Implement nullable shorthand for params

docs/tutorial.md

@@ -224,6 +224,9 @@ This also prevents chain assignments so use parentheses:
 
 ## Nullability
 
+Nullability is part of the type. `String` is non-null, `String?` is nullable. Use `!!` to assert non-null and throw
+`NullReferenceException` if the value is `null`.
+
 When the value is `null`, it might throws `NullReferenceException`, the name is somewhat a tradition. To avoid it
 one can check it against null or use _null coalescing_. The null coalescing means, if the operand (left) is null,
 the operation won't be performed and the result will be null. Here is the difference:
@@ -242,6 +245,9 @@ the operation won't be performed and the result will be null. Here is the differ
     assert( ref?() == null )
     >>> void
 
+Note: `?.` is still a typed operation. The receiver must have a compile-time type that declares the member; if the
+receiver is `Object`, cast it first or declare a more specific type.
+
 There is also "elvis operator", null-coalesce infix operator '?:' that returns rvalue if lvalue is `null`:
 
     null ?: "nothing"
@@ -425,8 +431,6 @@ Almost the same, using `val`:
     val foo = 1
     foo += 1 // this will throw exception
 
-# Constants
-
 Same as in kotlin:
 
     val HalfPi = π / 2
@@ -434,6 +438,133 @@ Same as in kotlin:
 Note using greek characters in identifiers! All letters allowed, but remember who might try to read your script, most
 likely will know some English, the rest is the pure uncertainty.
 
+# Types and inference
+
+Lyng uses Kotlin-style static types with inference. You can always write explicit types, but in most places the compiler
+can infer them from literals, defaults, and flow analysis.
+
+## Type annotations
+
+Use `:` to specify a type:
+
+    var x: Int = 10
+    val label: String = "count"
+    fun clamp(x: Int, min: Int, max: Int): Int { ... }
+
+`Object` is the top type. If you omit a type and there is no default value, the parameter is `Object` by default:
+
+    fun show(x) { println(x) }   // x is Object
+
+For nullable types, add `?`:
+
+    fun showMaybe(x: Object?) { ... }
+    fun parseInt(s: String?): Int? { ... }
+
+There is also a nullable shorthand for untyped parameters and constructor args: `x?` means `x: Object?`.
+It cannot be combined with an explicit type annotation.
+
+    class A(x?) { ... }     // x: Object?
+    fun f(x?) { x == null } // x: Object?
+
+`void` is a singleton value of the class `Void`. `Void` can be used as an explicit return type:
+
+    fun log(msg): Void { println(msg); void }
+
+`Null` is the class of `null`. It is a singleton type and mostly useful for type inference results.
+
+## Type inference
+
+The compiler infers types from:
+
+- literals: `1` is `Int`, `1.0` is `Real`, `"s"` is `String`, `'c'` is `Char`
+- defaults: `fun f(x=1, name="n")` infers `x: Int`, `name: String`
+- assignments: `val x = call()` uses the return type of `call`
+- returns and branches: the result type of a block is the last expression, and if any branch is nullable,
+  the inferred type becomes nullable
+- numeric ops: `Int` and `Real` stay `Int` when both sides are `Int`, and promote to `Real` on mixed arithmetic
+
+Examples:
+
+    fun inc(x=0) = x + 1        // (Int)->Int
+    fun maybe(flag) { if(flag) 1 else null } // ()->Int?
+
+Untyped locals are allowed, but their type is fixed on the first assignment:
+
+    var x
+    x = 1       // x becomes Int
+    x = "one"   // compile-time error
+
+    var y = null  // y is Object?
+    val z = null  // z is Null
+
+Empty list/map literals default to `List<Object>` and `Map<Object,Object>` until a more specific type is known:
+
+    val xs = []             // List<Object>
+    val ys: List<Int> = []  // List<Int>
+
+## Flow analysis
+
+Lyng uses flow analysis to narrow types inside branches:
+
+    fun len(x: String?): Int {
+        if( x == null ) return 0
+        // x is String (non-null) in this branch
+        return x.length
+    }
+
+`is` checks and `when` branches also narrow types:
+
+    fun kind(x: Object) {
+        if( x is Int ) return "int"
+        if( x is String ) return "string"
+        return "other"
+    }
+
+Narrowing is local to the branch; after the branch, the original type is restored.
+
+## Casts and unknown types
+
+Use `as` for explicit casts. The compiler inserts casts only when it can be valid and necessary. If a cast fails at
+runtime, it throws `ClassCastException`. If the value is nullable, `as T` implies a non-null assertion.
+
+Member access is resolved at compile time. Only members of `Object` are available on unknown types; non-Object members
+require an explicit cast:
+
+    fun f(x) {             // x is Object
+        x.toString()       // ok (Object member)
+        x.size()           // compile-time error
+        (x as List).size() // ok
+    }
+
+This avoids runtime name-resolution fallbacks; all symbols must be known at compile time.
+
+## Generics and bounds
+
+Generic parameters are declared with `<...>`:
+
+    fun id<T>(x: T): T = x
+    class Box<T>(val value: T)
+
+Bounds use `:` and can combine with `&` (intersection) and `|` (union):
+
+    fun sum<T: Int | Real>(x: T, y: T) = x + y
+    class Named<T: Iterable & Comparable>(val data: T)
+
+Type arguments are usually inferred from call sites:
+
+    val b = Box(10)     // Box<Int>
+    val s = id("ok")    // T is String
+
+## Variance
+
+Generic types are invariant by default, so `List<Int>` is not a `List<Object>`.
+Use declaration-site variance when you need it:
+
+    class Source<out T>(val value: T)
+    class Sink<in T> { fun accept(x: T) { ... } }
+
+`out` makes the type covariant (only produced), `in` makes it contravariant (only consumed).
+
 # Defining functions
 
     fun check(amount) {
@@ -1330,7 +1461,7 @@ than enum arrays, until `Lynon.encodeTyped` will be implemented.
     var result = null // here we will store the result
     >>> null
 
