feat: add arity getter to Fn class and rename numParams to arity

Add a public `arity` getter to the Fn class that returns the number of required arguments for a function. Internally rename the `numParams` field to `arity` across the codebase for consistency, update the C API parameter name from `numParams` to `arity` in `wrenDefineMethod` and `wrenDefineStaticMethod`, and adjust the error message in `callFunction` to use the new field name. Also update documentation to clarify that extra arguments are ignored rather than causing an error, and add a new test file `test/function/arity.wren` verifying arity values from 0 to 4.

src/wren_core.c

@@ -489,23 +489,6 @@ DEF_NATIVE(fiber_yield1)
   return PRIM_RUN_FIBER;
 }
 
-static PrimitiveResult callFunction(WrenVM* vm, Value* args, int numArgs)
-{
-  ObjFn* fn;
-  if (IS_CLOSURE(args[0]))
-  {
-    fn = AS_CLOSURE(args[0])->fn;
-  }
-  else
-  {
-    fn = AS_FN(args[0]);
-  }
-
-  if (numArgs < fn->numParams) RETURN_ERROR("Function expects more arguments.");
-
-  return PRIM_CALL;
-}
-
 DEF_NATIVE(fn_instantiate)
 {
   // Return the Fn class itself. When we then call "new" on it, it will
@@ -521,6 +504,28 @@ DEF_NATIVE(fn_new)
   RETURN_VAL(args[1]);
 }
 
+DEF_NATIVE(fn_arity)
+{
+  RETURN_NUM(AS_FN(args[0])->arity);
+}
+
+static PrimitiveResult callFunction(WrenVM* vm, Value* args, int numArgs)
+{
+  ObjFn* fn;
+  if (IS_CLOSURE(args[0]))
+  {
+    fn = AS_CLOSURE(args[0])->fn;
+  }
+  else
+  {
+    fn = AS_FN(args[0]);
+  }
+
+  if (numArgs < fn->arity) RETURN_ERROR("Function expects more arguments.");
+
+  return PRIM_CALL;
+}
+
 DEF_NATIVE(fn_call0) { return callFunction(vm, args, 0); }
 DEF_NATIVE(fn_call1) { return callFunction(vm, args, 1); }
 DEF_NATIVE(fn_call2) { return callFunction(vm, args, 2); }
@@ -1245,6 +1250,7 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->fnClass->obj.classObj, " instantiate", fn_instantiate);
   NATIVE(vm->fnClass->obj.classObj, "new ", fn_new);
 
+  NATIVE(vm->fnClass, "arity", fn_arity);
   NATIVE(vm->fnClass, "call", fn_call0);
   NATIVE(vm->fnClass, "call ", fn_call1);
   NATIVE(vm->fnClass, "call  ", fn_call2);
@@ -1262,7 +1268,6 @@ void wrenInitializeCore(WrenVM* vm)
   NATIVE(vm->fnClass, "call              ", fn_call14);
   NATIVE(vm->fnClass, "call               ", fn_call15);
   NATIVE(vm->fnClass, "call                ", fn_call16);
-  // TODO: "arity" getter.
   NATIVE(vm->fnClass, "toString", fn_toString);
 
   vm->nullClass = defineClass(vm, "Null");

src/wren_value.c

@@ -160,7 +160,7 @@ ObjFiber* wrenNewFiber(WrenVM* vm, Obj* fn)
 }
 
 ObjFn* wrenNewFunction(WrenVM* vm, Value* constants, int numConstants,
-                       int numUpvalues, int numParams,
+                       int numUpvalues, int arity,
                        uint8_t* bytecode, int bytecodeLength,
                        ObjString* debugSourcePath,
                        const char* debugName, int debugNameLength,
@@ -200,7 +200,7 @@ ObjFn* wrenNewFunction(WrenVM* vm, Value* constants, int numConstants,
   fn->constants = copiedConstants;
   fn->numUpvalues = numUpvalues;
   fn->numConstants = numConstants;
-  fn->numParams = numParams;
+  fn->arity = arity;
   fn->bytecodeLength = bytecodeLength;
   fn->debug = debug;
 

src/wren_value.h

@@ -246,7 +246,7 @@ typedef struct
   // The number of parameters this function expects. Used to ensure that .call
   // handles a mismatch between number of parameters and arguments. This will
   // only be set for fns, and not ObjFns that represent methods or scripts.
-  int numParams;
+  int arity;
   FnDebug* debug;
 } ObjFn;
 
@@ -571,7 +571,7 @@ ObjFiber* wrenNewFiber(WrenVM* vm, Obj* fn);
 // function will take over ownership of [bytecode] and [sourceLines]. It will
 // copy [constants] into its own array.
 ObjFn* wrenNewFunction(WrenVM* vm, Value* constants, int numConstants,
-                       int numUpvalues, int numParams,
+                       int numUpvalues, int arity,
                        uint8_t* bytecode, int bytecodeLength,
                        ObjString* debugSourcePath,
                        const char* debugName, int debugNameLength,