Implement restricted setter visibility (private set / protected set) for class fields and properties.

CHANGELOG.md

@@ -11,7 +11,22 @@
   - Integration: Updated TextMate grammar and IntelliJ plugin (highlighting + keywords).
   - Documentation: New "Properties" section in `docs/OOP.md`.
 
-- Docs: Scopes and Closures guidance
+- Language: Restricted Setter Visibility
+  - Support for `private set` and `protected set` modifiers on `var` fields and properties.
+  - Allows members to be publicly readable but only writable from within the declaring class or its subclasses.
+  - Enforcement at runtime: throws `AccessException` on unauthorized writes.
+  - Supported only for declarations in class bodies (fields and properties).
+  - Documentation: New "Restricted Setter Visibility" section in `docs/OOP.md`.
+
+- Language: Late-initialized `val` fields in classes
+  - Support for declaring `val` without an immediate initializer in class bodies.
+  - Compulsory initialization: every late-init `val` must be assigned at least once within the class body or an `init` block.
+  - Write-once enforcement: assigning to a `val` is allowed only if its current value is `Unset`.
+  - Access protection: reading a late-init `val` before it is assigned returns the `Unset` singleton; using `Unset` for most operations throws an `UnsetException`.
+  - Extension properties do not support late-init.
+  - Documentation: New "Late-initialized `val` fields" and "The `Unset` singleton" sections in `docs/OOP.md`.
+
+- Docs: OOP improvements
   - New page: `docs/scopes_and_closures.md` detailing `ClosureScope` resolution order, recursion‑safe helpers (`chainLookupIgnoreClosure`, `chainLookupWithMembers`, `baseGetIgnoreClosure`), cycle prevention, and capturing lexical environments for callbacks (`snapshotForClosure`).
   - Updated: `docs/advanced_topics.md` (link to the new page), `docs/parallelism.md` (closures in `launch`/`flow`), `docs/OOP.md` (visibility from closures with preserved `currentClassCtx`), `docs/exceptions_handling.md` (compatibility alias `SymbolNotFound`).
   - Tutorial: added quick link to Scopes and Closures.

docs/OOP.md

@@ -175,6 +175,44 @@ statements discussed later, there could be default values, ellipsis, etc.
 
 Note that unlike **Kotlin**, which uses `=` for named arguments, Lyng uses `:` to avoid ambiguity with assignment expressions.
 
+### Late-initialized `val` fields
+
+You can declare a `val` field without an immediate initializer if you provide an assignment for it within an `init` block or the class body. This is useful when the initial value depends on logic that cannot be expressed in a single expression.
+
+```kotlin
+class DataProcessor(data: Object) {
+    val result: Object
+    
+    init {
+        // Complex initialization logic
+        result = transform(data)
+    }
+}
+```
+
+Key rules for late-init `val`:
+- **Compile-time Check**: The compiler ensures that every `val` declared without an initializer in a class body has at least one assignment within that class body (including `init` blocks). Failing to do so results in a syntax error.
+- **Write-Once**: A `val` can only be assigned once. Even if it was declared without an initializer, once it is assigned a value (e.g., in `init`), any subsequent assignment will throw an `IllegalAssignmentException`.
+- **Access before Initialization**: If you attempt to read a late-init `val` before it has been assigned (for example, by calling a method in `init` that reads the field before its assignment), it will hold a special `Unset` value. Using `Unset` for most operations (like arithmetic or method calls) will throw an `UnsetException`.
+- **No Extensions**: Extension properties do not support late initialization as they do not have per-instance storage. Extension `val`s must always have an initializer or a `get()` accessor.
+
+### The `Unset` singleton
+
+The `Unset` singleton represents a field that has been declared but not yet initialized. While it can be compared and converted to a string, most other operations on it are forbidden to prevent accidental use of uninitialized data.
+
+```kotlin
+class T {
+    val x
+    fun check() {
+        if (x == Unset) println("Not ready")
+    }
+    init {
+        check() // Prints "Not ready"
+        x = 42
+    }
+}
+```
+
 ## Methods
 
 Functions defined inside a class body are methods, and unless declared
@@ -394,6 +432,69 @@ Are declared with var
     assert( p.isSpecial == true )
     >>> void
 
