add Optional type

common/src/commonMain/kotlin/dev/inmo/micro_utils/common/Optional.kt

@@ -0,0 +1,52 @@
+package dev.inmo.micro_utils.common
+
+import kotlinx.serialization.Serializable
+
+/**
+ * This type represents [T] as not only potentially nullable data, but also as a data which can not be presented. This
+ * type will be useful in cases when [T] is nullable and null as valuable data too in time of data absence should be
+ * presented by some third type.
+ *
+ * Let's imagine, you have nullable name in some database. In case when name is not nullable everything is clear - null
+ * will represent absence of row in the database. In case when name is nullable null will be a little bit dual-meaning,
+ * cause this null will say nothing about availability of the row (of course, it is exaggerated example)
+ *
+ * @see Optional.presented
+ * @see Optional.absent
+ * @see Optional.optional
+ * @see Optional.onPresented
+ * @see Optional.onAbsent
+ */
+@Serializable
+data class Optional<T> internal constructor(
+    internal val data: T?,
+    internal val dataPresented: Boolean
+) {
+    companion object {
+        /**
+         * Will create [Optional] with presented data
+         */
+        fun <T> presented(data: T) = Optional(data, true)
+        /**
+         * Will create [Optional] without data
+         */
+        fun <T> absent() = Optional<T>(null, false)
+    }
+}
+
+inline val <T> T.optional
+    get() = Optional.presented(this)
+
+/**
+ * Will call [block] when data presented ([Optional.dataPresented] == true)
+ */
+fun <T> Optional<T>.onPresented(block: (T) -> Unit): Optional<T> = apply {
+    if (dataPresented) { block(data as T) }
+}
+
+/**
+ * Will call [block] when data absent ([Optional.dataPresented] == false)
+ */
+fun <T> Optional<T>.onAbsent(block: () -> Unit): Optional<T> = apply {
+    if (!dataPresented) { block() }
+}