fix #19 set of null-coalesce operators

docs/exceptions_handling.md

@@ -131,7 +131,7 @@ _this functionality is not yet released_
 | class                      | notes                                                 |
 |----------------------------|-------------------------------------------------------|
 | Exception                  | root of al throwable objects                          |
-| NullPointerException       |                                                       |
+| NullReferenceException       |                                                       |
 | AssertionFailedException   |                                                       | 
 | ClassCastException         |                                                       |
 | IndexOutOfBoundsException  |                                                       |

docs/tutorial.md

@@ -118,6 +118,51 @@ These operators return rvalue, unmodifiable.
 
 ## Assignment return r-value!
 
+Naturally, assignment returns its value:
+
+    var x
+    x = 11
+    >>> 11
+
+rvalue means you cant assign the result if the assignment
+
+    var x
+    assertThrows { (x = 11) = 5 }
+    void
+    >>> void
+
+This also prevents chain assignments so use parentheses:
+
+    var x
+    var y
+    x = (y = 1)
+    >>> 1
+
+## Nullability
+
+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:
+
+    val ref = null
+    assertThrows {  ref.field }
+    assertThrows {  ref.method() }
+    assertThrows {  ref.array[1] }
+    assertThrows {  ref[1] }
+    assertThrows {  ref() }
+    
+    assert( ref?.field == null )
+    assert( ref?.method() == null )
+    assert( ref?.array?[1] == null )
+    assert( ref?[1] == null )
+    assert( ref?() == null )
+    >>> void
+
+There is also "elvis operator", null-coalesce infix operator '?:' that returns rvalue if lvalue is `null`:
+
+    null ?: "nothing"
+    >>> "nothing"
+
 ## Math
 
 It is rather simple, like everywhere else:

lynglib/build.gradle.kts

@@ -4,7 +4,7 @@ import org.jetbrains.kotlin.gradle.ExperimentalWasmDsl
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
 
 group = "net.sergeych"
-version = "0.6.1-SNAPSHOT"
+version = "0.6.-SNAPSHOT"
 
 buildscript {
     repositories {

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

@@ -121,7 +121,8 @@ class Compiler(
                     operand = Accessor { op.getter(it).value.logicalNot(it).asReadonly }
                 }
 
-                Token.Type.DOT -> {
+                Token.Type.DOT, Token.Type.NULL_COALESCE -> {
+                    var isOptional = t.type == Token.Type.NULL_COALESCE
                     operand?.let { left ->
                         // dotcall: calling method on the operand, if next is ID, "("
                         var isCall = false
@@ -138,6 +139,9 @@ class Compiler(
                                     operand = Accessor { context ->
                                         context.pos = next.pos
                                         val v = left.getter(context).value
+                                        if (v == ObjNull && isOptional)
+                                            ObjNull.asReadonly
+                                        else
                                             ObjRecord(
                                                 v.invokeInstanceMethod(
                                                     context,
@@ -148,7 +152,9 @@ class Compiler(
                                     }
                                 }
 
-                                Token.Type.LBRACE -> {
+
+                                Token.Type.LBRACE, Token.Type.NULL_COALESCE_BLOCKINVOKE -> {
+                                    isOptional = nt.type == Token.Type.NULL_COALESCE_BLOCKINVOKE
                                     // single lambda arg, like assertTrows { ... }
                                     cc.next()
                                     isCall = true
@@ -159,6 +165,9 @@ class Compiler(
                                     operand = Accessor { context ->
                                         context.pos = next.pos
                                         val v = left.getter(context).value
+                                        if (v == ObjNull && isOptional)
+                                            ObjNull.asReadonly
+                                        else
                                             ObjRecord(
                                                 v.invokeInstanceMethod(
                                                     context,
@@ -174,25 +183,30 @@ class Compiler(
                         }
                         if (!isCall) {
                             operand = Accessor({ context ->
-                                left.getter(context).value.readField(context, next.value)
+                                val x = left.getter(context).value
+                                if (x == ObjNull && isOptional) ObjNull.asReadonly
+                                else x.readField(context, next.value)
                             }) { cc, newValue ->
                                 left.getter(cc).value.writeField(cc, next.value, newValue)
                             }
                         }
-                    } ?: throw ScriptError(t.pos, "Expecting expression before dot")
+                    }
+
+                        ?: throw ScriptError(t.pos, "Expecting expression before dot")
                 }
 