+### Restricted Setter Visibility
+
+You can restrict the visibility of a `var` field's or property's setter by using `private set` or `protected set` modifiers. This allows the member to be publicly readable but only writable from within the class or its subclasses.
+
+#### On Fields
+
+```kotlin
+class SecretCounter {
+    var count = 0
+        private set // Can be read anywhere, but written only in SecretCounter
+        
+    fun increment() { count++ }
+}
+
+val c = SecretCounter()
+println(c.count) // OK
+c.count = 10     // Throws AccessException
+c.increment()    // OK
+```
+
+#### On Properties
+
+You can also apply restricted visibility to custom property setters:
+
+```kotlin
+class Person(private var _age: Int) {
+    var age
+        get() = _age
+        private set(v) { if (v >= 0) _age = v }
+}
+```
+
+#### Protected Setters and Inheritance
+
+A `protected set` allows subclasses to modify a field that is otherwise read-only to the public:
+
+```kotlin
+class Base {
+    var state = "initial"
+        protected set
+}
+
+class Derived : Base() {
+    fun changeState(newVal) {
+        state = newVal // OK: protected access from subclass
+    }
+}
+
+val d = Derived()
+println(d.state) // OK: "initial"
+d.changeState("updated")
+println(d.state) // OK: "updated"
+d.state = "bad"  // Throws AccessException: public write not allowed
+```
+
+### Key Rules and Limitations
+
+- **Only for `var`**: Restricted setter visibility cannot be used with `val` declarations, as they are inherently read-only. Attempting to use it with `val` results in a syntax error.
+- **Class Body Only**: These modifiers can only be used on members declared within the class body. They are not supported for primary constructor parameters.
+- **`private set`**: The setter is only accessible within the same class context (specifically, when `this` is an instance of that class).
+- **`protected set`**: The setter is accessible within the declaring class and all its transitive subclasses.
+- **Multiple Inheritance**: In MI scenarios, visibility is checked against the class that actually declared the member. Qualified access (e.g., `this@Base.field = value`) also respects restricted setter visibility.
+
 ### Private fields
 
 Private fields are visible only _inside the class instance_:

lynglib/src/commonMain/kotlin/net/sergeych/lyng/CodeContext.kt

@@ -20,5 +20,7 @@ package net.sergeych.lyng
 sealed class CodeContext {
     class Module(@Suppress("unused") val packageName: String?): CodeContext()
     class Function(val name: String): CodeContext()
-    class ClassBody(val name: String): CodeContext()
+    class ClassBody(val name: String): CodeContext() {
+        val pendingInitializations = mutableMapOf<String, Pos>()
+    }
 }

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Compiler.kt

@@ -138,9 +138,16 @@ class Compiler(
     private var lastParsedBlockRange: MiniRange? = null
 
     private suspend fun <T> inCodeContext(context: CodeContext, f: suspend () -> T): T {
-        return try {
+        codeContexts.add(context)
-            codeContexts.add(context)
+        try {
-            f()
+            val res = f()
+            if (context is CodeContext.ClassBody) {
+                if (context.pendingInitializations.isNotEmpty()) {
+                    val (name, pos) = context.pendingInitializations.entries.first()
+                    throw ScriptError(pos, "val '$name' must be initialized in the class body or init block")
+                }
+            }
+            return res
         } finally {
             codeContexts.removeLast()
         }
@@ -360,7 +367,21 @@ class Compiler(
             val rvalue = parseExpressionLevel(level + 1)
                 ?: throw ScriptError(opToken.pos, "Expecting expression")
 
-            lvalue = op.generate(opToken.pos, lvalue!!, rvalue)
+            val res = op.generate(opToken.pos, lvalue!!, rvalue)
+            if (opToken.type == Token.Type.ASSIGN) {
+                val ctx = codeContexts.lastOrNull()
+                if (ctx is CodeContext.ClassBody) {
+                    val target = lvalue
+                    val name = when (target) {
+                        is LocalVarRef -> target.name
+                        is FastLocalVarRef -> target.name
+                        is FieldRef -> if (target.target is LocalVarRef && target.target.name == "this") target.name else null
+                        else -> null
+                    }
+                    if (name != null) ctx.pendingInitializations.remove(name)
+                }
+            }
+            lvalue = res
         }
         return lvalue
     }
@@ -492,24 +513,32 @@ class Compiler(
                     // there could be terminal operators or keywords:// variable to read or like
                     when (t.value) {
                         in stopKeywords -> {
-                            if (operand != null) throw ScriptError(t.pos, "unexpected keyword")
+                            if (t.value == "init" && !(codeContexts.lastOrNull() is CodeContext.ClassBody && cc.peekNextNonWhitespace().type == Token.Type.LBRACE)) {
-                            // Allow certain statement-like constructs to act as expressions
+                                // Soft keyword: init is only a keyword in class body when followed by {
-                            // when they appear in expression position (e.g., `if (...) ... else ...`).
+                                cc.previous()
-                            // Other keywords should be handled by the outer statement parser.
+                                operand = parseAccessor()
-                            when (t.value) {
+                            } else {
-                                "if" -> {
+                                if (operand != null) throw ScriptError(t.pos, "unexpected keyword")
-                                    val s = parseIfStatement()
+                                // Allow certain statement-like constructs to act as expressions
-                                    operand = StatementRef(s)
+                                // when they appear in expression position (e.g., `if (...) ... else ...`).
-                                }
+                                // Other keywords should be handled by the outer statement parser.
-                                "when" -> {
+                                when (t.value) {
-                                    val s = parseWhenStatement()
+                                    "if" -> {
-                                    operand = StatementRef(s)
+                                        val s = parseIfStatement()
-                                }
+                                        operand = StatementRef(s)
-                                else -> {
+                                    }
-                                    // Do not consume the keyword as part of a term; backtrack
+
-                                    // and return null so outer parser handles it.
+                                    "when" -> {
-                                    cc.previous()
+                                        val s = parseWhenStatement()
-                                    return null
+                                        operand = StatementRef(s)
+                                    }
+
+                                    else -> {
+                                        // Do not consume the keyword as part of a term; backtrack
+                                        // and return null so outer parser handles it.
+                                        cc.previous()
+                                        return null
+                                    }
                                 }
                             }
                         }
@@ -1367,7 +1396,7 @@ class Compiler(
         }
 
