Update Coproducts

modules/core/arrow-generic/src/test/kotlin/arrow/generic/CoproductTest.kt

@@ -1,21 +1,16 @@
 package arrow.generic
 
-import arrow.core.None
-import arrow.core.Option
-import arrow.core.Some
-import arrow.core.some
+import arrow.core.*
 import arrow.generic.coproduct2.Coproduct2
 import arrow.generic.coproduct2.cop
 import arrow.generic.coproduct2.coproductOf
 import arrow.generic.coproduct2.fold
 import arrow.generic.coproduct2.select
 import arrow.generic.coproduct22.Coproduct22
-import arrow.generic.coproduct3.cop
-import arrow.generic.coproduct3.fold
-import arrow.generic.coproduct3.select
+import arrow.generic.coproduct3.*
 import arrow.test.UnitSpec
-import io.kotlintest.shouldBe
 import io.kotlintest.runner.junit4.KotlinTestRunner
+import io.kotlintest.shouldBe
 import org.junit.runner.RunWith
 
 @RunWith(KotlinTestRunner::class)
@@ -24,8 +19,13 @@ class CoproductTest : UnitSpec() {
     init {
 
         "Coproducts should be generated up to 22" {
-            class Proof2(f: Coproduct2<Unit, Unit>) { val x = f }
-            class Proof22(f: Coproduct22<Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit>) { val x = f }
+            class Proof2(f: Coproduct2<Unit, Unit>) {
+                val x = f
+            }
+
+            class Proof22(f: Coproduct22<Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit, Unit>) {
+                val x = f
+            }
         }
 
         "select should return None if value isn't correct type" {
@@ -82,5 +82,49 @@ class CoproductTest : UnitSpec() {
                     { "Third" }
             ) shouldBe "Third"
         }
+
+        "cop replacement functions are equivalent" {
+            val firstValue = "String"
+            val secondValue = 100L
+            val thirdValue = none<String>()
+
+            firstValue.cop<String, Long, Option<String>>() shouldBe firstValue.first<String, Long, Option<String>>()
+            secondValue.cop<String, Long, Option<String>>() shouldBe secondValue.second<String, Long, Option<String>>()
+            thirdValue.cop<String, Long, Option<String>>() shouldBe thirdValue.third<String, Long, Option<String>>()
+        }
+
+        "types can be inferred with the extension constructors" {
+            fun typeInferenceTest(input: String?): Coproduct3<String, Int, Option<String>> {
+                return when {
+                    input == null -> none<String>().third()
+                    input.length > 100 -> input.length.second()
+                    else -> input.first()
+                }
+            }
+
+            typeInferenceTest(null) shouldBe Third<String, Int, Option<String>>(none())
+        }
+
+        "types can be inferred with the ADT constructors" {
+            fun typeInferenceTest(input: String?): Coproduct3<String, Int, Option<String>> {
+                return when {
+                    input == null -> Third(none())
+                    input.length > 100 -> Second(input.length)
+                    else -> First(input)
+                }
+            }
+
+            typeInferenceTest(null) shouldBe none<String>().third<String, Int, Option<String>>()
+        }
+
+        "Coproduct data classes should be available" {
+            val firstValue = "String"
+            val secondValue = 100L
+            val thirdValue = none<String>()
+
+            First<String, Long, Option<String>>(firstValue) shouldBe firstValue.first<String, Long, Option<String>>()
+            Second<String, Long, Option<String>>(secondValue) shouldBe secondValue.second<String, Long, Option<String>>()
+            Third<String, Long, Option<String>>(thirdValue) shouldBe thirdValue.third<String, Long, Option<String>>()
+        }
     }
 }

modules/docs/arrow-docs/docs/docs/generic/coproduct/README.md

@@ -23,10 +23,11 @@ compile 'io.arrow-kt:arrow-generic:$arrow_version'
 
 Coproducts represent a sealed hierarchy of types where only one of the specified set of types exist every time. Conceptually, it's very similar to the stdlib `sealed` class, or to [Either]({{ '/docs/arrow/core/either' | relative_url }}) if we move on to Arrow data types. [Either]({{ '/docs/arrow/core/either' | relative_url }}) supports one of two values, an `Either<A, B>` has to contain an instance of `A` or `B`. We can extrapolate that concept to `N` number of types. So a `Coproduct5<A, B, C, D, E>` has to contain an instance of `A`, `B`, `C`, `D`, or `E`. For example, perhaps there's a search function for a Car Dealer app that can show `Dealership`s, `Car`s and `SalesPerson`s in a list, we could model that list of results as `List<Coproduct3<Dealership, Car, SalesPerson>`. The result would contain a list of heterogeneous elements but each element is one of `Dealership`, `Car` or `SalesPerson` and our UI can render the list elements based on those types.
 
