From 176cf8312da9c9ca6a698af6e5a11d665c804d62 Mon Sep 17 00:00:00 2001
From: skydoves <skydoves@naver.com>
Date: Sat, 27 Nov 2021 13:54:37 +0900
Subject: [PATCH 1/3] Bump AGP and kotlin versions

---
 dependencies.gradle                      | 6 +++---
 gradle/wrapper/gradle-wrapper.properties | 2 +-
 whatif/build.gradle                      | 5 ++++-
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/dependencies.gradle b/dependencies.gradle
index 2e74f7a..e08a1ca 100644
--- a/dependencies.gradle
+++ b/dependencies.gradle
@@ -5,14 +5,14 @@ ext.versions = [
     versionCode      : 11,
     versionName      : '1.1.0',
 
-    gradleBuildTool  : '4.1.3',
-    spotlessGradle   : '5.14.0',
+    gradleBuildTool  : '7.0.2',
+    spotlessGradle   : '5.15.0',
     ktlintGradle     : '0.41.0',
     dokkaGradle      : '1.4.30',
     binaryValidator  : '0.7.1',
     mavenPublish     : '0.15.1',
 
-    kotlin           : '1.4.31',
+    kotlin           : '1.5.30',
     androidxAppcompat: '1.3.0',
 
     // for demo
diff --git a/gradle/wrapper/gradle-wrapper.properties b/gradle/wrapper/gradle-wrapper.properties
index 64e9590..6a66379 100644
--- a/gradle/wrapper/gradle-wrapper.properties
+++ b/gradle/wrapper/gradle-wrapper.properties
@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.6.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.0.2-all.zip
diff --git a/whatif/build.gradle b/whatif/build.gradle
index ad35f82..5665a19 100644
--- a/whatif/build.gradle
+++ b/whatif/build.gradle
@@ -1,4 +1,3 @@
-apply plugin: 'java-library'
 apply plugin: 'kotlin'
 apply plugin: 'org.jetbrains.dokka'
 apply plugin: 'binary-compatibility-validator'
@@ -21,6 +20,10 @@ task coverageReport(type: JacocoReport, dependsOn: 'test') {
   }
 }
 
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
+  kotlinOptions.freeCompilerArgs += ["-Xopt-in=kotlin.contracts.ExperimentalContracts"]
+}
+
 dependencies {
   testImplementation "junit:junit:$versions.junit"
 }

From 79acdf039c0caf240a934944c1b81cb499a2c475 Mon Sep 17 00:00:00 2001
From: skydoves <skydoves@naver.com>
Date: Sat, 27 Nov 2021 13:55:05 +0900
Subject: [PATCH 2/3] Add contract on whatif module functions

---
 .../main/java/com/skydoves/whatif/WhatIf.kt   | 101 ++++++++++---
 .../java/com/skydoves/whatif/WhatIfArray.kt   | 137 +++++++++++++-----
 .../com/skydoves/whatif/WhatIfCollections.kt  | 122 ++++++++++++----
 .../java/com/skydoves/whatif/WhatIfString.kt  |  18 ++-
 4 files changed, 288 insertions(+), 90 deletions(-)

diff --git a/whatif/src/main/java/com/skydoves/whatif/WhatIf.kt b/whatif/src/main/java/com/skydoves/whatif/WhatIf.kt
index 92a09f1..5c0faed 100644
--- a/whatif/src/main/java/com/skydoves/whatif/WhatIf.kt
+++ b/whatif/src/main/java/com/skydoves/whatif/WhatIf.kt
@@ -20,6 +20,9 @@
 