         "init" -> {
-            if (codeContexts.lastOrNull() is CodeContext.ClassBody) {
+            if (codeContexts.lastOrNull() is CodeContext.ClassBody && cc.peekNextNonWhitespace().type == Token.Type.LBRACE) {
                 val block = parseBlock()
                 lastParsedBlockRange?.let { range ->
                     miniSink?.onInitDecl(MiniInitDecl(MiniRange(id.pos, range.end), id.pos))
@@ -2696,7 +2725,7 @@ class Compiler(
             cc.restorePos(markBeforeEq)
             cc.skipWsTokens()
             val next = cc.peekNextNonWhitespace()
-            if (next.isId("get") || next.isId("set")) {
+            if (next.isId("get") || next.isId("set") || next.isId("private") || next.isId("protected")) {
                 isProperty = true
                 cc.restorePos(markBeforeEq)
                 cc.skipWsTokens()
@@ -2714,7 +2743,13 @@ class Compiler(
             if (!isProperty && eqToken.type != Token.Type.ASSIGN) {
                 if (!isMutable && (declaringClassNameCaptured == null) && (extTypeName == null))
                     throw ScriptError(start, "val must be initialized")
-                else {
+                else if (!isMutable && declaringClassNameCaptured != null && extTypeName == null) {
+                    // lateinit val in class: track it
+                    (codeContexts.lastOrNull() as? CodeContext.ClassBody)?.pendingInitializations?.put(name, start)
+                    cc.restorePos(markBeforeEq)
+                    cc.skipWsTokens()
+                    setNull = true
+                } else {
                     cc.restorePos(markBeforeEq)
                     cc.skipWsTokens()
                     setNull = true
@@ -2753,8 +2788,8 @@ class Compiler(
 //            if (isDelegate) throw ScriptError(start, "static delegates are not yet implemented")
             currentInitScope += statement {
                 val initValue = initialExpression?.execute(this)?.byValueCopy() ?: ObjNull
-                (thisObj as ObjClass).createClassField(name, initValue, isMutable, visibility, pos)
+                (thisObj as ObjClass).createClassField(name, initValue, isMutable, visibility, null, pos)
-                addItem(name, isMutable, initValue, visibility, ObjRecord.Type.Field)
+                addItem(name, isMutable, initValue, visibility, null, ObjRecord.Type.Field)
                 ObjVoid
             }
             return NopStatement
@@ -2763,11 +2798,11 @@ class Compiler(
         // Check for accessors if it is a class member
         var getter: Statement? = null
         var setter: Statement? = null
+        var setterVisibility: Visibility? = null
         if (declaringClassNameCaptured != null || extTypeName != null) {
             while (true) {
-                val t = cc.peekNextNonWhitespace()
+                val t = cc.skipWsTokens()
                 if (t.isId("get")) {
-                    cc.skipWsTokens()
                     cc.next() // consume 'get'
                     cc.requireToken(Token.Type.LPAREN)
                     cc.requireToken(Token.Type.RPAREN)
@@ -2783,7 +2818,6 @@ class Compiler(
                         throw ScriptError(cc.current().pos, "Expected { or = after get()")
                     }
                 } else if (t.isId("set")) {
-                    cc.skipWsTokens()
                     cc.next() // consume 'set'
                     cc.requireToken(Token.Type.LPAREN)
                     val setArg = cc.requireToken(Token.Type.ID, "Expected setter argument name")
@@ -2809,6 +2843,46 @@ class Compiler(
                     } else {
                         throw ScriptError(cc.current().pos, "Expected { or = after set(...)")
                     }
+                } else if (t.isId("private") || t.isId("protected")) {
+                    val vis = if (t.isId("private")) Visibility.Private else Visibility.Protected
+                    val mark = cc.savePos()
+                    cc.next() // consume modifier
+                    if (cc.skipWsTokens().isId("set")) {
+                        cc.next() // consume 'set'
+                        setterVisibility = vis
+                        if (cc.skipWsTokens().type == Token.Type.LPAREN) {
+                            cc.next() // consume '('
+                            val setArg = cc.requireToken(Token.Type.ID, "Expected setter argument name")
+                            cc.requireToken(Token.Type.RPAREN)
+                            setter = if (cc.peekNextNonWhitespace().type == Token.Type.LBRACE) {
+                                cc.skipWsTokens()
+                                val body = parseBlock()
+                                statement(body.pos) { scope ->
+                                    val value = scope.args.list.firstOrNull() ?: ObjNull
+                                    scope.addItem(setArg.value, true, value, recordType = ObjRecord.Type.Argument)
+                                    body.execute(scope)
+                                }
+                            } else if (cc.peekNextNonWhitespace().type == Token.Type.ASSIGN) {
+                                cc.skipWsTokens()
+                                cc.next() // consume '='
+                                val expr = parseExpression() ?: throw ScriptError(
+                                    cc.current().pos,
+                                    "Expected setter expression"
+                                )
+                                val st = (expr as? Statement) ?: statement(expr.pos) { s -> expr.execute(s) }
+                                statement(st.pos) { scope ->
+                                    val value = scope.args.list.firstOrNull() ?: ObjNull
+                                    scope.addItem(setArg.value, true, value, recordType = ObjRecord.Type.Argument)
+                                    st.execute(scope)
+                                }
+                            } else {
+                                throw ScriptError(cc.current().pos, "Expected { or = after set(...)")
+                            }
+                        }
+                    } else {
+                        cc.restorePos(mark)
+                        break
+                    }
                 } else break
             }
             if (getter != null || setter != null) {
@@ -2817,11 +2891,13 @@ class Compiler(
                         throw ScriptError(start, "var property must have both get() and set()")
                     }
                 } else {
-                    if (setter != null)
+                    if (setter != null || setterVisibility != null)
-                        throw ScriptError(start, "val property cannot have a setter (name: $name)")
+                        throw ScriptError(start, "val property cannot have a setter or restricted visibility set (name: $name)")
                     if (getter == null)
                         throw ScriptError(start, "val property with accessors must have a getter (name: $name)")
                 }
+            } else if (setterVisibility != null && !isMutable) {
+                throw ScriptError(start, "val field cannot have restricted visibility set (name: $name)")
             }
         }
 
