diff --git a/compiler/erg_common/lib.rs b/compiler/erg_common/lib.rs
index 9e6520c1..eed6d0f1 100644
--- a/compiler/erg_common/lib.rs
+++ b/compiler/erg_common/lib.rs
@@ -13,9 +13,9 @@ pub mod levenshtein;
 pub mod macros;
 pub mod opcode;
 pub mod python_util;
-pub mod rccell;
 pub mod serialize;
 pub mod set;
+pub mod shared;
 pub mod stdin;
 pub mod str;
 pub mod traits;
diff --git a/compiler/erg_common/rccell.rs b/compiler/erg_common/shared.rs
similarity index 80%
rename from compiler/erg_common/rccell.rs
rename to compiler/erg_common/shared.rs
index 47ef43ef..986ab7f8 100644
--- a/compiler/erg_common/rccell.rs
+++ b/compiler/erg_common/shared.rs
@@ -4,42 +4,42 @@ use std::hash::{Hash, Hasher};
 use std::rc::Rc;
 
 #[derive(Debug)]
-pub struct RcCell<T: ?Sized>(Rc<RefCell<T>>);
+pub struct Shared<T: ?Sized>(Rc<RefCell<T>>);
 
-impl<T: PartialEq> PartialEq for RcCell<T> {
+impl<T: PartialEq> PartialEq for Shared<T> {
     #[inline]
     fn eq(&self, other: &Self) -> bool {
         self.0 == other.0
     }
 }
 
-impl<T: ?Sized> Clone for RcCell<T> {
-    fn clone(&self) -> RcCell<T> {
+impl<T: ?Sized> Clone for Shared<T> {
+    fn clone(&self) -> Shared<T> {
         Self(Rc::clone(&self.0))
     }
 }
 
-impl<T: Eq> Eq for RcCell<T> {}
+impl<T: Eq> Eq for Shared<T> {}
 
-impl<T: Hash> Hash for RcCell<T> {
+impl<T: Hash> Hash for Shared<T> {
     fn hash<H: Hasher>(&self, state: &mut H) {
         self.borrow().hash(state);
     }
 }
 
-impl<T: Default> Default for RcCell<T> {
+impl<T: Default> Default for Shared<T> {
     fn default() -> Self {
         Self::new(Default::default())
     }
 }
 
-impl<T: fmt::Display> fmt::Display for RcCell<T> {
+impl<T: fmt::Display> fmt::Display for Shared<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{}", self.borrow())
     }
 }
 
-impl<T> RcCell<T> {
+impl<T> Shared<T> {
     pub fn new(t: T) -> Self {
         Self(Rc::new(RefCell::new(t)))
     }
@@ -54,7 +54,7 @@ impl<T> RcCell<T> {
     }
 }
 
-impl<T: ?Sized> RcCell<T> {
+impl<T: ?Sized> Shared<T> {
     #[inline]
     pub fn copy(&self) -> Self {
         Self(self.0.clone())
@@ -89,7 +89,7 @@ impl<T: ?Sized> RcCell<T> {
     }
 }
 
-impl<T: Clone> RcCell<T> {
+impl<T: Clone> Shared<T> {
     #[inline]
     pub fn clone_inner(&self) -> T {
         self.borrow().clone()
diff --git a/compiler/erg_compiler/context/eval.rs b/compiler/erg_compiler/context/eval.rs
index 4bf9a0cf..9e10a86d 100644
--- a/compiler/erg_compiler/context/eval.rs
+++ b/compiler/erg_compiler/context/eval.rs
@@ -2,8 +2,8 @@ use std::mem;
 
