Add decimal module and operator interop registry

docs/Decimal.md

@@ -0,0 +1,250 @@
+# Decimal (`lyng.decimal`)
+
+`lyng.decimal` adds an arbitrary-precision decimal type to Lyng as a normal library module.
+
+Import it when you need decimal arithmetic that should not inherit `Real`'s binary floating-point behavior:
+
+```lyng
+import lyng.decimal
+```
+
+## What `BigDecimal` Is For
+
+Use `BigDecimal` when values are fundamentally decimal:
+
+- money
+- human-entered quantities
+- exact decimal text
+- predictable decimal rounding
+- user-facing formatting and tests
+
+Do not use it just because a number has a fractional part. `Real` is still the right type for ordinary double-precision numeric work.
+
+## Creating Decimal Values
+
+There are three supported conversions:
+
+```lyng
+import lyng.decimal
+
+val a = 1.d
+val b = 2.2.d
+val c = "2.2".d
+
+assertEquals("1", a.toStringExpanded())
+assertEquals("2.2", b.toStringExpanded())
+assertEquals("2.2", c.toStringExpanded())
+```
+
+The three forms mean different things:
+
+- `1.d`: convert `Int -> BigDecimal`
+- `2.2.d`: convert `Real -> BigDecimal`
+- `"2.2".d`: parse exact decimal text
+
+That distinction is intentional.
+
+### `Real.d` vs `"..." .d`
+
+`Real.d` preserves the current `Real` value. It does not pretend the source code was exact decimal text.
+
+```lyng
+import lyng.decimal
+
+assertEquals("0.30000000000000004", (0.1 + 0.2).d.toStringExpanded())
+assertEquals("0.3", "0.3".d.toStringExpanded())
+```
+
+This follows the "minimal confusion" rule:
+
+- if you start from a `Real`, you get a decimal representation of that `Real`
+- if you want exact decimal source text, use a `String`
+
+## Factory Functions
+
+The explicit factory methods are:
+
+```lyng
+import lyng.decimal
+
+BigDecimal.fromInt(10)
+BigDecimal.fromReal(2.5)
+BigDecimal.fromString("12.34")
+```
+
+These are equivalent to the conversion-property forms, but sometimes clearer in APIs or generated code.
+
+## Core Operations
+
+`BigDecimal` supports:
+
+- `+`
+- `-`
+- `*`
+- `/`
+- `%`
+- unary `-`
+- comparison operators
+- equality operators
+
+Examples:
+
+```lyng
+import lyng.decimal
+
+assertEquals("3.75", ("1.5".d + "2.25".d).toStringExpanded())
+assertEquals("1.25", ("2.5".d - "1.25".d).toStringExpanded())
+assertEquals("3.0", ("1.5".d * 2.d).toStringExpanded())
+assertEquals("0.5", (1.d / 2.d).toStringExpanded())
+assert("2.0".d > "1.5".d)
+assert("2.0".d == 2.d)
+```
+
+## Interoperability With `Int` and `Real`
+
+The decimal module registers mixed-operand operator bridges so both sides read naturally:
+
+```lyng
+import lyng.decimal
+
+assertEquals(3.d, 1 + 2.d)
+assertEquals(3.d, 2.d + 1)
+assertEquals(1.5.d, 1.d + 0.5)
+assertEquals(1.5.d, 0.5 + 1.d)
+assert(2 == 2.d)
+assert(3 > 2.d)
+```
+
+Without this registration mechanism, only the cases directly implemented on the left-hand class would work. The bridge fills the gap for expressions such as `Int + BigDecimal` and `Real + BigDecimal`.
+
+See [OperatorInterop.md](OperatorInterop.md) for the generic mechanism behind that.
+
+## String Representation
+
+Use `toStringExpanded()` when you want plain decimal output without scientific notation:
+
+```lyng
+import lyng.decimal
+
+assertEquals("12.34", "12.34".d.toStringExpanded())
+```
+
+This is the recommended representation for:
+
+- tests
+- user-visible diagnostics
+- decimal formatting checks
+
+## Conversions Back To Built-ins
+
+```lyng
+import lyng.decimal
+
+assertEquals(2, "2.9".d.toInt())
+assertEquals(2.9, "2.9".d.toReal())
+```
+
+Use `toReal()` only when you are willing to return to binary floating-point semantics.
+
+## Division Context
+
+Division is the operation where precision and rounding matter most.
+
+By default, decimal division uses:
+
+- precision: `34` significant digits
+- rounding: `HalfEven`
+
+Example:
+
+```lyng
+import lyng.decimal
+
+assertEquals("0.3333333333333333333333333333333333", (1.d / 3.d).toStringExpanded())
+assertEquals("0.6666666666666666666666666666666667", ("2".d / 3.d).toStringExpanded())
+```
+
+## `withDecimalContext(...)`
+
+Use `withDecimalContext(...)` to override decimal division rules inside a block:
+
+```lyng
+import lyng.decimal
+
+assertEquals(
+    "0.3333333333",
+    withDecimalContext(10) { (1.d / 3.d).toStringExpanded() }
+)
+```
+
+You can also pass an explicit context object:
+
+```lyng
+import lyng.decimal
+
+val ctx = DecimalContext(6, DecimalRounding.HalfAwayFromZero)
+assertEquals("0.666667", withDecimalContext(ctx) { ("2".d / 3.d).toStringExpanded() })
+```
+
+The context is dynamic and local to the block. After the block exits, the previous context is restored.
+
+## Rounding Modes
+
+Available rounding modes:
+
+- `HalfEven`
+- `HalfAwayFromZero`
+- `HalfTowardsZero`
+- `Ceiling`
+- `Floor`
+- `AwayFromZero`
+- `TowardsZero`
+
+Tie example at precision `2`:
+
+```lyng
+import lyng.decimal
+
+assertEquals("0.12", withDecimalContext(2, DecimalRounding.HalfEven) { (1.d / 8.d).toStringExpanded() })
+assertEquals("0.13", withDecimalContext(2, DecimalRounding.HalfAwayFromZero) { (1.d / 8.d).toStringExpanded() })
+assertEquals("0.12", withDecimalContext(2, DecimalRounding.HalfTowardsZero) { (1.d / 8.d).toStringExpanded() })
+assertEquals("0.13", withDecimalContext(2, DecimalRounding.Ceiling) { (1.d / 8.d).toStringExpanded() })
+assertEquals("0.12", withDecimalContext(2, DecimalRounding.Floor) { (1.d / 8.d).toStringExpanded() })
+```
+
+Negative values follow the same named policy in the obvious direction:
+
+```lyng
+import lyng.decimal
+
+assertEquals("-0.13", withDecimalContext(2, DecimalRounding.HalfAwayFromZero) { (-1.d / 8.d).toStringExpanded() })
+assertEquals("-0.12", withDecimalContext(2, DecimalRounding.HalfTowardsZero) { (-1.d / 8.d).toStringExpanded() })
+```
+
+## Recommended Usage Rules
+
+If you care about exact decimal source text:
+
+```lyng
+"12.34".d
+```
+
+If you intentionally convert an existing binary floating-point value:
+
+```lyng
+someReal.d
+```
+
+If you want local control over division:
+
+```lyng
+withDecimalContext(precision, rounding) { ... }
+```
+
+If you want custom mixed operators for your own type, follow the same pattern as Decimal and use the operator registry:
+
+- define the operators on your own class
+- choose a common class
+- register mixed operand bridges
+
+See [OperatorInterop.md](OperatorInterop.md).