@@ -2838,7 +2914,7 @@ class Compiler(
                 val type = context[extTypeName]?.value ?: context.raiseSymbolNotFound("class $extTypeName not found")
                 if (type !is ObjClass) context.raiseClassCastError("$extTypeName is not the class instance")
 
-                context.addExtension(type, name, ObjRecord(prop, isMutable = false, visibility = visibility, declaringClass = null, type = ObjRecord.Type.Property))
+                context.addExtension(type, name, ObjRecord(prop, isMutable = false, visibility = visibility, writeVisibility = setterVisibility, declaringClass = null, type = ObjRecord.Type.Property))
 
                 return@statement prop
             }
@@ -2872,6 +2948,7 @@ class Compiler(
                             isMutable,
                             prop,
                             visibility,
+                            setterVisibility,
                             recordType = ObjRecord.Type.Property
                         )
                         ObjVoid
@@ -2879,38 +2956,41 @@ class Compiler(
                     ObjVoid
                 } else {
                     // We are in instance scope already: perform initialization immediately
-                    context.addItem(storageName, isMutable, prop, visibility, recordType = ObjRecord.Type.Property)
+                    context.addItem(storageName, isMutable, prop, visibility, setterVisibility, recordType = ObjRecord.Type.Property)
                     prop
                 }
             } else {
-                if (declaringClassName != null && !isStatic) {
+                    val isLateInitVal = !isMutable && initialExpression == null && getter == null && setter == null
-                    val storageName = "$declaringClassName::$name"
+                    if (declaringClassName != null && !isStatic) {
-                    // If we are in class scope now (defining instance field), defer initialization to instance time
+                        val storageName = "$declaringClassName::$name"
-                    val isClassScope = context.thisObj is ObjClass && (context.thisObj !is ObjInstance)
+                        // If we are in class scope now (defining instance field), defer initialization to instance time
-                    if (isClassScope) {
+                        val isClassScope = context.thisObj is ObjClass && (context.thisObj !is ObjInstance)
-                        val cls = context.thisObj as ObjClass
+                        if (isClassScope) {
-                        // Defer: at instance construction, evaluate initializer in instance scope and store under mangled name
+                            val cls = context.thisObj as ObjClass
-                        val initStmt = statement(start) { scp ->
+                            // Defer: at instance construction, evaluate initializer in instance scope and store under mangled name
-                            val initValue = initialExpression?.execute(scp)?.byValueCopy() ?: ObjNull
+                            val initStmt = statement(start) { scp ->
-                            // Preserve mutability of declaration: do NOT use addOrUpdateItem here, as it creates mutable records
+                                val initValue =
-                            scp.addItem(storageName, isMutable, initValue, visibility, recordType = ObjRecord.Type.Field)
+                                    initialExpression?.execute(scp)?.byValueCopy() ?: if (isLateInitVal) ObjUnset else ObjNull
+                                // Preserve mutability of declaration: do NOT use addOrUpdateItem here, as it creates mutable records
+                                scp.addItem(storageName, isMutable, initValue, visibility, setterVisibility, recordType = ObjRecord.Type.Field)
+                                ObjVoid
+                            }
+                            cls.instanceInitializers += initStmt
                             ObjVoid
+                        } else {
+                            // We are in instance scope already: perform initialization immediately
+                            val initValue =
+                                initialExpression?.execute(context)?.byValueCopy() ?: if (isLateInitVal) ObjUnset else ObjNull
