
}
}
//Would be better to use ‘numbers.sum()’
val total = numbers.fold(0, Int::plus)
println("Total of numbers is: $total")
}
The Sample Re-Written in Kotlin
Points to note:
 No redundant class
 No semi-colons
 Type inference
 Both ‘val’ and ‘var’
 Helper functions
 String interpolation
 Simplified collections
 Interop with Java types
 Simpler use of FP

• What is it?
• Why does it matter?
Introducing Arrow

Languages Today Are Converging

The Great Scala vs. Kotlin Debate
I am an Enterprise
Coder. Like my father
before me
You fool. If only you
knew the power of
Monads!

How Much Functional Programming is ‘Just Right’?

Arrow is a functional programming library for Kotlin coders
• Launched in Jan when the two leading libraries joined forces
• To prevent the ‘Scalaz vs. Cats’ debate that exists in Scala
It is not a formal part of the Kotlin environment
• But is generating a lot of interest in the Kotlin community
• It has resulted in proposals for changes to the language
Learning Arrow can be a bit frustrating as:
• The documentation is incomplete and patchy
• Changes are still occurring between releases
• Sample code is hard to find….
I’m still learning what its all about!
• This is very much a report on my progress so far…
Introducing Arrow

Lets Do The Same Thing Many Times 

fun readPropertyA(name: String): Option<String> {
val result = System.getProperty(name)
return if(result != null) Some(result) else None
}
fun readPropertyB(name: String) =
Option.fromNullable(System.getProperty(name))
Reading Property Values Safely Via Option
<<abstract>>
Option
Some None
Empty SetSet of One

fun print1(input: Option<String>): Unit {
when(input) {
is None -> println("Nothing was found")
is Some -> println("'${input.t}' was found")
}
}
println("${input.getOrElse { "Nothing" }} was found")
}

val result = input.fold({ "Nothing" }, { it })
println("$result was found")
}
fun print4(input1: Option<String>, input2: Option<String>): Unit {
val result = input1.flatMap { first ->
input2.map { second ->
"$first and $second"
}
}
println("Results are ${result.getOrElse { "Nothing" }}")
}

print1(readPropertyA("java.version"))
print1(readPropertyA("wibble"))
printLine()
print2(readPropertyB("java.version"))
print2(readPropertyB("wibble"))
printLine()
print3(readPropertyA("java.version"))
print3(readPropertyA("wibble"))
printLine()
print4(
readPropertyA("java.version"),
readPropertyB("java.version")
)
printLine()
print4(
readPropertyA("java.version"),
readPropertyB("wibble")
)
printLine()
print4(readPropertyA("wibble"), readPropertyB("wibble"))
printLine()
print4(readPropertyA("wibble"), readPropertyB("wibble"))
}

'1.8.0_121' was found
Nothing was found
---------------
1.8.0_121 was found
Nothing was found
---------------
1.8.0_121 was found
Nothing was found
---------------
Results are 1.8.0_121 and 1.8.0_121
---------------
Results are Nothing
---------------
Results are Nothing
---------------
Results are Nothing

class Postcode(val input: String?) {
fun value() = Option.fromNullable(input)
}
class Address(val street: String, val postcode: Postcode?) {
fun location() = Option.fromNullable(postcode)
}
class Person(val name: String, val address: Address?) {
fun residence() = Option.fromNullable(address)
}
Using Option as a Monad

fun printPostcode(person: Person) {
val result = Option.monad().binding {
val address = person.residence().bind()
val location = address.location().bind()
location.value().bind()
}.fix()
println(result.fold( { "No postcode available" },
{ "Postcode of $it" }))
}

printPostcode(Person("Dave",
Address("10 Arcatia Road",
Postcode("ABC 123"))))
printPostcode(Person("Dave",
Address("10 Arcatia Road", null)))
printPostcode(Person("Dave", null))
}
Postcode of ABC 123
No postcode available
No postcode available

fun firstLine(path: String): Try<String> {
fun readFirstLine(path: String): String {
val reader = BufferedReader(FileReader(path))
return reader.use { it.readLine() }
}
return Try { readFirstLine(path) }
}
Reading the First Line from a File via Try
<<abstract>>
Try
Success Failure
Holds ErrorHolds Result