docs/OperatorInterop.md

@@ -0,0 +1,309 @@
+# Operator Interop Registry
+
+`lyng.operators` provides a runtime registry for mixed-class binary operators.
+
+Import it when you want expressions such as:
+
+- `1 + MyType(...)`
+- `2 < MyType(...)`
+- `3 == MyType(...)`
+
+to work without modifying the built-in `Int` or `Real` classes.
+
+```lyng
+import lyng.operators
+```
+
+## Why This Exists
+
+If your class defines:
+
+```lyng
+class Amount(val value: Int) {
+    fun plus(other: Amount) = Amount(value + other.value)
+}
+```
+
+then:
+
+```lyng
+Amount(1) + Amount(2)
+```
+
+works, because the left operand already knows how to add another `Amount`.
+
+But:
+
+```lyng
+1 + Amount(2)
+```
+
+does not naturally work, because `Int` has not been rewritten to know about `Amount`.
+
+The operator interop registry solves exactly that problem.
+
+## Mental Model
+
+Registration describes a mixed pair:
+
+- left class `L`
+- right class `R`
+- common class `C`
+
+When Lyng sees `L op R`, it:
+
+1. converts `L -> C`
+2. converts `R -> C`
+3. evaluates the operator as `C op C`
+
+So the registry is a bridge, not a separate arithmetic engine.
+
+## API
+
+```lyng
+OperatorInterop.register(
+    leftClass,
+    rightClass,
+    commonClass,
+    operators,
+    leftToCommon,
+    rightToCommon
+)
+```
+
+Parameters:
+
+- `leftClass`: original left operand class
+- `rightClass`: original right operand class
+- `commonClass`: class that will actually execute the operator methods
+- `operators`: list of operators enabled for this pair
+- `leftToCommon`: conversion from left operand to common class
+- `rightToCommon`: conversion from right operand to common class
+
+## Supported Operators
+
+`BinaryOperator` values:
+
+- `Plus`
+- `Minus`
+- `Mul`
+- `Div`
+- `Mod`
+- `Compare`
+- `Equals`
+
+Meaning:
+
+- `Compare` enables `<`, `<=`, `>`, `>=`, and `<=>`
+- `Equals` enables `==` and `!=`
+
+## Minimal Working Example
+
+```lyng
+package test.decimalbox
+import lyng.operators
+
+class DecimalBox(val value: Int) {
+    fun plus(other: DecimalBox) = DecimalBox(value + other.value)
+    fun minus(other: DecimalBox) = DecimalBox(value - other.value)
+    fun mul(other: DecimalBox) = DecimalBox(value * other.value)
+    fun div(other: DecimalBox) = DecimalBox(value / other.value)
+    fun mod(other: DecimalBox) = DecimalBox(value % other.value)
+    fun compareTo(other: DecimalBox) = value <=> other.value
+}
+
+OperatorInterop.register(
+    Int,
+    DecimalBox,
+    DecimalBox,
+    [
+        BinaryOperator.Plus,
+        BinaryOperator.Minus,
+        BinaryOperator.Mul,
+        BinaryOperator.Div,
+        BinaryOperator.Mod,
+        BinaryOperator.Compare,
+        BinaryOperator.Equals
+    ],
+    { x: Int -> DecimalBox(x) },
+    { x: DecimalBox -> x }
+)
+```
+
+Then:
+
+```lyng
+import test.decimalbox
+
+assertEquals(DecimalBox(3), 1 + DecimalBox(2))
+assertEquals(DecimalBox(1), 3 - DecimalBox(2))
+assertEquals(DecimalBox(8), 4 * DecimalBox(2))
+assertEquals(DecimalBox(4), 8 / DecimalBox(2))
+assertEquals(DecimalBox(1), 7 % DecimalBox(2))
+assert(1 < DecimalBox(2))
+assert(2 <= DecimalBox(2))
+assert(3 > DecimalBox(2))
+assert(2 == DecimalBox(2))
+assert(2 != DecimalBox(3))
+```
+
+## How Decimal Uses It
+
+`lyng.decimal` uses this same mechanism so that:
+
+```lyng
+import lyng.decimal
+
+1 + 2.d
+0.5 + 1.d
+2 == 2.d
+3 > 2.d
+```
+
+work naturally even though `Int` and `Real` themselves were not edited to know `BigDecimal`.
+
+The shape is:
+
+- `leftClass = Int` or `Real`
+- `rightClass = BigDecimal`
+- `commonClass = BigDecimal`
+- convert built-ins into `BigDecimal`
+- leave `BigDecimal` values unchanged
+
+## Step-By-Step Pattern For Your Own Type
+
+### 1. Pick the common class
+
+Choose one class that will be the actual arithmetic domain.
+
+For numeric-like types, that is usually your own class:
+
+```lyng
+class Rational(...)
+```
+
+### 2. Implement operators on that class
+
+The common class should define the operations you plan to register.
+
+Example:
+
+```lyng
+class Rational(val num: Int, val den: Int) {
+    fun plus(other: Rational) = Rational(num * other.den + other.num * den, den * other.den)
+    fun minus(other: Rational) = Rational(num * other.den - other.num * den, den * other.den)
+    fun mul(other: Rational) = Rational(num * other.num, den * other.den)
+    fun div(other: Rational) = Rational(num * other.den, den * other.num)
+    fun compareTo(other: Rational) = (num * other.den) <=> (other.num * den)
+
+    static fun fromInt(value: Int) = Rational(value, 1)
+}
+```
+
+### 3. Register the mixed pair
+
+```lyng
+import lyng.operators
+
+OperatorInterop.register(
+    Int,
+    Rational,
+    Rational,
+    [
+        BinaryOperator.Plus,
+        BinaryOperator.Minus,
+        BinaryOperator.Mul,
+        BinaryOperator.Div,
+        BinaryOperator.Compare,
+        BinaryOperator.Equals
+    ],
+    { x: Int -> Rational.fromInt(x) },
+    { x: Rational -> x }
+)
+```
+
+### 4. Use it
+
+```lyng
+assertEquals(Rational(3, 2), 1 + Rational(1, 2))
+assert(Rational(3, 2) == Rational(3, 2))
+assert(2 > Rational(3, 2))
+```
+
+## Registering More Than One Built-in Type
+
+If you want both `Int + MyType` and `Real + MyType`, register both pairs explicitly:
+
+```lyng
+OperatorInterop.register(
+    Int,
+    MyType,
+    MyType,
+    [BinaryOperator.Plus, BinaryOperator.Compare, BinaryOperator.Equals],
+    { x: Int -> MyType.fromInt(x) },
+    { x: MyType -> x }
+)
+
+OperatorInterop.register(
+    Real,
+    MyType,
+    MyType,
+    [BinaryOperator.Plus, BinaryOperator.Compare, BinaryOperator.Equals],
+    { x: Real -> MyType.fromReal(x) },
+    { x: MyType -> x }
+)
+```
+
+Each mixed pair is independent.
+
+## Pure Lyng Registration
+
+This mechanism is intentionally useful from pure Lyng code, not only from Kotlin-backed modules.
+
+That means you can:
+
+- declare a class in Lyng
+- define its operators in Lyng
+- register mixed operand bridges in Lyng
+
+without touching compiler internals.
+
+## Where To Register
+
+Register once during module initialization.
+
+Top-level module code is a good place:
+
+```lyng
+package my.rational
+import lyng.operators
+
+class Rational(...)
+
+OperatorInterop.register(...)
+```
+
+That keeps registration close to the type declaration and makes importing the module enough to activate the interop.
+
+## What Registration Does Not Do
+
+The registry does not:
+
+- invent operators your common class does not implement
+- change the original `Int`, `Real`, or other built-ins
+- automatically cover every class pair
+- replace normal method overload resolution when the left-hand class already knows what to do
+
+It only teaches Lyng how to bridge a specific mixed pair into a common class for the listed operators.
+
+## Recommended Design Rules
+
+If you want interop to feel natural:
+
+- choose one obvious common class
+- make conversions explicit and unsurprising
+- implement `compareTo` if you want ordering operators
+- register `Equals` whenever mixed equality should work
+- keep the registered operator list minimal and accurate
+
+For decimal-like semantics, also read [Decimal.md](Decimal.md).

docs/whats_new.md

@@ -5,6 +5,73 @@ For a programmer-focused migration summary, see `docs/whats_new_1_5.md`.
 
 ## Language Features
 
+### Decimal Arithmetic Module (`lyng.decimal`)
+Lyng now ships a first-class decimal module built as a regular extension library rather than a deep core special case.
+
+It provides:
+
+- `BigDecimal`
+- convenient `.d` conversions from `Int`, `Real`, and `String`
+- mixed arithmetic with `Int` and `Real`
+- local division precision and rounding control via `withDecimalContext(...)`
+
+```lyng
+import lyng.decimal
+
+assertEquals("3", (1 + 2.d).toStringExpanded())
+assertEquals("0.30000000000000004", (0.1 + 0.2).d.toStringExpanded())
+assertEquals("0.3", "0.3".d.toStringExpanded())
+
+assertEquals(
+    "0.3333333333",
+    withDecimalContext(10) { (1.d / 3.d).toStringExpanded() }
+)
+```
+
+The distinction between `Real -> Decimal` and exact decimal parsing is explicit by design:
+
+- `2.2.d` converts the current `Real` value
+- `"2.2".d` parses exact decimal text
+
+See [Decimal](Decimal.md).
+
+### Binary Operator Interop Registry
+Lyng now provides a general mechanism for mixed binary operators through `lyng.operators`.
+
+This solves cases like:
+
+- `Int + MyType`
+- `Real < MyType`
+- `Int == MyType`
+
+without requiring changes to built-in classes.
+
+```lyng
+import lyng.operators
+
+class DecimalBox(val value: Int) {
+    fun plus(other: DecimalBox) = DecimalBox(value + other.value)
+    fun compareTo(other: DecimalBox) = value <=> other.value
+}
+
+OperatorInterop.register(
+    Int,
+    DecimalBox,
+    DecimalBox,
+    [BinaryOperator.Plus, BinaryOperator.Compare, BinaryOperator.Equals],
+    { x: Int -> DecimalBox(x) },
+    { x: DecimalBox -> x }
+)
+
+assertEquals(DecimalBox(3), 1 + DecimalBox(2))
+assert(1 < DecimalBox(2))
+assert(2 == DecimalBox(2))
+```
+
+`lyng.decimal` uses this same mechanism internally to interoperate with `Int` and `Real`.
+
+See [Operator Interop Registry](OperatorInterop.md).
+
 ### Class Properties with Accessors
 Classes now support properties with custom `get()` and `set()` accessors. Properties in Lyng do **not** have automatic backing fields; they are pure accessors.
 

gradle/libs.versions.toml

@@ -8,6 +8,7 @@ android-compileSdk = "34"
 kotlinx-coroutines = "1.10.2"
 kotlinx-datetime = "0.6.1"
 mp_bintools = "0.3.2"
+ionspin-bignum = "0.3.10"
 firebaseCrashlyticsBuildtools = "3.0.3"
 okioVersion = "3.10.2"
 compiler = "3.2.0-alpha11"
@@ -22,6 +23,7 @@ kotlinx-coroutines-test = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-t
 kotlinx-coroutines-core = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-core", version.ref = "kotlinx-coroutines" }
 kotlinx-datetime = { module = "org.jetbrains.kotlinx:kotlinx-datetime", version.ref = "kotlinx-datetime" }
 mp_bintools = { module = "net.sergeych:mp_bintools", version.ref = "mp_bintools" }
+ionspin-bignum = { module = "com.ionspin.kotlin:bignum", version.ref = "ionspin-bignum" }
 firebase-crashlytics-buildtools = { group = "com.google.firebase", name = "firebase-crashlytics-buildtools", version.ref = "firebaseCrashlyticsBuildtools" }
 okio = { module = "com.squareup.okio:okio", version.ref = "okioVersion" }
 okio-fakefilesystem = { module = "com.squareup.okio:okio-fakefilesystem", version.ref = "okioVersion" }

lynglib/build.gradle.kts

@@ -101,6 +101,7 @@ kotlin {
                 //put your multiplatform dependencies here
                 api(libs.kotlinx.coroutines.core)
                 api(libs.mp.bintools)
+                implementation(libs.ionspin.bignum)
             }
         }
         val commonTest by getting {

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

@@ -4667,6 +4667,7 @@ class Compiler(
             }
             is MethodCallRef -> methodReturnTypeDeclByRef[ref]
             is CallRef -> callReturnTypeDeclByRef[ref]
+            is BinaryOpRef -> inferBinaryOpReturnTypeDecl(ref)
             is StatementRef -> (ref.statement as? ExpressionStatement)?.let { resolveReceiverTypeDecl(it.ref) }
             else -> null
         }
@@ -4730,6 +4731,7 @@ class Compiler(
             is ThisMethodSlotCallRef -> inferMethodCallReturnClass(ref.methodName())
             is QualifiedThisMethodSlotCallRef -> inferMethodCallReturnClass(ref.methodName())
             is CallRef -> inferCallReturnClass(ref)
+            is BinaryOpRef -> inferBinaryOpReturnClass(ref)
             is FieldRef -> {
                 val targetClass = resolveReceiverClassForMember(ref.target)
                 inferFieldReturnClass(targetClass, ref.name)
@@ -4744,10 +4746,105 @@ class Compiler(
         }
     }
 
