Support optional arguments by tracking separate arities

README.md

@@ -62,6 +62,20 @@ greet("user")
 # → Hello, user!
 ```
 
+Optional arguments are supported, though as of writing they must default to `None`:
+
+```py
+def greet(name=None):
+    if not name:
+        print "Hello, world!"
+    else:
+        print "Hello, " + name + "!"
+greet()
+gree("user")
+# → Hello, world!
+#   Hello, user!
+```
+
 ### Variables
 
 In a departure from Python, Kuroko has explicit variable declaration and traditional scoping rules. Variables are declared with the `let` keyword and take the value `None` if not defined at declaration time:

compiler.c

@@ -495,10 +495,16 @@ static void function(FunctionType type, size_t blockWidth) {
 				}
 				continue;
 			}
-			current->function->arity++;
-			if (current->function->arity > 255) errorAtCurrent("too many function parameters");
 			ssize_t paramConstant = parseVariable("Expect parameter name.");
 			defineVariable(paramConstant);
+			if (match(TOKEN_EQUAL)) {
+				consume(TOKEN_NONE,"Optional arguments can only be assigned the default value of None.");
+				current->function->defaultArgs++;
+				if (current->function->defaultArgs > 255) errorAtCurrent("too many function parameters");
+			} else {
+				current->function->requiredArgs++;
+				if (current->function->requiredArgs > 255) errorAtCurrent("too many function parameters");
+			}
 		} while (match(TOKEN_COMMA));
 	}
 	consume(TOKEN_RIGHT_PAREN, "Expected end of parameter list.");

object.c

@@ -109,7 +109,8 @@ void krk_printObject(FILE * f, KrkValue value) {
 
 KrkFunction * krk_newFunction() {
 	KrkFunction * function = ALLOCATE_OBJECT(KrkFunction, OBJ_FUNCTION);
-	function->arity = 0;
+	function->requiredArgs = 0;
+	function->defaultArgs = 0;
 	function->upvalueCount = 0;
 	function->name = NULL;
 	krk_initChunk(&function->chunk);

object.h

@@ -57,7 +57,8 @@ typedef struct KrkUpvalue {
 
 typedef struct {
 	KrkObj obj;
-	int arity;
+	short requiredArgs;
+	short defaultArgs;
 	size_t upvalueCount;
 	KrkChunk chunk;
 	KrkString * name;

vm.c

@@ -316,10 +316,14 @@ static KrkValue krk_set_tracing(int argc, KrkValue argv[]) {
 }
 
 static int call(KrkClosure * closure, int argCount) {
-	if (argCount != closure->function->arity) {
-		krk_runtimeError("Wrong number of arguments (%d expected, got %d)", closure->function->arity, argCount);
+	if (argCount < closure->function->requiredArgs || argCount > (closure->function->requiredArgs + closure->function->defaultArgs)) {
+		krk_runtimeError("Wrong number of arguments (at least %d expected, got %d)", closure->function->requiredArgs, argCount);
 		return 0;
 	}
+	while (argCount < (closure->function->requiredArgs + closure->function->defaultArgs)) {
+		krk_push(NONE_VAL());
+		argCount++;
+	}
 	if (vm.frameCount == FRAMES_MAX) {
 		krk_runtimeError("Too many call frames.");
 		return 0;
@@ -655,7 +659,7 @@ static KrkClosure * boundNative(NativeFn method, int arity) {
 
 	/* Build a function that calls it */
 	KrkFunction * methodWrapper = krk_newFunction();
-	methodWrapper->arity = arity; /* This is WITHOUT the self reference */
+	methodWrapper->requiredArgs = arity; /* This is WITHOUT the self reference */
 	krk_writeConstant(&methodWrapper->chunk, nativeFunction, 1);
 
 	/* Stack silliness */