Discovering Kotlin Contracts

Slides for me lightning talk on Kotlin Contracts @ GDG Hamburg Android

Transcript

1. Discovering Kotlin Contracts Nicola Corti @cortinico
2. 1.3
3. The Problem
4. The Problem @Test fun testMyTokenLength() { }
5. The Problem @Test fun testMyTokenLength() { val token : String? = getMyToken(); }
6. The Problem @Test fun testMyTokenLength() { val token : String? = getMyToken(); assertNotNull(token) }
7. The Problem @Test fun testMyTokenLength() { val token : String? = getMyToken(); assertNotNull(token) assertEquals(42, token.length) } Error(5, 22): Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type String?
8. The Problem @Test fun testMyTokenLength() { val token : String? = getMyToken(); assertNotNull(token) assertEquals(42, token?.length) }
9. Kotlin Contracts • The Compiler is not smart enough to
 understand the context • You know something that the compiler doesn’t know • We can provide this extra knowledge with a Kotlin Contract • The goal is to write better/cleaner code Experimental features
10. Syntax
11. Syntax fun Int?.isValid(): Boolean { return this != null && this != 0 }
12. Syntax fun Int?.isValid(): Boolean { contract { } return this != null && this != 0 }
13. Syntax fun Int?.isValid(): Boolean { contract { // Place your effects here } return this != null && this != 0 }
14. Effects fun isValid(number: Int?) : Boolean Int? Boolean Effect1, Effect2, Effect3
15. Returns fun returns(): SimpleEffect fun returns(value: Any?): SimpleEffect fun returnsNotNull(): SimpleEffect interface SimpleEffect : Effect { infix fun implies(boolExpr: Boolean): ConditionalEffect } ConditionalEffect
16. Syntax fun Int?.isValid(): Boolean { contract { } return this != null && this != 0 }
17. Syntax fun Int?.isValid(): Boolean { contract { returns(true) } return this != null && this != 0 }
18. Syntax fun Int?.isValid(): Boolean { contract { returns(true) implies } return this != null && this != 0 }
19. Syntax fun Int?.isValid(): Boolean { contract { returns(true) implies (this@isValid != null) } return this != null && this != 0 }
20. Syntax val aInt : Int? = getAnInt() if (aInt.isValid()){ } else { }
21. Syntax val aInt : Int? = getAnInt() if (aInt.isValid()){ // Here the compiler knows that `aInt` is not nullable // thanks to the contract on `isValid` aInt.absoluteValue } else { }
22. Syntax val aInt : Int? = getAnInt() if (aInt.isValid()){ // Here the compiler knows that `aInt` is not nullable // thanks to the contract on `isValid` aInt.absoluteValue } else { // Here the compiler has no extra information since // the `isValid` has only a `returns(true)` effect aInt?.absoluteValue }
23. The Problem @Test fun testMyTokenLenght() { val token : String? = getMyToken(); assertNotNull(token) assertEquals(42, token?.length) }
24. The Problem @Test fun testMyTokenLenght() { val token : String? = getMyToken(); assertNotNull(token) assertEquals(42, token.length) }
25. The Problem fun assertNotNull(actual: Any?) { }
26. The Problem fun assertNotNull(actual: Any?) { org.junit.Assert.assertNotNull(actual) }
27. The Problem fun assertNotNull(actual: Any?) { contract { returns() implies (actual != null) } org.junit.Assert.assertNotNull(actual) }
28. CallsInPlace • lambda will not be called after the call to owner-function is finished. • lambda will not be passed to another function that 
 doesn’t have a similar contract. • lambda will be called the specified amount of times (kind) fun <R> callsInPlace( lambda: Function<R>, kind: InvocationKind = InvocationKind.UNKNOWN ): CallsInPlace
29. CallsInPlace InvocationKind.UNKNOWN
30. CallsInPlace InvocationKind.UNKNOWN InvocationKind.AT_MOST_ONCE
31. CallsInPlace InvocationKind.UNKNOWN InvocationKind.AT_MOST_ONCE InvocationKind.AT_LEAST_ONCE
32. CallsInPlace InvocationKind.UNKNOWN InvocationKind.AT_MOST_ONCE InvocationKind.AT_LEAST_ONCE InvocationKind.EXACTLY_ONCE
33. CallsInPlace fun main(){ val x : Int? run { x = 10 } println(x) }
34. CallsInPlace fun main(){ val x : Int? run { x = 10 } println(x) } Error:(6, 18) Variable ‘x’ must be initialized Error:(4, 8) Captured values initialization is forbidden due to possible reassignment
35. CallsInPlace public inline fun <T, R> T.run(block: T.() -> R): R { return block() }
36. CallsInPlace public inline fun <T, R> T.run(block: T.() -> R): R { contract { } return block() }
37. CallsInPlace public inline fun <T, R> T.run(block: T.() -> R): R { contract { callsInPlace(block, InvocationKind.EXACTLY_ONCE) } return block() }
38. CallsInPlace fun main(){ val x : Int? run { x = 10 } println(x) }
39. Limitations • They are not verified. 
 You’re responsible for writing correct contracts. • returns(value) and implies allowed values are limited • Only to top-level and block functions.
40. Thanks! Nicola Corti @cortinico bit.ly/ktcontracts bit.ly/ktcontracts-slides