Add lyng.observable ObservableList hooks, flow events, and docs

docs/Collection.md

@@ -11,6 +11,7 @@ Concrete collection classes:
 
 - Mutable: [List], [Set], [Map]
 - Immutable: [ImmutableList], [ImmutableSet], [ImmutableMap]
+- Observable mutable lists (opt-in module): [ObservableList]
 
 | name                   | description                                          |
 |------------------------|------------------------------------------------------|
@@ -27,3 +28,4 @@ See [List], [Set], [Iterable] and [Efficient Iterables in Kotlin Interop](Effici
 [ImmutableList]: ImmutableList.md
 [ImmutableSet]: ImmutableSet.md
 [ImmutableMap]: ImmutableMap.md
+[ObservableList]: ObservableList.md

docs/List.md

@@ -2,6 +2,7 @@
 
 Mutable list of any objects.
 For immutable list values, see [ImmutableList].
+For observable mutable lists and change hooks, see [ObservableList].
 
 It's class in Lyng is `List`:
 
@@ -180,6 +181,50 @@ for `sort()` will be `sort { a, b -> a <=> b }
 
 It inherits from [Iterable] too and thus all iterable methods are applicable to any list.
 
+## Observable list hooks
+
+Observable hooks are provided by module `lyng.observable` and are opt-in:
+
+    import lyng.observable
+
+    val src = [1,2,3]
+    val xs = src.observable()
+    assert(xs is ObservableList<Int>)
+
+    var before = 0
+    var after = 0
+    xs.beforeChange { before++ }
+    xs.onChange { after++ }
+
+    xs += 4
+    xs[0] = 100
+    assertEquals([100,2,3,4], xs)
+    assertEquals(2, before)
+    assertEquals(2, after)
+    >>> void
+
+`beforeChange` runs before mutation commit and may reject it by throwing exception (typically `ChangeRejectionException` from the same module):
+
+    import lyng.observable
+    val xs = [1,2].observable()
+    xs.beforeChange { throw ChangeRejectionException("read only") }
+    assertThrows(ChangeRejectionException) { xs += 3 }
+    assertEquals([1,2], xs)
+    >>> void
+
+`changes()` returns `Flow<ListChange<T>>` of committed events:
+
+    import lyng.observable
+    val xs = [10,20].observable()
+    val it = xs.changes().iterator()
+    xs += 30
+    assert(it.hasNext())
+    val e = it.next()
+    assert(e is ListInsert<Int>)
+    assertEquals([30], (e as ListInsert<Int>).values)
+    it.cancelIteration()
+    >>> void
+
 ## Member inherited from Array
 
 | name             | meaning                        | type  |
@@ -199,3 +244,4 @@ It inherits from [Iterable] too and thus all iterable methods are applicable to
 
 [Iterable]: Iterable.md
 [ImmutableList]: ImmutableList.md
+[ObservableList]: ObservableList.md

docs/ObservableList.md

@@ -0,0 +1,70 @@
+# ObservableList module
+
+`ObservableList` lives in explicit module `lyng.observable`.
+
+Import it first:
+
+    import lyng.observable
+    >>> void
+
+Create from a regular mutable list:
+
+    import lyng.observable
+    val xs = [1,2,3].observable()
+    assert(xs is ObservableList<Int>)
+    assertEquals([1,2,3], xs)
+    >>> void
+
+## Hook flow
+
+Event order is:
+1. `beforeChange(change)` listeners
+2. mutation commit
+3. `onChange(change)` listeners
+4. `changes()` flow emission
+
+Rejection is done by throwing in `beforeChange`.
+
+    import lyng.observable
+    val xs = [1,2].observable()
+    xs.beforeChange {
+        throw ChangeRejectionException("no mutation")
+    }
+    assertThrows(ChangeRejectionException) { xs += 3 }
+    assertEquals([1,2], xs)
+    >>> void
+
+## Subscriptions
+
+`beforeChange` and `onChange` return `Subscription`.
+Call `cancel()` to unsubscribe.
+
+    import lyng.observable
+    val xs = [1].observable()
+    var hits = 0
+    val sub = xs.onChange { hits++ }
+    xs += 2
+    sub.cancel()
+    xs += 3
+    assertEquals(1, hits)
+    >>> void
+
+## Change events
+
+`changes()` returns `Flow<ListChange<T>>` with concrete event classes:
+- `ListInsert`
+- `ListSet`
+- `ListRemove`
+- `ListClear`
+- `ListReorder`
+
+    import lyng.observable
+    val xs = [10,20].observable()
+    val it = xs.changes().iterator()
+    xs[1] = 200
+    val ev = it.next()
+    assert(ev is ListSet<Int>)
+    assertEquals(20, (ev as ListSet<Int>).oldValue)
+    assertEquals(200, ev.newValue)
+    it.cancelIteration()
+    >>> void

