Add flatten, flatMap, and filterNotNull functions with corresponding tests

lynglib/src/commonTest/kotlin/StdlibTest.kt

@@ -91,4 +91,19 @@ class StdlibTest {
             assertEquals([1,2], (1..8).dropLast(6).toList() )
         """.trimIndent())
     }
+
+    @Test
+    fun testFlattenAndFilter() = runTest {
+        eval("""
+            assertEquals([1,2,3,4,5,6], [1,3,5].map { [it, it+1] }.flatten() )
+            assertEquals([1,3,5], [null,1,null, 3,5].filterNotNull().toList())
+        """)
+    }
+
+    @Test
+    fun testFlatMap() = runTest {
+        eval("""
+            assertEquals([1,2,3,4,5,6], [1,3,5].flatMap { [it,it+1] }.toList() )
+        """)
+    }
 }

lynglib/stdlib/lyng/root.lyng

@@ -27,6 +27,14 @@ fun Iterable.filter(predicate) {
     }
 }
 
+/*
+filter out all null elements from this collection (Iterable); collection of
+non-null elements is returned
+*/
+fun Iterable.filterNotNull() {
+    filter { it != null }
+}
+
 /* Skip the first N elements of this iterable. */
 fun Iterable.drop(n) {
     var cnt = 0
@@ -179,6 +187,26 @@ fun Iterable.shuffled() {
     toList().apply { shuffle() }
 }
 
+/*
+Returns a single list of all elements from all collections in the given collection.
+@return List
+*/
+fun Iterable.flatten() {
+    val result = []
+    forEach { i ->
+        i.forEach { result.add(it) }
+    }
+    result
+}
+
+/*
+Returns a single list of all elements yielded from results of transform function being
+invoked on each element of original collection.
+*/
+fun Iterable.flatMap(transform): List {
+    map(transform).flatten()
+}
+
 /* Return string representation like [a,b,c]. */
 fun List.toString() {
     "[" + joinToString(",") + "]"