 package com.skydoves.whatif
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 /**
  * WhatIf is kotlin extensions for expressing a single if-else statement, nullable and boolean.
  */
@@ -38,7 +41,9 @@ public inline fun <T> T.whatIf(
   given: (T) -> Boolean?,
   whatIf: () -> Unit
 ): T {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   if (given(this) == true) {
     whatIf()
   }
@@ -62,7 +67,10 @@ public inline fun <T> T.whatIf(
   whatIf: () -> Unit,
   whatIfNot: () -> Unit
 ): T {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (given(this) == true) {
     whatIf()
   } else {
@@ -86,7 +94,9 @@ public inline fun <T> T.whatIf(
   given: Boolean?,
   whatIf: T.() -> Unit
 ): T {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   if (given == true) {
     this.apply { whatIf() }
   }
@@ -111,7 +121,10 @@ public inline fun <T> T.whatIf(
   whatIf: T.() -> Unit,
   whatIfNot: T.() -> Unit
 ): T {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (given == true) {
     this.apply { whatIf() }
   } else {
@@ -135,7 +148,9 @@ public inline fun <T> T.whatIf(
   given: () -> Boolean?,
   whatIfDo: T.() -> Unit
 ): T {
-
+  contract {
+    callsInPlace(whatIfDo, InvocationKind.AT_MOST_ONCE)
+  }
   return this.whatIf(
     given = given,
     whatIfDo = whatIfDo,
@@ -161,7 +176,10 @@ public inline fun <T> T.whatIf(
   whatIfDo: T.() -> Unit,
   whatIfNot: T.() -> Unit
 ): T {
-
+  contract {
+    callsInPlace(whatIfDo, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (given() == true) {
     this.whatIfDo()
   } else {
@@ -185,7 +203,9 @@ public inline fun <T, R> T.whatIfMap(
   default: R,
   whatIf: (T) -> R
 ): R {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   return this.whatIfMap(
     whatIf = whatIf,
     whatIfNot = { default }
@@ -207,7 +227,10 @@ public inline fun <T, R> T.whatIfMap(
   whatIf: (T) -> R,
   whatIfNot: (T) -> R
 ): R {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null) {
     return whatIf(this)
   }
@@ -232,7 +255,9 @@ public inline fun <T, R> T.whatIfMap(
   default: R,
   whatIf: (T) -> R
 ): R {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   return this.whatIfMap(
     given = given,
     whatIf = whatIf,
@@ -258,7 +283,10 @@ public inline fun <T, R> T.whatIfMap(
   whatIf: (T) -> R,
   whatIfNot: (T) -> R
 ): R {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (given == true) {
     return whatIf(this)
   }
@@ -290,7 +318,9 @@ public inline fun <T, R> T.whatIfLet(
   default: R,
   whatIf: (T) -> R
 ): R {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   if (given == true) {
     return whatIf(this)
   }
@@ -322,7 +352,10 @@ public inline fun <T, R> T.whatIfLet(
   whatIf: (T) -> R,
   whatIfNot: (T) -> R
 ): R {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (given == true) {
     return whatIf(this)
   }
@@ -341,7 +374,9 @@ public inline fun <T, R> T.whatIfLet(
 public inline fun <T> T?.whatIfNotNull(
   whatIf: (T) -> Unit
 ): T? {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.EXACTLY_ONCE)
+  }
   return this.whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = { }
@@ -363,7 +398,10 @@ public inline fun <T> T?.whatIfNotNull(
   whatIf: (T) -> Unit,
   whatIfNot: () -> Unit
 ): T? {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null) {
     whatIf(this)
     return this
@@ -393,7 +431,9 @@ public inline fun <T> T?.whatIfNotNull(
 public inline fun <reified R> Any?.whatIfNotNullAs(
   whatIf: (R) -> Unit
 ): Any? {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   return whatIfNotNullAs(
     whatIf = whatIf,
     whatIfNot = { }
@@ -426,7 +466,10 @@ public inline fun <reified R> Any?.whatIfNotNullAs(
   whatIf: (R) -> Unit,
   whatIfNot: () -> Unit
 ): Any? {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null && this is R) {
     whatIf(this as R)
     return this
@@ -451,7 +494,10 @@ public inline fun <T, R> T?.whatIfNotNullWith(
   whatIf: (T) -> R,
   whatIfNot: (T?) -> R
 ): R {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null) {
     return whatIf(this)
   }
@@ -470,7 +516,9 @@ public inline fun <T, R> T?.whatIfNotNullWith(
 public inline fun Boolean?.whatIf(
   whatIf: () -> Unit
 ): Boolean? {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   return this.whatIf(
     whatIf = whatIf,
     whatIfNot = { }
@@ -492,7 +540,10 @@ public inline fun Boolean?.whatIf(
   whatIf: () -> Unit,
   whatIfNot: () -> Unit
 ): Boolean? {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this == true) {
     whatIf()
   } else {
@@ -513,7 +564,9 @@ public inline fun Boolean?.whatIf(
 public inline fun Boolean?.whatIfElse(
   whatIf: () -> Unit
 ): Boolean? {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   if (this == false) {
     whatIf()
   }
@@ -534,7 +587,9 @@ public inline fun Boolean?.whatIfAnd(
   predicate: Boolean?,
   whatIf: () -> Unit
 ): Boolean? {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   if (this == true && predicate == true) {
     whatIf()
   }
@@ -555,7 +610,9 @@ public inline fun Boolean?.whatIfOr(
   predicate: Boolean?,
   whatIf: () -> Unit
 ): Boolean? {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   if (this == true || predicate == true) {
     whatIf()
   }
diff --git a/whatif/src/main/java/com/skydoves/whatif/WhatIfArray.kt b/whatif/src/main/java/com/skydoves/whatif/WhatIfArray.kt
index fd20e22..07c0181 100644
--- a/whatif/src/main/java/com/skydoves/whatif/WhatIfArray.kt
+++ b/whatif/src/main/java/com/skydoves/whatif/WhatIfArray.kt
@@ -19,6 +19,9 @@
 
