Add compiler contracts

See: https://kotlinlang.org/docs/reference/whatsnew13.html#contracts

build.gradle.kts

@@ -1,7 +1,6 @@
 import com.jfrog.bintray.gradle.BintrayExtension
 import com.jfrog.bintray.gradle.tasks.BintrayUploadTask
 import org.jetbrains.dokka.gradle.DokkaTask
-import org.jetbrains.kotlin.gradle.plugin.KotlinPluginWrapper
 import org.jetbrains.kotlin.gradle.plugin.KotlinSourceSet
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
@@ -22,9 +21,10 @@ allprojects {
         jcenter()
     }
 
-    plugins.withType<KotlinPluginWrapper> {
+    tasks.withType<KotlinCompile> {
-        tasks.withType<KotlinCompile> {
+        kotlinOptions {
-            kotlinOptions.jvmTarget = "1.8"
+            jvmTarget = "1.8"
+            freeCompilerArgs = listOf("-Xuse-experimental=kotlin.contracts.ExperimentalContracts")
         }
     }
 }

src/main/kotlin/com/github/michaelbull/result/And.kt

@@ -1,5 +1,8 @@
 package com.github.michaelbull.result
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 @Deprecated("Use lazy-evaluating variant instead", ReplaceWith("and { result }"))
 infix fun <V, E> Result<V, E>.and(result: Result<V, E>): Result<V, E> {
     return and { result }
@@ -11,6 +14,10 @@ infix fun <V, E> Result<V, E>.and(result: Result<V, E>): Result<V, E> {
  * - Rust: [Result.and](https://doc.rust-lang.org/std/result/enum.Result.html#method.and)
  */
 inline infix fun <V, E> Result<V, E>.and(result: () -> Result<V, E>): Result<V, E> {
+    contract {
+        callsInPlace(result, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> result()
         is Err -> this
@@ -25,6 +32,10 @@ inline infix fun <V, E> Result<V, E>.and(result: () -> Result<V, E>): Result<V,
  * - Rust: [Result.and_then](https://doc.rust-lang.org/std/result/enum.Result.html#method.and_then)
  */
 inline infix fun <V, E, U> Result<V, E>.andThen(transform: (V) -> Result<U, E>): Result<U, E> {
+    contract {
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> transform(value)
         is Err -> this

src/main/kotlin/com/github/michaelbull/result/Get.kt

@@ -1,5 +1,8 @@
 package com.github.michaelbull.result
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 /**
  * Returns the [value][Ok.value] if this [Result] is [Ok], otherwise `null`.
  *
@@ -7,6 +10,11 @@ package com.github.michaelbull.result
  * - Rust: [Result.ok](https://doc.rust-lang.org/std/result/enum.Result.html#method.ok)
  */
 fun <V, E> Result<V, E>.get(): V? {
+    contract {
+        returnsNotNull() implies (this@get is Ok<V>)
+        returns(null) implies (this@get is Err<E>)
+    }
+
     return when (this) {
         is Ok -> value
         is Err -> null
@@ -19,6 +27,11 @@ fun <V, E> Result<V, E>.get(): V? {
  * - Rust: [Result.err](https://doc.rust-lang.org/std/result/enum.Result.html#method.err)
  */
 fun <V, E> Result<V, E>.getError(): E? {
+    contract {
+        returns(null) implies (this@getError is Ok<V>)
+        returnsNotNull() implies (this@getError is Err<E>)
+    }
+
     return when (this) {
         is Ok -> null
         is Err -> error
@@ -41,6 +54,10 @@ infix fun <V, E> Result<V, E>.getOr(default: V): V {
  * @return The [value][Ok.value] if [Ok], otherwise [default].
  */
 inline infix fun <V, E> Result<V, E>.getOr(default: () -> V): V {
+    contract {
+        callsInPlace(default, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> value
         is Err -> default()
@@ -61,6 +78,10 @@ infix fun <V, E> Result<V, E>.getErrorOr(default: E): E {
  * @return The [error][Err.error] if [Err], otherwise [default].
  */
 inline infix fun <V, E> Result<V, E>.getErrorOr(default: () -> E): E {
+    contract {
+        callsInPlace(default, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> default()
         is Err -> error
@@ -75,6 +96,10 @@ inline infix fun <V, E> Result<V, E>.getErrorOr(default: () -> E): E {
  * - Rust: [Result.unwrap_or_else](https://doc.rust-lang.org/src/core/result.rs.html#735-740)
  */
 inline infix fun <V, E> Result<V, E>.getOrElse(transform: (E) -> V): V {
+    contract {
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> value
         is Err -> transform(error)
@@ -86,6 +111,10 @@ inline infix fun <V, E> Result<V, E>.getOrElse(transform: (E) -> V): V {
  * [transformation][transform] of the [value][Ok.value].
  */
 inline infix fun <V, E> Result<V, E>.getErrorOrElse(transform: (V) -> E): E {
+    contract {
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> transform(value)
         is Err -> error