+                            // Preserve mutability of declaration: create record with correct mutability
+                            context.addItem(storageName, isMutable, initValue, visibility, setterVisibility, recordType = ObjRecord.Type.Field)
+                            initValue
                         }
-                        cls.instanceInitializers += initStmt
-                        ObjVoid
                     } else {
-                        // We are in instance scope already: perform initialization immediately
+                        // Not in class body: regular local/var declaration
                         val initValue = initialExpression?.execute(context)?.byValueCopy() ?: ObjNull
-                        // Preserve mutability of declaration: create record with correct mutability
+                        context.addItem(name, isMutable, initValue, visibility, recordType = ObjRecord.Type.Field)
-                        context.addItem(storageName, isMutable, initValue, visibility, recordType = ObjRecord.Type.Field)
                         initValue
                     }
-                } else {
-                    // Not in class body: regular local/var declaration
-                    val initValue = initialExpression?.execute(context)?.byValueCopy() ?: ObjNull
-                    context.addItem(name, isMutable, initValue, visibility, recordType = ObjRecord.Type.Field)
-                    initValue
-                }
             }
         }
     }
@@ -3213,7 +3293,10 @@ class Compiler(
          * The keywords that stop processing of expression term
          */
         val stopKeywords =
-            setOf("do", "break", "continue", "return", "if", "when", "do", "while", "for", "class")
+            setOf(
+                "break", "continue", "return", "if", "when", "do", "while", "for", "class",
+                "private", "protected", "val", "var", "fun", "fn", "static", "init", "enum"
+            )
     }
 }
 

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Scope.kt

@@ -262,6 +262,9 @@ open class Scope(
 
     fun raiseClassCastError(msg: String): Nothing = raiseError(ObjClassCastException(this, msg))
 
+    fun raiseUnset(message: String = "property is unset (not initialized)"): Nothing =
+        raiseError(ObjUnsetException(this, message))
+
     @Suppress("unused")
     fun raiseSymbolNotFound(name: String): Nothing =
         raiseError(ObjSymbolNotDefinedException(this, "symbol is not defined: $name"))
@@ -431,6 +434,7 @@ open class Scope(
         name: String,
         value: Obj,
         visibility: Visibility = Visibility.Public,
+        writeVisibility: Visibility? = null,
         recordType: ObjRecord.Type = ObjRecord.Type.Other
     ): ObjRecord =
         objects[name]?.let {
@@ -445,17 +449,18 @@ open class Scope(
                 callScope.localBindings[name] = it
             }
             it
-        } ?: addItem(name, true, value, visibility, recordType)
+        } ?: addItem(name, true, value, visibility, writeVisibility, recordType)
 
     fun addItem(
         name: String,
         isMutable: Boolean,
         value: Obj,
         visibility: Visibility = Visibility.Public,
+        writeVisibility: Visibility? = null,
         recordType: ObjRecord.Type = ObjRecord.Type.Other,
         declaringClass: net.sergeych.lyng.obj.ObjClass? = currentClassCtx
     ): ObjRecord {
-        val rec = ObjRecord(value, isMutable, visibility, declaringClass = declaringClass, type = recordType)
+        val rec = ObjRecord(value, isMutable, visibility, writeVisibility, declaringClass = declaringClass, type = recordType)
         objects[name] = rec
         // Index this binding within the current frame to help resolve locals across suspension
         localBindings[name] = rec

lynglib/src/commonMain/kotlin/net/sergeych/lyng/Script.kt

@@ -60,6 +60,7 @@ class Script(
 
         internal val rootScope: Scope = Scope(null).apply {
             ObjException.addExceptionsToContext(this)
+            addConst("Unset", ObjUnset)
             addFn("print") {
                 for ((i, a) in args.withIndex()) {
                     if (i > 0) print(' ' + a.toString(this).value)