fun print1(input: Try<String>): Unit {
when(input) {
is Success -> println("Read '${input.value}'")
is Failure -> println("Threw '${input.exception.message}'")
}
}
val result = input.fold( { "Threw '${it.message}'" },
{ "Read '$it'" })
println(result)
}
input.map { println("Read '$it'") }
input.recover { println("Threw '${it.message}'") }
}

fun print4(input: String) {
fun fullPath(str: String) = "data/$str"
val finalResult = firstLine(fullPath(input)).flatMap { one ->
firstLine(fullPath(one)).flatMap { two ->
firstLine(fullPath(two)).flatMap { three ->
firstLine(fullPath(three)).flatMap { four ->
firstLine(fullPath(four)).map { result ->
result
}
}
}
}
}
val message = finalResult.fold({ it.message }, { it })
println("Path navigation produced '$message'")
}
Traversing Across Files

print1(firstLine("data/input4.txt"))
print1(firstLine("foobar.txt"))
printLine()
printLine()
printLine()
print4("input.txt")
print4("foobar.txt")
}

Read 'Fortune favors the prepared mind'
Threw 'foobar.txt (No such file or directory)'
---------------
---------------
---------------
Path navigation produced 'Fortune favors the prepared mind'
Path navigation produced 'data/foobar.txt (No such file or directory)'

fun readFromFiles(input: String): String? {
val result = Try.monad().binding {
val one = firstLine(fullPath(input)).bind()
val two = firstLine(fullPath(one)).bind()
val three = firstLine(fullPath(two)).bind()
val four = firstLine(fullPath(three)).bind()
firstLine(fullPath(four)).bind()
}.fix()
return result.fold({ it.message }, { it })
}

println("Path navigation produced '${readFromFiles("input.txt")}'")
println("Path navigation produced '${readFromFiles("foobar")}'")
}
Path navigation produced 'Fortune favors the prepared mind'
Path navigation produced 'data/foobar (No such file or directory)'

fun genNumber() : Either<Int, Int> {
val number = (random() * 100).toInt()
return if(number % 2 == 0) Right(number)
else Left(number)
}
Using the Either Type
<<abstract>>
Either
Left Right
Happy PathAlternative

val results = (1 .. 10).map {
genNumber().flatMap { first ->
genNumber().map { second ->
Pair(first, second)
}
}
}
results.forEach { result ->
val msg = result.fold(
{ "Odd number $it" },
{ "Even numbers ${it.first} and ${it.second}" }
)
println(msg)
}
}
Using the Either Type Even numbers 50 and 40
Odd number 77
Even numbers 52 and 32
Odd number 25
Odd number 89
Even numbers 80 and 54
Odd number 65
Odd number 1
Odd number 1
Odd number 33

fun checkNum(number:Int) : Either<Int, Int> {
return if(number % 2 == 0) Right(number) else Left(number)
}
class EitherTest : ShouldSpec() {
init {
should("be able to detect Right") {
checkNum(4).shouldBeRight(4)
}
should("be able to detect Left") {
checkNum(5).shouldBeLeft(5)
}
}
}
All Arrow Types are Supported in KotlinTest

Our Sample Problem
Whats your ID?
ab12
How old are you?
19
Where do you work?
HR
Error: Bad ID
Whats your ID?
AB12
How old are you?
19
Where do you work?
IT
Result: AB12 of age 19 working in IT
Whats your ID?
ab12
How old are you?
14
Where do you work?
Mars
Error: Bad Dept Bad ID Bad Age

class Employee(val id: String, val age: Int, val dept: String) {
override fun toString() = "$id of age $age working in $dept"
}
fun askQuestion(question: String): String {
println(question)
return readLine() ?: ""
}
Reading and Validating Information
<<abstract>>
Validated
Invalid Valid
ResultMessage

fun checkID(): Validated<String, String> {
val regex = Regex("[A-Z]{2}[0-9]{2}")
val response = askQuestion("Whats your ID?")
return if(regex.matches(response)) Valid(response)
else Invalid("Bad ID")
}
fun checkAge(): Validated<String, Int> {
val response = askQuestion("How old are you?").toInt()
return if(response > 16) Valid(response) else Invalid("Bad Age")
}
fun checkDept(): Validated<String, String> {
val depts = listOf("HR", "Sales", "IT")
val response = askQuestion("Where do you work?")
return if(depts.contains(response)) Valid(response)
else Invalid("Bad Dept")
}

val sg = object : Semigroup<String> {
override fun String.combine(b: String) = "$this $b"
}
val id = checkID()
val age = checkAge()
val dept = checkDept()
val result = Validated.applicative(sg)
.map(id, age, dept, {
(a,b,c) -> Employee(a,b,c)
})
.fix()
println(result.fold({ "Error: $it" }, {"Result: $it"} ))
}

