ifEmpty and isBlank extensions

- Introduce ifEmpty extension for Collections, Maps, Arrays, Sequences
- Introduce ifEmpty and isBlank for CharSequences

#KT-15695 Fixed

libraries/stdlib/src/kotlin/collections/Arrays.kt

@@ -53,3 +53,15 @@ public inline fun Array<*>?.isNullOrEmpty(): Boolean {
 
     return this == null || this.isEmpty()
 }
+
+/**
+ * Returns this array if it's not empty
+ * or the result of calling [defaultValue] function if the array is empty.
+ *
+ * @sample samples.collections.Arrays.Usage.arrayIfEmpty
+ */
+@SinceKotlin("1.3")
+@kotlin.internal.InlineOnly
+@Suppress("UPPER_BOUND_CANNOT_BE_ARRAY")
+public inline fun <C, R> C.ifEmpty(defaultValue: () -> R): R where C : Array<*>, C : R =
+    if (isEmpty()) defaultValue() else this

libraries/stdlib/src/kotlin/collections/Collections.kt

@@ -197,6 +197,18 @@ public inline fun <T> Collection<T>?.orEmpty(): Collection<T> = this ?: emptyLis
 @kotlin.internal.InlineOnly
 public inline fun <T> List<T>?.orEmpty(): List<T> = this ?: emptyList()
 
+/**
+ * Returns this collection if it's not empty
+ * or the result of calling [defaultValue] function if the collection is empty.
+ *
+ * @sample samples.collections.Collections.Collections.collectionIfEmpty
+ */
+@SinceKotlin("1.3")
+@kotlin.internal.InlineOnly
+public inline fun <C, R> C.ifEmpty(defaultValue: () -> R): R where C : Collection<*>, C : R =
+    if (isEmpty()) defaultValue() else this
+
+
 /**
  * Checks if all elements in the specified collection are contained in this collection.
  *

libraries/stdlib/src/kotlin/collections/Maps.kt

@@ -162,6 +162,17 @@ public inline fun <K, V> Map<out K, V>?.isNullOrEmpty(): Boolean {
 @kotlin.internal.InlineOnly
 public inline fun <K, V> Map<K, V>?.orEmpty(): Map<K, V> = this ?: emptyMap()
 
+/**
+ * Returns this map if it's not empty
+ * or the result of calling [defaultValue] function if the map is empty.
+ *
+ * @sample samples.collections.Maps.Usage.mapIfEmpty
+ */
+@SinceKotlin("1.3")
+@kotlin.internal.InlineOnly
+public inline fun <M, R> M.ifEmpty(defaultValue: () -> R): R where M : Map<*, *>, M : R =
+    if (isEmpty()) defaultValue() else this
+
 /**
  * Checks if the map contains the given key.
  *

libraries/stdlib/src/kotlin/collections/Sequences.kt

@@ -9,6 +9,7 @@
 package kotlin.sequences
 
 import kotlin.*
+import kotlin.coroutines.experimental.buildSequence
 
 /**
  * Given an [iterator] function constructs a [Sequence] that returns values through the [Iterator]
@@ -55,6 +56,23 @@ private object EmptySequence : Sequence<Nothing>, DropTakeSequence<Nothing> {
 @kotlin.internal.InlineOnly
 public inline fun <T> Sequence<T>?.orEmpty(): Sequence<T> = this ?: emptySequence()
 
+
+/**
+ * Returns a sequence that iterates through the elements either of this sequence
+ * or, if this sequence turns out to be empty, of the sequence returned by [defaultValue] function.
+ *
+ * @sample samples.collections.Sequences.Usage.sequenceIfEmpty
+ */
+@SinceKotlin("1.3")
+public fun <T> Sequence<T>.ifEmpty(defaultValue: () -> Sequence<T>): Sequence<T> = buildSequence {
+    val iterator = this@ifEmpty.iterator()
+    if (iterator.hasNext()) {
+        yieldAll(iterator)
+    } else {
+        yieldAll(defaultValue())
+    }
+}
+
 /**
  * Returns a sequence of all elements from all sequences in this sequence.
  *

libraries/stdlib/src/kotlin/text/Strings.kt

@@ -276,6 +276,28 @@ public operator fun CharSequence.iterator(): CharIterator = object : CharIterato
 @kotlin.internal.InlineOnly
 public inline fun String?.orEmpty(): String = this ?: ""
 
+/**
+ * Returns this char sequence if it's not empty
+ * or the result of calling [defaultValue] function if the char sequence is empty.
+ *
+ * @sample samples.text.Strings.stringIfEmpty
+ */
+@SinceKotlin("1.3")
+@kotlin.internal.InlineOnly
+public inline fun <C, R> C.ifEmpty(defaultValue: () -> R): R where C : CharSequence, C : R =
+    if (isEmpty()) defaultValue() else this
+
+/**
+ * Returns this char sequence if it is not empty and doesn't consist solely of whitespace characters,
+ * or the result of calling [defaultValue] function otherwise.
+ *
+ * @sample samples.text.Strings.stringIfBlank
+ */
+@SinceKotlin("1.3")
+@kotlin.internal.InlineOnly
+public inline fun <C, R> C.ifBlank(defaultValue: () -> R): R where C : CharSequence, C : R =
+    if (isBlank()) defaultValue() else this
+
 /**
  * Returns the range of valid character indices for this char sequence.
  */