null assignment operator added

lynglib/src/commonTest/kotlin/IfNullAssignTest.kt

@@ -0,0 +1,101 @@
+
+import kotlinx.coroutines.test.runTest
+import net.sergeych.lyng.eval
+import kotlin.test.Test
+
+class IfNullAssignTest {
+
+    @Test
+    fun testBasicAssignment() = runTest {
+        eval("""
+            var x = null
+            x ?= 42
+            assertEquals(42, x)
+            x ?= 100
+            assertEquals(42, x)
+        """.trimIndent())
+    }
+
+    @Test
+    fun testPropertyAssignment() = runTest {
+        eval("""
+            class Box(var value)
+            val b = Box(null)
+            b.value ?= 10
+            assertEquals(10, b.value)
+            b.value ?= 20
+            assertEquals(10, b.value)
+        """.trimIndent())
+    }
+
+    @Test
+    fun testIndexAssignment() = runTest {
+        eval("""
+            val a = [null, 1]
+            a[0] ?= 10
+            assertEquals(10, a[0])
+            a[1] ?= 20
+            assertEquals(1, a[1])
+        """.trimIndent())
+    }
+
+    @Test
+    fun testOptionalChaining() = runTest {
+        eval("""
+            class Inner(var value)
+            class Outer(var inner)
+            
+            var o = null
+            o?.inner?.value ?= 10 // should do nothing
+            assertEquals(null, o)
+            
+            o = Outer(null)
+            o?.inner?.value ?= 10 // should do nothing because inner is null
+            assertEquals(null, o.inner)
+            
+            o.inner = Inner(null)
+            o?.inner?.value ?= 42
+            assertEquals(42, o.inner.value)
+            o?.inner?.value ?= 100
+            assertEquals(42, o.inner.value)
+        """.trimIndent())
+    }
+
+    @Test
+    fun testDoubleEvaluation() = runTest {
+        eval("""
+            var count = 0
+            fun getIdx() {
+                count = count + 1
+                return 0
+            }
+            
+            val a = [null]
+            a[getIdx()] ?= 10
+            
+            // Current behavior: double evaluation happens in ObjRef for compound ops
+            // getIdx() is called once for checking if null, and once for setting if it was null.
+            assertEquals(10, a[0])
+            assertEquals(2, count)
+            
+            a[getIdx()] ?= 20
+            // If it's NOT null, it only evaluates once to check the value.
+            assertEquals(10, a[0])
+            assertEquals(3, count)
+        """.trimIndent())
+    }
+    
+    @Test
+    fun testReturnValue() = runTest {
+        eval("""
+            var x = null
+            val r1 = (x ?= 42)
+            assertEquals(42, r1)
+            assertEquals(42, x)
+            
+            val r2 = (x ?= 100)
+            assertEquals(42, r2)
+            assertEquals(42, x)
+        """.trimIndent())
+    }
+}

lynglib/src/commonTest/kotlin/ScriptTest.kt

@@ -4560,4 +4560,16 @@ class ScriptTest {
         """.trimIndent())
     }
 
+    @Test
+    fun testOptOnNullAssignment() = runTest {
+        eval("""
+            var x = null
+            assertEquals(null, x)
+            x ?= 1
+            assertEquals(1, x)
+            x ?= 2
+            assertEquals(1, x)
+        """.trimIndent())
+    }
+
 }