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

@@ -4704,6 +4704,22 @@ class Compiler(
             name == "toImmutableMap" && receiver is TypeDecl.Generic && base == "Iterable" -> {
                 TypeDecl.Generic("ImmutableMap", listOf(TypeDecl.TypeAny, TypeDecl.TypeAny), false)
             }
+            name == "observable" && receiver is TypeDecl.Generic && (base == "List" || base == "ObservableList") -> {
+                val arg = receiver.args.firstOrNull() ?: TypeDecl.TypeAny
+                TypeDecl.Generic("ObservableList", listOf(arg), false)
+            }
+            name == "changes" && receiver is TypeDecl.Generic && base == "ObservableList" -> {
+                val arg = receiver.args.firstOrNull() ?: TypeDecl.TypeAny
+                TypeDecl.Generic("Flow", listOf(TypeDecl.Generic("ListChange", listOf(arg), false)), false)
+            }
+            name == "changes" && receiver is TypeDecl.Generic && base == "Observable" -> {
+                val arg = receiver.args.firstOrNull() ?: TypeDecl.TypeAny
+                TypeDecl.Generic("Flow", listOf(arg), false)
+            }
+            (name == "beforeChange" || name == "onChange") && receiver is TypeDecl.Generic &&
+                (base == "Observable" || base == "ObservableList") -> {
+                TypeDecl.Simple("Subscription", false)
+            }
             name == "toMap" && receiver is TypeDecl.Generic && base == "Iterable" -> {
                 TypeDecl.Generic("Map", listOf(TypeDecl.TypeAny, TypeDecl.TypeAny), false)
             }
@@ -4794,6 +4810,9 @@ class Compiler(
         "toImmutableList" -> ObjImmutableList.type
         "toImmutableSet" -> ObjImmutableSet.type
         "toImmutableMap" -> ObjImmutableMap.type
+        "observable" -> ObjObservableList.type
+        "beforeChange", "onChange" -> ObjSubscription.type
+        "changes" -> ObjFlow.type
         "toImmutable" -> Obj.rootObjectType
         "toMutable" -> ObjMutableBuffer.type
         "seq" -> ObjFlow.type
@@ -8322,6 +8341,16 @@ class Compiler(
             "Regex" -> ObjRegex.type
             "RegexMatch" -> ObjRegexMatch.type
             "MapEntry" -> ObjMapEntry.type
+            "Observable" -> ObjObservable
+            "Subscription" -> ObjSubscription.type
+            "ListChange" -> ObjListChange.type
+            "ListSet" -> ObjListSetChange.type
+            "ListInsert" -> ObjListInsertChange.type
+            "ListRemove" -> ObjListRemoveChange.type
+            "ListClear" -> ObjListClearChange.type
+            "ListReorder" -> ObjListReorderChange.type
+            "ObservableList" -> ObjObservableList.type
+            "ChangeRejectionException" -> ObjChangeRejectionExceptionClass
             "Exception" -> ObjException.Root
             "Instant" -> ObjInstant.type
             "DateTime" -> ObjDateTime.type

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

@@ -25,6 +25,7 @@ import net.sergeych.lyng.bytecode.CmdVm
 import net.sergeych.lyng.miniast.*
 import net.sergeych.lyng.obj.*
 import net.sergeych.lyng.pacman.ImportManager
+import net.sergeych.lyng.stdlib_included.observableLyng
 import net.sergeych.lyng.stdlib_included.rootLyng
 import net.sergeych.lynon.ObjLynonClass
 import net.sergeych.mp_tools.globalDefer
@@ -593,6 +594,19 @@ class Script(
                     module.eval(Source("lyng.stdlib", rootLyng))
                     ObjKotlinIterator.bindTo(module.requireClass("KotlinIterator"))
                 }
+                addPackage("lyng.observable") { module ->
+                    module.addConst("Observable", ObjObservable)
+                    module.addConst("Subscription", ObjSubscription.type)
+                    module.addConst("ListChange", ObjListChange.type)
+                    module.addConst("ListSet", ObjListSetChange.type)
+                    module.addConst("ListInsert", ObjListInsertChange.type)
+                    module.addConst("ListRemove", ObjListRemoveChange.type)
+                    module.addConst("ListClear", ObjListClearChange.type)
+                    module.addConst("ListReorder", ObjListReorderChange.type)
+                    module.addConst("ObservableList", ObjObservableList.type)
+                    module.addConst("ChangeRejectionException", ObjChangeRejectionExceptionClass)
+                    module.eval(Source("lyng.observable", observableLyng))
+                }
                 addPackage("lyng.buffer") {
                     it.addConstDoc(
                         name = "Buffer",

lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt

@@ -4440,6 +4440,7 @@ class BytecodeCompiler(
         val initClass = when (localTarget?.name) {
             "List" -> ObjList.type
             "ImmutableList" -> ObjImmutableList.type
+            "ObservableList" -> ObjObservableList.type
             "Map" -> ObjMap.type
             "ImmutableMap" -> ObjImmutableMap.type
             "Set" -> ObjSet.type
@@ -7326,6 +7327,16 @@ class BytecodeCompiler(
             "Regex" -> ObjRegex.type
             "RegexMatch" -> ObjRegexMatch.type
             "MapEntry" -> ObjMapEntry.type
+            "Observable" -> ObjObservable
+            "Subscription" -> ObjSubscription.type
+            "ListChange" -> ObjListChange.type
+            "ListSet" -> ObjListSetChange.type
+            "ListInsert" -> ObjListInsertChange.type
+            "ListRemove" -> ObjListRemoveChange.type
+            "ListClear" -> ObjListClearChange.type
+            "ListReorder" -> ObjListReorderChange.type
+            "ObservableList" -> ObjObservableList.type
+            "ChangeRejectionException" -> ObjChangeRejectionExceptionClass
             "Instant" -> ObjInstant.type
             "DateTime" -> ObjDateTime.type
             "Duration" -> ObjDuration.type
@@ -7389,6 +7400,9 @@ class BytecodeCompiler(
         "toImmutableSet" -> ObjImmutableSet.type
         "toMap" -> ObjMap.type
         "toImmutableMap" -> ObjImmutableMap.type
+        "observable" -> ObjObservableList.type
+        "beforeChange", "onChange" -> ObjSubscription.type
+        "changes" -> ObjFlow.type
         "joinToString" -> ObjString.type
         "now",
         "truncateToSecond",

lynglib/src/commonMain/kotlin/net/sergeych/lyng/miniast/StdlibDocsBootstrap.kt

@@ -47,6 +47,10 @@ object StdlibDocsBootstrap {
             @Suppress("UNUSED_VARIABLE")
             val _immutableSet = net.sergeych.lyng.obj.ObjImmutableSet.type
             @Suppress("UNUSED_VARIABLE")
+            val _observableList = net.sergeych.lyng.obj.ObjObservableList.type
+            @Suppress("UNUSED_VARIABLE")
+            val _subscription = net.sergeych.lyng.obj.ObjSubscription.type
+            @Suppress("UNUSED_VARIABLE")
             val _int = net.sergeych.lyng.obj.ObjInt.type
             @Suppress("UNUSED_VARIABLE")
             val _real = net.sergeych.lyng.obj.ObjReal.type

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

@@ -29,8 +29,8 @@ import net.sergeych.lynon.LynonDecoder
 import net.sergeych.lynon.LynonEncoder
 import net.sergeych.lynon.LynonType
 
-class ObjList(val list: MutableList<Obj> = mutableListOf()) : Obj() {
+open class ObjList(val list: MutableList<Obj> = mutableListOf()) : Obj() {
-    private fun shouldTreatAsSingleElement(scope: Scope, other: Obj): Boolean {
+    protected open fun shouldTreatAsSingleElement(scope: Scope, other: Obj): Boolean {
         if (!other.isInstanceOf(ObjIterable)) return true
         val declaredElementType = scope.declaredListElementTypeForValue(this)
         if (declaredElementType != null && matchesTypeDecl(scope, other, declaredElementType)) {
@@ -95,7 +95,7 @@ class ObjList(val list: MutableList<Obj> = mutableListOf()) : Obj() {
         }
     }
 
-    override suspend fun putAt(scope: Scope, index: Obj, newValue: Obj) {
+    open override suspend fun putAt(scope: Scope, index: Obj, newValue: Obj) {
         list[index.toInt()] = newValue
     }
 
@@ -149,7 +149,7 @@ class ObjList(val list: MutableList<Obj> = mutableListOf()) : Obj() {
         }
 
 
-    override suspend fun plusAssign(scope: Scope, other: Obj): Obj {
+    open override suspend fun plusAssign(scope: Scope, other: Obj): Obj {
         if (other is ObjList) {
             list.addAll(other.list)
         } else if (!shouldTreatAsSingleElement(scope, other) && other.isInstanceOf(ObjIterable)) {
@@ -180,7 +180,7 @@ class ObjList(val list: MutableList<Obj> = mutableListOf()) : Obj() {
         return ObjList(out)
     }
 
-    override suspend fun minusAssign(scope: Scope, other: Obj): Obj {
+    open override suspend fun minusAssign(scope: Scope, other: Obj): Obj {
         if (shouldTreatAsSingleElement(scope, other)) {
             list.remove(other)
             return this
@@ -221,7 +221,7 @@ class ObjList(val list: MutableList<Obj> = mutableListOf()) : Obj() {
         }
     }
 
-    override val objClass: ObjClass
+    open override val objClass: ObjClass
         get() = type
 