 use erg_common::dict::Dict;
 use erg_common::error::Location;
-use erg_common::rccell::RcCell;
 use erg_common::set::Set;
+use erg_common::shared::Shared;
 use erg_common::traits::{Locational, Stream};
 use erg_common::vis::Field;
 use erg_common::{dict, fn_name, option_enum_unwrap, set};
@@ -326,7 +326,12 @@ impl Context {
     fn eval_const_normal_record(&self, record: &NormalRecord) -> EvalResult<ValueObj> {
         let mut attrs = vec![];
         // HACK: should avoid cloning
-        let mut record_ctx = Context::instant(Str::ever("<unnamed record>"), 2, self.clone());
+        let mut record_ctx = Context::instant(
+            Str::ever("<unnamed record>"),
+            2,
+            self.mod_cache.clone(),
+            self.clone(),
+        );
         for attr in record.attrs.iter() {
             let name = attr.sig.ident().map(|i| i.inspect());
             let elem = record_ctx.eval_const_block(&attr.body.block, name)?;
@@ -431,7 +436,7 @@ impl Context {
         match (lhs, rhs) {
             (TyParam::Value(ValueObj::Mut(lhs)), TyParam::Value(rhs)) => self
                 .eval_bin(op, lhs.borrow().clone(), rhs.clone())
-                .map(|v| TyParam::Value(ValueObj::Mut(RcCell::new(v)))),
+                .map(|v| TyParam::Value(ValueObj::Mut(Shared::new(v)))),
             (TyParam::Value(lhs), TyParam::Value(rhs)) => self
                 .eval_bin(op, lhs.clone(), rhs.clone())
                 .map(TyParam::value),
@@ -459,7 +464,7 @@ impl Context {
             Pos => todo!(),
             Neg => todo!(),
             Invert => todo!(),
-            Mutate => Ok(ValueObj::Mut(RcCell::new(val))),
+            Mutate => Ok(ValueObj::Mut(Shared::new(val))),
             other => todo!("{other}"),
         }
     }
diff --git a/compiler/erg_compiler/context/initialize/mod.rs b/compiler/erg_compiler/context/initialize/mod.rs
index 5efe5b19..44786884 100644
--- a/compiler/erg_compiler/context/initialize/mod.rs
+++ b/compiler/erg_compiler/context/initialize/mod.rs
@@ -20,6 +20,7 @@ use erg_parser::ast::VarName;
 use crate::context::initialize::const_func::{class_func, inherit_func, inheritable_func};
 use crate::context::instantiate::{ConstTemplate, TyVarContext};
 use crate::context::{ClassDefType, Context, ContextKind, DefaultInfo, ParamSpec, TraitInstance};
+use crate::mod_cache::SharedModuleCache;
 use crate::varinfo::{Mutability, VarInfo, VarKind};
 use DefaultInfo::*;
 use Mutability::*;
@@ -172,26 +173,26 @@ impl Context {
     // 型境界はすべて各サブルーチンで定義する
     // push_subtype_boundなどはユーザー定義APIの型境界決定のために使用する
     fn init_builtin_traits(&mut self) {
-        let unpack = Self::mono_trait("Unpack", Self::TOP_LEVEL);
-        let inheritable_type = Self::mono_trait("InheritableType", Self::TOP_LEVEL);
-        let named = Self::mono_trait("Named", Self::TOP_LEVEL);
-        let mut mutable = Self::mono_trait("Mutable", Self::TOP_LEVEL);
+        let unpack = Self::mono_trait("Unpack", None, Self::TOP_LEVEL);
+        let inheritable_type = Self::mono_trait("InheritableType", None, Self::TOP_LEVEL);
+        let named = Self::mono_trait("Named", None, Self::TOP_LEVEL);
+        let mut mutable = Self::mono_trait("Mutable", None, Self::TOP_LEVEL);
         let proj = mono_proj(mono_q("Self"), "ImmutType");
         let f_t = func(vec![param_t("old", proj.clone())], None, vec![], proj);
         let t = pr1_met(ref_mut(mono_q("Self"), None), f_t, NoneType);
         let t = quant(t, set! { subtypeof(mono_q("Self"), mono("Immutizable")) });
         mutable.register_builtin_decl("update!", t, Public);
         // REVIEW: Immutatable?
-        let mut immutizable = Self::mono_trait("Immutizable", Self::TOP_LEVEL);
+        let mut immutizable = Self::mono_trait("Immutizable", None, Self::TOP_LEVEL);
         immutizable.register_superclass(mono("Mutable"), &mutable);
         immutizable.register_builtin_decl("ImmutType", Type, Public);
         // REVIEW: Mutatable?
-        let mut mutizable = Self::mono_trait("Mutizable", Self::TOP_LEVEL);
+        let mut mutizable = Self::mono_trait("Mutizable", None, Self::TOP_LEVEL);
         mutizable.register_builtin_decl("MutType!", Type, Public);
-        let mut in_ = Self::poly_trait("In", vec![PS::t("T", NonDefault)], Self::TOP_LEVEL);
+        let mut in_ = Self::poly_trait("In", vec![PS::t("T", NonDefault)], None, Self::TOP_LEVEL);
         let params = vec![PS::t("T", NonDefault)];
-        let input = Self::poly_trait("Input", params.clone(), Self::TOP_LEVEL);
