implement __functor

fix-vm/src/instructions/calls.rs

@@ -85,8 +85,30 @@ impl<'gc> crate::Vm<'gc> {
                 new_app.args[position] = arg;
                 self.push(Value::new_gc(Gc::new(mc, new_app)))
             }
+        } else if let Some(attrs) = func.as_gc::<AttrSet>()
+            && let Some(functor) = attrs.lookup(self.functor_sym)
+        {
+            // f arg  =>  (f.__functor f) arg
+            //
+            // Stage the work for `CallFunctor1` so retries during force are
+            // safe: the stack invariant `[..., orig_arg, self, functor]`
+            // holds every time control re-enters phase 1.
+            self.call_depth -= 1;
+            self.call_stack.push(CallFrame {
+                pc: resume_pc,
+                thunk: None,
+                env: self.env,
+            });
+            self.push(arg);
+            self.push(func.relax());
+            self.push(functor);
+            reader.set_pc(PrimOpPhase::CallFunctor1.ip() as usize);
+            return Step::Continue(());
         } else {
-            todo!("call other types: {func:?}")
+            return self.finish_err(Error::eval_error(format!(
+                "attempt to call something which is not a function but {}",
+                func.ty()
+            )));
         }
         Step::Continue(())
     }