Contracts and Inline classes

Kotlin contracts and inline classes

Transcript

1. Contracts and Inline Classes Dan Rusu and Roushd Ahmed May 2 2 , 2 0 1 9
2. FR Refreshments • Provided by Faire • We use Kotlin for backend and Android development 2
3. FR Contracts • Introduced in Kotlin 1.3 (currently experimental) • The goal of contracts is to give semantic information to the compiler • Enables extra patterns • Reduces checks and casts in the codebase • Can be enabled in Gradle using: 3 compileKotlin { kotlinOptions { freeCompilerArgs { "-Xuse-experimental=kotlin.contracts.ExperimentalContracts" } } }
4. FR Assigning val from lambda 4 inline fun logAndRun(action: String, run: () -> Unit) { logger.info(“starting $action”) run() } fun setup() { val connection: Connection logAndRun(“connection setup”) { // Does not compile: // Initialization forbidden due to possible reassignment connection = establishConnection() } … }
5. FR Assigning val from lambda (cont’d) 5 inline fun logAndRun(action: String, run: () -> Unit) { contract { callsInPlace(run, InvocationKind.EXACTLY_ONCE) } logger.info(“starting $action”) run() } fun setup() { val connection: Connection logAndRun(“connection setup”) { // Initialization is now valid connection = establishConnection() } … }
6. FR Assigning var from lambda 6 inline fun autoRetry(run: () -> Unit) { var numTries = 0 while(numTries <= MAX_RETRIES) { try { run() return } catch (e: Exception) { numTries++ if (numTries > MAX_RETRIES) throw e } } } fun generateToken(prefix: String) { var token: String autoRetry { token = prefix + Random.nextInt(0, 100000) validateUniqueToken(token) } // Compiler error: token must be initialized println(“Generated token: $token”) }
7. FR Assigning var from lambda (cont’d) 7 inline fun autoRetry(run: () -> Unit) { contract { callsInPlace(run, InvocationKind.AT_LEAST_ONCE) } … // auto retry logic from previous slide run() } fun generateToken(prefix: String) { var token: String autoRetry { token = prefix + Random.nextInt(0, 100000) validateUniqueToken(token) } // Compiler knows that token is initialized println(“Generated token: $token”) }
8. FR Smart cast with nullable types 8 data class UserRequest(val userName: String?, val isManager: Boolean?) data class User(val userName: String, val isManager: Boolean) fun createUser(request: UserRequest): User { if (request.userName == null || request.userName.isBlank()) { throw IllegalArgumentException("Missing User Name") } if (request.isManager == null ) { throw IllegalArgumentException("Unknown if Manager or Code Monkey") } // userName and isManager is smart cast to non-null types return User(request.userName, request.isManager) }
9. FR Smart cast with nullable types (cont’d) 9 fun createUser(request: UserRequest): User { validateUserRequest(request) // Does not compile // userName and isManager no longer smart cast to non-null types return User(request.userName, request.isManager) } fun validateUserRequest(request: UserRequest) { if (request.userName == null || request.userName.isBlank()) { throw IllegalArgumentException("Missing User Name") } if (request.isManager == null) { throw IllegalArgumentException("Unknown if Manager or Code Monkey") } }
10. FR Smart cast with nullable types (cont’d) 10 fun createUser(request: UserRequest): User { validateUserRequest(request.userName, request.isManager) return User(request.userName, request.isManager) } fun validateUserRequest(userName: String?, isManager: Boolean?) { contract { returns() implies (userName != null && isManager != null) } if (userName == null || userName.isBlank()) { throw IllegalArgumentException("Missing User Name") } if (isManager == null) { throw IllegalArgumentException("Unknown if Master or Code Monkey") } }
11. FR Smart cast to instance type 11 open class SessionUser(var isActive: Boolean = true) data class Robot(val robotName: String) : SessionUser() data class User(val userName: String, val isManager: Boolean) : SessionUser() { fun isPrivileged(): Boolean = isManager } fun SessionUser.isActiveUser(): Boolean = this is User && this.isActive fun isPrivilegedSession(sessionUser: SessionUser): Boolean { if (sessionUser.isActiveUser()) { // Note isPrivileged only exists on User type return (sessionUser as User).isPrivileged() } return false }
