Solution: Logger
class PetStore(
private val database: Database,
) {
context(Logger)
fun addPet(
addPetRequest: AddPetRequest,
): Pet? {
logInfo("Adding pet with name ${addPetRequest.name}")
return try {
database.addPet(addPetRequest)
.also { logInfo("Added pet with id ${it.id}") }
} catch (e: InsertionConflictException) {
logWarning("There already is " +
"pet named ${addPetRequest.name}")
null
} catch (e: Exception) {
logError("Failed to add " +
"pet with name ${addPetRequest.name}")
null
}
}
}
I needed to break the function definition to fit the page. In actual code, I would not do that.
Example solution in playground
import org.junit.After
import org.junit.Test
import kotlin.random.Random
import kotlin.test.assertEquals
class PetStore(
private val database: Database,
) {
context(Logger)
fun addPet(
addPetRequest: AddPetRequest,
): Pet? {
logInfo("Adding pet with name ${addPetRequest.name}")
return try {
database.addPet(addPetRequest)
.also { logInfo("Added pet with id ${it.id}") }
} catch (e: InsertionConflictException) {
logWarning("There already is " +
"pet named ${addPetRequest.name}")
null
} catch (e: Exception) {
logError("Failed to add " +
"pet with name ${addPetRequest.name}")
null
}
}
}
data class AddPetRequest(val name: String)
data class Pet(val id: Int, val name: String)
class InsertionConflictException : Exception()
interface Database {
fun addPet(addPetRequest: AddPetRequest): Pet
}
interface Logger {
fun logInfo(message: String)
fun logWarning(message: String)
fun logError(message: String)
}
fun main(): Unit = with(ConsoleLogger()) {
val database = RandomDatabase()
val petStore = PetStore(database)
petStore.addPet(AddPetRequest("Fluffy"))
// [INFO] - Adding pet with name Fluffy
// [INFO] - Added pet with id -81731626
// or
// [WARNING] - There already is pet named Fluffy
// or
// [ERROR] - Failed to add pet with name Fluffy
}
class RandomDatabase : Database {
override fun addPet(addPetRequest: AddPetRequest): Pet =
when {
Random.nextBoolean() ->
Pet(1234, addPetRequest.name)
Random.nextBoolean() ->
throw InsertionConflictException()
else -> throw Exception()
}
}
class ConsoleLogger : Logger {
override fun logInfo(message: String) {
println("[INFO] - $message")
}
override fun logWarning(message: String) {
println("[WARNING] - $message")
}
override fun logError(message: String) {
println("[ERROR] - $message")
}
}
class PetStoreTest {
private val database = FakeDatabase()
private val petStore = PetStore(database)
private val logger = FakeLogger()
@After
fun tearDown() {
database.clear()
logger.clear()
}
@Test
fun `should add pet`() {
val pet = with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
val expected = Pet(0, "Fluffy")
assertEquals(expected, pet)
assertEquals(expected, database.getPets().single())
}
@Test
fun `should return null when database failing`() {
database.startFailing()
val pet = with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(null, pet)
assertEquals(emptyList<Pet>(), database.getPets())
}
@Test
fun `should return null when conflict`() {
database.addPet(AddPetRequest("Fluffy"))
val pet = with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(null, pet)
}
@Test
fun `should log info when added pet`() {
with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(
listOf(
FakeLogger.Level.INFO to "Adding pet with name Fluffy",
FakeLogger.Level.INFO to "Added pet with id 0",
),
logger.getMessages()
)
}
@Test
fun `should log warning when adding conflict`() {
database.addPet(AddPetRequest("Fluffy"))
with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(
listOf(
FakeLogger.Level.INFO to "Adding pet with name Fluffy",
FakeLogger.Level.WARNING to "There already is pet named Fluffy",
),
logger.getMessages()
)
}
@Test
fun `should log error when database error`() {
database.startFailing()
with(logger) {
petStore.addPet(AddPetRequest("Fluffy"))
}
assertEquals(
listOf(
FakeLogger.Level.INFO to "Adding pet with name Fluffy",
FakeLogger.Level.ERROR to "Failed to add pet with name Fluffy",
),
logger.getMessages()
)
}
}
class FakeLogger : Logger {
private val messages = mutableListOf<Pair<Level, String>>()
fun clear() {
messages.clear()
}
fun getMessages(): List<Pair<Level, String>> = messages.toList()
override fun logInfo(message: String) {
messages.add(Level.INFO to message)
}
override fun logWarning(message: String) {
messages.add(Level.WARNING to message)
}
override fun logError(message: String) {
messages.add(Level.ERROR to message)
}
enum class Level {
INFO, WARNING, ERROR
}
}
class FakeDatabase : Database {
private val pets = mutableListOf<Pet>()
private var shouldFail = false
fun startFailing() {
shouldFail = true
}
fun clear() {
pets.clear()
shouldFail = false
}
override fun addPet(addPetRequest: AddPetRequest): Pet {
if (pets.any { it.name == addPetRequest.name }) throw InsertionConflictException()
if (shouldFail) throw Exception()
val pet = Pet(pets.size, addPetRequest.name)
pets.add(pet)
return pet
}
fun getPets(): List<Pet> = pets.toList()
}