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/Obj.kt

@@ -416,7 +416,7 @@ open class Obj {
 
         val decl = field.declaringClass
         val caller = scope.currentClassCtx
-        if (!canAccessMember(field.visibility, decl, caller))
+        if (!canAccessMember(field.effectiveWriteVisibility, decl, caller))
             scope.raiseError(ObjAccessException(scope, "can't assign field ${name}: not visible (declared in ${decl?.className ?: "?"}, caller ${caller?.className ?: "?"})"))
         if (field.value is ObjProperty) {
             (field.value as ObjProperty).callSetter(scope, this, newValue, decl)
@@ -665,6 +665,50 @@ object ObjNull : Obj() {
     }
 }
 
+@Serializable
+@SerialName("unset")
+object ObjUnset : Obj() {
+    override suspend fun compareTo(scope: Scope, other: Obj): Int = if (other === this) 0 else -1
+    override fun equals(other: Any?): Boolean = other === this
+    override fun toString(): String = "Unset"
+
+    override suspend fun readField(scope: Scope, name: String): ObjRecord = scope.raiseUnset()
+    override suspend fun invokeInstanceMethod(
+        scope: Scope,
+        name: String,
+        args: Arguments,
+        onNotFoundResult: (suspend () -> Obj?)?
+    ): Obj = scope.raiseUnset()
+
+    override suspend fun getAt(scope: Scope, index: Obj): Obj = scope.raiseUnset()
+    override suspend fun putAt(scope: Scope, index: Obj, newValue: Obj) = scope.raiseUnset()
+    override suspend fun callOn(scope: Scope): Obj = scope.raiseUnset()
+    override suspend fun plus(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun minus(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun negate(scope: Scope): Obj = scope.raiseUnset()
+    override suspend fun mul(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun div(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun mod(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun logicalNot(scope: Scope): Obj = scope.raiseUnset()
+    override suspend fun logicalAnd(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun logicalOr(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun operatorMatch(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun bitAnd(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun bitOr(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun bitXor(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun shl(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun shr(scope: Scope, other: Obj): Obj = scope.raiseUnset()
+    override suspend fun bitNot(scope: Scope): Obj = scope.raiseUnset()
+
+    override val objClass: ObjClass by lazy {
+        object : ObjClass("Unset") {
+            override suspend fun deserialize(scope: Scope, decoder: LynonDecoder, lynonType: LynonType?): Obj {
+                return ObjUnset
+            }
+        }
+    }
+}
+
 /**
  * TODO: get rid of it. Maybe we ise some Lyng inheritance instead
  */

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjClass.kt

@@ -345,6 +345,7 @@ open class ObjClass(
         initialValue: Obj,
         isMutable: Boolean = false,
         visibility: Visibility = Visibility.Public,
+        writeVisibility: Visibility? = null,
         pos: Pos = Pos.builtIn,
         declaringClass: ObjClass? = this
     ) {
@@ -353,7 +354,7 @@ open class ObjClass(
         if (existingInSelf != null && existingInSelf.isMutable == false)
             throw ScriptError(pos, "$name is already defined in $objClass")
         // Install/override in this class
-        members[name] = ObjRecord(initialValue, isMutable, visibility, declaringClass = declaringClass)
+        members[name] = ObjRecord(initialValue, isMutable, visibility, writeVisibility, declaringClass = declaringClass)
         // Structural change: bump layout version for PIC invalidation
         layoutVersion += 1
     }
@@ -368,13 +369,14 @@ open class ObjClass(
         initialValue: Obj,
         isMutable: Boolean = false,
         visibility: Visibility = Visibility.Public,
+        writeVisibility: Visibility? = null,
         pos: Pos = Pos.builtIn
     ) {
         initClassScope()
         val existing = classScope!!.objects[name]
         if (existing != null)
             throw ScriptError(pos, "$name is already defined in $objClass or one of its supertypes")
-        classScope!!.addItem(name, isMutable, initialValue, visibility)
+        classScope!!.addItem(name, isMutable, initialValue, visibility, writeVisibility)
         // Structural change: bump layout version for PIC invalidation
         layoutVersion += 1
     }
@@ -383,11 +385,12 @@ open class ObjClass(
         name: String,
         isOpen: Boolean = false,
         visibility: Visibility = Visibility.Public,
+        writeVisibility: Visibility? = null,
         declaringClass: ObjClass? = this,
         code: suspend Scope.() -> Obj
     ) {
         val stmt = statement { code() }
-        createField(name, stmt, isOpen, visibility, Pos.builtIn, declaringClass)
+        createField(name, stmt, isOpen, visibility, writeVisibility, Pos.builtIn, declaringClass)
     }
 
     fun addConst(name: String, value: Obj) = createField(name, value, isMutable = false)
@@ -397,12 +400,13 @@ open class ObjClass(
         getter: (suspend Scope.() -> Obj)? = null,
         setter: (suspend Scope.(Obj) -> Unit)? = null,
         visibility: Visibility = Visibility.Public,
+        writeVisibility: Visibility? = null,
         declaringClass: ObjClass? = this
     ) {
         val g = getter?.let { statement { it() } }
         val s = setter?.let { statement { it(requiredArg(0)); ObjVoid } }
         val prop = ObjProperty(name, g, s)
-        members[name] = ObjRecord(prop, false, visibility, declaringClass, type = ObjRecord.Type.Property)
+        members[name] = ObjRecord(prop, false, visibility, writeVisibility, declaringClass, type = ObjRecord.Type.Property)
         layoutVersion += 1
     }
 