     override suspend fun toKotlin(scope: Scope): Any {

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

@@ -0,0 +1,589 @@
+/*
+ * Copyright 2026 Sergey S. Chernov real.sergeych@gmail.com
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package net.sergeych.lyng.obj
+
+import kotlinx.coroutines.channels.Channel
+import kotlinx.coroutines.channels.ReceiveChannel
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
+import net.sergeych.lyng.Arguments
+import net.sergeych.lyng.Scope
+import net.sergeych.lyng.asFacade
+import net.sergeych.lyng.miniast.ParamDoc
+import net.sergeych.lyng.miniast.TypeGenericDoc
+import net.sergeych.lyng.miniast.addFnDoc
+import net.sergeych.lyng.miniast.addPropertyDoc
+import net.sergeych.lyng.miniast.type
+
+val ObjObservable = ObjClass("Observable").apply {
+    addFn("beforeChange") { raiseNotImplemented("Observable.beforeChange") }
+    addFn("onChange") { raiseNotImplemented("Observable.onChange") }
+    addFn("changes") { raiseNotImplemented("Observable.changes") }
+}
+
+val ObjChangeRejectionExceptionClass = ObjException.Companion.ExceptionClass("ChangeRejectionException", ObjException.Root)
+
+open class ObjListChange : Obj() {
+    override val objClass: ObjClass get() = type
+
+    companion object {
+        val type = ObjClass("ListChange")
+    }
+}
+
+class ObjListSetChange(val index: Int, val oldValue: Obj, val newValue: Obj) : ObjListChange() {
+    override val objClass: ObjClass get() = type
+
+    companion object {
+        val type = ObjClass("ListSet", ObjListChange.type).apply {
+            addPropertyDoc("index", "Changed index.", type("lyng.Int"), moduleName = "lyng.observable", getter = { thisAs<ObjListSetChange>().index.toObj() })
+            addPropertyDoc("oldValue", "Value before assignment.", type("lyng.Any"), moduleName = "lyng.observable", getter = { thisAs<ObjListSetChange>().oldValue })
+            addPropertyDoc("newValue", "Assigned value.", type("lyng.Any"), moduleName = "lyng.observable", getter = { thisAs<ObjListSetChange>().newValue })
+        }
+    }
+}
+
+class ObjListInsertChange(val index: Int, val values: ObjList) : ObjListChange() {
+    override val objClass: ObjClass get() = type
+
+    companion object {
+        val type = ObjClass("ListInsert", ObjListChange.type).apply {
+            addPropertyDoc("index", "Insertion index.", type("lyng.Int"), moduleName = "lyng.observable", getter = { thisAs<ObjListInsertChange>().index.toObj() })
+            addPropertyDoc(
+                "values",
+                "Inserted values.",
+                TypeGenericDoc(type("lyng.List"), listOf(type("lyng.Any"))),
+                moduleName = "lyng.observable",
+                getter = { thisAs<ObjListInsertChange>().values }
+            )
+        }
+    }
+}
+
+class ObjListRemoveChange(val index: Int, val oldValue: Obj) : ObjListChange() {
+    override val objClass: ObjClass get() = type
+
+    companion object {
+        val type = ObjClass("ListRemove", ObjListChange.type).apply {
+            addPropertyDoc("index", "Removed index.", type("lyng.Int"), moduleName = "lyng.observable", getter = { thisAs<ObjListRemoveChange>().index.toObj() })
+            addPropertyDoc("oldValue", "Removed value.", type("lyng.Any"), moduleName = "lyng.observable", getter = { thisAs<ObjListRemoveChange>().oldValue })
+        }
+    }
+}
+
+class ObjListClearChange(val oldValues: ObjList) : ObjListChange() {
+    override val objClass: ObjClass get() = type
+
+    companion object {
+        val type = ObjClass("ListClear", ObjListChange.type).apply {
+            addPropertyDoc(
+                "oldValues",
+                "All list values before clear.",
+                TypeGenericDoc(type("lyng.List"), listOf(type("lyng.Any"))),
+                moduleName = "lyng.observable",
+                getter = { thisAs<ObjListClearChange>().oldValues }
+            )
+        }
+    }
+}
+
+class ObjListReorderChange(val oldValues: ObjList, val newValues: Obj) : ObjListChange() {
+    override val objClass: ObjClass get() = type
+
+    companion object {
+        val type = ObjClass("ListReorder", ObjListChange.type).apply {
+            addPropertyDoc(
+                "oldValues",
+                "Values before reorder.",
+                TypeGenericDoc(type("lyng.List"), listOf(type("lyng.Any"))),
+                moduleName = "lyng.observable",
+                getter = { thisAs<ObjListReorderChange>().oldValues }
+            )
+            addPropertyDoc(
+                "newValues",
+                "Values after reorder (or Unset in beforeChange when unknown before commit).",
+                type("lyng.Any"),
+                moduleName = "lyng.observable",
+                getter = { thisAs<ObjListReorderChange>().newValues }
+            )
+        }
+    }
+}
+
+class ObjSubscription(private val onCancel: suspend () -> Unit) : Obj() {
+    private val access = Mutex()
+    private var cancelled = false
+
+    override val objClass: ObjClass get() = type
+
+    suspend fun cancel() {
+        var action: (suspend () -> Unit)? = null
+        access.withLock {
+            if (!cancelled) {
+                cancelled = true
+                action = onCancel
+            }
+        }
+        action?.invoke()
+    }
+
+    companion object {
+        val type = object : ObjClass("Subscription") {
+            override suspend fun callOn(scope: Scope): Obj = scope.raiseIllegalArgument("Subscription constructor is not available")
+        }.apply {
+            addFnDoc(
+                "cancel",
+                "Unsubscribe listener. Safe to call multiple times.",
+                returns = type("lyng.Void"),
+                moduleName = "lyng.observable"
+            ) {
+                thisAs<ObjSubscription>().cancel()
+                ObjVoid
+            }
+        }
+    }
+}
+
+private class ObjObservableListFlowProducer(
+    private val owner: ObjObservableList,
+    private val subscriptionId: Long,
+    private val channel: ReceiveChannel<Obj>
+) : Obj() {
+    override val objClass: ObjClass get() = Obj.rootObjectType
+
+    override suspend fun callOn(scope: Scope): Obj {
+        val builder = scope.thisObj as? ObjFlowBuilder
+            ?: scope.raiseIllegalState("flow builder is not available")
+        try {
+            for (change in channel) {
+                builder.invokeInstanceMethod(scope, "emit", change)
+            }
+        } finally {
+            owner.unsubscribeChangeChannel(subscriptionId)
+        }
+        return ObjVoid
+    }
+}
+
+class ObjObservableList(list: MutableList<Obj> = mutableListOf()) : ObjList(list) {
+    private val access = Mutex()
+    private var nextSubscriptionId: Long = 1L
+    private val beforeListeners = linkedMapOf<Long, Obj>()
+    private val afterListeners = linkedMapOf<Long, Obj>()
+    private val changeChannels = linkedMapOf<Long, Channel<Obj>>()
+
+    override val objClass: ObjClass get() = type
+
+    private fun nextId(): Long = nextSubscriptionId++
+
+    private suspend fun addBeforeListener(listener: Obj): ObjSubscription {
+        val id = access.withLock {
+            val id = nextId()
+            beforeListeners[id] = listener
+            id
+        }
+        return ObjSubscription {
+            access.withLock { beforeListeners.remove(id) }
+        }
+    }
+
+    private suspend fun addAfterListener(listener: Obj): ObjSubscription {
+        val id = access.withLock {
+            val id = nextId()
+            afterListeners[id] = listener
+            id
+        }
+        return ObjSubscription {
+            access.withLock { afterListeners.remove(id) }
+        }
+    }
+
+    private suspend fun notifyListeners(scope: Scope, listeners: Collection<Obj>, change: Obj) {
+        val facade = scope.asFacade()
+        for (listener in listeners) {
