parTraverseEitherN.kt

@file:Suppress("RedundantSuspendModifier")

import arrow.core.Either
import arrow.core.computations.either
import arrow.fx.coroutines.parTraverseEitherN
import kotlinx.coroutines.delay
import java.io.IOException
import kotlin.time.ExperimentalTime
import kotlin.time.TimeSource

suspend fun throwToTest(id: String) =
    if (id == "post#5") throw IOException("Failed to fetch post with id=$id ...") else Unit

data class User(val id: String, val username: String, val postIds: List<String>)
data class Post(val id: String, val title: String)
sealed interface Error {
    val cause: Throwable

    data class RemoteError(override val cause: Throwable) : Error
    data class LocalError(override val cause: Throwable) : Error
}

suspend fun putStrLn(message: Any?) = println(message)

suspend fun fetchUser(userId: String): Either<Error.RemoteError, User> = Either.catch {
    putStrLn("fetchUser userId=$userId")
    delay(500)
    User(
        id = userId,
        username = "Username",
        postIds = List(32) { "post#$it" }
    )
}.mapLeft(Error::RemoteError)

suspend fun fetchPost(postId: String): Either<Error.RemoteError, Post> = Either.catch {
    putStrLn("fetchPost postId=$postId")
    throwToTest(postId)
    delay(500)
    Post(id = postId, title = "Title of $postId")
}.mapLeft(Error::RemoteError)

suspend fun savePosts(posts: List<Post>): Either<Error.LocalError, Unit> = Either.catch {
    delay(500)
    putStrLn("Saved ${posts.size}")
}.mapLeft(Error::LocalError)

suspend fun program(userId: String): Either<Error, List<Post>> = either {
    val user: User = fetchUser(userId).bind()
    val posts: List<Post> = user.postIds.parTraverseEitherN(4, ::fetchPost).bind()
    savePosts(posts).bind()
    posts
}

@OptIn(ExperimentalTime::class)
suspend fun main() {
    val mark = TimeSource.Monotonic.markNow()
    program("user#01").fold(ifLeft = { putStrLn("Failure: $it") }, ifRight = { putStrLn("Success: ${it.size}") })
    putStrLn(mark.elapsedNow())
}

// build.gradle.kts
//
// plugins {
//     kotlin("jvm") version "1.5.0-M2"
//     application
// }
//
// dependencies {
//     val arrowVersion = "0.13.1"
//     implementation("io.arrow-kt:arrow-core:$arrowVersion")
//     implementation("io.arrow-kt:arrow-fx-coroutines:$arrowVersion")
//     implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.4.3")
// }