+    private fun typeDeclOfClass(objClass: ObjClass): TypeDecl = TypeDecl.Simple(objClass.className, false)
+
+    private fun binaryOpMethodName(op: BinOp): String? = when (op) {
+        BinOp.PLUS -> "plus"
+        BinOp.MINUS -> "minus"
+        BinOp.STAR -> "mul"
+        BinOp.SLASH -> "div"
+        BinOp.PERCENT -> "mod"
+        else -> null
+    }
+
+    private fun interopOperatorFor(op: BinOp): InteropOperator? = when (op) {
+        BinOp.PLUS -> InteropOperator.Plus
+        BinOp.MINUS -> InteropOperator.Minus
+        BinOp.STAR -> InteropOperator.Mul
+        BinOp.SLASH -> InteropOperator.Div
+        BinOp.PERCENT -> InteropOperator.Mod
+        BinOp.LT, BinOp.LTE, BinOp.GT, BinOp.GTE -> InteropOperator.Compare
+        BinOp.EQ, BinOp.NEQ -> InteropOperator.Equals
+        else -> null
+    }
+
+    private fun sameClassArithmeticFallback(leftClass: ObjClass, rightClass: ObjClass, op: BinOp): TypeDecl? {
+        if (leftClass !== rightClass) return null
+        val methodName = binaryOpMethodName(op) ?: return null
+        if (leftClass.getInstanceMemberOrNull(methodName, includeAbstract = true) == null) return null
+        return typeDeclOfClass(leftClass)
+    }
+
+    private fun inferBinaryOpReturnTypeDecl(ref: BinaryOpRef): TypeDecl? {
+        val leftClass = resolveReceiverClassForMember(ref.left) ?: inferObjClassFromRef(ref.left)
+        val rightClass = resolveReceiverClassForMember(ref.right) ?: inferObjClassFromRef(ref.right)
+        val boolType = typeDeclOfClass(ObjBool.type)
+        val intType = typeDeclOfClass(ObjInt.type)
+        val realType = typeDeclOfClass(ObjReal.type)
+        val stringType = typeDeclOfClass(ObjString.type)
+
+        when (ref.op) {
+            BinOp.OR, BinOp.AND,
+            BinOp.EQARROW, BinOp.EQ, BinOp.NEQ, BinOp.REF_EQ, BinOp.REF_NEQ, BinOp.MATCH, BinOp.NOTMATCH,
+            BinOp.LTE, BinOp.LT, BinOp.GTE, BinOp.GT,
+            BinOp.IN, BinOp.NOTIN,
+            BinOp.IS, BinOp.NOTIS -> return boolType
+            else -> {}
+        }
+
+        if (leftClass == null || rightClass == null) return null
+
+        val leftIsInt = leftClass == ObjInt.type
+        val rightIsInt = rightClass == ObjInt.type
+        val leftIsReal = leftClass == ObjReal.type
+        val rightIsReal = rightClass == ObjReal.type
+        val leftIsNumeric = leftIsInt || leftIsReal
+        val rightIsNumeric = rightIsInt || rightIsReal
+
+        return when (ref.op) {
+            BinOp.PLUS -> when {
+                leftIsInt && rightIsInt -> intType
+                leftIsNumeric && rightIsNumeric -> realType
+                leftClass == ObjString.type -> stringType
+                interopOperatorFor(ref.op)?.let {
+                    OperatorInteropRegistry.commonClassFor(leftClass, rightClass, it)
+                } != null -> typeDeclOfClass(
+                    OperatorInteropRegistry.commonClassFor(leftClass, rightClass, interopOperatorFor(ref.op)!!)!!
+                )
+                else -> binaryOpMethodName(ref.op)?.let { classMethodReturnTypeDecl(leftClass, it) }
+                    ?: sameClassArithmeticFallback(leftClass, rightClass, ref.op)
+            }
+            BinOp.MINUS, BinOp.STAR, BinOp.SLASH, BinOp.PERCENT -> when {
+                leftIsInt && rightIsInt -> intType
+                leftIsNumeric && rightIsNumeric -> realType
+                ref.op == BinOp.STAR && leftClass == ObjString.type && rightIsNumeric -> stringType
+                interopOperatorFor(ref.op)?.let {
+                    OperatorInteropRegistry.commonClassFor(leftClass, rightClass, it)
+                } != null -> typeDeclOfClass(
+                    OperatorInteropRegistry.commonClassFor(leftClass, rightClass, interopOperatorFor(ref.op)!!)!!
+                )
+                else -> binaryOpMethodName(ref.op)?.let { classMethodReturnTypeDecl(leftClass, it) }
+                    ?: sameClassArithmeticFallback(leftClass, rightClass, ref.op)
+            }
+            BinOp.BAND, BinOp.BOR, BinOp.BXOR, BinOp.SHL, BinOp.SHR ->
+                if (leftIsInt && rightIsInt) intType else null
+            else -> null
+        }
+    }
+
     private fun inferBinaryOpReturnClass(ref: BinaryOpRef): ObjClass? {
+        inferBinaryOpReturnTypeDecl(ref)?.let { declared ->
+            resolveTypeDeclObjClass(declared)?.let { return it }
+        }
         val leftClass = resolveReceiverClassForMember(ref.left) ?: inferObjClassFromRef(ref.left)
         val rightClass = resolveReceiverClassForMember(ref.right) ?: inferObjClassFromRef(ref.right)
         if (leftClass == null || rightClass == null) return null
+        interopOperatorFor(ref.op)?.let { op ->
+            OperatorInteropRegistry.commonClassFor(leftClass, rightClass, op)?.let { return it }
+        }
+        sameClassArithmeticFallback(leftClass, rightClass, ref.op)?.let { declared ->
+            resolveTypeDeclObjClass(declared)?.let { return it }
+        }
         return when (ref.op) {
             BinOp.PLUS, BinOp.MINUS -> when {
                 leftClass == ObjInstant.type && rightClass == ObjInstant.type && ref.op == BinOp.MINUS -> ObjDuration.type

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

@@ -0,0 +1,109 @@
+/*
+ * 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
+
+import net.sergeych.lyng.obj.Obj
+import net.sergeych.lyng.obj.ObjClass
+import net.sergeych.lyng.obj.ObjInt
+import net.sergeych.lyng.obj.ObjVoid
+
+internal enum class InteropOperator(val memberName: String?) {
+    Plus("plus"),
+    Minus("minus"),
+    Mul("mul"),
+    Div("div"),
+    Mod("mod"),
+    Compare("compareTo"),
+    Equals("equals");
+
+    companion object {
+        fun fromName(name: String): InteropOperator =
+            entries.firstOrNull { it.name == name }
+                ?: throw IllegalArgumentException("unknown interop operator: $name")
+    }
+}
+
+private data class InteropRule(
+    val commonClass: ObjClass,
+    val operators: Set<InteropOperator>,
+    val leftToCommon: Obj,
+    val rightToCommon: Obj
+)
+
+internal data class PromotedOperands(val left: Obj, val right: Obj)
+
+internal object OperatorInteropRegistry {
+    private val rules = mutableMapOf<Pair<ObjClass, ObjClass>, InteropRule>()
+
+    fun register(
+        leftClass: ObjClass,
+        rightClass: ObjClass,
+        commonClass: ObjClass,
+        operatorNames: List<String>,
+        leftToCommon: Obj,
+        rightToCommon: Obj
+    ) {
+        val operators = operatorNames.mapTo(linkedSetOf(), InteropOperator::fromName)
+        rules[leftClass to rightClass] = InteropRule(commonClass, operators, leftToCommon, rightToCommon)
+        if (leftClass !== rightClass) {
+            rules[rightClass to leftClass] = InteropRule(commonClass, operators, rightToCommon, leftToCommon)
+        }
+    }
+
+    suspend fun promote(scope: Scope, left: Obj, right: Obj, operator: InteropOperator): PromotedOperands? {
+        val leftValue = unwrap(left)
+        val rightValue = unwrap(right)
+        val rule = rules[leftValue.objClass to rightValue.objClass] ?: return null
+        if (operator !in rule.operators) return null
+        val promotedLeft = rule.leftToCommon.invoke(scope, ObjVoid, Arguments(leftValue))
+        val promotedRight = rule.rightToCommon.invoke(scope, ObjVoid, Arguments(rightValue))
+        if (promotedLeft.objClass !== rule.commonClass || promotedRight.objClass !== rule.commonClass) {
+            scope.raiseIllegalState(
+                "Operator interop promotion must return ${rule.commonClass.className}, " +
+                    "got ${promotedLeft.objClass.className} and ${promotedRight.objClass.className}"
+            )
+        }
+        return PromotedOperands(promotedLeft, promotedRight)
+    }
+
+    suspend fun invokeBinary(scope: Scope, left: Obj, right: Obj, operator: InteropOperator): Obj? {
+        val promoted = promote(scope, left, right, operator) ?: return null
+        val memberName = operator.memberName ?: return null
+        return promoted.left.invokeInstanceMethod(scope, memberName, Arguments(promoted.right))
+    }
+
+    suspend fun invokeCompare(scope: Scope, left: Obj, right: Obj): Int? {
+        val promoted = promote(scope, left, right, InteropOperator.Compare) ?: return null
+        return promoted.left.invokeInstanceMethod(scope, "compareTo", Arguments(promoted.right))
+            .cast<ObjInt>(scope)
+            .value
+            .toInt()
+    }
+
+    fun commonClassFor(leftClass: ObjClass, rightClass: ObjClass, operator: InteropOperator): ObjClass? {
+        val rule = rules[leftClass to rightClass] ?: return null
+        if (operator !in rule.operators) return null
+        return rule.commonClass
+    }
+
+    private suspend fun unwrap(obj: Obj): Obj = when (obj) {
+        is FrameSlotRef -> obj.read()
+        is RecordSlotRef -> obj.read()
+        else -> obj
+    }
+}

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

@@ -27,7 +27,9 @@ 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.decimalLyng
 import net.sergeych.lyng.stdlib_included.observableLyng
+import net.sergeych.lyng.stdlib_included.operatorsLyng
 import net.sergeych.lyng.stdlib_included.rootLyng
 import net.sergeych.lynon.ObjLynonClass
 import net.sergeych.mp_tools.globalDefer
@@ -727,6 +729,36 @@ class Script(
                     module.addConst("ChangeRejectionException", ObjChangeRejectionExceptionClass)
                     module.eval(Source("lyng.observable", observableLyng))
                 }
+                addPackage("lyng.operators") { module ->
+                    module.eval(Source("lyng.operators", operatorsLyng))
+                    module.bindObject("OperatorInterop") {
+                        addFun("register") {
+                            val leftClass = requiredArg<ObjClass>(0)
+                            val rightClass = requiredArg<ObjClass>(1)
+                            val commonClass = requiredArg<ObjClass>(2)
+                            val operators = requiredArg<ObjList>(3).list.map { value ->
+                                val entry = value as? ObjEnumEntry
+                                    ?: requireScope().raiseIllegalArgument(
+                                        "OperatorInterop.register expects BinaryOperator enum entries"
+                                    )
+                                entry.name.value
+                            }
+                            OperatorInteropRegistry.register(
+                                leftClass = leftClass,
+                                rightClass = rightClass,
+                                commonClass = commonClass,
+                                operatorNames = operators,
+                                leftToCommon = args[4],
+                                rightToCommon = args[5]
+                            )
+                            ObjVoid
+                        }
+                    }
+                }
+                addPackage("lyng.decimal") { module ->
+                    module.eval(Source("lyng.decimal", decimalLyng))
+                    ObjBigDecimalSupport.bindTo(module)
+                }
                 addPackage("lyng.buffer") {
                     it.addConstDoc(
                         name = "Buffer",

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

@@ -25,6 +25,8 @@ import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonNull
 import kotlinx.serialization.serializer
 import net.sergeych.lyng.*
+import net.sergeych.lyng.InteropOperator
+import net.sergeych.lyng.OperatorInteropRegistry
 import net.sergeych.lyng.miniast.ParamDoc
 import net.sergeych.lyng.miniast.addFnDoc
 import net.sergeych.lyng.miniast.type
@@ -175,7 +177,8 @@ open class Obj {
         if (other === this) return 0
         if (other === ObjNull || other === ObjUnset || other === ObjVoid) return 2
         return invokeInstanceMethod(scope, "compareTo", Arguments(other)) {
-            scope.raiseNotImplemented("compareTo for ${objClass.className}")
+            OperatorInteropRegistry.invokeCompare(scope, this, other)?.let { ObjInt.of(it.toLong()) }
+                ?: scope.raiseNotImplemented("compareTo for ${objClass.className}")
         }.cast<ObjInt>(scope).toInt()
     }
 