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjException.kt

@@ -194,7 +194,9 @@ open class ObjException(
                 "AccessException",
                 "UnknownException",
                 "NotFoundException",
-                "IllegalOperationException"
+                "IllegalOperationException",
+                "UnsetException",
+                "SyntaxError"
             )) {
                 scope.addConst(name, getOrCreateExceptionClass(name))
             }
@@ -245,3 +247,6 @@ class ObjIllegalOperationException(scope: Scope, message: String = "Operation is
 
 class ObjNotFoundException(scope: Scope, message: String = "not found") :
     ObjException("NotFoundException", scope, message)
+
+class ObjUnsetException(scope: Scope, message: String = "property is unset (not initialized)") :
+    ObjException("UnsetException", scope, message)

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjInstance.kt

@@ -97,8 +97,8 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
             val decl = f.declaringClass
             if (scope.thisObj !== this || scope.currentClassCtx == null) {
                 val caller = scope.currentClassCtx
-                if (!canAccessMember(f.visibility, decl, caller))
+                if (!canAccessMember(f.effectiveWriteVisibility, decl, caller))
-                    ObjIllegalAssignmentException(
+                    ObjAccessException(
                         scope,
                         "can't assign to field $name (declared in ${decl?.className ?: "?"})"
                     ).raise()
@@ -108,7 +108,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                 prop.callSetter(scope, this, newValue, decl)
                 return
             }
-            if (!f.isMutable) ObjIllegalAssignmentException(scope, "can't reassign val $name").raise()
+            if (!f.isMutable && f.value !== ObjUnset) ObjIllegalAssignmentException(scope, "can't reassign val $name").raise()
             if (f.value.assign(scope, newValue) == null)
                 f.value = newValue
             return
@@ -131,8 +131,8 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
             }
             if (scope.thisObj !== this || scope.currentClassCtx == null) {
                 val caller = scope.currentClassCtx
-                if (!canAccessMember(rec.visibility, declaring, caller))
+                if (!canAccessMember(rec.effectiveWriteVisibility, declaring, caller))
-                    ObjIllegalAssignmentException(
+                    ObjAccessException(
                         scope,
                         "can't assign to field $name (declared in ${declaring?.className ?: "?"})"
                     ).raise()
@@ -142,7 +142,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                 prop.callSetter(scope, this, newValue, declaring)
                 return
             }
-            if (!rec.isMutable) ObjIllegalAssignmentException(scope, "can't reassign val $name").raise()
+            if (!rec.isMutable && rec.value !== ObjUnset) ObjIllegalAssignmentException(scope, "can't reassign val $name").raise()
             if (rec.value.assign(scope, newValue) == null)
                 rec.value = newValue
             return
@@ -350,12 +350,12 @@ class ObjQualifiedView(val instance: ObjInstance, private val startClass: ObjCla
         instance.instanceScope.objects[mangled]?.let { f ->
             val decl = f.declaringClass ?: startClass
             val caller = scope.currentClassCtx
-            if (!canAccessMember(f.visibility, decl, caller))
+            if (!canAccessMember(f.effectiveWriteVisibility, decl, caller))
-                ObjIllegalAssignmentException(
+                ObjAccessException(
                     scope,
                     "can't assign to field $name (declared in ${decl.className})"
                 ).raise()
-            if (!f.isMutable) ObjIllegalAssignmentException(scope, "can't reassign val $name").raise()
+            if (!f.isMutable && f.value !== ObjUnset) ObjIllegalAssignmentException(scope, "can't reassign val $name").raise()
             if (f.value.assign(scope, newValue) == null) f.value = newValue
             return
         }
@@ -364,12 +364,12 @@ class ObjQualifiedView(val instance: ObjInstance, private val startClass: ObjCla
             instance.instanceScope[name]?.let { f ->
                 val decl = f.declaringClass ?: instance.objClass.findDeclaringClassOf(name)
                 val caller = scope.currentClassCtx
-                if (!canAccessMember(f.visibility, decl, caller))
+                if (!canAccessMember(f.effectiveWriteVisibility, decl, caller))
-                    ObjIllegalAssignmentException(
+                    ObjAccessException(
                         scope,
                         "can't assign to field $name (declared in ${decl?.className ?: "?"})"
                     ).raise()
-                if (!f.isMutable) ObjIllegalAssignmentException(scope, "can't reassign val $name").raise()
+                if (!f.isMutable && f.value !== ObjUnset) ObjIllegalAssignmentException(scope, "can't reassign val $name").raise()
                 if (f.value.assign(scope, newValue) == null) f.value = newValue
                 return
             }
@@ -377,8 +377,8 @@ class ObjQualifiedView(val instance: ObjInstance, private val startClass: ObjCla
         val r = memberFromAncestor(name) ?: scope.raiseError("no such field: $name")
         val decl = r.declaringClass ?: startClass
         val caller = scope.currentClassCtx
-        if (!canAccessMember(r.visibility, decl, caller))
+        if (!canAccessMember(r.effectiveWriteVisibility, decl, caller))
-            ObjIllegalAssignmentException(scope, "can't assign to field $name (declared in ${decl.className})").raise()
+            ObjAccessException(scope, "can't assign to field $name (declared in ${decl.className})").raise()
         if (!r.isMutable) scope.raiseError("can't assign to read-only field: $name")
         if (r.value.assign(scope, newValue) == null) r.value = newValue
     }