                 Token.Type.COLONCOLON -> {
                     operand = parseScopeOperator(operand, cc)
                 }
 
-                Token.Type.LPAREN -> {
+                Token.Type.LPAREN, Token.Type.NULL_COALESCE_INVOKE -> {
                     operand?.let { left ->
                         // this is function call from <left>
                         operand = parseFunctionCall(
                             cc,
                             left,
                             false,
+                            t.type == Token.Type.NULL_COALESCE_INVOKE
                         )
                     } ?: run {
                         // Expression in parentheses
@@ -205,15 +219,18 @@ class Compiler(
                     }
                 }
 
-                Token.Type.LBRACKET -> {
+                Token.Type.LBRACKET, Token.Type.NULL_COALESCE_INDEX -> {
                     operand?.let { left ->
                         // array access
+                        val isOptional = t.type == Token.Type.NULL_COALESCE_INDEX
                         val index = parseStatement(cc) ?: throw ScriptError(t.pos, "Expecting index expression")
                         cc.skipTokenOfType(Token.Type.RBRACKET, "missing ']' at the end of the list literal")
                         operand = Accessor({ cxt ->
                             val i = (index.execute(cxt) as? ObjInt)?.value?.toInt()
                                 ?: cxt.raiseError("index must be integer")
-                            left.getter(cxt).value.getAt(cxt, i).asMutable
+                            val x = left.getter(cxt).value
+                            if( x == ObjNull && isOptional) ObjNull.asReadonly
+                            else x.getAt(cxt, i).asMutable
                         }) { cxt, newValue ->
                             val i = (index.execute(cxt) as? ObjInt)?.value?.toInt()
                                 ?: cxt.raiseError("index must be integer")
@@ -337,10 +354,15 @@ class Compiler(
                     }
                 }
 
-                Token.Type.LBRACE -> {
+                Token.Type.LBRACE, Token.Type.NULL_COALESCE_BLOCKINVOKE -> {
                     operand = operand?.let { left ->
                         cc.previous()
-                        parseFunctionCall(cc, left, blockArgument = true)
+                        parseFunctionCall(
+                            cc,
+                            left,
+                            blockArgument = true,
+                            t.type == Token.Type.NULL_COALESCE_BLOCKINVOKE
+                        )
                     } ?: parseLambdaExpression(cc)
                 }
 
@@ -590,7 +612,12 @@ class Compiler(
     }
 
 
-    private fun parseFunctionCall(cc: CompilerContext, left: Accessor, blockArgument: Boolean): Accessor {
+    private fun parseFunctionCall(
+        cc: CompilerContext,
+        left: Accessor,
+        blockArgument: Boolean,
+        isOptional: Boolean
+    ): Accessor {
         // insofar, functions always return lvalue
         var detectedBlockArgument = blockArgument
         val args = if (blockArgument) {
@@ -607,6 +634,7 @@ class Compiler(
 
         return Accessor { context ->
             val v = left.getter(context)
+            if (v.value == ObjNull && isOptional) return@Accessor v.value.asReadonly
             v.value.callOn(
                 context.copy(
                     context.pos,
@@ -1600,6 +1628,9 @@ class Compiler(
             Operator.simple(Token.Type.NOTIN, lastPrty) { c, a, b -> ObjBool(!b.contains(c, a)) },
             Operator.simple(Token.Type.IS, lastPrty) { c, a, b -> ObjBool(a.isInstanceOf(b)) },
             Operator.simple(Token.Type.NOTIS, lastPrty) { c, a, b -> ObjBool(!a.isInstanceOf(b)) },
+
+            Operator.simple(Token.Type.ELVIS, ++lastPrty) { c, a, b -> if( a == ObjNull) b else a },
+
             // shuttle <=> 6
             Operator.simple(Token.Type.SHUTTLE, ++lastPrty) { c, a, b ->
                 ObjInt(a.compareTo(c, b).toLong())

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

@@ -16,7 +16,7 @@ class Context(
     fun raiseNotImplemented(what: String = "operation"): Nothing = raiseError("$what is not implemented")
 