@@ -284,7 +287,8 @@ open class Obj {
         }
         if (self !== this) return self.plus(scope, otherValue)
         return invokeInstanceMethod(scope, "plus", Arguments(otherValue)) {
-            scope.raiseNotImplemented("plus for ${objClass.className}")
+            OperatorInteropRegistry.invokeBinary(scope, self, otherValue, InteropOperator.Plus)
+                ?: scope.raiseNotImplemented("plus for ${objClass.className}")
         }
     }
 
@@ -301,7 +305,8 @@ open class Obj {
         }
         if (self !== this) return self.minus(scope, otherValue)
         return invokeInstanceMethod(scope, "minus", Arguments(otherValue)) {
-            scope.raiseNotImplemented("minus for ${objClass.className}")
+            OperatorInteropRegistry.invokeBinary(scope, self, otherValue, InteropOperator.Minus)
+                ?: scope.raiseNotImplemented("minus for ${objClass.className}")
         }
     }
 
@@ -324,7 +329,8 @@ open class Obj {
         }
         if (self !== this) return self.mul(scope, otherValue)
         return invokeInstanceMethod(scope, "mul", Arguments(otherValue)) {
-            scope.raiseNotImplemented("mul for ${objClass.className}")
+            OperatorInteropRegistry.invokeBinary(scope, self, otherValue, InteropOperator.Mul)
+                ?: scope.raiseNotImplemented("mul for ${objClass.className}")
         }
     }
 
@@ -341,7 +347,8 @@ open class Obj {
         }
         if (self !== this) return self.div(scope, otherValue)
         return invokeInstanceMethod(scope, "div", Arguments(otherValue)) {
-            scope.raiseNotImplemented("div for ${objClass.className}")
+            OperatorInteropRegistry.invokeBinary(scope, self, otherValue, InteropOperator.Div)
+                ?: scope.raiseNotImplemented("div for ${objClass.className}")
         }
     }
 
@@ -358,7 +365,8 @@ open class Obj {
         }
         if (self !== this) return self.mod(scope, otherValue)
         return invokeInstanceMethod(scope, "mod", Arguments(otherValue)) {
-            scope.raiseNotImplemented("mod for ${objClass.className}")
+            OperatorInteropRegistry.invokeBinary(scope, self, otherValue, InteropOperator.Mod)
+                ?: scope.raiseNotImplemented("mod for ${objClass.className}")
         }
     }
 

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

@@ -0,0 +1,313 @@
+/*
+ * 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 com.ionspin.kotlin.bignum.decimal.BigDecimal as IonBigDecimal
+import com.ionspin.kotlin.bignum.decimal.DecimalMode
+import com.ionspin.kotlin.bignum.decimal.RoundingMode
+import com.ionspin.kotlin.bignum.integer.BigInteger
+import net.sergeych.lyng.Arguments
+import net.sergeych.lyng.FrameSlotRef
+import net.sergeych.lyng.InteropOperator
+import net.sergeych.lyng.ModuleScope
+import net.sergeych.lyng.OperatorInteropRegistry
+import net.sergeych.lyng.RecordSlotRef
+import net.sergeych.lyng.Scope
+import net.sergeych.lyng.ScopeFacade
+import net.sergeych.lyng.TypeDecl
+import net.sergeych.lyng.miniast.addPropertyDoc
+import net.sergeych.lyng.miniast.type
+import net.sergeych.lyng.requiredArg
+
+object ObjBigDecimalSupport {
+    private const val decimalContextVar = "__lyng_decimal_context__"
+    // For Real -> BigDecimal, preserve the actual IEEE-754 Double value using a
+    // round-trip-safe precision. This intentionally does not try to recover source text.
+    private val realConversionMode = DecimalMode(17L, RoundingMode.ROUND_HALF_TO_EVEN)
+    // Division needs an explicit stopping rule for non-terminating results. Use a
+    // decimal128-like default context until Lyng exposes per-operation contexts.
+    private val defaultDivisionMode = DecimalMode(34L, RoundingMode.ROUND_HALF_TO_EVEN)
+    private val zero: IonBigDecimal = IonBigDecimal.ZERO
+    private val decimalTypeDecl = TypeDecl.Simple("lyng.decimal.BigDecimal", false)
+    private object BoundMarker
+    private data class DecimalRuntimeContext(
+        val precision: Long,
+        val rounding: RoundingMode
+    ) : Obj()
+
+    suspend fun bindTo(module: ModuleScope) {
+        val decimalClass = module.requireClass("BigDecimal")
+        if (decimalClass.kotlinClassData === BoundMarker) return
+        decimalClass.kotlinClassData = BoundMarker
+        decimalClass.isAbstract = false
+        val hooks = decimalClass.bridgeInitHooks ?: mutableListOf<suspend (ScopeFacade, ObjInstance) -> Unit>().also {
+            decimalClass.bridgeInitHooks = it
+        }
+        hooks += { _, instance ->
+            instance.kotlinInstanceData = zero
+        }
+        decimalClass.addFn("plus") {
+            newInstance(decimalClass, valueOf(thisObj).plus(coerceArg(requireScope(), args.firstAndOnly())))
+        }
+        decimalClass.addFn("minus") {
+            newInstance(decimalClass, valueOf(thisObj).minus(coerceArg(requireScope(), args.firstAndOnly())))
+        }
+        decimalClass.addFn("mul") {
+            newInstance(decimalClass, valueOf(thisObj).times(coerceArg(requireScope(), args.firstAndOnly())))
+        }
+        decimalClass.addFn("div") {
+            newInstance(decimalClass, divideWithContext(valueOf(thisObj), coerceArg(requireScope(), args.firstAndOnly()), currentDivisionMode(requireScope())))
+        }
+        decimalClass.addFn("mod") {
+            newInstance(decimalClass, valueOf(thisObj).rem(coerceArg(requireScope(), args.firstAndOnly())))
+        }
+        decimalClass.addFn("compareTo") {
+            ObjInt.of(valueOf(thisObj).compareTo(coerceArg(requireScope(), args.firstAndOnly())).toLong())
+        }
+        decimalClass.addFn("negate") {
+            newInstance(decimalClass, valueOf(thisObj).unaryMinus())
+        }
+        decimalClass.addFn("toInt") {
+            ObjInt.of(valueOf(thisObj).longValue(false))
+        }
+        decimalClass.addFn("toReal") {
+            ObjReal.of(valueOf(thisObj).doubleValue(false))
+        }
+        decimalClass.addFn("toStringExpanded") {
+            ObjString(valueOf(thisObj).toStringExpanded())
+        }
+        decimalClass.addClassFn("fromInt") {
+            val value = requiredArg<ObjInt>(0).value
+            newInstance(decimalClass, IonBigDecimal.fromLongAsSignificand(value))
+        }
+        decimalClass.addClassFn("fromReal") {
+            val value = requiredArg<ObjReal>(0).value
+            newInstance(decimalClass, IonBigDecimal.fromDouble(value, realConversionMode))
+        }
+        decimalClass.addClassFn("fromString") {
+            val value = requiredArg<ObjString>(0).value
+            try {
+                newInstance(decimalClass, IonBigDecimal.parseStringWithMode(value))
+            } catch (e: Throwable) {
+                requireScope().raiseIllegalArgument("invalid BigDecimal string: $value")
+            }
+        }
+        module.addFn("withDecimalContext") {
+            val (context, block) = when (args.list.size) {
+                2 -> {
+                    val first = args[0]
+                    val block = args[1]
+                    if (first is ObjInt) {
+                        DecimalRuntimeContext(first.value, RoundingMode.ROUND_HALF_TO_EVEN) to block
+                    } else {
+                        normalizeContext(requireScope(), first) to block
+                    }
+                }
+                3 -> {
+                    val precision = requiredArg<ObjInt>(0).value
+                    val rounding = roundingModeFromObj(requireScope(), args[1])
+                    DecimalRuntimeContext(precision, rounding) to args[2]
+                }
+                else -> requireScope().raiseIllegalArgument("withDecimalContext expects (context, block), (precision, block), or (precision, rounding, block)")
+            }
+            val child = requireScope().createChildScope()
+            child.addConst(decimalContextVar, context)
+            block.callOn(child)
+        }
+        registerBuiltinConversions(decimalClass)
+        registerInterop(decimalClass)
+    }
+
+    private fun valueOf(obj: Obj): IonBigDecimal {
+        val instance = obj as? ObjInstance ?: error("BigDecimal receiver must be an object instance")
+        return instance.kotlinInstanceData as? IonBigDecimal ?: zero
+    }
+
+    private suspend fun currentDivisionMode(scope: Scope): DecimalMode {
+        val context = findContextObject(scope) ?: return defaultDivisionMode
+        return DecimalMode(context.precision, context.rounding)
+    }
+
+    private fun divideWithContext(left: IonBigDecimal, right: IonBigDecimal, mode: DecimalMode): IonBigDecimal {
+        if (mode.decimalPrecision <= 0L) {
+            return stripMode(left.divide(right, mode))
+        }
+        val exactLeft = stripMode(left)
+        val exactRight = stripMode(right)
+        val guardMode = DecimalMode(mode.decimalPrecision + 2, RoundingMode.TOWARDS_ZERO)
+        var guarded = stripMode(exactLeft.divide(exactRight, guardMode))
+        val hasMoreTail = !stripMode(exactLeft - stripMode(guarded * exactRight)).isZero()
+        if (hasMoreTail && isHalfRounding(mode.roundingMode) && looksLikeExactHalf(guarded, mode.decimalPrecision)) {
+            guarded = nudgeLastDigitAwayFromZero(guarded)
+        }
+        return stripMode(guarded.roundSignificand(mode))
+    }
+
+    private suspend fun ScopeFacade.newInstance(decimalClass: ObjClass, value: IonBigDecimal): ObjInstance {
+        val instance = call(decimalClass) as? ObjInstance
+            ?: raiseIllegalState("BigDecimal() did not return an object instance")
+        instance.kotlinInstanceData = value
+        return instance
+    }
+
+    private fun coerceArg(scope: Scope, value: Obj): IonBigDecimal = when (value) {
+        is ObjInt -> IonBigDecimal.fromLongAsSignificand(value.value)
+        is ObjReal -> IonBigDecimal.fromDouble(value.value, realConversionMode)
+        is ObjInstance -> {
+            if (value.objClass.className != "BigDecimal") {
+                scope.raiseIllegalArgument("expected BigDecimal-compatible value, got ${value.objClass.className}")
+            }
+            value.kotlinInstanceData as? IonBigDecimal ?: zero
+        }
+        else -> scope.raiseIllegalArgument("expected BigDecimal-compatible value, got ${value.objClass.className}")
+    }
+
+    private suspend fun normalizeContext(scope: Scope, value: Obj): DecimalRuntimeContext {
+        val instance = value as? ObjInstance
+            ?: scope.raiseClassCastError("withDecimalContext expects DecimalContext as the first argument")
+        if (instance.objClass.className != "DecimalContext") {
+            scope.raiseClassCastError("withDecimalContext expects DecimalContext as the first argument")
+        }
+        return decimalRuntimeContextFromInstance(scope, instance)
+    }
+
+    private fun findContextObject(scope: Scope): DecimalRuntimeContext? {
+        var current: Scope? = scope
+        while (current != null) {
+            val record = current.objects[decimalContextVar] ?: current.localBindings[decimalContextVar]
+            val value = when (val raw = record?.value) {
+                is FrameSlotRef -> raw.peekValue()
+                is RecordSlotRef -> raw.peekValue()
+                else -> raw
+            }
+            when (value) {
+                is DecimalRuntimeContext -> return value
+            }
+            current = current.parent
+        }
+        return null
+    }
+
+    private suspend fun decimalRuntimeContextFromInstance(scope: Scope, context: ObjInstance): DecimalRuntimeContext {
+        val precision = context.readField(scope, "precision").value as? ObjInt
+            ?: scope.raiseClassCastError("DecimalContext.precision must be Int")
+        if (precision.value <= 0L) {
+            scope.raiseIllegalArgument("DecimalContext precision must be positive")
+        }
+        val rounding = roundingModeFromObj(scope, context.readField(scope, "rounding").value)
+        return DecimalRuntimeContext(precision.value, rounding)
+    }
+
+    private fun stripMode(value: IonBigDecimal): IonBigDecimal =
+        IonBigDecimal.fromBigIntegerWithExponent(value.significand, value.exponent)
+
+    private fun isHalfRounding(mode: RoundingMode): Boolean = when (mode) {
+        RoundingMode.ROUND_HALF_TO_EVEN,
+        RoundingMode.ROUND_HALF_AWAY_FROM_ZERO,
+        RoundingMode.ROUND_HALF_TOWARDS_ZERO,
+        RoundingMode.ROUND_HALF_CEILING,
+        RoundingMode.ROUND_HALF_FLOOR,
+        RoundingMode.ROUND_HALF_TO_ODD -> true
+        else -> false
+    }
+
+    private fun looksLikeExactHalf(value: IonBigDecimal, targetPrecision: Long): Boolean {
+        val digits = value.significand.abs().toString(10)
+        if (digits.length <= targetPrecision) return false
+        val discarded = digits.substring(targetPrecision.toInt())
+        return discarded[0] == '5' && discarded.drop(1).all { it == '0' }
+    }
+
+    private fun nudgeLastDigitAwayFromZero(value: IonBigDecimal): IonBigDecimal {
+        val ulpExponent = value.exponent - value.precision + 1
+        val ulp = IonBigDecimal.fromBigIntegerWithExponent(BigInteger.ONE, ulpExponent)
+        return if (value.significand.signum() < 0) value - ulp else value + ulp
+    }
+
+    private fun roundingModeFromObj(scope: Scope, value: Obj): RoundingMode {
+        val entry = value as? ObjEnumEntry ?: scope.raiseClassCastError("DecimalContext.rounding must be DecimalRounding")
+        return when (entry.name.value) {
+            "HalfEven" -> RoundingMode.ROUND_HALF_TO_EVEN
+            "HalfAwayFromZero" -> RoundingMode.ROUND_HALF_AWAY_FROM_ZERO
+            "HalfTowardsZero" -> RoundingMode.ROUND_HALF_TOWARDS_ZERO
+            "Ceiling" -> RoundingMode.CEILING
+            "Floor" -> RoundingMode.FLOOR
+            "AwayFromZero" -> RoundingMode.AWAY_FROM_ZERO
+            "TowardsZero" -> RoundingMode.TOWARDS_ZERO
+            else -> scope.raiseIllegalArgument("unsupported DecimalRounding: ${entry.name.value}")
+        }
+    }
+
+    private fun registerBuiltinConversions(decimalClass: ObjClass) {
+        ObjInt.type.addPropertyDoc(
+            name = "d",
+            doc = "Convert this integer to a BigDecimal.",
+            type = type("lyng.decimal.BigDecimal"),
+            moduleName = "lyng.decimal",
+            getter = { newInstance(decimalClass, IonBigDecimal.fromLongAsSignificand(thisAs<ObjInt>().value)) }
+        )
+        ObjInt.type.members["d"] = ObjInt.type.members.getValue("d").copy(typeDecl = decimalTypeDecl)
+        ObjReal.type.addPropertyDoc(
+            name = "d",
+            doc = "Convert this real number to a BigDecimal by preserving the current IEEE-754 value with 17 significant digits and half-even rounding.",
+            type = type("lyng.decimal.BigDecimal"),
+            moduleName = "lyng.decimal",
+            getter = { newInstance(decimalClass, IonBigDecimal.fromDouble(thisAs<ObjReal>().value, realConversionMode)) }
+        )
+        ObjReal.type.members["d"] = ObjReal.type.members.getValue("d").copy(typeDecl = decimalTypeDecl)
+        ObjString.type.addPropertyDoc(
+            name = "d",
+            doc = "Parse this string as a BigDecimal.",
+            type = type("lyng.decimal.BigDecimal"),
+            moduleName = "lyng.decimal",
+            getter = {
+                val value = thisAs<ObjString>().value
+                try {
+                    newInstance(decimalClass, IonBigDecimal.parseStringWithMode(value))
+                } catch (e: Throwable) {
+                    requireScope().raiseIllegalArgument("invalid BigDecimal string: $value")
+                }
+            }
+        )
+        ObjString.type.members["d"] = ObjString.type.members.getValue("d").copy(typeDecl = decimalTypeDecl)
+    }
+
+    private fun registerInterop(decimalClass: ObjClass) {
+        OperatorInteropRegistry.register(
+            leftClass = ObjInt.type,
+            rightClass = decimalClass,
+            commonClass = decimalClass,
+            operatorNames = listOf(
+                InteropOperator.Plus.name,
+                InteropOperator.Minus.name,
+                InteropOperator.Mul.name,
+                InteropOperator.Div.name,
+                InteropOperator.Mod.name,
+                InteropOperator.Compare.name,
+                InteropOperator.Equals.name
+            ),
+            leftToCommon = ObjExternCallable.fromBridge {
+                val value = requiredArg<ObjInt>(0).value
+                newInstance(decimalClass, IonBigDecimal.fromLongAsSignificand(value))
+            },
+            rightToCommon = ObjExternCallable.fromBridge {
+                requiredArg<Obj>(0)
+            }
+        )
+    }
+}