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRecord.kt

@@ -26,12 +26,14 @@ data class ObjRecord(
     var value: Obj,
     val isMutable: Boolean,
     val visibility: Visibility = Visibility.Public,
+    val writeVisibility: Visibility? = null,
     /** If non-null, denotes the class that declared this member (field/method). */
     val declaringClass: ObjClass? = null,
     var importedFrom: Scope? = null,
     val isTransient: Boolean = false,
     val type: Type = Type.Other,
 ) {
+    val effectiveWriteVisibility: Visibility get() = writeVisibility ?: visibility
     enum class Type(val comparable: Boolean = false,val serializable: Boolean = false) {
         Field(true, true),
         @Suppress("unused")

lynglib/src/commonMain/kotlin/net/sergeych/lyng/obj/ObjRef.kt

@@ -442,9 +442,9 @@ class ConstRef(private val record: ObjRecord) : ObjRef {
  * Reference to an object's field with optional chaining.
  */
 class FieldRef(
-    private val target: ObjRef,
+    val target: ObjRef,
-    private val name: String,
+    val name: String,
-    private val isOptional: Boolean,
+    val isOptional: Boolean,
 ) : ObjRef {
     // 4-entry PIC for reads/writes (guarded by PerfFlags.FIELD_PIC)
     // Reads

lynglib/src/commonTest/kotlin/OOTest.kt

@@ -418,4 +418,101 @@ class OOTest {
             
         """.trimIndent())
     }
+
+    @Test
+    fun testLateInitValsInClasses() = runTest {
+        assertFails {
+            eval("""
+                class T {
+                    val x
+                }
+            """)
+        }
+
+        assertFails {
+            eval("val String.late")
+        }
+
+        eval("""
+            // but we can "late-init" them in init block:
+            class OK {
+                val x
+                
+                init {
+                    x = "foo"
+                }
+            }
+            val ok = OK()
+            assertEquals("foo", ok.x)
+            
+            // they can't be reassigned:
+            assertThrows(IllegalAssignmentException) {
+                ok.x = "bar"
+            }
+            
+            // To test access before init, we need a trick:
+            class AccessBefore {
+                val x
+                fun readX() = x
+                init {
+                    assertEquals(x, Unset)
+                    // if we call readX() here, x is Unset. 
+                    // Just reading it is fine, but using it should throw:
+                    assertThrows(UnsetException) { readX() + 1 }
+                    x = 42
+                }
+            }
+            AccessBefore()
+        """.trimIndent())
+    }
+
+    @Test
+    fun testPrivateSet() = runTest {
+        eval("""
+            class A {
+                var y = 100
+                    private set
+                fun setValue(newValue) { y = newValue }
+            }
+            assertEquals(100, A().y)
+            assertThrows(AccessException) { A().y = 200 }
+            val a = A()
+            a.setValue(200)
+            assertEquals(200, a.y)
+            
+            class B(initial) {
+                var y = initial
+                    protected set
+            }
+            class C(initial) : B(initial) {
+                fun setBValue(v) { y = v }
+            }
+            val c = C(10)
+            assertEquals(10, c.y)
+            assertThrows(AccessException) { c.y = 20 }
+            c.setBValue(30)
+            assertEquals(30, c.y)
+            
+            class D {
+                private var _y = 0
+                var y 
+                    get() = _y
+                    private set(v) { _y = v }
+                fun setY(v) { y = v }
+            }
+            val d = D()
+            assertEquals(0, d.y)
+            assertThrows(AccessException) { d.y = 10 }
+            d.setY(20)
+            assertEquals(20, d.y)
+        """)
+    }
+
+    @Test
+    fun testValPrivateSetError() = runTest {
+        assertFails {
+            eval("class E { val x = 1 private set }")
+        }
+    }
+
 }