Arrow in Kotlin

Issue: BAEL-1495

kotlin-libraries/pom.xml

@@ -87,6 +87,13 @@
             <artifactId>h2</artifactId>
             <version>${h2database.version}</version>
         </dependency>
+        <!-- https://mvnrepository.com/artifact/io.arrow-kt/arrow-core -->
+        <dependency>
+            <groupId>io.arrow-kt</groupId>
+            <artifactId>arrow-core</artifactId>
+            <version>0.7.3</version>
+        </dependency>
+
     </dependencies>
 
     <properties>

kotlin-libraries/src/main/kotlin/com/baeldung/kotlin/arrow/FunctionalErrorHandlingWithEither.kt

@@ -0,0 +1,54 @@
+package com.baeldung.kotlin.arrow
+
+import arrow.core.Either
+import arrow.core.filterOrElse
+import kotlin.math.sqrt
+
+class FunctionalErrorHandlingWithEither {
+
+    sealed class ComputeProblem {
+        object OddNumber : ComputeProblem()
+        object NotANumber : ComputeProblem()
+    }
+
+    fun parseInput(s : String) : Either<ComputeProblem, Int> = Either.cond(s.toIntOrNull() != null, {-> s.toInt()}, {->ComputeProblem.NotANumber} )
+
+    fun isEven(x : Int) : Boolean = x % 2 == 0
+
+    fun biggestDivisor(x: Int) : Int = biggestDivisor(x, 2)
+
+    fun biggestDivisor(x : Int, y : Int) : Int {
+        if(x == y){
+            return 1;
+        }
+        if(x % y == 0){
+            return x / y;
+        }
+        return biggestDivisor(x, y+1)
+    }
+
+    fun isSquareNumber(x : Int) : Boolean {
+        val sqrt: Double = sqrt(x.toDouble())
+        return sqrt % 1.0 == 0.0
+    }
+
+    fun computeWithEither(input : String) : Either<ComputeProblem, Boolean> {
+        return parseInput(input)
+                .filterOrElse(::isEven) {->ComputeProblem.OddNumber}
+                .map (::biggestDivisor)
+                .map (::isSquareNumber)
+    }
+
+    fun computeWithEitherClient(input : String) {
+        val computeWithEither = computeWithEither(input)
+
+        when(computeWithEither){
+            is Either.Right -> "The greatest divisor is square number: ${computeWithEither.b}"
+            is Either.Left -> when(computeWithEither.a){
+                is ComputeProblem.NotANumber -> "Wrong input! Not a number!"
+                is ComputeProblem.OddNumber -> "It is an odd number!"
+            }
+        }
+    }
+
+}

kotlin-libraries/src/main/kotlin/com/baeldung/kotlin/arrow/FunctionalErrorHandlingWithOption.kt

@@ -0,0 +1,46 @@
+package com.baeldung.kotlin.arrow
+
+import arrow.core.None
+import arrow.core.Option
+import arrow.core.Some
+import kotlin.math.sqrt
+
+class FunctionalErrorHandlingWithOption {
+
+    fun parseInput(s : String) : Option<Int> = Option.fromNullable(s.toIntOrNull())
+
+    fun isEven(x : Int) : Boolean = x % 2 == 0
+
+    fun biggestDivisor(x: Int) : Int = biggestDivisor(x, 2)
+
+    fun biggestDivisor(x : Int, y : Int) : Int {
+        if(x == y){
+            return 1;
+        }
+        if(x % y == 0){
+            return x / y;
+        }
+        return biggestDivisor(x, y+1)
+    }
+
+    fun isSquareNumber(x : Int) : Boolean {
+        val sqrt: Double = sqrt(x.toDouble())
+        return sqrt % 1.0 == 0.0
+    }
+
+    fun computeWithOption(input : String) : Option<Boolean> {
+        return parseInput(input)
+                .filter(::isEven)
+                .map(::biggestDivisor)
+                .map(::isSquareNumber)
+    }
+
+    fun computeWithOptionClient(input : String) : String{
+        val computeOption = computeWithOption(input)
+
+        return when(computeOption){
+            is None -> "Not an even number!"
+            is Some -> "The greatest divisor is square number: ${computeOption.t}"
+        }
+    }
+}

kotlin-libraries/src/test/kotlin/com/baeldung/kotlin/arrow/FunctionalDataTypes.kt