-# Integral data types
+# Built-in types
 
 | type   | description                     | literal samples     |
 |--------|---------------------------------|---------------------|
@@ -1340,6 +1471,7 @@ than enum arrays, until `Lynon.encodeTyped` will be implemented.
 | Char   | single unicode character        | `'S'`, `'\n'`       |
 | String | unicode string, no limits       | "hello" (see below) |
 | List   | mutable list                    | [1, "two", 3]       |
+| Object | top type for all values         |                     |
 | Void   | no value could exist, singleton | void                |
 | Null   | missing value, singleton        | null                |
 | Fn     | callable type                   |                     |

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

@@ -2641,8 +2641,31 @@ class Compiler(
                         access
                     }
 
+                    // Nullable shorthand: "x?" means Object? (or makes explicit type nullable)
+                    var nullableHint = false
+                    val nullableHintPos = cc.savePos()
+                    cc.skipWsTokens()
+                    if (cc.peekNextNonWhitespace().type == Token.Type.QUESTION) {
+                        cc.nextNonWhitespace()
+                        nullableHint = true
+                    } else {
+                        cc.restorePos(nullableHintPos)
+                    }
+
                     // type information (semantic + mini syntax)
-                    val (typeInfo, miniType) = parseTypeDeclarationWithMini()
+                    if (nullableHint) {
+                        val afterHint = cc.savePos()
+                        cc.skipWsTokens()
+                        if (cc.peekNextNonWhitespace().type == Token.Type.COLON) {
+                            throw ScriptError(t.pos, "nullable shorthand '?' cannot be combined with explicit type")
+                        }
+                        cc.restorePos(afterHint)
+                    }
+                    var (typeInfo, miniType) = parseTypeDeclarationWithMini()
+                    if (nullableHint) {
+                        typeInfo = makeTypeDeclNullable(typeInfo)
+                        miniType = miniType?.let { makeMiniTypeNullable(it) }
+                    }
 
                     var defaultValue: Statement? = null
                     cc.ifNextIs(Token.Type.ASSIGN) {
@@ -2713,6 +2736,31 @@ class Compiler(
         return parseTypeUnionWithMini()
     }
 
+    private fun makeTypeDeclNullable(type: TypeDecl): TypeDecl {
+        if (type.isNullable) return type
+        return when (type) {
+            TypeDecl.TypeAny -> TypeDecl.TypeNullableAny
+            TypeDecl.TypeNullableAny -> type
+            is TypeDecl.Function -> type.copy(nullable = true)
+            is TypeDecl.TypeVar -> type.copy(nullable = true)
+            is TypeDecl.Union -> type.copy(nullable = true)
+            is TypeDecl.Intersection -> type.copy(nullable = true)
+            is TypeDecl.Simple -> TypeDecl.Simple(type.name, true)
+            is TypeDecl.Generic -> TypeDecl.Generic(type.name, type.args, true)
+        }
+    }
+
+    private fun makeMiniTypeNullable(type: MiniTypeRef): MiniTypeRef {
+        return when (type) {
+            is MiniTypeName -> type.copy(nullable = true)
+            is MiniGenericType -> type.copy(nullable = true)
+            is MiniFunctionType -> type.copy(nullable = true)
+            is MiniTypeVar -> type.copy(nullable = true)
+            is MiniTypeUnion -> type.copy(nullable = true)
+            is MiniTypeIntersection -> type.copy(nullable = true)
+        }
+    }
+
     private fun parseTypeUnionWithMini(): Pair<TypeDecl, MiniTypeRef> {
         var left = parseTypeIntersectionWithMini()
         val options = mutableListOf(left)

lynglib/src/commonTest/kotlin/TypesTest.kt

@@ -146,4 +146,24 @@ class TypesTest {
             println("Result: "+limit)
         """.trimIndent())
     }
+
+    @Test
+    fun testNullableHints() = runTest {
+        eval("""
+            // nullable, without type os Object?
+            class N(x=null)
+            assertEquals(null, N().x)
+            assertEquals("foo", N("foo").x)
+            // nullable shortcut (x?)is same as (var x: Object?)
+            class A(x?)
+            assertEquals(null, A(null).x)
+            assertEquals("ok", A("ok").x)
+            
+            // same in function: x? is a shortcut for (x: Object?)
+            fun f(x?) = x?.let { x + "!" }
+            assertEquals(null, f(null))
+            assertEquals("ok!", f("ok"))
+        """.trimIndent()
+        )
+    }
 }