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

@@ -364,6 +364,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
         onNotFoundResult: (suspend () -> Obj?)?
     ): Obj {
         val caller = scope.currentClassCtx
+        val methodScope = scope.applyClosure(instanceScope)
         
         // Fast path for public members when outside any class context
         if (caller == null) {
@@ -373,7 +374,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                     if (rec.type == ObjRecord.Type.Property) {
                         if (args.isEmpty()) return (rec.value as ObjProperty).callGetter(scope, this, decl)
                     } else if (rec.type == ObjRecord.Type.Fun) {
-                        return rec.value.invoke(instanceScope, this, args, decl)
+                        return rec.value.invoke(methodScope, this, args, decl)
                     }
                 }
             }
@@ -384,7 +385,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                         if (rec.type == ObjRecord.Type.Property) {
                             if (args.isEmpty()) return (rec.value as ObjProperty).callGetter(scope, this, decl)
                         } else if (rec.type == ObjRecord.Type.Fun) {
-                            return rec.value.invoke(instanceScope, this, args, decl)
+                            return rec.value.invoke(methodScope, this, args, decl)
                         }
                     }
                 }
@@ -394,7 +395,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                         if (rec.type == ObjRecord.Type.Property) {
                             if (args.isEmpty()) return (rec.value as ObjProperty).callGetter(scope, this, decl)
                         } else if (rec.type == ObjRecord.Type.Fun) {
-                            return rec.value.invoke(instanceScope, this, args, decl)
+                            return rec.value.invoke(methodScope, this, args, decl)
                         }
                     }
                 }
@@ -409,7 +410,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                     if (rec.type == ObjRecord.Type.Property) {
                         if (args.isEmpty()) return (rec.value as ObjProperty).callGetter(scope, this, c)
                     } else if (rec.type == ObjRecord.Type.Fun) {
-                        return rec.value.invoke(instanceScope, this, args, c)
+                        return rec.value.invoke(methodScope, this, args, c)
                     }
                 }
             }
@@ -418,7 +419,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                     if (rec.type == ObjRecord.Type.Property) {
                         if (args.isEmpty()) return (rec.value as ObjProperty).callGetter(scope, this, c)
                     } else if (rec.type == ObjRecord.Type.Fun) {
-                        return rec.value.invoke(instanceScope, this, args, c)
+                        return rec.value.invoke(methodScope, this, args, c)
                     }
                 }
             }
@@ -427,7 +428,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                     if (rec.type == ObjRecord.Type.Property) {
                         if (args.isEmpty()) return (rec.value as ObjProperty).callGetter(scope, this, c)
                     } else if (rec.type == ObjRecord.Type.Fun) {
-                        return rec.value.invoke(instanceScope, this, args, c)
+                        return rec.value.invoke(methodScope, this, args, c)
                     }
                 }
             }
@@ -439,7 +440,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                 val decl = rec.declaringClass ?: objClass.findDeclaringClassOf(name) ?: objClass
                 val effectiveCaller = caller ?: if (scope.thisObj === this) objClass else null
                 if (canAccessMember(rec.visibility, decl, effectiveCaller, name)) {
-                    return rec.value.invoke(instanceScope, this, args, decl)
+                    return rec.value.invoke(methodScope, this, args, decl)
                 }
             }
         }
@@ -474,7 +475,7 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
                 if (args.isEmpty()) return (rec.value as ObjProperty).callGetter(scope, this, decl)
             } else if (rec.type == ObjRecord.Type.Fun) {
                 return rec.value.invoke(
-                    instanceScope,
+                    methodScope,
                     this,
                     args,
                     decl
@@ -586,8 +587,10 @@ class ObjInstance(override val objClass: ObjClass) : Obj() {
     }
 
     override suspend fun compareTo(scope: Scope, other: Obj): Int {
-        if (other !is ObjInstance) return -1
+        if (other !is ObjInstance || other.objClass != objClass) {
-        if (other.objClass != objClass) return -1
+            OperatorInteropRegistry.invokeCompare(scope, this, other)?.let { return it }
+            return -1
+        }
         for (f in comparableVars) {
             val a = f.value.value
             val b = other.instanceScope.objects[f.key]?.value ?: scope.raiseError("Internal error: field ${f.key} not found in other instance")

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

@@ -19,6 +19,8 @@ package net.sergeych.lyng.obj
 
 import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonPrimitive
+import net.sergeych.lyng.InteropOperator
+import net.sergeych.lyng.OperatorInteropRegistry
 import net.sergeych.lyng.Scope
 import net.sergeych.lyng.miniast.addFnDoc
 import net.sergeych.lynon.LynonDecoder
@@ -54,10 +56,11 @@ class ObjInt(val value: Long, override val isConst: Boolean = false) : Obj(), Nu
     }
 
     override suspend fun compareTo(scope: Scope, other: Obj): Int {
-        if (other !is Numeric) return -2
         return if (other is ObjInt) {
             value.compareTo(other.value)
         } else {
+            OperatorInteropRegistry.invokeCompare(scope, this, other)?.let { return it }
+            if (other !is Numeric) return -2
             doubleValue.compareTo(other.doubleValue)
         }
     }
@@ -70,28 +73,39 @@ class ObjInt(val value: Long, override val isConst: Boolean = false) : Obj(), Nu
         if (other is ObjInt)
             of(this.value + other.value)
         else
+            OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Plus)
+                ?:
             ObjReal.of(this.doubleValue + other.toDouble())
 
     override suspend fun minus(scope: Scope, other: Obj): Obj =
         if (other is ObjInt)
             of(this.value - other.value)
         else
+            OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Minus)
+                ?:
             ObjReal.of(this.doubleValue - other.toDouble())
 
     override suspend fun mul(scope: Scope, other: Obj): Obj =
         if (other is ObjInt) {
             of(this.value * other.value)
-        } else ObjReal.of(this.value * other.toDouble())
+        } else {
+            OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Mul)
+                ?: ObjReal.of(this.value * other.toDouble())
+        }
 
     override suspend fun div(scope: Scope, other: Obj): Obj =
         if (other is ObjInt)
             of(this.value / other.value)
-        else ObjReal.of(this.value / other.toDouble())
+        else
+            OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Div)
+                ?: ObjReal.of(this.value / other.toDouble())
 
     override suspend fun mod(scope: Scope, other: Obj): Obj =
         if (other is ObjInt)
             of(this.value % other.value)
-        else ObjReal.of(this.value.toDouble() % other.toDouble())
+        else
+            OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Mod)
+                ?: ObjReal.of(this.value.toDouble() % other.toDouble())
 
     /**
      * Numbers are now immutable, so we can't do in-place assignment.