+            facade.call(listener, Arguments(change))
+        }
+    }
+
+    private suspend fun snapshotBeforeListeners(): List<Obj> = access.withLock { beforeListeners.values.toList() }
+    private suspend fun snapshotAfterListeners(): List<Obj> = access.withLock { afterListeners.values.toList() }
+
+    private suspend fun notifyBefore(scope: Scope, change: Obj) {
+        notifyListeners(scope, snapshotBeforeListeners(), change)
+    }
+
+    private suspend fun notifyAfter(scope: Scope, change: Obj) {
+        notifyListeners(scope, snapshotAfterListeners(), change)
+    }
+
+    private suspend fun emitCommittedChange(change: Obj) {
+        val channels = access.withLock { changeChannels.toMap() }
+        val dead = mutableListOf<Long>()
+        for ((id, channel) in channels) {
+            if (channel.trySend(change).isFailure) {
+                dead += id
+            }
+        }
+        if (dead.isNotEmpty()) {
+            access.withLock {
+                for (id in dead) {
+                    changeChannels.remove(id)?.close()
+                }
+            }
+        }
+    }
+
+    private suspend fun mutate(scope: Scope, beforeChange: Obj, mutation: suspend () -> Obj?): Obj? {
+        notifyBefore(scope, beforeChange)
+        val committed = mutation()
+        if (committed != null) {
+            notifyAfter(scope, committed)
+            emitCommittedChange(committed)
+        }
+        return committed
+    }
+
+    private fun snapshotList(): ObjList = ObjList(list.toMutableList())
+
+    private suspend fun createChangesFlow(scope: Scope): ObjFlow {
+        val channel = Channel<Obj>(Channel.UNLIMITED)
+        val id = access.withLock {
+            val id = nextId()
+            changeChannels[id] = channel
+            id
+        }
+        val producer = ObjObservableListFlowProducer(this, id, channel)
+        return ObjFlow(producer, scope)
+    }
+
+    internal suspend fun unsubscribeChangeChannel(id: Long) {
+        access.withLock {
+            changeChannels.remove(id)?.close()
+        }
+    }
+
+    override suspend fun putAt(scope: Scope, index: Obj, newValue: Obj) {
+        val idx = index.toInt()
+        val old = list[idx]
+        val change = ObjListSetChange(idx, old, newValue)
+        mutate(scope, change) {
+            list[idx] = newValue
+            change
+        }
+    }
+
+    override suspend fun plusAssign(scope: Scope, other: Obj): Obj {
+        when {
+            other is ObjList -> {
+                if (other.list.isEmpty()) return this
+                val insert = ObjListInsertChange(list.size, ObjList(other.list.toMutableList()))
+                mutate(scope, insert) {
+                    list.addAll(other.list)
+                    insert
+                }
+            }
+
+            !shouldTreatAsSingleElement(scope, other) && other.isInstanceOf(ObjIterable) -> {
+                val otherList = (other.invokeInstanceMethod(scope, "toList") as ObjList).list
+                if (otherList.isEmpty()) return this
+                val insert = ObjListInsertChange(list.size, ObjList(otherList.toMutableList()))
+                mutate(scope, insert) {
+                    list.addAll(otherList)
+                    insert
+                }
+            }
+
+            else -> {
+                val insert = ObjListInsertChange(list.size, ObjList(mutableListOf(other)))
+                mutate(scope, insert) {
+                    list.add(other)
+                    insert
+                }
+            }
+        }
+        return this
+    }
+
+    override suspend fun minusAssign(scope: Scope, other: Obj): Obj {
+        if (shouldTreatAsSingleElement(scope, other)) {
+            val index = list.indexOf(other)
+            if (index < 0) return this
+            val old = list[index]
+            val remove = ObjListRemoveChange(index, old)
+            mutate(scope, remove) {
+                list.removeAt(index)
+                remove
+            }
+            return this
+        }
+        if (other.isInstanceOf(ObjIterable)) {
+            val toRemove = mutableSetOf<Obj>()
+            other.enumerate(scope) {
+                toRemove += it
+                true
+            }
+            val oldValues = snapshotList()
+            val filtered = list.filterNot { toRemove.contains(it) }
+            if (filtered.size == list.size) return this
+            val committed = if (filtered.isEmpty()) {
+                ObjListClearChange(oldValues)
+            } else {
+                ObjListReorderChange(oldValues, ObjList(filtered.toMutableList()))
+            }
+            mutate(scope, committed) {
+                list.clear()
+                list.addAll(filtered)
+                committed
+            }
+            return this
+        }
+        val index = list.indexOf(other)
+        if (index < 0) return this
+        val old = list[index]
+        val remove = ObjListRemoveChange(index, old)
+        mutate(scope, remove) {
+            list.removeAt(index)
+            remove
+        }
+        return this
+    }
+
+    companion object {
+        val type = object : ObjClass("ObservableList", ObjList.type) {
+            override suspend fun callOn(scope: Scope): Obj {
+                return ObjObservableList(scope.args.list.toMutableList())
+            }
+        }.apply {
+            addFnDoc(
+                "beforeChange",
+                "Subscribe to pre-commit list changes. Throw to reject mutation.",
+                params = listOf(ParamDoc("listener", type("lyng.Any"))),
+                returns = type("lyng.observable.Subscription"),
+                moduleName = "lyng.observable"
+            ) {
+                thisAs<ObjObservableList>().addBeforeListener(requireOnlyArg())
+            }
+            addFnDoc(
+                "onChange",
+                "Subscribe to post-commit list changes.",
+                params = listOf(ParamDoc("listener", type("lyng.Any"))),
+                returns = type("lyng.observable.Subscription"),
+                moduleName = "lyng.observable"
+            ) {
+                thisAs<ObjObservableList>().addAfterListener(requireOnlyArg())
+            }
+            addFnDoc(
+                "changes",
+                "Get a flow of committed list changes.",
+                returns = TypeGenericDoc(type("lyng.Flow"), listOf(type("lyng.observable.ListChange"))),
+                moduleName = "lyng.observable"
+            ) {
+                thisAs<ObjObservableList>().createChangesFlow(requireScope())
+            }
+            addFnDoc(
+                "observable",
+                "Observable list is already observable; returns this.",
+                returns = type("lyng.observable.ObservableList"),
+                moduleName = "lyng.observable"
+            ) { thisObj }
+            addFnDoc(
+                "add",
+                "Append one or more elements and emit ListInsert.",
+                moduleName = "lyng.observable"
+            ) {
+                val self = thisAs<ObjObservableList>()
+                if (args.isEmpty()) return@addFnDoc ObjVoid
+                val inserted = ObjList(args.list.toMutableList())
+                val change = ObjListInsertChange(self.list.size, inserted)
+                self.mutate(requireScope(), change) {
+                    self.list.addAll(args.list)
+                    change
+                }
+                ObjVoid
+            }
+            addFnDoc(
+                "insertAt",
+                "Insert one or more values at index and emit ListInsert.",
+                params = listOf(ParamDoc("index", type("lyng.Int"))),
+                moduleName = "lyng.observable"
+            ) {
+                if (args.size < 2) raiseError("insertAt takes 2+ arguments")