 package com.skydoves.whatif
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 /**
  * An expression for invoking [whatIf] when the [Array] is not null and not empty.
  *
@@ -30,12 +33,15 @@ package com.skydoves.whatif
 @WhatIfInlineOnly
 public inline fun <T> Array<out T>?.whatIfNotNullOrEmpty(
   whatIf: (Array<out T>) -> Unit
-): Array<out T>? = apply {
-
+): Array<out T>? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -52,13 +58,17 @@ public inline fun <T> Array<out T>?.whatIfNotNullOrEmpty(
 public inline fun <T> Array<out T>?.whatIfNotNullOrEmpty(
   whatIf: (Array<out T>) -> Unit,
   whatIfNot: () -> Unit
-): Array<out T>? = apply {
-
+): Array<out T>? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (!this.isNullOrEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -72,8 +82,10 @@ public inline fun <T> Array<out T>?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun ByteArray?.whatIfNotNullOrEmpty(
   whatIf: (ByteArray) -> Unit
-): ByteArray? = apply {
-
+): ByteArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   return this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
@@ -94,13 +106,17 @@ public inline fun ByteArray?.whatIfNotNullOrEmpty(
 public inline fun ByteArray?.whatIfNotNullOrEmpty(
   whatIf: (ByteArray) -> Unit,
   whatIfNot: () -> Unit
-): ByteArray? = apply {
-
+): ByteArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null && this.isNotEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -114,12 +130,15 @@ public inline fun ByteArray?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun ShortArray?.whatIfNotNullOrEmpty(
   whatIf: (ShortArray) -> Unit
-): ShortArray? = apply {
-
+): ShortArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -136,13 +155,17 @@ public inline fun ShortArray?.whatIfNotNullOrEmpty(
 public inline fun ShortArray?.whatIfNotNullOrEmpty(
   whatIf: (ShortArray) -> Unit,
   whatIfNot: () -> Unit
-): ShortArray? = apply {
-
+): ShortArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null && this.isNotEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -156,12 +179,15 @@ public inline fun ShortArray?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun IntArray?.whatIfNotNullOrEmpty(
   whatIf: (IntArray) -> Unit
-): IntArray? = apply {
-
+): IntArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -178,13 +204,17 @@ public inline fun IntArray?.whatIfNotNullOrEmpty(
 public inline fun IntArray?.whatIfNotNullOrEmpty(
   whatIf: (IntArray) -> Unit,
   whatIfNot: () -> Unit
-): IntArray? = apply {
-
+): IntArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null && this.isNotEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -198,12 +228,15 @@ public inline fun IntArray?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun LongArray?.whatIfNotNullOrEmpty(
   whatIf: (LongArray) -> Unit
-): LongArray? = apply {
-
+): LongArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -220,13 +253,17 @@ public inline fun LongArray?.whatIfNotNullOrEmpty(
 public inline fun LongArray?.whatIfNotNullOrEmpty(
   whatIf: (LongArray) -> Unit,
   whatIfNot: () -> Unit
-): LongArray? = apply {
-
+): LongArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null && this.isNotEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -240,12 +277,15 @@ public inline fun LongArray?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun FloatArray?.whatIfNotNullOrEmpty(
   whatIf: (FloatArray) -> Unit
-): FloatArray? = apply {
-
+): FloatArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -262,13 +302,17 @@ public inline fun FloatArray?.whatIfNotNullOrEmpty(
 public inline fun FloatArray?.whatIfNotNullOrEmpty(
   whatIf: (FloatArray) -> Unit,
   whatIfNot: () -> Unit
-): FloatArray? = apply {
-
+): FloatArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null && this.isNotEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -282,12 +326,15 @@ public inline fun FloatArray?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun DoubleArray?.whatIfNotNullOrEmpty(
   whatIf: (DoubleArray) -> Unit
-): DoubleArray? = apply {
-
+): DoubleArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -304,13 +351,17 @@ public inline fun DoubleArray?.whatIfNotNullOrEmpty(
 public inline fun DoubleArray?.whatIfNotNullOrEmpty(
   whatIf: (DoubleArray) -> Unit,
   whatIfNot: () -> Unit
-): DoubleArray? = apply {
-
+): DoubleArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null && this.isNotEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -324,12 +375,15 @@ public inline fun DoubleArray?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun BooleanArray?.whatIfNotNullOrEmpty(
   whatIf: (BooleanArray) -> Unit
-): BooleanArray? = apply {
-
+): BooleanArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -346,13 +400,17 @@ public inline fun BooleanArray?.whatIfNotNullOrEmpty(
 public inline fun BooleanArray?.whatIfNotNullOrEmpty(
   whatIf: (BooleanArray) -> Unit,
   whatIfNot: () -> Unit
-): BooleanArray? = apply {
-
+): BooleanArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null && this.isNotEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -366,12 +424,15 @@ public inline fun BooleanArray?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun CharArray?.whatIfNotNullOrEmpty(
   whatIf: (CharArray) -> Unit
-): CharArray? = apply {
-
+): CharArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -388,11 +449,15 @@ public inline fun CharArray?.whatIfNotNullOrEmpty(
 public inline fun CharArray?.whatIfNotNullOrEmpty(
   whatIf: (CharArray) -> Unit,
   whatIfNot: () -> Unit
-): CharArray? = apply {
-
+): CharArray? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (this != null && this.isNotEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
diff --git a/whatif/src/main/java/com/skydoves/whatif/WhatIfCollections.kt b/whatif/src/main/java/com/skydoves/whatif/WhatIfCollections.kt
index ab5039a..af4f85e 100644
--- a/whatif/src/main/java/com/skydoves/whatif/WhatIfCollections.kt
+++ b/whatif/src/main/java/com/skydoves/whatif/WhatIfCollections.kt
@@ -19,6 +19,9 @@
 
