add letIf

README.md

@@ -21,7 +21,7 @@ in the stdlib such as:
 - [isNotNullAndNotEmpty/Blank](https://github.com/robstoll/kbox/tree/main/src/commonMain/kotlin/ch/tutteli/kbox/isNotNullAndNot.kt)
 - [identity](https://github.com/robstoll/kbox/tree/main/src/commonMain/kotlin/ch/tutteli/kbox/identity.kt)
 - [Tuple4(Like) - Tuple9(Like)](https://github.com/robstoll/kbox/tree/main/src/commonMain/generated/kotlin/ch/tutteli/kbox)  
-  (including functions mapComponentX, append, glue)
+  (including functions mapA1 - mapA9, append, glue)
 - [Triple/Pair.mapFirst/Second/Third](https://github.com/robstoll/kbox/tree/main/src/commonMain/generated/kotlin/ch/tutteli/kbox/tupleMap.kt#L12)
 - [toVararg](https://github.com/robstoll/kbox/tree/main/src/commonMain/generated/kotlin/ch/tutteli/kbox/toVararg.kt), [mapVararg](https://github.com/robstoll/kbox/tree/main/src/commonMain/generated/kotlin/ch/tutteli/kbox/mapVararg.kt), [varargToList (glue)](https://github.com/robstoll/kbox/tree/main/src/commonMain/generated/kotlin/ch/tutteli/kbox/varargToList.kt)
 

src/commonMain/kotlin/ch/tutteli/kbox/takeIf.kt

@@ -1,12 +1,23 @@
 package ch.tutteli.kbox
 
+import kotlin.contracts.ExperimentalContracts
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
 /**
  * Calls the [provider] only if the [predicate] holds, returns `null` otherwise.
  *
- * This function shall complement `takeIf` from the stdlib of Kotlin for cases where the cost to call the provider
+ * This function shall complement [kotlin.takeIf] for cases where the cost to call the provider
  * is high or the code involved to define the provider is long and a postfix `takeIf` is less readable
  * than stating it at the beginning.
  *
  * @since 1.1.0
  */
-inline fun <R> takeIf(predicate: Boolean, provider: () -> R): R? = if (predicate) provider() else null
+//TODO move to xIf with 4.0.0
+@OptIn(ExperimentalContracts::class)
+inline fun <R> takeIf(predicate: Boolean, provider: () -> R): R? {
+    contract {
+        callsInPlace(provider, InvocationKind.AT_MOST_ONCE)
+    }
+    return if (predicate) provider() else null
+}

src/commonMain/kotlin/ch/tutteli/kbox/xIf.kt

@@ -0,0 +1,21 @@
+package ch.tutteli.kbox
+
+import kotlin.contracts.ExperimentalContracts
+import kotlin.contracts.InvocationKind
+import kotlin.contracts.contract
+
+/**
+ * Delegates to [let] in case the given [predicate] holds, returns `this` otherwise.
+ *
+ * This function shall complement [let] for cases such as invoking a builder method conditionally which without [letIf]
+ * would be written as follows `if (predicate) t.let{...} else t`
+ *
+ * @since 3.2.0
+ */
+@OptIn(ExperimentalContracts::class)
+inline fun <T : U, R : U, U> T.letIf(predicate: Boolean, block: (T) -> R): U {
+    contract {
+        callsInPlace(block, InvocationKind.AT_MOST_ONCE)
+    }
+    return if (predicate) let(block) else this
+}