src/main/kotlin/com/github/michaelbull/result/Map.kt

@@ -1,5 +1,8 @@
 package com.github.michaelbull.result
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 /**
  * Maps this [Result<V, E>][Result] to [Result<U, E>][Result] by either applying the [transform]
  * function to the [value][Ok.value] if this [Result] is [Ok], or returning this [Err].
@@ -9,6 +12,10 @@ package com.github.michaelbull.result
  * - Rust: [Result.map](https://doc.rust-lang.org/std/result/enum.Result.html#method.map)
  */
 inline infix fun <V, E, U> Result<V, E>.map(transform: (V) -> U): Result<U, E> {
+    contract {
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> Ok(transform(value))
         is Err -> this
@@ -24,6 +31,10 @@ inline infix fun <V, E, U> Result<V, E>.map(transform: (V) -> U): Result<U, E> {
  * - Rust: [Result.map_err](https://doc.rust-lang.org/std/result/enum.Result.html#method.map_err)
  */
 inline infix fun <V, E, F> Result<V, E>.mapError(transform: (E) -> F): Result<V, F> {
+    contract {
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> this
         is Err -> Err(transform(error))
@@ -54,10 +65,12 @@ inline infix fun <V, E, U> Result<Iterable<V>, E>.mapAll(transform: (V) -> Resul
  * - Elm: [Result.Extra.mapBoth](http://package.elm-lang.org/packages/elm-community/result-extra/2.2.0/Result-Extra#mapBoth)
  * - Haskell: [Data.Either.either](https://hackage.haskell.org/package/base-4.10.0.0/docs/Data-Either.html#v:either)
  */
-inline fun <V, E, U> Result<V, E>.mapBoth(
+inline fun <V, E, U> Result<V, E>.mapBoth(success: (V) -> U, failure: (E) -> U): U {
-    success: (V) -> U,
+    contract {
-    failure: (E) -> U
+        callsInPlace(success, InvocationKind.AT_MOST_ONCE)
-): U {
+        callsInPlace(failure, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> success(value)
         is Err -> failure(error)
@@ -74,10 +87,12 @@ inline fun <V, E, U> Result<V, E>.mapBoth(
  * - Elm: [Result.Extra.mapBoth](http://package.elm-lang.org/packages/elm-community/result-extra/2.2.0/Result-Extra#mapBoth)
  * - Haskell: [Data.Either.either](https://hackage.haskell.org/package/base-4.10.0.0/docs/Data-Either.html#v:either)
  */
-inline fun <V, E, U> Result<V, E>.fold(
+inline fun <V, E, U> Result<V, E>.fold(success: (V) -> U, failure: (E) -> U): U {
-    success: (V) -> U,
+    contract {
-    failure: (E) -> U
+        callsInPlace(success, InvocationKind.AT_MOST_ONCE)
-): U {
+        callsInPlace(failure, InvocationKind.AT_MOST_ONCE)
+    }
+
     return mapBoth(success, failure)
 }
 