@@ -0,0 +1,143 @@
+package com.baeldung.kotlin.arrow
+
+import arrow.core.*
+import org.junit.Assert
+import org.junit.Test
+
+class FunctionalDataTypes {
+
+    @Test
+    fun whenIdCreated_thanValueIsPresent(){
+        val id = Id("foo")
+        val justId = Id.just("foo");
+
+        Assert.assertEquals("foo", id.extract())
+        Assert.assertEquals(justId, id)
+    }
+
+    fun length(s : String) : Int = s.length
+
+    fun isBigEnough(i : Int) : Boolean = i > 10
+
+    @Test
+    fun whenIdCreated_thanMapIsAssociative(){
+        val foo = Id("foo")
+
+        val map1 = foo.map(::length)
+                .map(::isBigEnough)
+        val map2 = foo.map { s -> isBigEnough(length(s)) }
+
+        Assert.assertEquals(map1, map2)
+    }
+
+    fun lengthId(s : String) : Id<Int> = Id.just(length(s))
+
+    fun isBigEnoughId(i : Int) : Id<Boolean> = Id.just(isBigEnough(i))
+
+    @Test
+    fun whenIdCreated_thanFlatMapIsAssociative(){
+        val bar = Id("bar")
+
+        val flatMap = bar.flatMap(::lengthId)
+            .flatMap(::isBigEnoughId)
+        val flatMap1 = bar.flatMap { s -> lengthId(s).flatMap(::isBigEnoughId) }
+
+        Assert.assertEquals(flatMap, flatMap1)
+    }
+
+    @Test
+    fun whenOptionCreated_thanValueIsPresent(){
+        val factory = Option.just(42)
+        val constructor = Option(42)
+        val emptyOptional = Option.empty<Integer>()
+        val fromNullable = Option.fromNullable(null)
+
+        Assert.assertEquals(42, factory.getOrElse { -1 })
+        Assert.assertEquals(factory, constructor)
+        Assert.assertEquals(emptyOptional, fromNullable)
+    }
+
+    @Test
+    fun whenOptionCreated_thanConstructorDifferFromFactory(){
+        val constructor : Option<String?> = Option(null)
+        val fromNullable : Option<String?> = Option.fromNullable(null)
+
+        try{
+            constructor.map { s -> s!!.length }
+            Assert.fail()
+        } catch (e : KotlinNullPointerException){
+            fromNullable.map { s->s!!.length }
+        }
+        Assert.assertNotEquals(constructor, fromNullable)
+    }
+
+    fun wrapper(x : Integer?) : Option<Int> = if (x == null) Option.just(-1) else Option.just(x.toInt())
+
+    @Test
+    fun whenOptionFromNullableCreated_thanItBreaksLeftIdentity(){
+        val optionFromNull = Option.fromNullable(null)
+
+        Assert.assertNotEquals(optionFromNull.flatMap(::wrapper), wrapper(null))
+    }
+
+    @Test
+    fun whenEitherCreated_thanOneValueIsPresent(){
+        val rightOnly : Either<String,Int> = Either.right(42)
+        val leftOnly : Either<String,Int> = Either.left("foo")
+
+        Assert.assertTrue(rightOnly.isRight())
+        Assert.assertTrue(leftOnly.isLeft())
+        Assert.assertEquals(42, rightOnly.getOrElse { -1 })
+        Assert.assertEquals(-1, leftOnly.getOrElse { -1 })
+
+        Assert.assertEquals(0, rightOnly.map { it % 2 }.getOrElse { -1 })
+        Assert.assertEquals(-1, leftOnly.map { it % 2 }.getOrElse { -1 })
+        Assert.assertTrue(rightOnly.flatMap { Either.Right(it % 2) }.isRight())
+        Assert.assertTrue(leftOnly.flatMap { Either.Right(it % 2) }.isLeft())
+    }
+
+    @Test
+    fun whenEvalNowUsed_thenMapEvaluatedLazily(){
+        val now = Eval.now(1)
+        Assert.assertEquals(1, now.value())
+
+        var counter : Int = 0
+        val map = now.map { x -> counter++; x+1 }
+        Assert.assertEquals(0, counter)
+        
+        val value = map.value()
+        Assert.assertEquals(2, value)
+        Assert.assertEquals(1, counter)
+    }
+
+    @Test
+    fun whenEvalLaterUsed_theResultIsMemoized(){
+        var counter : Int = 0
+        val later = Eval.later { counter++; counter }
+        Assert.assertEquals(0, counter)
+
+        val firstValue = later.value()
+        Assert.assertEquals(1, firstValue)
+        Assert.assertEquals(1, counter)
+
+        val secondValue = later.value()
+        Assert.assertEquals(1, secondValue)
+        Assert.assertEquals(1, counter)
+    }
+
+    @Test
+    fun whenEvalAlwaysUsed_theResultIsNotMemoized(){
+        var counter : Int = 0
+        val later = Eval.always { counter++; counter }
+        Assert.assertEquals(0, counter)
+
+        val firstValue = later.value()
+        Assert.assertEquals(1, firstValue)
+        Assert.assertEquals(1, counter)
+
+        val secondValue = later.value()
+        Assert.assertEquals(2, secondValue)
+        Assert.assertEquals(2, counter)
+    }
+
+}