 package com.skydoves.whatif
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 /**
  * An expression for invoking [whatIf] when the [List] is not null and not empty.
  *
@@ -30,12 +33,15 @@ package com.skydoves.whatif
 @WhatIfInlineOnly
 public inline fun <T> List<T>?.whatIfNotNullOrEmpty(
   whatIf: (List<T>) -> Unit
-): List<T>? = apply {
-
+): List<T>? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -52,13 +58,17 @@ public inline fun <T> List<T>?.whatIfNotNullOrEmpty(
 public inline fun <T> List<T>?.whatIfNotNullOrEmpty(
   whatIf: (List<T>) -> Unit,
   whatIfNot: () -> Unit
-): List<T>? = apply {
-
+): List<T>? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (!this.isNullOrEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -72,12 +82,15 @@ public inline fun <T> List<T>?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun <T> Set<T>?.whatIfNotNullOrEmpty(
   whatIf: (Set<T>) -> Unit
-): Set<T>? = apply {
-
+): Set<T>? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -94,13 +107,17 @@ public inline fun <T> Set<T>?.whatIfNotNullOrEmpty(
 public inline fun <T> Set<T>?.whatIfNotNullOrEmpty(
   whatIf: (Set<T>) -> Unit,
   whatIfNot: () -> Unit
-): Set<T>? = apply {
-
+): Set<T>? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (!this.isNullOrEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -114,12 +131,15 @@ public inline fun <T> Set<T>?.whatIfNotNullOrEmpty(
 @WhatIfInlineOnly
 public inline fun <T, R> Map<T, R>?.whatIfNotNullOrEmpty(
   whatIf: (Map<T, R>) -> Unit
-): Map<T, R>? = apply {
-
+): Map<T, R>? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -136,13 +156,17 @@ public inline fun <T, R> Map<T, R>?.whatIfNotNullOrEmpty(
 public inline fun <T, R> Map<T, R>?.whatIfNotNullOrEmpty(
   whatIf: (Map<T, R>) -> Unit,
   whatIfNot: () -> Unit
-): Map<T, R>? = apply {
-
+): Map<T, R>? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (!this.isNullOrEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }
 
