diff --git a/src/obj/builtin.rs b/src/obj/builtin.rs
index 1aaf830..45852e9 100644
--- a/src/obj/builtin.rs
+++ b/src/obj/builtin.rs
@@ -13,12 +13,13 @@ pub static PRINTLN_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new
             .expect("no __str__ or __repr__ member")
     };
     let return_value = vm.call(to_string, vec![]);
-    let str_ref: &StrRef = std::any::Any::downcast_ref(&return_value).expect("to_string to return str value");
     {
-        read_obj!(let str_obj = str_ref);
+        read_obj!(let str_obj = return_value);
+        let str_obj: &Str = str_obj.as_any().downcast_ref().unwrap();
         println!("{}", str_obj.value());
     }
 
+    vm.push(NIL_NAME.sym_ref());
     Signal::Return
 }));
 
@@ -32,9 +33,9 @@ pub static PRINT_BUILTIN_FUN: Lazy<NativeFunRef> = Lazy::new(|| NativeFun::new_o
             .expect("no __str__ or __repr__ member")
     };
     let return_value = vm.call(to_string, vec![]);
-    let str_ref: &StrRef = std::any::Any::downcast_ref(&return_value).expect("to_string to return str value");
     {
-        read_obj!(let str_obj = str_ref);
+        read_obj!(let str_obj = return_value);
+        let str_obj: &Str = str_obj.as_any().downcast_ref().unwrap();
         print!("{}", str_obj.value());
     }
 
diff --git a/src/obj/fun.rs b/src/obj/fun.rs
index b717537..2c365d4 100644
--- a/src/obj/fun.rs
+++ b/src/obj/fun.rs
@@ -11,6 +11,61 @@ pub trait Fun {
     fn create_frame(&self, callee: ObjRef, vm: &Vm, _args: Vec<ObjRef>) -> Frame;
 }
 
+pub type MethodRef = ObjRef<Method>;
+
+#[derive(Scan, Debug)]
+pub struct Method {
+    vtable: Vtable,
+    attrs: Attrs,
+}
+
+impl Method {
+    pub fn new_obj(owner: ObjRef, function: ObjRef) -> MethodRef {
+        self_referring_obj! {
+            Self {
+                vtable: Default::default(),
+                attrs: attrs! {
+                    SELF_MEMBER_NAME.sym => owner,
+                    FUNC_MEMBER_NAME.sym => function,
+                }
+            },
+            vtable: |obj_ref: ObjRef| vtable! {
+                CALL_MEMBER_NAME.sym => obj_ref.clone(),
+            }
+        }
+    }
+}
+
+impl Obj for Method {
+    fn vtable(&self) -> &Vtable {
+        &self.vtable
+    }
+
+    fn attrs(&self) -> &Attrs {
+        &self.attrs
+    }
+
+    fn attrs_mut(&mut self) -> Option<&mut Attrs> {
+        Some(&mut self.attrs)
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any { self }
+
+    fn as_fun(&self) -> Option<&dyn Fun> { Some(self) }
+}
+
+impl Fun for Method {
+    fn create_frame(&self, _callee: ObjRef, vm: &Vm, mut args: Vec<ObjRef>) -> Frame {
+        // insert self argument
+        args.insert(0, self.get_attr(SELF_MEMBER_NAME.sym).unwrap());
+        // get function and use its create_frame method with the updated args
+        let fun_ref = self.get_attr(FUNC_MEMBER_NAME.sym).unwrap();
+        read_obj!(let fun_obj = fun_ref);
+        let fun = fun_obj.as_fun().unwrap();
+        fun.create_frame(fun_ref.clone(), vm, args)
+    }
+}
+
 //
 // struct UserFun
 //
diff --git a/src/obj/int.rs b/src/obj/int.rs
index cc66a64..e1b47a3 100644
--- a/src/obj/int.rs
+++ b/src/obj/int.rs
@@ -1,4 +1,5 @@
-use crate::obj::prelude::*;
+use crate::obj::{prelude::*, reserved::*};
+use once_cell::sync::Lazy;
 use shredder::Scan;
 use std::fmt::{Debug, Formatter, self};
 
@@ -13,14 +14,18 @@ pub struct Int {
 
 impl Int {
     pub fn new_obj(value: i64) -> ObjRef<Self> {
-        // TODO : vtable for Int
-        let obj_ref = ObjRef::new(Self {
-            value,
-            vtable: Default::default(),
-            attrs: Default::default(),
-        });
-
-        obj_ref
+        // TODO : cache int values
+        self_referring_obj! {
+            Self {
+                value,
+                vtable: Default::default(),
+                attrs: Default::default(),
+            },
+            vtable: |obj_ref: ObjRef| vtable! {
+                // TODO needs Method type so this function is given an argument when called
+                STR_MEMBER_NAME.sym => Method::new_obj(obj_ref.clone(), INT_STR_FUN.clone()),
+            }
+        }
     }
 
     pub fn value(&self) -> i64 {
@@ -37,3 +42,15 @@ impl Debug for Int {
             .finish()
     }
 }
+
+static INT_STR_FUN: Lazy<NativeFunRef> = Lazy::new(|| {
+    NativeFun::new_obj(|_callee, vm, args| {
+        read_obj!(let int_obj = &args[0]);
+        if let Some(int_obj) = std::any::Any::downcast_ref::<Int>(int_obj.as_any()) {
+            vm.push(Str::new_obj(int_obj.value.to_string()));
+        } else {
+            panic!("{:?} is not an int", int_obj)
+        }
+        crate::vm::signal::Signal::Return
+    })
+});
diff --git a/src/vm/mod.rs b/src/vm/mod.rs
index 82cd5a3..21f96ec 100644
--- a/src/vm/mod.rs
+++ b/src/vm/mod.rs
@@ -149,9 +149,11 @@ impl<'c> Vm<'c> {
                 let frame = callee_obj.as_fun()
                     .expect("callable function")
                     .create_frame(callee.clone(), self, args);
+                // Jump to the first address of the new function call if it's a user function
+                if let Frame::User(_) = &frame {
+                    self.set_pc(0);
+                }
                 self.frames_mut().push(frame);
-                // Jump to the first address of the new function call
-                self.set_pc(0);
             }
             Signal::Return => {
                 // 1. pop return value