+                val self = thisAs<ObjObservableList>()
+                val index = requiredArg<ObjInt>(0).value.toInt()
+                val values = args.list.drop(1)
+                val change = ObjListInsertChange(index, ObjList(values.toMutableList()))
+                self.mutate(requireScope(), change) {
+                    var i = index
+                    for (v in values) self.list.add(i++, v)
+                    change
+                }
+                ObjVoid
+            }
+            addFnDoc(
+                "removeAt",
+                "Remove element at index or range and emit remove/reorder/clear changes.",
+                params = listOf(ParamDoc("start", type("lyng.Int")), ParamDoc("end", type("lyng.Int"))),
+                moduleName = "lyng.observable"
+            ) {
+                val self = thisAs<ObjObservableList>()
+                val start = requiredArg<ObjInt>(0).value.toInt()
+                if (args.size == 2) {
+                    val end = requireOnlyArg<ObjInt>().value.toInt()
+                    val oldValues = self.snapshotList()
+                    val newValues = self.list.toMutableList().apply { subList(start, end).clear() }
+                    if (newValues == self.list) return@addFnDoc self
+                    val change = if (newValues.isEmpty()) ObjListClearChange(oldValues)
+                    else ObjListReorderChange(oldValues, ObjList(newValues.toMutableList()))
+                    self.mutate(requireScope(), change) {
+                        self.list.subList(start, end).clear()
+                        change
+                    }
+                } else {
+                    val old = self.list[start]
+                    val change = ObjListRemoveChange(start, old)
+                    self.mutate(requireScope(), change) {
+                        self.list.removeAt(start)
+                        change
+                    }
+                }
+                self
+            }
+            addFnDoc(
+                "removeLast",
+                "Remove last element(s) and emit remove/reorder/clear changes.",
+                params = listOf(ParamDoc("count", type("lyng.Int"))),
+                moduleName = "lyng.observable"
+            ) {
+                val self = thisAs<ObjObservableList>()
+                if (self.list.isEmpty()) return@addFnDoc self
+                if (args.isNotEmpty()) {
+                    val count = requireOnlyArg<ObjInt>().value.toInt()
+                    if (count <= 0) return@addFnDoc self
+                    val oldValues = self.snapshotList()
+                    val newValues = if (count >= self.list.size) emptyList() else self.list.subList(0, self.list.size - count).toList()
+                    val change = if (newValues.isEmpty()) ObjListClearChange(oldValues)
+                    else ObjListReorderChange(oldValues, ObjList(newValues.toMutableList()))
+                    self.mutate(requireScope(), change) {
+                        if (count >= self.list.size) self.list.clear()
+                        else self.list.subList(self.list.size - count, self.list.size).clear()
+                        change
+                    }
+                } else {
+                    val index = self.list.lastIndex
+                    val old = self.list[index]
+                    val change = ObjListRemoveChange(index, old)
+                    self.mutate(requireScope(), change) {
+                        self.list.removeAt(index)
+                        change
+                    }
+                }
+                self
+            }
+            addFnDoc(
+                "removeRange",
+                "Remove a range and emit reorder/clear changes.",
+                params = listOf(ParamDoc("range")),
+                moduleName = "lyng.observable"
+            ) {
+                val self = thisAs<ObjObservableList>()
+                val before = self.snapshotList()
+                val copy = self.list.toMutableList()
+                val range = requiredArg<Obj>(0)
+                if (range is ObjRange) {
+                    val index = range
+                    when {
+                        index.start is ObjInt && index.end is ObjInt -> {
+                            if (index.isEndInclusive) copy.subList(index.start.toInt(), index.end.toInt() + 1)
+                            else copy.subList(index.start.toInt(), index.end.toInt())
+                        }
+
+                        index.isOpenStart && !index.isOpenEnd -> {
+                            if (index.isEndInclusive) copy.subList(0, index.end!!.toInt() + 1)
+                            else copy.subList(0, index.end!!.toInt())
+                        }
+
+                        index.isOpenEnd && !index.isOpenStart -> {
+                            copy.subList(index.start!!.toInt(), copy.size)
+                        }
+
+                        index.isOpenStart && index.isOpenEnd -> {
+                            copy
+                        }
+
+                        else -> {
+                            throw RuntimeException("Can't apply range for index: $index")
+                        }
+                    }.clear()
+                } else {
+                    val start = range.toInt()
+                    val end = requiredArg<ObjInt>(1).value.toInt() + 1
+                    copy.subList(start, end).clear()
+                }
+                if (copy == self.list) return@addFnDoc self
+                val change = if (copy.isEmpty()) ObjListClearChange(before)
+                else ObjListReorderChange(before, ObjList(copy.toMutableList()))
+                self.mutate(requireScope(), change) {
+                    self.list.clear()
+                    self.list.addAll(copy)
+                    change
+                }
+                self
+            }
+            addFnDoc(
+                "sortWith",
+                "Sort list in place and emit ListReorder.",
+                params = listOf(ParamDoc("comparator")),
+                moduleName = "lyng.observable"
+            ) {
+                val self = thisAs<ObjObservableList>()
+                if (self.list.size < 2) return@addFnDoc ObjVoid
+                val comparator = requireOnlyArg<Obj>()
+                val oldValues = self.snapshotList()
+                val beforeChange = ObjListReorderChange(oldValues, ObjUnset)
+                self.notifyBefore(requireScope(), beforeChange)
+                self.quicksort { a, b -> call(comparator, Arguments(a, b)).toInt() }
+                val newValues = self.snapshotList()
+                if (oldValues.list != newValues.list) {
+                    val committed = ObjListReorderChange(oldValues, newValues)
+                    self.notifyAfter(requireScope(), committed)
+                    self.emitCommittedChange(committed)
+                }
+                ObjVoid
+            }
+            addFnDoc(
+                "shuffle",
+                "Shuffle list in place and emit ListReorder.",
+                moduleName = "lyng.observable"
+            ) {
+                val self = thisAs<ObjObservableList>()
+                if (self.list.size < 2) return@addFnDoc ObjVoid
+                val oldValues = self.snapshotList()
+                val beforeChange = ObjListReorderChange(oldValues, ObjUnset)
+                self.notifyBefore(requireScope(), beforeChange)
+                self.list.shuffle()
+                val newValues = self.snapshotList()
+                if (oldValues.list != newValues.list) {
+                    val committed = ObjListReorderChange(oldValues, newValues)
+                    self.notifyAfter(requireScope(), committed)
+                    self.emitCommittedChange(committed)
+                }
+                ObjVoid
+            }
+        }
+    }
+}