     @Suppress("unused")
-    fun raiseNPE(): Nothing = raiseError(ObjNullPointerException(this))
+    fun raiseNPE(): Nothing = raiseError(ObjNullReferenceException(this))
 
     @Suppress("unused")
     fun raiseIndexOutOfBounds(message: String = "Index out of bounds"): Nothing =

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

@@ -289,6 +289,26 @@ object ObjNull : Obj() {
         return other is ObjNull || other == null
     }
 
+    override suspend fun readField(context: Context, name: String): ObjRecord {
+        context.raiseNPE()
+    }
+
+    override suspend fun invokeInstanceMethod(context: Context, name: String, args: Arguments): Obj {
+        context.raiseNPE()
+    }
+
+    override suspend fun getAt(context: Context, index: Int): Obj {
+        context.raiseNPE()
+    }
+
+    override suspend fun putAt(context: Context, index: Int, newValue: Obj) {
+        context.raiseNPE()
+    }
+
+    override suspend fun callOn(context: Context): Obj {
+        context.raiseNPE()
+    }
+
     override fun toString(): String = "null"
 }
 
@@ -383,7 +403,7 @@ open class ObjException(exceptionClass: ExceptionClass, val context: Context, va
             context.addConst("Exception", Root)
             existingErrorClasses["Exception"] = Root
             for (name in listOf(
-                "NullPointerException",
+                "NullReferenceException",
                 "AssertionFailedException",
                 "ClassCastException",
                 "IndexOutOfBoundsException",
@@ -400,7 +420,7 @@ open class ObjException(exceptionClass: ExceptionClass, val context: Context, va
     }
 }
 
-class ObjNullPointerException(context: Context) : ObjException("NullPointerException", context, "object is null")
+class ObjNullReferenceException(context: Context) : ObjException("NullReferenceException", context, "object is null")
 
 class ObjAssertionFailedException(context: Context, message: String) :
     ObjException("AssertionFailedException", context, message)

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

@@ -267,6 +267,21 @@ private class Parser(fromPos: Pos) {
                 Token(value.toString(), start, Token.Type.CHAR)
             }
 
+            '?' -> {
+                when(currentChar.also { pos.advance() }) {
+                    ':' -> Token("??", from, Token.Type.ELVIS)
+                    '?' -> Token("??", from, Token.Type.ELVIS)
+                    '.' -> Token("?.", from, Token.Type.NULL_COALESCE)
+                    '[' -> Token("?(", from, Token.Type.NULL_COALESCE_INDEX)
+                    '(' -> Token("?(", from, Token.Type.NULL_COALESCE_INVOKE)
+                    '{' -> Token("?{", from, Token.Type.NULL_COALESCE_BLOCKINVOKE)
+                    else -> {
+                        pos.back()
+                        Token("?", from, Token.Type.QUESTION)
+                    }
+                }
+            }
+
             else -> {
                 // text infix operators:
                 // Labels processing is complicated!

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

@@ -21,6 +21,11 @@ data class Token(val value: String, val pos: Pos, val type: Type) {
         ELLIPSIS, DOTDOT, DOTDOTLT,
         NEWLINE,
         EOF,
+        NULL_COALESCE,
+        ELVIS,
+        NULL_COALESCE_INDEX,
+        NULL_COALESCE_INVOKE,
+        NULL_COALESCE_BLOCKINVOKE,
     }
 
     companion object {

lynglib/src/commonTest/kotlin/ScriptTest.kt

@@ -2113,4 +2113,24 @@ class ScriptTest {
     """.trimIndent()
         )
     }
+
+    @Test
+    fun testNull1() = runTest {
+        eval("""
+            var s = null
+            assertThrows { s.length }
+            assertThrows { s.size() }
+            
+            assertEquals( null, s?.size() )
+            assertEquals( null, s?.length )
+            assertEquals( null, s?.length ?{ "test" } )
+            assertEquals( null, s?[1] )
+            assertEquals( null, s ?{ "test" } )
+            assertEquals( null, s.test ?{ "test" } )
+            
+            s = "xx"
+            assert(s.lower().size == 2)
+            assert(s.length == 2)
+        """.trimIndent())
+    }
 }