@@ -87,10 +102,12 @@ inline fun <V, E, U> Result<V, E>.fold(
  *
  * - Haskell: [Data.Bifunctor.Bimap](https://hackage.haskell.org/package/base-4.10.0.0/docs/Data-Bifunctor.html#v:bimap)
  */
-inline fun <V, E, U, F> Result<V, E>.mapEither(
+inline fun <V, E, U, F> Result<V, E>.mapEither(success: (V) -> U, failure: (E) -> F): Result<U, F> {
-    success: (V) -> U,
+    contract {
-    failure: (E) -> F
+        callsInPlace(success, InvocationKind.AT_MOST_ONCE)
-): Result<U, F> {
+        callsInPlace(failure, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> Ok(success(value))
         is Err -> Err(failure(error))
@@ -106,5 +123,9 @@ inline fun <V, E, U, F> Result<V, E>.mapEither(
  * - Scala: [Either.flatMap](http://www.scala-lang.org/api/2.12.0/scala/util/Either.html#flatMap[AA>:A,Y](f:B=>scala.util.Either[AA,Y]):scala.util.Either[AA,Y])
  */
 inline infix fun <V, E, U> Result<V, E>.flatMap(transform: (V) -> Result<U, E>): Result<U, E> {
+    contract {
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return andThen(transform)
 }

src/main/kotlin/com/github/michaelbull/result/On.kt

@@ -1,9 +1,16 @@
 package com.github.michaelbull.result
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 /**
  * Invokes an [action] if this [Result] is [Ok].
  */
 inline infix fun <V, E> Result<V, E>.onSuccess(action: (V) -> Unit): Result<V, E> {
+    contract {
+        callsInPlace(action, InvocationKind.AT_MOST_ONCE)
+    }
+
     if (this is Ok) {
         action(value)
     }
@@ -15,6 +22,10 @@ inline infix fun <V, E> Result<V, E>.onSuccess(action: (V) -> Unit): Result<V, E
  * Invokes an [action] if this [Result] is [Err].
  */
 inline infix fun <V, E> Result<V, E>.onFailure(action: (E) -> Unit): Result<V, E> {
+    contract {
+        callsInPlace(action, InvocationKind.AT_MOST_ONCE)
+    }
+
     if (this is Err) {
         action(error)
     }

src/main/kotlin/com/github/michaelbull/result/Or.kt

@@ -1,5 +1,8 @@
 package com.github.michaelbull.result
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 @Deprecated("Use lazy-evaluating variant instead", ReplaceWith("or { result }"))
 infix fun <V, E> Result<V, E>.or(result: Result<V, E>): Result<V, E> {
     return or { result }
@@ -11,6 +14,10 @@ infix fun <V, E> Result<V, E>.or(result: Result<V, E>): Result<V, E> {
  * - Rust: [Result.or](https://doc.rust-lang.org/std/result/enum.Result.html#method.or)
  */
 inline infix fun <V, E> Result<V, E>.or(result: () -> Result<V, E>): Result<V, E> {
+    contract {
+        callsInPlace(result, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> this
         is Err -> result()
@@ -24,6 +31,10 @@ inline infix fun <V, E> Result<V, E>.or(result: () -> Result<V, E>): Result<V, E
  * - Rust: [Result.or_else](https://doc.rust-lang.org/std/result/enum.Result.html#method.or_else)
  */
 inline infix fun <V, E> Result<V, E>.orElse(transform: (E) -> Result<V, E>): Result<V, E> {
+    contract {
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> this
         is Err -> transform(error)
@@ -35,6 +46,10 @@ inline infix fun <V, E> Result<V, E>.orElse(transform: (E) -> Result<V, E>): Res
  * otherwise this [Ok].
  */
 inline infix fun <V, E> Result<V, E>.recover(transform: (E) -> V): Ok<V> {
+    contract {
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         is Ok -> this
         is Err -> Ok(transform(error))