lynglib/src/commonMain/kotlin/net/sergeych/lyng/stdlib_included/observable_lyng.kt

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2026 Sergey S. Chernov real.sergeych@gmail.com
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package net.sergeych.lyng.stdlib_included
+
+@Suppress("Unused", "MemberVisibilityCanBePrivate")
+internal val observableLyng = """
+package lyng.observable
+
+extern class ChangeRejectionException : Exception
+
+extern class Subscription {
+    fun cancel(): Void
+}
+
+extern class Observable<Change> {
+    fun beforeChange(listener: (Change)->Void): Subscription
+    fun onChange(listener: (Change)->Void): Subscription
+    fun changes(): Flow<Change>
+}
+
+extern class ListChange<T>
+
+extern class ListSet<T> : ListChange<T> {
+    val index: Int
+    val oldValue: Object
+    val newValue: Object
+}
+
+extern class ListInsert<T> : ListChange<T> {
+    val index: Int
+    val values: List<T>
+}
+
+extern class ListRemove<T> : ListChange<T> {
+    val index: Int
+    val oldValue: Object
+}
+
+extern class ListClear<T> : ListChange<T> {
+    val oldValues: List<T>
+}
+
+extern class ListReorder<T> : ListChange<T> {
+    val oldValues: List<T>
+    val newValues: Object
+}
+
+extern class ObservableList<T> : List<T> {
+    fun beforeChange(listener: (ListChange<T>)->Void): Subscription
+    fun onChange(listener: (ListChange<T>)->Void): Subscription
+    fun changes(): Flow<ListChange<T>>
+}
+
+fun List<T>.observable(): ObservableList<T> {
+    if( this is ObservableList<T> ) this
+    else ObservableList(...this)
+}
+""".trimIndent()