kotlin-libraries/src/test/kotlin/com/baeldung/kotlin/arrow/FunctionalErrorHandlingWithEitherTest.kt

@@ -0,0 +1,68 @@
+package com.baeldung.kotlin.arrow
+
+import arrow.core.Either
+import com.baeldung.kotlin.arrow.FunctionalErrorHandlingWithEither.ComputeProblem.NotANumber
+import com.baeldung.kotlin.arrow.FunctionalErrorHandlingWithEither.ComputeProblem.OddNumber
+import org.junit.Assert
+import org.junit.Test
+
+class FunctionalErrorHandlingWithEitherTest {
+
+    val operator = FunctionalErrorHandlingWithEither()
+
+    @Test
+    fun givenInvalidInput_whenComputeInvoked_NotANumberIsPresent(){
+        val computeWithEither = operator.computeWithEither("bar")
+
+        Assert.assertTrue(computeWithEither.isLeft())
+        when(computeWithEither){
+            is Either.Left -> when(computeWithEither.a){
+                NotANumber -> "Ok."
+                else -> Assert.fail()
+            }
+            else -> Assert.fail()
+        }
+    }
+
+    @Test
+    fun givenOddNumberInput_whenComputeInvoked_OddNumberIsPresent(){
+        val computeWithEither = operator.computeWithEither("121")
+
+        Assert.assertTrue(computeWithEither.isLeft())
+        when(computeWithEither){
+            is Either.Left -> when(computeWithEither.a){
+                OddNumber -> "Ok."
+                else -> Assert.fail()
+            }
+            else -> Assert.fail()
+        }
+    }
+
+    @Test
+    fun givenEvenNumberWithoutSquare_whenComputeInvoked_OddNumberIsPresent(){
+        val computeWithEither = operator.computeWithEither("100")
+
+        Assert.assertTrue(computeWithEither.isRight())
+        when(computeWithEither){
+            is Either.Right -> when(computeWithEither.b){
+                false -> "Ok."
+                else -> Assert.fail()
+            }
+            else -> Assert.fail()
+        }
+    }
+
+    @Test
+    fun givenEvenNumberWithSquare_whenComputeInvoked_OddNumberIsPresent(){
+        val computeWithEither = operator.computeWithEither("98")
+
+        Assert.assertTrue(computeWithEither.isRight())
+        when(computeWithEither){
+            is Either.Right -> when(computeWithEither.b){
+                true -> "Ok."
+                else -> Assert.fail()
+            }
+            else -> Assert.fail()
+        }
+    }
+}

kotlin-libraries/src/test/kotlin/com/baeldung/kotlin/arrow/FunctionalErrorHandlingWithOptionTest.kt

@@ -0,0 +1,34 @@
+package com.baeldung.kotlin.arrow
+
+import org.junit.Assert
+import org.junit.Test
+
+class FunctionalErrorHandlingWithOptionTest {
+
+    val operator = FunctionalErrorHandlingWithOption()
+
+    @Test
+    fun givenInvalidInput_thenErrorMessageIsPresent(){
+        val useComputeOption = operator.computeWithOptionClient("foo")
+        Assert.assertEquals("Not an even number!", useComputeOption)
+    }
+
+    @Test
+    fun givenOddNumberInput_thenErrorMessageIsPresent(){
+        val useComputeOption = operator.computeWithOptionClient("539")
+        Assert.assertEquals("Not an even number!",useComputeOption)
+    }
+
+    @Test
+    fun givenEvenNumberInputWithNonSquareNum_thenFalseMessageIsPresent(){
+        val useComputeOption = operator.computeWithOptionClient("100")
+        Assert.assertEquals("The greatest divisor is square number: false",useComputeOption)
+    }
+
+    @Test
+    fun givenEvenNumberInputWithSquareNum_thenTrueMessageIsPresent(){
+        val useComputeOption = operator.computeWithOptionClient("242")
+        Assert.assertEquals("The greatest divisor is square number: true",useComputeOption)
+    }
+
+}