12. FR Smart cast to instance type (cont’d) 12 fun SessionUser.isActiveUser(): Boolean { contract { returns(true) implies (this@isActiveUser is User) } return this is User && this.isActive } fun isPrivilegedSession(sessionUser: SessionUser): Boolean { if (sessionUser.isActiveUser()) { // Note sessionUser smart cast to User type return sessionUser.isPrivileged() } return false }
13. FR Multiple smart casts 13 fun SessionUser.isUser(): Boolean { contract { returns(true) implies (this@isUser is User) returns(false) implies (this@isUser is Robot) } return this is User } fun isPrivilegedSession(sessionUser: SessionUser): Boolean { if (sessionUser.isUser()) { return sessionUser.isPrivileged() // smart cast to User type } else { sessionUser.terminate() // smart cast to Robot type } return false }
14. FR Inline Classes • Introduced in Kotlin 1.3 (currently experimental) • Zero-cost abstraction • Safer and more robust code without sacrificing performance / scalability • Can be enabled in Gradle using: 14 compileKotlin { kotlinOptions { freeCompilerArgs += "-XXLanguage:+InlineClasses" } }
15. FR Motivation • In 1999, after 10 months of travel to Mars, the $125 million Nasa orbiter burned and broke into pieces. Caused by accidentally mixing imperial and metric units • Wrapping values in hot sections of code can have severe impacts on real-time latency-sensitive requirements • We need stronger types that are enforced at compile time without introducing additional memory, performance, or scalability concerns • We need to reason about code and make assumptions: • E.g. Age has already been validated so a negative value won’t accidentally sneak through • E.g. Length is in meters • E.g. Id refers to a customer (it’s not just any number or an id for a different type) • E.g. Unsigned types make more sense in some areas such as distances 15
16. FR Nasa Orbiter 16 inline class Meter(val value: Double) inline class Foot(val value: Double) val Int.meters get() = Meter(toDouble()) val Int.feet get() = Foot(toDouble()) fun shouldFireReverseThrusters(distanceRemaining: Meter): Boolean { TODO() } fun visitMars() { // Does not compile. Type mismatch, required Meter, found Foot if (shouldFireReverseThrusters(5000.feet)) … // Allowed if (shouldFireReverseThrusters(5000.meters)) … }
17. FR Validation 17 inline class Age @Deprecated("Don't call the constructor directly", ReplaceWith("value.toAge()")) constructor(val value: Int) fun Int.toAge(): Age { require(this in 1..100) { "Age $this is out of range"} @Suppress("DEPRECATION") return Age(this) } operator fun Int.minus(age: Age) = this - age.value // Formula is safe since we know that age will never be negative fun computeRations(age: Age): Int = 100 - age
18. FR State Transition Enforcement 18 class Person(val name: String) inline class SittingPerson(private val person: Person) { fun walk() = WalkingPerson(person) } inline class WalkingPerson(private val person: Person) { fun sit() = SittingPerson(person) fun startRunning() = RunningPerson(person) } inline class RunningPerson(private val person: Person) { fun walk() = WalkingPerson(person) } fun createPerson() = SittingPerson(Person("Dan")) … var person = createPerson() person.startRunning() // Does not compile, need to walk before you can run person.walk().startRunning().walk().sit() // Allowed
19. FR Summary • Compiled to underlying type • Can store a single value • No backing fields (no delegation e.g. lateinit) • No initializer block • Can be addressed by deprecating constructor and creating a single creation function • Cannot participate in class hierarchy • But can implement interfaces • Cannot be used from Java • Are sometimes auto-boxed (e.g. when storing in collections) • Referential equality is prohibited due to this 19
20. Thank You!