+{: data-executable='true'}
+
 ```kotlin:ank
 import arrow.generic.*
-import arrow.generic.coproduct3.Coproduct3
-import arrow.generic.coproduct3.fold
+import arrow.generic.coproduct3.*
 
 fun toDisplayValues(items: List<Coproduct3<Car, Dealership, Salesperson>>): List<String> {
   return items.map {
@@ -37,6 +38,18 @@ fun toDisplayValues(items: List<Coproduct3<Car, Dealership, Salesperson>>): List
     )
   }
 }
+
+fun main() {
+    println(
+        toDisplayValues(
+            listOf<Coproduct3<Car, Dealership, Salesperson>>(
+                Car(Speed(100)).first(),
+                Dealership("Cedar Falls, Iowa").second(),
+                Salesperson("Car McCarface").third()
+            )
+        )
+    )
+}
 ```
 
 Let's say we have an api. Our api operates under the following conditions:
@@ -45,73 +58,123 @@ Let's say we have an api. Our api operates under the following conditions:
 
 Because we have some common errors that every endpoint can return to us, we can define a sealed class of these and call them `CommonServerError` because they make sense to be sealed together for reusability. Likewise, we can logically group our specific errors into `RegistrationError` and we have `Registration` as our success type.
 
-With Coproducts, we're able to define a result for this api call as `Coproduct3<CommonServerError, RegistrationError, Registration>`. We've been able to compose these results without having to write our own sealed class containing all the common errors for each endpoint.
+The most obvious approach would be to use Kotlin's `sealed class` to create a return type for this api call:
 
-#### Extensions
+```kotlin:ank
+sealed class ApiResult {
+  data class CommonServerError(val value: CommonServerError): ApiResult()
+  data class RegistrationError(val value: RegistrationError): ApiResult()
+  data class Registration(val value: Registration): ApiResult()
+}
+```
+
+Immediately we can observe there's boilerplate to this approach. We need to make a data class that just holds a single value to combine these results to a common type. Any time we need to add to ApiResult we need to go through and add another wrapping class to conform it to this type.
 
-##### coproductOf
+Upon using it we can also observe that there's unwrapping that needs to take place to actually use the values:
+
+```kotlin:ank
+fun handleResult(apiResult: ApiResult): String {
+  return when (apiResult) {
+    is ApiResult.CommonServerError -> "Common: ${apiResult.value}"
+    is ApiResult.RegistrationError -> "RegistrationError: ${apiResult.value}"
+    is ApiResult.Registration -> "Registration: ${apiResult.value}"
+  }
+}
+```
+
+With Coproducts, we're able to define a result for this api call as `typealias ApiResult = Coproduct3<CommonServerError, RegistrationError, Registration>`. We've been able to compose these results without having to write our own sealed class containing all the common errors for each endpoint. This lets us flatten a layer of boilerplate by abstracting the sealed hierarchy and lets us freely compose types from different domain types.
+
+#### Constructors
 
 So now that we've got our api response modeled, we need to be able to create an instance of `Coproduct3`.
 
+{: data-executable='true'}
+
 ```kotlin:ank
 import arrow.generic.*
-import arrow.generic.coproduct3.coproductOf
+import arrow.generic.coproduct3.First
+
 
-val apiResult = coproductOf<CommonServerError, RegistrationError, Registration>(ServerError) //Returns Coproduct3<CommonServerError, RegistrationError, Registration>
+fun main() {
+    println(First<CommonServerError, RegistrationError, Registration>(ServerError))
+}
 ```
 
-There are `coproductOf` constructor functions for each Coproduct regardless of the arity (number of types). All we have to do is pass in our value and have the correct type parameters on it, the value must be a type declared on the function call.
+Coproducts are backed by a sealed class hierarchy and we can use the data classes to create Coproducts. The class names resemble the index of the generic, for example, Coproduct3<A, B, C> has First, Second and Third. First references the `A`, Second references the `B` and so forth.
 
 If we pass in a value that doesn't correspond to any types on the Coproduct, it won't compile:
 
+{: data-executable='true'}
+
 ```kotlin:ank
 import arrow.generic.*
-import arrow.generic.coproduct3.coproductOf
+import arrow.generic.coproduct3.First
 
-//val apiResult = coproductOf<String, RegistrationError, Registration>(ServerError)
-//error: type mismatch: inferred type is ServerError but String was expected
+fun main() {
+//    println(
+//        First<String, RegistrationError, Registration>(ServerError)
+//    )
+}
 ```
 
-##### cop
+#### Extensions
+
+##### constructors
 
 You might be saying "That's great and all but passing in values as parameters is so Java, I want something more Kotlin!". Well look no further, just like [Either]({{ '/docs/arrow/core/either' | relative_url }})'s `left()` and `right()` extension methods, Coproducts can be created with an extension method on any type:
 
+{: data-executable='true'}
+
+
 ```kotlin:ank
 import arrow.generic.*
-import arrow.generic.coproduct3.cop
+import arrow.generic.coproduct3.*
 
-val apiResult = ServerError.cop<CommonServerError, RegistrationError, Registration>() //Returns Coproduct3<CommonServerError, RegistrationError, Registration>
+fun main() {
+    println(ServerError.first<CommonServerError, RegistrationError, Registration>())
+    println(CarAlreadyRegistered.second<CommonServerError, RegistrationError, Registration>())
+}
 ```
 