Our example of Validated contained a bug…
• We assumed the value given for ‘age’ would be an integer
Composing Types
Whats your ID?
AB12
How old are you?
abc
Exception in thread "main" java.lang.NumberFormatException: For input string: "abc"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Integer.parseInt(Integer.java:580)
at java.lang.Integer.parseInt(Integer.java:615)
at arrow.types.validated.ProgramKt.checkAge(Program.kt:22)
at arrow.types.validated.ProgramKt.main(Program.kt:37)

fun checkAge(): Validated<String, Int> {
val response = askQuestion("How old are you?").toInt()
return if(response > 16) Valid(response)
else Invalid("Bad Age")
}
Composing Types

We can solve the problem by composing types together
• Our methods should return a Try<Validated<T,U>>
Composing Types

fun checkID(): Try<Validated<String, String>> {
val regex = Regex("[A-Z]{2}[0-9]{2}")
val response = askQuestion("Whats your ID?")
return Try { if(regex.matches(response)) Valid(response)
else Invalid("Bad ID") }
}
fun checkAge(): Try<Validated<String, Int>> {
val response = Try { askQuestion("How old are you?").toInt() }
return response.map({ num -> if(num > 16) Valid(num)
else Invalid("Bad Age")})
}
Composing Types

fun checkSalary(): Try<Validated<String, Double>> {
val response = Try { askQuestion("What is your salary?")
.toDouble() }
return response.map({ num -> if(num > 15000.0) Valid(num)
else Invalid("Bad Salary") })
}
fun checkDept(): Try<Validated<String, String>> {
val depts = listOf("HR", "Sales", "IT")
val response = askQuestion("Where do you work?")
return Try { if(depts.contains(response)) Valid(response)
else Invalid("Bad Dept") }
}
Composing Types

fun success(emp: Employee)
= println("Created $emp")
fun exception(ex: Throwable)
= println("Exception occurred - ${ex.message}")
fun invalid(msg: String?)
= println("Validation error occurred - $msg")
class Employee(val id: String, val age: Int,
val dept: String, val salary: Double) {
constructor(t: Tuple4<String,Int,String,Double>) :
this(t.a,t.b,t.c,t.d)
override fun toString()
= "$id of age $age working in $dept earning $salary"
}
Composing Types

Composing Types
Try<Validated<T,U>>
Failure
Success<Validated<T,U>>
exception()
Valid<U>
Invalid<T>
success()
invalid()

val app = Validated.applicative(object : Semigroup<String> {
override fun String.combine(b: String) = "$this $b"
})
Try.monad().binding {
val id = checkID().bind()
val age = checkAge().bind()
val dept = checkDept().bind()
val salary = checkSalary().bind()
app.map(id, age, dept, salary, ::Employee).fix()
}.fix().fold(
{exception(it)},
{it.fold(::invalid,::success)})
}
Composing Types

Composing Types
Whats your ID?
AB12
How old are you?
abc
Exception occurred - For input string: "abc"
Whats your ID?
AB12
How old are you?
21
Where do you work?
Sales
What is your salary?
abc
Exception occurred - For input string: "abc"
Whats your ID?
foo
How old are you?
21
Where do you work?
Sales
30000.0
Validation error occurred - Bad ID
Whats your ID?
AB12
How old are you?
21
Where do you work?
foo
30000.0
Validation error occurred - Bad Dept

Composing Types
Whats your ID?
AB12
How old are you?
21
Where do you work?
Sales
30000.0
Created AB12 of age 21 working in Sales earning 30000.0

fun demo1() {
val addNums = { no1: Int, no2: Int ->
println("Adding $no1 to $no2")
no1 + no2
}
val addSeven = addNums.partially2(7)
val result = addSeven(3)
println(result)
}
Partial Invocation
Adding 3 to 7
10

fun demo2() {
no1 + no2
}
val addSeven = addNums.partially2(7)
val addSevenToThree = addSeven.partially1(3)
val result = addSevenToThree()
println(result)
}
Partial Invocation
Adding 3 to 7
10