 /**
@@ -158,13 +182,16 @@ public inline fun <T, R> Map<T, R>?.whatIfNotNullOrEmpty(
 public inline fun <reified T : MutableCollection<E>, reified E> T.addWhatIfNotNull(
   element: E?,
   whatIf: (T) -> Unit
-): T = apply {
-
+): T {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.addWhatIfNotNull(
     element = element,
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -182,8 +209,11 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.addWhatIfNotNu
   element: E?,
   whatIf: (T) -> Unit,
   whatIfNot: (T) -> Unit
-): T = apply {
-
+): T {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   element?.whatIfNotNullAs<E>(
     whatIf = {
       add(it)
@@ -193,6 +223,7 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.addWhatIfNotNu
       whatIfNot(this)
     }
   )
+  return this
 }
 
 /**
@@ -208,13 +239,16 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.addWhatIfNotNu
 public inline fun <reified T : MutableCollection<E>, reified E> T.addAllWhatIfNotNull(
   element: Collection<E>?,
   whatIf: (T) -> Unit
-): T = apply {
-
+): T {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.addAllWhatIfNotNull(
     element = element,
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -232,8 +266,11 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.addAllWhatIfNo
   element: Collection<E>?,
   whatIf: (T) -> Unit,
   whatIfNot: (T) -> Unit
-): T = apply {
-
+): T {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   element?.whatIfNotNull(
     whatIf = {
       addAll(it)
@@ -243,6 +280,7 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.addAllWhatIfNo
       whatIfNot(this)
     }
   )
+  return this
 }
 
 /**
@@ -258,13 +296,16 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.addAllWhatIfNo
 public inline fun <reified T : MutableCollection<E>, reified E> T.removeWhatIfNotNull(
   element: E?,
   whatIf: (T) -> Unit
-): T = apply {
-
+): T {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.removeWhatIfNotNull(
     element = element,
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -282,8 +323,11 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.removeWhatIfNo
   element: E?,
   whatIf: (T) -> Unit,
   whatIfNot: (T) -> Unit
-): T = apply {
-
+): T {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   element?.whatIfNotNullAs<E>(
     whatIf = {
       remove(it)
@@ -293,6 +337,7 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.removeWhatIfNo
       whatIfNot(this)
     }
   )
+  return this
 }
 
 /**
@@ -308,13 +353,16 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.removeWhatIfNo
 public inline fun <reified T : MutableCollection<E>, reified E> T.removeAllWhatIfNotNull(
   element: Collection<E>?,
   whatIf: (T) -> Unit
-): T = apply {
-
+): T {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.removeAllWhatIfNotNull(
     element = element,
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -332,8 +380,11 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.removeAllWhatI
   element: Collection<E>?,
   whatIf: (T) -> Unit,
   whatIfNot: (T) -> Unit
-): T = apply {
-
+): T {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   element?.whatIfNotNull(
     whatIf = {
       removeAll(it)
@@ -343,6 +394,7 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.removeAllWhatI
       whatIfNot(this)
     }
   )
+  return this
 }
 
 /**
@@ -357,6 +409,9 @@ public inline fun <reified T : MutableCollection<E>, reified E> T.removeAllWhatI
 public inline fun Iterable<Boolean?>.whatIfAnd(
   whatIf: () -> Unit
 ): Iterable<Boolean?> {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   return this.whatIfAnd(
     whatIf = whatIf,
     whatIfNot = { }
@@ -377,6 +432,10 @@ public inline fun Iterable<Boolean?>.whatIfAnd(
   whatIf: () -> Unit,
   whatIfNot: (() -> Unit)
 ): Iterable<Boolean?> {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   var predicate: Boolean? = null
 
   this.forEach {
@@ -408,6 +467,9 @@ public inline fun Iterable<Boolean?>.whatIfAnd(
 public inline fun Iterable<Boolean?>.whatIfOr(
   whatIf: () -> Unit
 ): Iterable<Boolean?> {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   return this.whatIfOr(
     whatIf = whatIf,
     whatIfNot = { }
@@ -428,6 +490,10 @@ public inline fun Iterable<Boolean?>.whatIfOr(
   whatIf: () -> Unit,
   whatIfNot: (() -> Unit)
 ): Iterable<Boolean?> {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   var predicate: Boolean? = null
 