-All we have to do is provide the type parameters and we can make a Coproduct using the `cop` extension method. Just like `coproductOf`, if the type of the value isn't in the type parameters of the method call, it won't compile:
+All we have to do is provide the type parameters and we can make a Coproduct using the extension methods. Just like the data classes, if the type of the value isn't in the type parameters of the method call, or it's not in the correct type parameter index, it won't compile:
+
+{: data-executable='true'}
 
 ```kotlin:ank
 import arrow.generic.*
-import arrow.generic.coproduct3.cop
+import arrow.generic.coproduct3.first
 
-//val apiResult = ServerError.cop<String, RegistrationError, Registration>()
-//error: type mismatch: inferred type is ServerError but String was expected
+fun main() {
+//    println(
+//        "String".first<CommonServerError, RegistrationError, Registration>()
+//    )
+}
 ```
 
 ##### fold
 
 Obviously, we're not just modeling errors for fun, we're going to handle them! All Coproducts have `fold` which allows us to condense the Coproduct down to a single type. For example, we could handle errors as such in a UI:
 
+{: data-executable='true'}
+
 ```kotlin:ank
 import arrow.generic.*
-import arrow.generic.coproduct3.Coproduct3
-import arrow.generic.coproduct3.fold
+import arrow.generic.coproduct3.*
 
 fun handleCommonError(commonError: CommonServerError) {
+    println("Encountered a common error $commonError")
 }
 
 fun showCarAlreadyRegistered() {
+    println("Car is already registered")
 }
 
 fun callPolice() {
+    println("That car is stolen!!!!1!")
 }
 
 fun showCarSuccessfullyRegistered(car: Car) {
+    println("Successfully Registered!")
 }
 
 fun renderApiResult(apiResult: Coproduct3<CommonServerError, RegistrationError, Registration>) = apiResult.fold(
@@ -128,14 +191,19 @@ fun renderApiResult(apiResult: Coproduct3<CommonServerError, RegistrationError,
                 showCarSuccessfullyRegistered(registeredCar)
             }
     )
+
+fun main() {
+    renderApiResult(Registration(Car(Speed(100))).third())
+}
 ```
 
 This example returns `Unit` because all of these are side effects, let's say our application was built for a command line and we just have to show a `String` for the result of the call (if only it was always that easy):
 
+{: data-executable='true'}
+
 ```kotlin:ank
 import arrow.generic.*
-import arrow.generic.coproduct3.Coproduct3
-import arrow.generic.coproduct3.fold
+import arrow.generic.coproduct3.*
 
 fun renderApiResult(apiResult: Coproduct3<CommonServerError, RegistrationError, Registration>): String = apiResult.fold(
             { commonError ->
@@ -155,6 +223,10 @@ fun renderApiResult(apiResult: Coproduct3<CommonServerError, RegistrationError,
                 "Successfully Registered: $registration.car"
             }
     )
+
+fun main() {
+    println(renderApiResult(Registration(Car(Speed(100))).third()))
+}
 ```
 
 Here we're able to return the result of the `fold` and since it's exhaustively evaluated, we're forced to handle all cases! Neat! Let's say we also want to store the `Registration` object into our database when we successfully register a car. We don't really want to have to `fold` over every single case just to handle something for the `Registration`, this is where `select<T>` comes to the rescue!
@@ -163,10 +235,13 @@ Here we're able to return the result of the `fold` and since it's exhaustively e
 
 We're able to take a Coproduct and `select` the type we care about from it. `select` returns an `Option`, if the value of the Coproduct was for the type you're trying to `select`, you'll get `Some`, if it was not the type used with `select`, you'll get `None`.
 
+{: data-executable='true'}
+
 ```kotlin:ank
 import arrow.generic.*
 import arrow.generic.coproduct3.Coproduct3
 import arrow.generic.coproduct3.select
+import arrow.generic.coproduct3.first
 
 fun handleApiResult(
         database: Database,
@@ -178,16 +253,28 @@ fun handleApiResult(
                     { database.insertRegistration(it) }
             )
 }
+
+fun main() {
+    println(
+        ServerError.first<CommonServerError, RegistrationError, Registration>()
+            .select<RegistrationError>()
+    )
+}
 ```
 
 `select` can only be called with a type that exists on the Coproduct, if the type doesn't exist, it won't compile:
+
+{: data-executable='true'}
+
 ```kotlin:ank
 import arrow.generic.*
 import arrow.generic.coproduct3.Coproduct3
 import arrow.generic.coproduct3.select
 
-fun handleApiResult(apiResult: Coproduct3<CommonServerError, RegistrationError, Registration>): Unit {
-//    apiResult.select<String>()
-//error: type mismatch: inferred type is Coproduct3<CommonServerError, RegistrationError, Registration> but Coproduct3<String, *, *> was expected
+fun main() {
+//    println(
+//        ServerError.first<CommonServerError, RegistrationError, Registration>()
+//                    .select<String>()
+//    )
 }
 ```