fun demo3() {
no1 + no2
}
val addSeven = addNums.reverse().partially2(7)
val result = addSeven(3)
println(result)
}
Partial Invocation
Adding 7 to 3
10

fun grep(path: String, regex: Regex, action: (String) -> Unit) {
reader.use {
it.lines()
.filter { regex.matches(it) }
.forEach(action)
}
}
val grepLambda = { a: String, b: Regex, c: (String) -> Unit ->
grep(a, b, c)
}
fun printLine() = println("-------------")
Here’s Something More Useful

val filePath = "data/grepInput.txt"
val regex = "[A-Z]{2}[0-9]{2}".toRegex()
grep(filePath, regex, ::println)
printLine()
grepLambda(filePath, regex, ::println)
printLine()
AB12
CD34
EF56
-------------
AB12
CD34
EF56
-------------

val grepAndPrint = grepLambda.partially3(::println)
grepAndPrint(filePath, regex)
printLine()
val sb = StringBuilder()
val grepAndConcat = grepLambda.partially3 {sb.append(it)}
grepAndConcat(filePath, regex)
println(sb.toString())
printLine()
val grepAndPrintRegex = grepAndPrint.partially2(regex)
grepAndPrintRegex(filePath)
AB12
CD34
EF56
-------------
AB12CD34EF56
-------------
AB12
CD34
EF56

val addThree = { a:Int, b: Int, c:Int ->
println("Adding $a, $b and $c")
a + b + c
}
fun printLine() = println("--------------------")
println(addThree(10,20,40))
printLine()
val f1 = addThree.curried()
val f2 = f1(10)
val f3 = f2(20)
val result = f3(40)
println(result)
printLine()
println(f1(10)(20)(40))
printLine()
val f4 = addThree.reverse().curried()
println(f4(10)(20)(40))
println()
}
The Basic Syntax of Currying
Adding 10, 20 and 40
70
--------------------
70
--------------------
70
--------------------
70

fun grep(path: String, regex: Regex, action: (String) -> Unit) {
reader.use {
it.lines()
.filter { regex.matches(it) }
.forEach(action)
}
}
val grepLambda = { a: String, b: Regex, c: (String) -> Unit ->
grep(a,b,c) }
fun printLine() = println("-------------")
A More Useful Example

val filePath = "data/grepInput.txt"
val regex1 = "[A-Z]{2}[0-9]{2}".toRegex()
val regex2 = "[a-z]{2}[0-9]{2}".toRegex()
val f1 = grepLambda.curried()
val grepInFile = f1(filePath)
val grepRegex1 = grepInFile(regex1)
val grepRegex2 = grepInFile(regex2)
grepRegex1(::println)
printLine()
grepRegex2(::println)
}
A More Useful Example
AB12
CD34
EF56
-------------
ab12
cd34
ef56

val source = { name: String -> "data/$name" }
val allLines = { path: String ->
reader.use {
it.lines().toList()
}
}
val findMatches = { input: List<String> ->
val regex = "[A-Z]{2}[0-9]{2}".toRegex()
input.filter(regex::matches)
}
Composing Functions Together

val composed1 = findMatches compose allLines compose source
println(composed1("grepInput.txt"))
val composed2 = source forwardCompose allLines forwardCompose findMatches
val composed3 = source andThen allLines andThen findMatches
}
Composing Functions Together
[AB12, CD34, EF56]
[AB12, CD34, EF56]
[AB12, CD34, EF56]

@optics
data class Postcode(val value: String) {
override fun toString() = "$value"
}
@optics
data class Address(val street: String, val postcode: Postcode) {
override fun toString() = "$street ($postcode)"
}
@optics
data class Person(val name: String, val address: Address) {
override fun toString() = "$name living at $address"
}
Copying Immutable Structures With Lenses

val oldPerson = Person("Dave",
Address("10 Arcatia Road",
Postcode("BT26 ABC")))
println(oldPerson)
val personAddressPostcode
= personAddress() compose addressPostcode() compose postcodeValue()
val newPerson
= personAddressPostcode.modify(oldPerson, { _ -> "BT37 DEF" })
println(newPerson)
}
Copying Immutable Structures With Lenses
Dave living at 10 Arcatia Road (BT26 ABC)
Dave living at 10 Arcatia Road (BT37 DEF)

Coding in Kotlin with Arrow NIDC 2018

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