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

@@ -19,6 +19,8 @@ package net.sergeych.lyng.obj
 
 import kotlinx.serialization.json.JsonElement
 import kotlinx.serialization.json.JsonPrimitive
+import net.sergeych.lyng.InteropOperator
+import net.sergeych.lyng.OperatorInteropRegistry
 import net.sergeych.lyng.Scope
 import net.sergeych.lyng.miniast.addConstDoc
 import net.sergeych.lyng.miniast.addFnDoc
@@ -40,6 +42,8 @@ data class ObjReal(val value: Double) : Obj(), Numeric {
     override fun byValueCopy(): Obj = this
 
     override suspend fun compareTo(scope: Scope, other: Obj): Int {
+        if (other is ObjReal) return value.compareTo(other.value)
+        OperatorInteropRegistry.invokeCompare(scope, this, other)?.let { return it }
         if (other !is Numeric) return -2
         return value.compareTo(other.doubleValue)
     }
@@ -63,19 +67,24 @@ data class ObjReal(val value: Double) : Obj(), Numeric {
     }
 
     override suspend fun plus(scope: Scope, other: Obj): Obj =
-        of(this.value + other.toDouble())
+        OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Plus)
+            ?: of(this.value + other.toDouble())
 
     override suspend fun minus(scope: Scope, other: Obj): Obj =
-        of(this.value - other.toDouble())
+        OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Minus)
+            ?: of(this.value - other.toDouble())
 
     override suspend fun mul(scope: Scope, other: Obj): Obj =
-        of(this.value * other.toDouble())
+        OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Mul)
+            ?: of(this.value * other.toDouble())
 
     override suspend fun div(scope: Scope, other: Obj): Obj =
-        of(this.value / other.toDouble())
+        OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Div)
+            ?: of(this.value / other.toDouble())
 
     override suspend fun mod(scope: Scope, other: Obj): Obj =
-        of(this.value % other.toDouble())
+        OperatorInteropRegistry.invokeBinary(scope, this, other, InteropOperator.Mod)
+            ?: of(this.value % other.toDouble())
 
     /**
      * Returns unboxed Double value

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

@@ -0,0 +1,222 @@
+/*
+ * 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 decimalLyng = """
+package lyng.decimal
+
+/**
+ * Rounding policies used by [DecimalContext] and `withDecimalContext(...)`.
+ *
+ * These modes currently affect decimal division. They are designed to be explicit and readable in Lyng code.
+ *
+ * Common examples at precision `2`:
+ * - `HalfEven`: `1.d / 8.d -> 0.12`
+ * - `HalfAwayFromZero`: `1.d / 8.d -> 0.13`, `-1.d / 8.d -> -0.13`
+ * - `HalfTowardsZero`: `1.d / 8.d -> 0.12`, `-1.d / 8.d -> -0.12`
+ * - `Ceiling`: rounds toward positive infinity
+ * - `Floor`: rounds toward negative infinity
+ * - `AwayFromZero`: always increases magnitude when rounding is needed
+ * - `TowardsZero`: always truncates toward zero
+ */
+enum DecimalRounding {
+    HalfEven,
+    HalfAwayFromZero,
+    HalfTowardsZero,
+    Ceiling,
+    Floor,
+    AwayFromZero,
+    TowardsZero
+}
+
+/**
+ * Dynamic decimal arithmetic settings.
+ *
+ * A decimal context is not attached permanently to a `BigDecimal` value. Instead, it is applied dynamically
+ * inside `withDecimalContext(...)`, which makes the rule local to a block of code.
+ *
+ * Default context:
+ * - precision: `34` significant digits
+ * - rounding: `DecimalRounding.HalfEven`
+ *
+ * Example:
+ *
+ *     import lyng.decimal
+ *
+ *     (1.d / 3.d).toStringExpanded()
+ *     >>> "0.3333333333333333333333333333333333"
+ *
+ *     withDecimalContext(10) { (1.d / 3.d).toStringExpanded() }
+ *     >>> "0.3333333333"
+ *
+ *     withDecimalContext(2, DecimalRounding.HalfAwayFromZero) { (1.d / 8.d).toStringExpanded() }
+ *     >>> "0.13"
+ */
+class DecimalContext(
+    val precision: Int = 34,
+    val rounding: DecimalRounding = DecimalRounding.HalfEven
+)
+
+/**
+ * Arbitrary-precision decimal value.
+ *
+ * `BigDecimal` is intended for decimal arithmetic where binary floating-point (`Real`) is the wrong tool:
+ * - money
+ * - human-entered decimal values
+ * - ratios that should round in decimal, not in binary
+ * - reproducible decimal formatting
+ *
+ * Creating values:
+ *
+ * - `1.d` converts `Int -> BigDecimal`
+ * - `2.2.d` converts `Real -> BigDecimal` by preserving the current IEEE-754 value
+ * - `"2.2".d` parses exact decimal text
+ * - `BigDecimal.fromInt(...)`, `fromReal(...)`, `fromString(...)` are explicit factory forms
+ *
+ * Important distinction:
+ *
+ * - `2.2.d` means "take the current `Real` value and convert it"
+ * - `"2.2".d` means "parse this exact decimal literal text"
+ *
+ * Therefore:
+ *
+ *     import lyng.decimal
+ *
+ *     2.2.d.toStringExpanded()
+ *     >>> "2.2"
+ *
+ *     (0.1 + 0.2).d.toStringExpanded()
+ *     >>> "0.30000000000000004"
+ *
+ *     "0.3".d.toStringExpanded()
+ *     >>> "0.3"
+ *
+ * Mixed arithmetic:
+ *
+ * `BigDecimal` defines its own operators against decimal-compatible values, and the decimal module also registers
+ * interop bridges so built-in left-hand operands work naturally:
+ *
+ *     import lyng.decimal
+ *
+ *     1.d + 2
+ *     >>> 3.d
+ *
+ *     1 + 2.d
+ *     >>> 3.d
+ *
+ *     0.5 + 1.d
+ *     >>> 1.5.d
+ *
+ * Precision and rounding:
+ *
+ * - division uses the default decimal context unless overridden
+ * - use `withDecimalContext(...)` to apply a local precision/rounding policy
+ *
+ * Exact decimal literal style:
+ *
+ * If you want the source text itself to be the decimal value, use a string:
+ *
+ *     "2.2".d
+ *
+ * That is the precise form. `2.2.d` remains a `Real -> BigDecimal` conversion by design.
+ */
+extern class BigDecimal() {
+    /** Add another decimal-compatible value. */
+    extern fun plus(other: Object): BigDecimal
+    /** Subtract another decimal-compatible value. */
+    extern fun minus(other: Object): BigDecimal
+    /** Multiply by another decimal-compatible value. */
+    extern fun mul(other: Object): BigDecimal
+    /**
+     * Divide by another decimal-compatible value.
+     *
+     * Division uses the current decimal context:
+     * - by default: `34` significant digits, `HalfEven`
+     * - inside `withDecimalContext(...)`: the context active for the current block
+     */
+    extern fun div(other: Object): BigDecimal
+    /** Remainder with another decimal-compatible value. */
+    extern fun mod(other: Object): BigDecimal
+    /** Compare with another decimal-compatible value. */
+    extern fun compareTo(other: Object): Int
+    /** Unary minus. */
+    extern fun negate(): BigDecimal
+    /** Convert to `Int` by dropping the fractional part according to backend conversion rules. */
+    extern fun toInt(): Int
+    /** Convert to `Real`. */
+    extern fun toReal(): Real
+    /**
+     * Convert to a plain decimal string without scientific notation.
+     *
+     * This is the preferred representation for user-facing decimal tests and diagnostics.
+     */
+    extern fun toStringExpanded(): String
+
+    /** Create a decimal from an `Int`. */
+    static extern fun fromInt(value: Int): BigDecimal
+    /**
+     * Create a decimal from a `Real`.
+     *
+     * This preserves the current IEEE-754 value using a round-trip-safe decimal conversion.
+     * It does not try to recover the original source text.
+     */
+    static extern fun fromReal(value: Real): BigDecimal
+    /** Parse exact decimal text. */
+    static extern fun fromString(value: String): BigDecimal
+}
+
+/**
+ * Run [block] with the provided decimal context.
+ *
+ * This is the main way to control decimal division precision and rounding locally without changing global behavior.
+ *
+ * Example:
+ *
+ *     import lyng.decimal
+ *
+ *     withDecimalContext(10) {
+ *         (1.d / 3.d).toStringExpanded()
+ *     }
+ *     >>> "0.3333333333"
+ *
+ * Contexts are dynamic and block-local. After the block finishes, the previous context is restored.
+ */
+extern fun withDecimalContext<T>(context: DecimalContext, block: ()->T): T
+
+/**
+ * Convenience overload for changing only precision.
+ *
+ * Equivalent to `withDecimalContext(DecimalContext(precision, DecimalRounding.HalfEven), block)`.
+ */
+extern fun withDecimalContext<T>(precision: Int, block: ()->T): T
+
+/**
+ * Convenience overload for changing precision and rounding explicitly.
+ *
+ * Example:
+ *
+ *     import lyng.decimal
+ *
+ *     withDecimalContext(2, DecimalRounding.HalfAwayFromZero) {
+ *         (1.d / 8.d).toStringExpanded()
+ *     }
+ *     >>> "0.13"
+ */
+extern fun withDecimalContext<T>(precision: Int, rounding: DecimalRounding, block: ()->T): T
+""".trimIndent()