   this.forEach {
diff --git a/whatif/src/main/java/com/skydoves/whatif/WhatIfString.kt b/whatif/src/main/java/com/skydoves/whatif/WhatIfString.kt
index 56023d1..f10bc51 100644
--- a/whatif/src/main/java/com/skydoves/whatif/WhatIfString.kt
+++ b/whatif/src/main/java/com/skydoves/whatif/WhatIfString.kt
@@ -19,6 +19,9 @@
 
 package com.skydoves.whatif
 
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 /**
  * An expression for invoking [whatIf] when the [String] is not null and not empty.
  *
@@ -30,12 +33,15 @@ package com.skydoves.whatif
 @WhatIfInlineOnly
 public inline fun String?.whatIfNotNullOrEmpty(
   whatIf: (String) -> Unit
-): String? = apply {
-
+): String? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfNotNullOrEmpty(
     whatIf = whatIf,
     whatIfNot = { }
   )
+  return this
 }
 
 /**
@@ -52,11 +58,15 @@ public inline fun String?.whatIfNotNullOrEmpty(
 public inline fun String?.whatIfNotNullOrEmpty(
   whatIf: (String) -> Unit,
   whatIfNot: () -> Unit
-): String? = apply {
-
+): String? {
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (!this.isNullOrEmpty()) {
     whatIf(this)
   } else {
     whatIfNot()
   }
+  return this
 }

From 79ce1380a7a5a28ca9c842a21b45edaba59c860b Mon Sep 17 00:00:00 2001
From: skydoves <skydoves@naver.com>
Date: Sat, 27 Nov 2021 14:01:41 +0900
Subject: [PATCH 3/3] Add contract on whatif-android module

---
 whatif-android/build.gradle                   |  5 +-
 .../skydoves/whatif_android/WhatIfActivity.kt | 74 +++++++++++++++----
 .../skydoves/whatif_android/WhatIfFragment.kt | 33 +++++++--
 .../whatif_android/WhatIfFragmentActivity.kt  | 14 ++++
 whatif/build.gradle                           |  5 +-
 .../main/java/com/skydoves/whatif/WhatIf.kt   |  2 +-
 6 files changed, 107 insertions(+), 26 deletions(-)

diff --git a/whatif-android/build.gradle b/whatif-android/build.gradle
index 2611b17..f84926a 100644
--- a/whatif-android/build.gradle
+++ b/whatif-android/build.gradle
@@ -24,7 +24,10 @@ android {
 }
 
 tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
-  kotlinOptions.freeCompilerArgs += ["-Xexplicit-api=strict"]
+  kotlinOptions.freeCompilerArgs += [
+      "-Xexplicit-api=strict",
+      "-Xopt-in=kotlin.contracts.ExperimentalContracts"
+  ]
 }
 
 dependencies {
diff --git a/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfActivity.kt b/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfActivity.kt
index 2e2eb4b..1b55803 100644
--- a/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfActivity.kt
+++ b/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfActivity.kt
@@ -27,6 +27,8 @@ import android.os.Parcelable
 import com.skydoves.whatif.whatIfNotNull
 import com.skydoves.whatif.whatIfNotNullAs
 import java.io.Serializable
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
 
