Add Obj.__init__ and Obj.__call__, fix how __call__ works on Ty

* Obj.__call__ should generically call Obj.__init__, passing along the
  arguments given to __call__
* Ty::call function (in Rust) sets up the stack frame correctly now.
  Before, the only arguments visible to a `Ty.__call__` function would
  have been the values passed to it. However, the Ty.__call__ also needs
  *itself* passed to the __call__ function so we can retrieve the
  __init__ function and eventually end up calling that.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj.rs

@@ -452,6 +452,54 @@ impl Object for Obj {
     impl_base_obj!(Obj);
 }
 
+//
+// Obj methods
+//
+
+impl Obj {
+    pub(crate) fn do_call(vm: &mut Vm, state: FunctionState) -> FunctionResult {
+        let ty = vm.frame_stack()[0].clone();
+
+        match state {
+            FunctionState::Begin => {
+                let argc = vm.frame_stack().len() - 1;
+
+                let mut obj = Obj::new();
+                obj.set_attr("__ty__", ty.clone());
+                obj.instantiate();
+
+                let obj_ptr = make_ptr(obj);
+                let init = obj_ptr
+                    .borrow()
+                    .get_vtable_attr(obj_ptr.clone(), "__init__")
+                    .expect("no __init__");
+
+                vm.push(obj_ptr);
+                vm.push(init.clone());
+                // duplicate arguments that were pushed to the frame
+                for i in 0..argc {
+                    let arg = vm.frame_stack()[i + 1].clone();
+                    vm.push(arg);
+                }
+
+                init.borrow().call(vm, argc as Argc);
+
+                FunctionResult::Yield(0)
+            }
+            FunctionState::Resume(0) => {
+                vm.pop();
+                FunctionResult::Return
+            }
+            _ => unreachable!(),
+        }
+    }
+
+    pub(crate) fn init(_vm: &mut Vm, _state: FunctionState) -> FunctionResult {
+        // no-op
+        Nil::create().into()
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Nil
 ////////////////////////////////////////////////////////////////////////////////

src/obj/macros.rs

@@ -1,13 +1,15 @@
 macro_rules! impl_base_obj {
     ($base_name:ident, $type_name:ident) => {
         fn instantiate(&mut self) {
-            let ty = $crate::obj::BUILTINS.with_borrow(|builtins| {
-                builtins
-                    .get(stringify!($type_name))
-                    .expect(concat!("no ", stringify!($type_name)))
-                    .clone()
-            });
-            self.set_attr("__ty__", ty);
+            if self.get_attr("__ty__").is_none() {
+                let ty = $crate::obj::BUILTINS.with_borrow(|builtins| {
+                    builtins
+                        .get(stringify!($type_name))
+                        .expect(concat!("no ", stringify!($type_name)))
+                        .clone()
+                });
+                self.set_attr("__ty__", ty);
+            }
             self.$base_name.instantiate();
         }
 

src/obj/ty.rs

@@ -77,21 +77,37 @@ impl Object for Ty {
         // If you do e.g. `Int.__call__`, Int is an object, so it should be doing `__call__` as a
         // vtable value.
 
-        if cfg!(debug_assertions) {
-            let index = vm.stack().len() - 1 - argc as usize;
-            let this = vm.stack()[index].clone();
-            assert!(
-                std::ptr::addr_eq(&*this.borrow(), self),
-                "calling {}.__call__ on type that is not ourselves",
-                self.ty_name()
-            );
-        }
+        let index = vm.stack().len() - 1 - argc as usize;
+        let this = vm.stack()[index].clone();
+
+        assert!(
+            std::ptr::addr_eq(&*this.borrow(), self),
+            "calling {}.__call__ on type that is not ourselves",
+            self.ty_name()
+        );
+
+        // Additionally, we want to duplicate the type argument on the stack.
+        //
+        // When you call a new constructor, e.g. `MyType(arg1, arg2, arg3)`, the stack will look like this:
+        //
+        //     -- stack top
+        //     arg3
+        //     arg2
+        //     arg1
+        //     <Ty MyType>  <---- NOTE - this is what we are actually calling, and is being treated as a function
+        //     ...
+        //     -- stack bottom
+        //
+        // With a custom type, we want to be getting the __init__ function off of the type. Since
+        // we don't have access to the function in the call stack (in this case, it's our type),
+        // we duplicate the type to be the first argument.
+        vm.stack_mut().insert(index, this);
 
         let function = self
             .vtable
             .get("__call__")
             .expect("Why does a type not have a __call__ member?");
-        function.borrow().call(vm, argc);
+        function.borrow().call(vm, argc + 1);
     }
 
     fn arity(&self) -> Option<Argc> {
@@ -185,7 +201,9 @@ pub fn init_types() {
             len => BuiltinFunction::create("len", BaseObj::not_implemented_un, 1),
         },
         Obj {
-            //__call__ => BuiltinFunction::create("__call__",
+            // Constructor
+            __call__ => BuiltinFunction::create("__call__", Obj::do_call, 1),
+            __init__ => BuiltinFunction::create("__init__", Obj::init, 1),
             // Methods
         },
         List {