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

@@ -0,0 +1,191 @@
+/*
+ * 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 operatorsLyng = """
+package lyng.operators
+
+/**
+ * Binary operators that can be bridged between two different operand classes.
+ *
+ * Registering a pair means:
+ * - the runtime can evaluate `left op right` when `left` has class `L` and `right` has class `R`
+ * - both operands are first converted to a shared "common" class `C`
+ * - the actual operator implementation is then looked up on `C`
+ *
+ * This is primarily useful when:
+ * - you add a new numeric-like type in a library or in pure Lyng code
+ * - your type already implements operators against itself
+ * - you also want existing left-hand types such as `Int` or `Real` to work when your type is on the right
+ *
+ * Example:
+ *
+ *     import lyng.operators
+ *
+ *     class DecimalBox(val value: Int) {
+ *         fun plus(other: DecimalBox) = DecimalBox(value + other.value)
+ *         fun compareTo(other: DecimalBox) = value <=> other.value
+ *     }
+ *
+ *     OperatorInterop.register(
+ *         Int,
+ *         DecimalBox,
+ *         DecimalBox,
+ *         [BinaryOperator.Plus, BinaryOperator.Compare, BinaryOperator.Equals],
+ *         { x: Int -> DecimalBox(x) },
+ *         { x: DecimalBox -> x }
+ *     )
+ *
+ * After registration:
+ * - `1 + DecimalBox(2)` works
+ * - `1 < DecimalBox(2)` works
+ * - `2 == DecimalBox(2)` works
+ *
+ * But this registration does not replace methods on the original classes. It only teaches
+ * Lyng how to bridge a mixed pair into a common class for the listed operators.
+ */
+enum BinaryOperator {
+    /** `a + b` */
+    Plus,
+    /** `a - b` */
+    Minus,
+    /** `a * b` */
+    Mul,
+    /** `a / b` */
+    Div,
+    /** `a % b` */
+    Mod,
+    /**
+     * Ordering comparisons.
+     *
+     * Registering `Compare` enables `<`, `<=`, `>`, `>=`, and the shuttle operator `<=>`
+     * for the mixed operand pair.
+     */
+    Compare,
+    /**
+     * Equality comparisons.
+     *
+     * Registering `Equals` enables `==` and `!=` for the mixed operand pair.
+     */
+    Equals
+}
+
+/**
+ * Runtime registry for mixed-class binary operators.
+ *
+ * `register(L, R, C, ...)` defines how Lyng should evaluate expressions where:
+ * - the left operand has class `L`
+ * - the right operand has class `R`
+ * - the actual operation should be executed as if both were values of class `C`
+ *
+ * The registry is symmetric for the converted values, but not for the original syntax.
+ * Its job is specifically to fill the gap where your custom type appears on the right:
+ *
+ * - `myDecimal + 1` usually already works if `BigDecimal.plus(Int)` exists
+ * - `1 + myDecimal` needs registration because `Int` itself is not rewritten
+ *
+ * Typical pattern for a custom type:
+ *
+ *     import lyng.operators
+ *
+ *     class Rational(val num: Int, val den: Int) {
+ *         fun plus(other: Rational) = Rational(num * other.den + other.num * den, den * other.den)
+ *         fun minus(other: Rational) = Rational(num * other.den - other.num * den, den * other.den)
+ *         fun mul(other: Rational) = Rational(num * other.num, den * other.den)
+ *         fun div(other: Rational) = Rational(num * other.den, den * other.num)
+ *         fun compareTo(other: Rational) = (num * other.den) <=> (other.num * den)
+ *
+ *         static fun fromInt(value: Int) = Rational(value, 1)
+ *     }
+ *
+ *     OperatorInterop.register(
+ *         Int,
+ *         Rational,
+ *         Rational,
+ *         [
+ *             BinaryOperator.Plus,
+ *             BinaryOperator.Minus,
+ *             BinaryOperator.Mul,
+ *             BinaryOperator.Div,
+ *             BinaryOperator.Compare,
+ *             BinaryOperator.Equals
+ *         ],
+ *         { x: Int -> Rational.fromInt(x) },
+ *         { x: Rational -> x }
+ *     )
+ *
+ * Then:
+ * - `1 + Rational(1, 2)` works
+ * - `3 > Rational(5, 2)` works
+ * - `2 == Rational(2, 1)` works
+ *
+ * Decimal uses the same mechanism internally to make `Int + BigDecimal` and `Real + BigDecimal`
+ * work without changing the built-in `Int` or `Real` classes.
+ */
+extern object OperatorInterop {
+    /**
+     * Register a mixed-operand operator bridge.
+     *
+     * @param leftClass class of the original left operand
+     * @param rightClass class of the original right operand
+     * @param commonClass class that will actually execute the operator methods
+     * @param operators operators supported for this mixed pair
+     * @param leftToCommon conversion from `L` to `C`
+     * @param rightToCommon conversion from `R` to `C`
+     *
+     * Requirements for `commonClass`:
+     * - if you register `Plus`, `C` should implement `fun plus(other: C): C` or equivalent accepted result type
+     * - if you register `Minus`, `C` should implement `fun minus(other: C): ...`
+     * - if you register `Mul`, `C` should implement `fun mul(other: C): ...`
+     * - if you register `Div`, `C` should implement `fun div(other: C): ...`
+     * - if you register `Mod`, `C` should implement `fun mod(other: C): ...`
+     * - if you register `Compare`, `C` should implement `fun compareTo(other: C): Int`
+     *
+     * `Equals` reuses comparison/equality semantics of the promoted values.
+     *
+     * Registration is usually done once at module initialization time:
+     *
+     *     package my.rational
+     *     import lyng.operators
+     *
+     *     class Rational(val num: Int, val den: Int) {
+     *         fun plus(other: Rational) = Rational(num * other.den + other.num * den, den * other.den)
+     *         fun compareTo(other: Rational) = (num * other.den) <=> (other.num * den)
+     *         static fun fromInt(value: Int) = Rational(value, 1)
+     *     }
+     *
+     *     OperatorInterop.register(
+     *         Int,
+     *         Rational,
+     *         Rational,
+     *         [BinaryOperator.Plus, BinaryOperator.Compare, BinaryOperator.Equals],
+     *         { x: Int -> Rational.fromInt(x) },
+     *         { x: Rational -> x }
+     *     )
+     */
+    extern fun register<L, R, C>(
+        leftClass: Class<L>,
+        rightClass: Class<R>,
+        commonClass: Class<C>,
+        operators: List<BinaryOperator>,
+        leftToCommon: (L)->C,
+        rightToCommon: (R)->C
+    ): Void
+}
+""".trimIndent()

lynglib/src/commonTest/kotlin/net/sergeych/lyng/BigDecimalModuleTest.kt

@@ -0,0 +1,138 @@
+/*
+ * 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
+
+import kotlinx.coroutines.test.runTest
+import kotlin.test.Test
+
+class BigDecimalModuleTest {
+    @Test
+    fun testDecimalModuleFactoriesAndConversions() = runTest {
+        val scope = Script.newScope()
+        scope.eval(
+            """
+            import lyng.decimal
+
+            assertEquals("12.34", BigDecimal.fromString("12.34").toStringExpanded())
+            assertEquals("1", BigDecimal.fromInt(1).toStringExpanded())
+            assertEquals("2.5", "2.5".d.toStringExpanded())
+            assertEquals("1", 1.d.toStringExpanded())
+            assertEquals("2.2", 2.2.d.toStringExpanded())
+            assertEquals("3", (1 + 2).d.toStringExpanded())
+            assertEquals("1.5", (1 + 0.5).d.toStringExpanded())
+            assertEquals("3", (1 + 2.d).toStringExpanded())
+            assertEquals("3", (2.d + 1).toStringExpanded())
+            assertEquals(2.5, "2.5".d.toReal())
+            assertEquals(2, "2.5".d.toInt())
+            assertEquals(2.2, 2.2.d.toReal())
+            assertEquals("0.30000000000000004", (0.1 + 0.2).d.toStringExpanded())
+            """.trimIndent()
+        )
+    }
+
+    @Test
+    fun testDecimalModuleMixedIntOperators() = runTest {
+        val scope = Script.newScope()
+        scope.eval(
+            """
+            import lyng.decimal
+
+            assertEquals(3.d, 1 + 2.d)
+            assertEquals(3.d, 2.d + 1)
+            assertEquals(1.d, 3 - 2.d)
+            assertEquals(1.d, 3.d - 2)
+            assertEquals(8.d, 4 * 2.d)
+            assertEquals(8.d, 4.d * 2)
+            assertEquals(4.d, 8 / 2.d)
+            assertEquals(4.d, 8.d / 2)
+            assertEquals(1.d, 7 % 2.d)
+            assertEquals(1.d, 7.d % 2)
+            assert(1 < 2.d)
+            assert(2 <= 2.d)
+            assert(3 > 2.d)
+            assert(3.d > 2)
+            assert(2 == 2.d)
+            assert(2.d == 2)
+            """.trimIndent()
+        )
+    }
+
+    @Test
+    fun testDecimalDivisionUsesDefaultContext() = runTest {
+        val scope = Script.newScope()
+        scope.eval(
+            """
+            import lyng.decimal
+
+            assertEquals("0.125", (1.d / 8.d).toStringExpanded())
+            assertEquals("0.3333333333333333333333333333333333", (1.d / 3.d).toStringExpanded())
+            assertEquals("0.6666666666666666666666666666666667", ("2".d / 3.d).toStringExpanded())
+            """.trimIndent()
+        )
+    }
+
+    @Test
+    fun testWithDecimalContextOverridesDivisionContext() = runTest {
+        val scope = Script.newScope()
+        scope.eval(
+            """
+            import lyng.decimal
+
+            assertEquals("0.3333333333333333333333333333333333", (1.d / 3.d).toStringExpanded())
+            assertEquals("0.3333333333", withDecimalContext(10) { (1.d / 3.d).toStringExpanded() })
+            assertEquals("0.666667", withDecimalContext(6) { ("2".d / 3.d).toStringExpanded() })
+            assertEquals("0.666667", withDecimalContext(DecimalContext(6)) { ("2".d / 3.d).toStringExpanded() })
+            assertEquals("0.12", withDecimalContext(2) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("0.13", withDecimalContext(2, DecimalRounding.HalfAwayFromZero) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("0.13", withDecimalContext(DecimalContext(2, DecimalRounding.HalfAwayFromZero)) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("0.3333333333333333333333333333333333", (1.d / 3.d).toStringExpanded())
+            """.trimIndent()
+        )
+    }
+
+    @Test
+    fun testDecimalDivisionRoundingMatrix() = runTest {
+        val scope = Script.newScope()
+        scope.eval(
+            """
+            import lyng.decimal
+
+            assertEquals("0.12", withDecimalContext(2, DecimalRounding.HalfEven) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("-0.12", withDecimalContext(2, DecimalRounding.HalfEven) { (-1.d / 8.d).toStringExpanded() })
+
+            assertEquals("0.13", withDecimalContext(2, DecimalRounding.HalfAwayFromZero) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("-0.13", withDecimalContext(2, DecimalRounding.HalfAwayFromZero) { (-1.d / 8.d).toStringExpanded() })
+
+            assertEquals("0.12", withDecimalContext(2, DecimalRounding.HalfTowardsZero) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("-0.12", withDecimalContext(2, DecimalRounding.HalfTowardsZero) { (-1.d / 8.d).toStringExpanded() })
+
+            assertEquals("0.13", withDecimalContext(2, DecimalRounding.Ceiling) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("-0.12", withDecimalContext(2, DecimalRounding.Ceiling) { (-1.d / 8.d).toStringExpanded() })
+
+            assertEquals("0.12", withDecimalContext(2, DecimalRounding.Floor) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("-0.13", withDecimalContext(2, DecimalRounding.Floor) { (-1.d / 8.d).toStringExpanded() })
+
+            assertEquals("0.13", withDecimalContext(2, DecimalRounding.AwayFromZero) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("-0.13", withDecimalContext(2, DecimalRounding.AwayFromZero) { (-1.d / 8.d).toStringExpanded() })
+
+            assertEquals("0.12", withDecimalContext(2, DecimalRounding.TowardsZero) { (1.d / 8.d).toStringExpanded() })
+            assertEquals("-0.12", withDecimalContext(2, DecimalRounding.TowardsZero) { (-1.d / 8.d).toStringExpanded() })
+            """.trimIndent()
+        )
+    }
+}