 /**
  * An expression for invoking [whatIf] when the [Activity]'s intent extras is not null and not empty.
@@ -38,7 +40,9 @@ import java.io.Serializable
 public inline fun Activity.whatIfHasExtras(
   whatIf: (Bundle) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfHasExtras(
     whatIf = whatIf,
     whatIfNot = { }
@@ -58,7 +62,10 @@ public inline fun Activity.whatIfHasExtras(
   whatIf: (Bundle) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.extras.whatIfNotNull(
     whatIf = {
       if (!it.isEmpty) {
@@ -81,7 +88,9 @@ public inline fun Activity.whatIfHasExtras(
   name: String,
   whatIf: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfHasExtras(
     name = name,
     whatIf = whatIf,
@@ -104,7 +113,10 @@ public inline fun Activity.whatIfHasExtras(
   whatIf: () -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   if (intent.hasExtra(name)) {
     whatIf()
   } else {
@@ -124,7 +136,9 @@ public inline fun Activity.whatIfHasStringExtra(
   name: String,
   whatIf: (String) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getStringExtra(name).whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = { }
@@ -146,7 +160,10 @@ public inline fun Activity.whatIfHasStringExtra(
   whatIf: (String) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getStringExtra(name).whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = whatIfNot
@@ -165,7 +182,9 @@ public inline fun Activity.whatIfHasCharSequenceExtra(
   name: String,
   whatIf: (CharSequence) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getCharSequenceExtra(name).whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = { }
@@ -187,7 +206,10 @@ public inline fun Activity.whatIfHasCharSequenceExtra(
   whatIf: (CharSequence) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getCharSequenceExtra(name).whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = whatIfNot
@@ -206,7 +228,9 @@ public inline fun <reified T : Serializable> Activity.whatIfHasSerializableExtra
   name: String,
   whatIf: (T) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getSerializableExtra(name).whatIfNotNullAs(
     whatIf = whatIf,
     whatIfNot = { }
@@ -228,7 +252,10 @@ public inline fun <reified T : Serializable> Activity.whatIfHasSerializableExtra
   whatIf: (T) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getSerializableExtra(name).whatIfNotNullAs(
     whatIf = whatIf,
     whatIfNot = whatIfNot
@@ -247,7 +274,9 @@ public inline fun <reified T : Parcelable> Activity.whatIfHasParcelableExtra(
   name: String,
   whatIf: (T) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getParcelableExtra<T>(name).whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = { }
@@ -269,7 +298,10 @@ public inline fun <reified T : Parcelable> Activity.whatIfHasParcelableExtra(
   whatIf: (T) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getParcelableExtra<T>(name).whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = whatIfNot
@@ -288,7 +320,9 @@ public inline fun <reified T : Parcelable> Activity.whatIfHasParcelableArrayList
   name: String,
   whatIf: (ArrayList<T>) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getParcelableArrayListExtra<T>(name).whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = { }
@@ -310,7 +344,10 @@ public inline fun <reified T : Parcelable> Activity.whatIfHasParcelableArrayList
   whatIf: (ArrayList<T>) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.getParcelableArrayListExtra<T>(name).whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = whatIfNot
@@ -327,7 +364,9 @@ public inline fun <reified T : Parcelable> Activity.whatIfHasParcelableArrayList
 public inline fun Activity.whatIfHasDeepLinkUri(
   whatIf: (Uri) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this.whatIfHasDeepLinkUri(whatIf, { })
 }
 
@@ -344,6 +383,9 @@ public inline fun Activity.whatIfHasDeepLinkUri(
   whatIf: (Uri) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this.intent.data.whatIfNotNull(whatIf, whatIfNot)
 }
diff --git a/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfFragment.kt b/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfFragment.kt
index 13df157..11c959f 100644
--- a/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfFragment.kt
+++ b/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfFragment.kt
@@ -27,6 +27,8 @@ import androidx.fragment.app.Fragment
 import androidx.fragment.app.FragmentActivity
 import com.skydoves.whatif.whatIfNotNull
 import com.skydoves.whatif.whatIfNotNullAs
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
 
 /**
  * @author skydoves (Jaewoong Eum)
@@ -40,7 +42,9 @@ import com.skydoves.whatif.whatIfNotNullAs
 public inline fun Fragment?.whatIfNotNullContext(
   whatIf: (Context) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this?.context.whatIfNotNull {
     whatIf(it)
   }
@@ -80,7 +84,9 @@ public inline fun Fragment?.whatIfNotNullContext(
 public inline fun Fragment?.whatIfNotNullActivity(
   whatIf: (FragmentActivity) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   this?.activity.whatIfNotNull {
     whatIf(it)
   }
@@ -101,7 +107,10 @@ public inline fun Fragment?.whatIfNotNullActivity(
   whatIf: (FragmentActivity) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this?.activity.whatIfNotNull(
     whatIf = whatIf,
     whatIfNot = whatIfNot
@@ -120,7 +129,9 @@ public inline fun Fragment?.whatIfNotNullActivity(
 public inline fun Fragment?.whatIfHasArguments(
   whatIf: (Bundle) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   whatIfHasArguments(whatIf) { }
 }
 
@@ -138,7 +149,10 @@ public inline fun Fragment?.whatIfHasArguments(
   whatIf: (Bundle) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this?.arguments.whatIfNotNull(whatIf, whatIfNot)
 }
 