src/main/kotlin/com/github/michaelbull/result/Result.kt

@@ -1,5 +1,8 @@
 package com.github.michaelbull.result
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 /**
  * [Result] is a type that represents either success ([Ok]) or failure ([Err]).
  *
@@ -39,6 +42,10 @@ data class Err<out E>(val error: E) : Result<Nothing, E>()
  * non-null, otherwise the supplied [error].
  */
 inline infix fun <V, E> V?.toResultOr(error: () -> E): Result<V, E> {
+    contract {
+        callsInPlace(error, InvocationKind.AT_MOST_ONCE)
+    }
+
     return when (this) {
         null -> Err(error())
         else -> Ok(this)

src/main/kotlin/com/github/michaelbull/result/Unwrap.kt

@@ -1,5 +1,8 @@
 package com.github.michaelbull.result
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 class UnwrapException(message: String) : Exception(message)
 
 /**
@@ -10,6 +13,10 @@ class UnwrapException(message: String) : Exception(message)
  * @throws UnwrapException if the [Result] is an [Err], with a message containing the [error][Err.error].
  */
 fun <V, E> Result<V, E>.unwrap(): V {
+    contract {
+        returns() implies (this@unwrap is Ok<V>)
+    }
+
     return when (this) {
         is Ok -> value
         is Err -> throw UnwrapException("called Result.wrap on an Err value $error")
@@ -18,6 +25,10 @@ fun <V, E> Result<V, E>.unwrap(): V {
 
 @Deprecated("Use lazy-evaluating variant instead", ReplaceWith("expect { message }"))
 infix fun <V, E> Result<V, E>.expect(message: String): V {
+    contract {
+        returns() implies (this@expect is Ok<V>)
+    }
+
     return expect { message }
 }
 
@@ -30,6 +41,11 @@ infix fun <V, E> Result<V, E>.expect(message: String): V {
  * @throws UnwrapException if the [Result] is an [Err], with the specified [message].
  */
 inline infix fun <V, E> Result<V, E>.expect(message: () -> Any): V {
+    contract {
+        callsInPlace(message, InvocationKind.AT_MOST_ONCE)
+        returns() implies (this@expect is Ok<V>)
+    }
+
     return when (this) {
         is Ok -> value
         is Err -> throw UnwrapException("${message()} $error")
@@ -44,6 +60,10 @@ inline infix fun <V, E> Result<V, E>.expect(message: () -> Any): V {
  * @throws UnwrapException if the [Result] is [Ok], with a message containing the [value][Ok.value].
  */
 fun <V, E> Result<V, E>.unwrapError(): E {
+    contract {
+        returns() implies (this@unwrapError is Err<E>)
+    }
+
     return when (this) {
         is Ok -> throw UnwrapException("called Result.unwrapError on an Ok value $value")
         is Err -> error
@@ -52,6 +72,10 @@ fun <V, E> Result<V, E>.unwrapError(): E {
 
 @Deprecated("Use lazy-evaluating variant instead", ReplaceWith("expectError { message }"))
 infix fun <V, E> Result<V, E>.expectError(message: String): E {
+    contract {
+        returns() implies (this@expectError is Err<E>)
+    }
+
     return expectError { message }
 }
 
@@ -64,6 +88,11 @@ infix fun <V, E> Result<V, E>.expectError(message: String): E {
  * @throws UnwrapException if the [Result] is [Ok], with the specified [message].
  */
 inline infix fun <V, E> Result<V, E>.expectError(message: () -> Any): E {
+    contract {
+        callsInPlace(message, InvocationKind.AT_MOST_ONCE)
+        returns() implies (this@expectError is Err<E>)
+    }
+
     return when (this) {
         is Ok -> throw UnwrapException("${message()} $value")
         is Err -> error

src/main/kotlin/com/github/michaelbull/result/Zip.kt

@@ -1,5 +1,8 @@
 package com.github.michaelbull.result
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 private typealias Producer<T, E> = () -> Result<T, E>
 