lynglib/src/commonTest/kotlin/net/sergeych/lyng/OperatorInteropTest.kt

@@ -0,0 +1,111 @@
+/*
+ * 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
+
+import kotlinx.coroutines.test.runTest
+import kotlin.test.Test
+
+class OperatorInteropTest {
+    @Test
+    fun testPureLyngOperatorInteropRegistration() = runTest {
+        val im = Script.defaultImportManager.copy()
+        im.addPackage("test.decimalbox") { scope ->
+            scope.eval(
+                """
+                package test.decimalbox
+                import lyng.operators
+
+                class DecimalBox(val value: Int) {
+                    fun plus(other: DecimalBox) = DecimalBox(value + other.value)
+                    fun minus(other: DecimalBox) = DecimalBox(value - other.value)
+                    fun mul(other: DecimalBox) = DecimalBox(value * other.value)
+                    fun div(other: DecimalBox) = DecimalBox(value / other.value)
+                    fun mod(other: DecimalBox) = DecimalBox(value % other.value)
+                    fun compareTo(other: DecimalBox) = value <=> other.value
+                }
+
+                OperatorInterop.register(
+                    Int,
+                    DecimalBox,
+                    DecimalBox,
+                    [BinaryOperator.Plus, BinaryOperator.Minus, BinaryOperator.Mul, BinaryOperator.Div, BinaryOperator.Mod, BinaryOperator.Compare, BinaryOperator.Equals],
+                    { x: Int -> DecimalBox(x) },
+                    { x: DecimalBox -> x }
+                )
+                """.trimIndent()
+            )
+        }
+
+        val scope = im.newStdScope()
+        scope.eval(
+            """
+            import test.decimalbox
+
+            assertEquals(DecimalBox(3), 1 + DecimalBox(2))
+            assertEquals(DecimalBox(1), 3 - DecimalBox(2))
+            assertEquals(DecimalBox(8), 4 * DecimalBox(2))
+            assertEquals(DecimalBox(4), 8 / DecimalBox(2))
+            assertEquals(DecimalBox(1), 7 % DecimalBox(2))
+            assert(1 < DecimalBox(2))
+            assert(2 <= DecimalBox(2))
+            assert(3 > DecimalBox(2))
+            assert(2 >= DecimalBox(2))
+            assert(2 == DecimalBox(2))
+            assert(2 != DecimalBox(3))
+            """.trimIndent()
+        )
+    }
+
+    @Test
+    fun testRealInteropRegistrationUsesTopLevelModuleCode() = runTest {
+        val im = Script.defaultImportManager.copy()
+        im.addPackage("test.realbox") { scope ->
+            scope.eval(
+                """
+                package test.realbox
+                import lyng.operators
+
+                class RealBox(val value: Real) {
+                    fun plus(other: RealBox) = RealBox(value + other.value)
+                    fun compareTo(other: RealBox) = value <=> other.value
+                }
+
+                OperatorInterop.register(
+                    Real,
+                    RealBox,
+                    RealBox,
+                    [BinaryOperator.Plus, BinaryOperator.Compare, BinaryOperator.Equals],
+                    { x: Real -> RealBox(x) },
+                    { x: RealBox -> x }
+                )
+                """.trimIndent()
+            )
+        }
+
+        val scope = im.newStdScope()
+        scope.eval(
+            """
+            import test.realbox
+
+            assertEquals(RealBox(1.75), 0.5 + RealBox(1.25))
+            assert(1.5 < RealBox(2.0))
+            assert(2.0 == RealBox(2.0))
+            """.trimIndent()
+        )
+    }
+}

site/src/jsMain/kotlin/HomePage.kt

@@ -27,74 +27,85 @@ import org.jetbrains.compose.web.dom.*
 fun HomePage() {
     val samples = remember {
         listOf(
+            """
+            // Decimal arithmetic with explicit local precision
+            import lyng.decimal
+
+            val exact = "0.3".d
+            val fromReal = (0.1 + 0.2).d
+
+            println(exact.toStringExpanded())
+            println(fromReal.toStringExpanded())
+            println(withDecimalContext(10) { (1.d / 3.d).toStringExpanded() })
+            """.trimIndent(),
+            """
+            // Mixed operators for your own type
+            import lyng.operators
+
+            class Coins(val amount: Int) {
+                fun plus(other: Coins) = Coins(amount + other.amount)
+                fun compareTo(other: Coins) = amount <=> other.amount
+            }
+
+            OperatorInterop.register(
+                Int,
+                Coins,
+                Coins,
+                [BinaryOperator.Plus, BinaryOperator.Compare, BinaryOperator.Equals],
+                { x: Int -> Coins(x) },
+                { x: Coins -> x }
+            )
+
+            println(1 + Coins(2))
+            println(3 > Coins(2))
+            """.trimIndent(),
+            """
+            // Non-local returns from closures
+            fun findFirst<T>(list: Iterable<T>, predicate: (T)->Bool): T? {
+                list.forEach {
+                    if (predicate(it)) return@findFirst it
+                }
+                null
+            }
+
+            val found: Int? = findFirst([1, 5, 8, 12]) { it > 10 }
+            println("Found: " + found)
+            """.trimIndent(),
+            """
+            // Implicit coroutines: parallelism without ceremony
+            import lyng.time
+
+            val d1 = launch {
+                delay(100.milliseconds)
+                "Task A finished"
+            }
+            val d2 = launch {
+                delay(50.milliseconds)
+                "Task B finished"
+            }
+
+            println(d1.await())
+            println(d2.await())
+            """.trimIndent(),
             """
             // Everything is an expression
             val x: Int = 10
             val status: String = if (x > 0) "Positive" else "Zero or Negative"
 
-            // Even loops return values!
             val result = for (i in 1..5) {
                 if (i == 3) break "Found 3!"
             } else "Not found"
 
-            println("Result: " + result)
+            println(status)
+            println(result)
             """.trimIndent(),
             """
-            // Functional power with generics and collections
+            // Functional collections with strict static types
             val squares: List<Int> = (1..10)
                 .filter { it % 2 == 0 }
                 .map { it * it }
 
             println("Even squares: " + squares)
-            // Output: [4, 16, 36, 64, 100]
-            """.trimIndent(),
-            """
-            // Generics and type aliases
-            type Num = Int | Real
-            
-            class Box<out T: Num>(val value: T) {
-                fun get(): T = value
-            }
-            
-            val intBox = Box(42)
-            val realBox = Box(3.14)
-            println("Boxes: " + intBox.get() + ", " + realBox.get())
-            """.trimIndent(),
-            """
-            // Strict compile-time types and symbol resolution
-            fun greet(name: String, count: Int) {
-                for (i in 1..count) {
-                    println("Hello, " + name + "!")
-                }
-            }
-            
-            greet("Lyng", 3)
-            // greet(10, "error") // This would be a compile-time error!
-            """.trimIndent(),
-            """
-            // Flexible map literals and shorthands
-            val id = 101
-            val name = "Lyng"
-            val base = { id:, name: } // Shorthand for id: id, name: name
-
-            val full = { ...base, version: "1.5.0-SNAPSHOT", status: "active" }
-            println(full)
-            """.trimIndent(),
-            """
-            // Modern null safety
-            var config: Map<String, Int>? = null
-            config ?= { timeout: 30 } // Assign only if null
-
-            val timeout = config?["timeout"] ?: 60
-            println("Timeout is: " + timeout)
-            """.trimIndent(),
-            """
-            // Destructuring with splat operator
-            val [first, middle..., last] = [1, 2, 3, 4, 5, 6]
-
-            println("First: " + first)
-            println("Middle: " + middle)
-            println("Last: " + last)
             """.trimIndent(),
             """
             // Diamond-safe Multiple Inheritance (C3 MRO)
@@ -123,18 +134,6 @@ fun HomePage() {
             println([10, 20, 30].second)
             """.trimIndent(),
             """
-            // Non-local returns from closures
-            fun findFirst<T>(list: Iterable<T>, predicate: (T)->Bool): T? {
-                list.forEach {
-                    if (predicate(it)) return@findFirst it
-                }
-                null
-            }
-
-            val found: Int? = findFirst([1, 5, 8, 12]) { it > 10 }
-            println("Found: " + found)
-            """.trimIndent(),
-            """
             // Easy operator overloading
             class Vector(val x: Real, val y: Real) {
                 fun plus(other: Vector): Vector = Vector(x + other.x, y + other.y)
@@ -156,20 +155,13 @@ fun HomePage() {
             println("User name: " + u.name)
             """.trimIndent(),
             """
-            // Implicit coroutines: parallelism without ceremony
+            // Flexible map literals and shorthands
-            import lyng.time
+            val id = 101
+            val name = "Lyng"
+            val base = { id:, name: }
+            val full = { ...base, status: "active", tags: ["typed", "portable"] }
 
-            val d1 = launch {
+            println(full)
-                delay(100.milliseconds)
-                "Task A finished"
-            }
-            val d2 = launch {
-                delay(50.milliseconds)
-                "Task B finished"
-            }
-
-            println(d1.await())
-            println(d2.await())
             """.trimIndent()
         )
     }
@@ -195,6 +187,8 @@ fun HomePage() {
             Div({ classes("d-flex", "justify-content-center", "gap-2", "flex-wrap", "mb-4") }) {
                 // Benefits pills
                 listOf(
+                    "Decimal arithmetic",
+                    "Operator interop",
                     "Strict static typing",
                     "Generics & Type Aliases",
                     "Implicit coroutines",
@@ -233,6 +227,26 @@ fun HomePage() {
         }
     }
 
+    Div({ classes("row", "g-3", "mb-4") }) {
+        listOf(
+            Triple("New: Decimal", "Exact decimal values, local precision control, and mixed operators with Int and Real.", "#/docs/Decimal.md"),
+            Triple("New: Operator Interop", "Teach Lyng how your custom types interact with built-ins on the left-hand side.", "#/docs/OperatorInterop.md"),
+            Triple("What Changed", "Recent language, stdlib, and tooling improvements in one place.", "#/docs/whats_new.md")
+        ).forEach { (title, text, href) ->
+            Div({ classes("col-12", "col-lg-4") }) {
+                A(attrs = {
+                    classes("text-decoration-none")
+                    attr("href", href)
+                }) {
+                    Div({ classes("h-100", "p-3", "border", "rounded-3", "bg-body-tertiary") }) {
+                        H3({ classes("h5", "mb-2", "text-body") }) { Text(title) }
+                        P({ classes("mb-0", "text-muted") }) { Text(text) }
+                    }
+                }
+            }
+        }
+    }
+
     // Code sample slideshow
     Div({
         classes("markdown-body", "mt-0", "slide-container", "position-relative")