lynglib/src/commonTest/kotlin/ObservableListTest.kt

@@ -0,0 +1,117 @@
+/*
+ * Copyright 2026 Sergey S. Chernov
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+import kotlinx.coroutines.test.runTest
+import net.sergeych.lyng.eval
+import kotlin.test.Test
+
+class ObservableListTest {
+    @Test
+    fun observableHooksAndSubscriptionCancel() = runTest {
+        eval(
+            """
+            import lyng.observable
+
+            val src = [1,2]
+            val xs = src.observable()
+            assert(xs is ObservableList<Int>)
+            assert(src == [1,2])
+            assert(xs == [1,2])
+
+            var before = 0
+            var after = 0
+            val beforeSub = xs.beforeChange { before++ }
+            val afterSub = xs.onChange { after++ }
+
+            xs += 3
+            assertEquals([1,2,3], xs)
+            assertEquals(1, before)
+            assertEquals(1, after)
+
+            afterSub.cancel()
+            xs[0] = 100
+            assertEquals([100,2,3], xs)
+            assertEquals(2, before)
+            assertEquals(1, after)
+
+            beforeSub.cancel()
+            xs.removeAt(0)
+            assertEquals([2,3], xs)
+            assertEquals(2, before)
+            assertEquals(1, after)
+            """
+        )
+    }
+
+    @Test
+    fun observableBeforeChangeRejectsMutation() = runTest {
+        eval(
+            """
+            import lyng.observable
+
+            val xs = [1,2].observable()
+            var after = 0
+            xs.beforeChange {
+                throw ChangeRejectionException("no changes accepted")
+            }
+            xs.onChange { after++ }
+
+            assertThrows(ChangeRejectionException) {
+                xs += 3
+            }
+            assertEquals([1,2], xs)
+            assertEquals(0, after)
+            """
+        )
+    }
+
+    @Test
+    fun observableChangesFlowEmitsCommittedEvents() = runTest {
+        eval(
+            """
+            import lyng.observable
+
+            val xs = [10,20].observable()
+            val it = xs.changes().iterator()
+
+            xs += 30
+            assert(it.hasNext())
+            val e1 = it.next()
+            assert(e1 is ListInsert<Int>)
+            assertEquals(2, (e1 as ListInsert<Int>).index)
+            assertEquals([30], e1.values)
+
+            xs[1] = 200
+            assert(it.hasNext())
+            val e2 = it.next()
+            assert(e2 is ListSet<Int>)
+            assertEquals(1, (e2 as ListSet<Int>).index)
+            assertEquals(20, e2.oldValue)
+            assertEquals(200, e2.newValue)
+
+            xs.removeAt(0)
+            assert(it.hasNext())
+            val e3 = it.next()
+            assert(e3 is ListRemove<Int>)
+            assertEquals(0, (e3 as ListRemove<Int>).index)
+            assertEquals(10, e3.oldValue)
+
+            it.cancelIteration()
+            """
+        )
+    }
+}