 /**
@@ -13,6 +16,12 @@ inline fun <T1, T2, E, V> zip(
     result2: Producer<T2, E>,
     transform: (T1, T2) -> V
 ): Result<V, E> {
+    contract {
+        callsInPlace(result1, InvocationKind.EXACTLY_ONCE)
+        callsInPlace(result2, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return result1().flatMap { v1 ->
         result2().map { v2 ->
             transform(v1, v2)
@@ -32,6 +41,13 @@ inline fun <T1, T2, T3, E, V> zip(
     result3: Producer<T3, E>,
     transform: (T1, T2, T3) -> V
 ): Result<V, E> {
+    contract {
+        callsInPlace(result1, InvocationKind.EXACTLY_ONCE)
+        callsInPlace(result2, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(result3, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return result1().flatMap { v1 ->
         result2().flatMap { v2 ->
             result3().map { v3 ->
@@ -54,6 +70,14 @@ inline fun <T1, T2, T3, T4, E, V> zip(
     result4: Producer<T4, E>,
     transform: (T1, T2, T3, T4) -> V
 ): Result<V, E> {
+    contract {
+        callsInPlace(result1, InvocationKind.EXACTLY_ONCE)
+        callsInPlace(result2, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(result3, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(result4, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return result1().flatMap { v1 ->
         result2().flatMap { v2 ->
             result3().flatMap { v3 ->
@@ -79,6 +103,15 @@ inline fun <T1, T2, T3, T4, T5, E, V> zip(
     result5: Producer<T5, E>,
     transform: (T1, T2, T3, T4, T5) -> V
 ): Result<V, E> {
+    contract {
+        callsInPlace(result1, InvocationKind.EXACTLY_ONCE)
+        callsInPlace(result2, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(result3, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(result4, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(result5, InvocationKind.AT_MOST_ONCE)
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
     return result1().flatMap { v1 ->
         result2().flatMap { v2 ->
             result3().flatMap { v3 ->

src/test/kotlin/com/github/michaelbull/result/ResultIteratorTest.kt

@@ -1,6 +1,10 @@
 package com.github.michaelbull.result
 
-import kotlin.test.*
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
 
 class ResultIteratorTest {
     class HasNext {
@@ -36,7 +40,11 @@ class ResultIteratorTest {
         @Test
         fun throwsExceptionIfUnyieldedAndErr() {
             val iterator = Err("hello").iterator()
-            assertFailsWith<NoSuchElementException> { iterator.next() }
+
+            assertFailsWith<NoSuchElementException> {
+                @Suppress("IMPLICIT_NOTHING_AS_TYPE_PARAMETER")
+                iterator.next()
+            }
         }
 
         @Test

src/test/kotlin/com/github/michaelbull/result/UnwrapTest.kt

@@ -17,6 +17,7 @@ class UnwrapTest {
         @Test
         fun throwsExceptionIfErr() {
             assertFailsWith<UnwrapException>("called Result.wrap on an Err value 5000") {
+                @Suppress("IMPLICIT_NOTHING_AS_TYPE_PARAMETER")
                 Err(5000).unwrap()
             }
         }
@@ -38,6 +39,7 @@ class UnwrapTest {
             }
 
             assertFailsWith<UnwrapException>("the year should be 1994") {
+                @Suppress("IMPLICIT_NOTHING_AS_TYPE_PARAMETER")
                 Err(1994).expect { message }
             }
         }
@@ -47,6 +49,7 @@ class UnwrapTest {
         @Test
         fun throwsExceptionIfOk() {
             assertFailsWith<UnwrapException>("called Result.unwrapError on an Ok value example") {
+                @Suppress("IMPLICIT_NOTHING_AS_TYPE_PARAMETER")
                 Ok("example").unwrapError()
             }
         }
@@ -68,6 +71,7 @@ class UnwrapTest {
             }
 
             assertFailsWith<UnwrapException>("the year should be 2010") {
+                @Suppress("IMPLICIT_NOTHING_AS_TYPE_PARAMETER")
                 Ok(2010).expectError { message }
             }
         }