@@ -154,7 +168,9 @@ public inline fun Fragment?.whatIfHasArguments(
 public inline fun <reified T> Fragment?.whatIfFindParentInterface(
   whatIf: (T) -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+  }
   whatIfFindParentInterface(whatIf) { }
 }
 
@@ -172,6 +188,9 @@ public inline fun <reified T> Fragment?.whatIfFindParentInterface(
   whatIf: (T) -> Unit,
   whatIfNot: () -> Unit
 ) {
-
+  contract {
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
+    callsInPlace(whatIfNot, InvocationKind.AT_MOST_ONCE)
+  }
   this?.activity.whatIfNotNullAs(whatIf, whatIfNot)
 }
diff --git a/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfFragmentActivity.kt b/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfFragmentActivity.kt
index f3ff91c..ccbc933 100644
--- a/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfFragmentActivity.kt
+++ b/whatif-android/src/main/java/com/skydoves/whatif_android/WhatIfFragmentActivity.kt
@@ -20,6 +20,8 @@ import androidx.annotation.IdRes
 import androidx.fragment.app.Fragment
 import androidx.fragment.app.FragmentActivity
 import com.skydoves.whatif.whatIfNotNullAs
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
 
 /**
  * @author skydoves (Jaewoong Eum)
@@ -35,6 +37,9 @@ public inline fun <reified T : Fragment> FragmentActivity.whatIfFindFragment(
   @IdRes id: Int,
   whatIf: (T) -> Unit
 ) {
+  contract {
+    callsInPlace(whatIf, InvocationKind.EXACTLY_ONCE)
+  }
   whatIfFindFragment(id, whatIf, {})
 }
 
@@ -55,6 +60,9 @@ public inline fun <reified T : Fragment> FragmentActivity.whatIfFindFragment(
   whatIf: (T) -> Unit,
   whatIfNot: () -> Unit
 ) {
+  contract {
+    callsInPlace(whatIf, InvocationKind.EXACTLY_ONCE)
+  }
   supportFragmentManager.findFragmentById(id).whatIfNotNullAs(whatIf, whatIfNot)
 }
 
@@ -72,6 +80,9 @@ public inline fun <reified T : Fragment> FragmentActivity.whatIfFindFragment(
   tag: String?,
   whatIf: (T) -> Unit
 ) {
+  contract {
+    callsInPlace(whatIf, InvocationKind.EXACTLY_ONCE)
+  }
   whatIfFindFragment(tag, whatIf, {})
 }
 
@@ -92,5 +103,8 @@ public inline fun <reified T : Fragment> FragmentActivity.whatIfFindFragment(
   whatIf: (T) -> Unit,
   whatIfNot: () -> Unit
 ) {
+  contract {
+    callsInPlace(whatIf, InvocationKind.EXACTLY_ONCE)
+  }
   supportFragmentManager.findFragmentByTag(tag).whatIfNotNullAs(whatIf, whatIfNot)
 }
diff --git a/whatif/build.gradle b/whatif/build.gradle
index 5665a19..72b031e 100644
--- a/whatif/build.gradle
+++ b/whatif/build.gradle
@@ -21,7 +21,10 @@ task coverageReport(type: JacocoReport, dependsOn: 'test') {
 }
 
 tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
-  kotlinOptions.freeCompilerArgs += ["-Xopt-in=kotlin.contracts.ExperimentalContracts"]
+  kotlinOptions.freeCompilerArgs += [
+      "-Xexplicit-api=strict",
+      "-Xopt-in=kotlin.contracts.ExperimentalContracts"
+  ]
 }
 
 dependencies {
diff --git a/whatif/src/main/java/com/skydoves/whatif/WhatIf.kt b/whatif/src/main/java/com/skydoves/whatif/WhatIf.kt
index 5c0faed..ca0b8a2 100644
--- a/whatif/src/main/java/com/skydoves/whatif/WhatIf.kt
+++ b/whatif/src/main/java/com/skydoves/whatif/WhatIf.kt
@@ -375,7 +375,7 @@ public inline fun <T> T?.whatIfNotNull(
   whatIf: (T) -> Unit
 ): T? {
   contract {
-    callsInPlace(whatIf, InvocationKind.EXACTLY_ONCE)
+    callsInPlace(whatIf, InvocationKind.AT_MOST_ONCE)
   }
   return this.whatIfNotNull(
     whatIf = whatIf,