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Discovering Kotlin Contracts Nicola Corti @cortinico

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1.3

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The Problem

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The Problem @Test fun testMyTokenLength() { }

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The Problem @Test fun testMyTokenLength() { val token : String? = getMyToken(); }

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The Problem @Test fun testMyTokenLength() { val token : String? = getMyToken(); assertNotNull(token) }

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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?

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The Problem @Test fun testMyTokenLength() { val token : String? = getMyToken(); assertNotNull(token) assertEquals(42, token?.length) }

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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

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Syntax

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Syntax fun Int?.isValid(): Boolean { return this != null && this != 0 }

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Syntax fun Int?.isValid(): Boolean { contract { } return this != null && this != 0 }

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Syntax fun Int?.isValid(): Boolean { contract { // Place your effects here } return this != null && this != 0 }

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Effects fun isValid(number: Int?) : Boolean Int? Boolean Effect1, Effect2, Effect3

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Returns fun returns(): SimpleEffect fun returns(value: Any?): SimpleEffect fun returnsNotNull(): SimpleEffect interface SimpleEffect : Effect { infix fun implies(boolExpr: Boolean): ConditionalEffect } ConditionalEffect

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Syntax fun Int?.isValid(): Boolean { contract { } return this != null && this != 0 }

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Syntax fun Int?.isValid(): Boolean { contract { returns(true) } return this != null && this != 0 }

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Syntax fun Int?.isValid(): Boolean { contract { returns(true) implies } return this != null && this != 0 }

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Syntax fun Int?.isValid(): Boolean { contract { returns(true) implies (this@isValid != null) } return this != null && this != 0 }

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Syntax val aInt : Int? = getAnInt() if (aInt.isValid()){ } else { }

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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 { }

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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 }

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The Problem @Test fun testMyTokenLenght() { val token : String? = getMyToken(); assertNotNull(token) assertEquals(42, token?.length) }

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The Problem @Test fun testMyTokenLenght() { val token : String? = getMyToken(); assertNotNull(token) assertEquals(42, token.length) }

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The Problem fun assertNotNull(actual: Any?) { }

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The Problem fun assertNotNull(actual: Any?) { org.junit.Assert.assertNotNull(actual) }

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The Problem fun assertNotNull(actual: Any?) { contract { returns() implies (actual != null) } org.junit.Assert.assertNotNull(actual) }

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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 callsInPlace( lambda: Function, kind: InvocationKind = InvocationKind.UNKNOWN ): CallsInPlace

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CallsInPlace InvocationKind.UNKNOWN

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CallsInPlace InvocationKind.UNKNOWN InvocationKind.AT_MOST_ONCE

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CallsInPlace InvocationKind.UNKNOWN InvocationKind.AT_MOST_ONCE InvocationKind.AT_LEAST_ONCE

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CallsInPlace InvocationKind.UNKNOWN InvocationKind.AT_MOST_ONCE InvocationKind.AT_LEAST_ONCE InvocationKind.EXACTLY_ONCE

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CallsInPlace fun main(){ val x : Int? run { x = 10 } println(x) }

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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

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CallsInPlace public inline fun T.run(block: T.() -> R): R { return block() }

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CallsInPlace public inline fun T.run(block: T.() -> R): R { contract { } return block() }

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CallsInPlace public inline fun T.run(block: T.() -> R): R { contract { callsInPlace(block, InvocationKind.EXACTLY_ONCE) } return block() }

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CallsInPlace fun main(){ val x : Int? run { x = 10 } println(x) }

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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.

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Thanks! Nicola Corti @cortinico bit.ly/ktcontracts bit.ly/ktcontracts-slides