-        let output = Self::poly_trait("Output", params, Self::TOP_LEVEL);
+        let input = Self::poly_trait("Input", params.clone(), None, Self::TOP_LEVEL);
+        let output = Self::poly_trait("Output", params, None, Self::TOP_LEVEL);
         in_.register_superclass(poly("Input", vec![ty_tp(mono_q("T"))]), &input);
         let op_t = fn1_met(mono_q("T"), mono_q("I"), Bool);
         let op_t = quant(
@@ -202,7 +203,7 @@ impl Context {
         // Erg does not have a trait equivalent to `PartialEq` in Rust
         // This means, Erg's `Float` cannot be compared with other `Float`
         // use `l - r < EPSILON` to check if two floats are almost equal
-        let mut eq = Self::poly_trait("Eq", vec![PS::t("R", WithDefault)], Self::TOP_LEVEL);
+        let mut eq = Self::poly_trait("Eq", vec![PS::t("R", WithDefault)], None, Self::TOP_LEVEL);
         eq.register_superclass(poly("Output", vec![ty_tp(mono_q("R"))]), &output);
         // __eq__: |Self <: Eq()| Self.(Self) -> Bool
         let op_t = fn1_met(mono_q("Self"), mono_q("R"), Bool);
@@ -214,8 +215,12 @@ impl Context {
             },
         );
         eq.register_builtin_decl("__eq__", op_t, Public);
-        let mut partial_ord =
-            Self::poly_trait("PartialOrd", vec![PS::t("R", WithDefault)], Self::TOP_LEVEL);
+        let mut partial_ord = Self::poly_trait(
+            "PartialOrd",
+            vec![PS::t("R", WithDefault)],
+            None,
+            Self::TOP_LEVEL,
+        );
         partial_ord.register_superclass(poly("Eq", vec![ty_tp(mono_q("R"))]), &eq);
         let op_t = fn1_met(mono_q("Self"), mono_q("R"), option(mono("Ordering")));
         let op_t = quant(
@@ -226,17 +231,17 @@ impl Context {
             },
         );
         partial_ord.register_builtin_decl("__partial_cmp__", op_t, Public);
-        let mut ord = Self::mono_trait("Ord", Self::TOP_LEVEL);
+        let mut ord = Self::mono_trait("Ord", None, Self::TOP_LEVEL);
         ord.register_superclass(poly("Eq", vec![ty_tp(mono("Self"))]), &eq);
         ord.register_superclass(poly("PartialOrd", vec![ty_tp(mono("Self"))]), &partial_ord);
         // FIXME: poly trait
-        let num = Self::mono_trait("Num", Self::TOP_LEVEL);
+        let num = Self::mono_trait("Num", None, Self::TOP_LEVEL);
         /* vec![
             poly("Add", vec![]),
             poly("Sub", vec![]),
             poly("Mul", vec![]),
         ], */
-        let mut seq = Self::poly_trait("Seq", vec![PS::t("T", NonDefault)], Self::TOP_LEVEL);
+        let mut seq = Self::poly_trait("Seq", vec![PS::t("T", NonDefault)], None, Self::TOP_LEVEL);
         seq.register_superclass(poly("Output", vec![ty_tp(mono_q("T"))]), &output);
         let self_t = mono_q("Self");
         let t = fn0_met(self_t.clone(), Nat);
@@ -256,7 +261,7 @@ impl Context {
         let r_bound = static_instance("R", Type);
         let params = vec![PS::t("R", WithDefault)];
         let ty_params = vec![ty_tp(mono_q("R"))];
-        let mut add = Self::poly_trait("Add", params.clone(), Self::TOP_LEVEL);
+        let mut add = Self::poly_trait("Add", params.clone(), None, Self::TOP_LEVEL);
         // Rについて共変(__add__の型とは関係ない)
         add.register_superclass(poly("Output", vec![ty_tp(mono_q("R"))]), &output);
         let self_bound = subtypeof(mono_q("Self"), poly("Add", ty_params.clone()));
@@ -268,7 +273,7 @@ impl Context {
         let op_t = quant(op_t, set! {r_bound.clone(), self_bound});
         add.register_builtin_decl("__add__", op_t, Public);
         add.register_builtin_decl("Output", Type, Public);
-        let mut sub = Self::poly_trait("Sub", params.clone(), Self::TOP_LEVEL);
+        let mut sub = Self::poly_trait("Sub", params.clone(), None, Self::TOP_LEVEL);
         sub.register_superclass(poly("Output", vec![ty_tp(mono_q("R"))]), &output);
         let op_t = fn1_met(
             mono_q("Self"),
@@ -279,7 +284,7 @@ impl Context {
         let op_t = quant(op_t, set! {r_bound.clone(), self_bound});
         sub.register_builtin_decl("__sub__", op_t, Public);
         sub.register_builtin_decl("Output", Type, Public);
-        let mut mul = Self::poly_trait("Mul", params.clone(), Self::TOP_LEVEL);
+        let mut mul = Self::poly_trait("Mul", params.clone(), None, Self::TOP_LEVEL);
         mul.register_superclass(poly("Output", vec![ty_tp(mono_q("R"))]), &output);
         let op_t = fn1_met(
             mono_q("Self"),
@@ -290,7 +295,7 @@ impl Context {
         let op_t = quant(op_t, set! {r_bound.clone(), self_bound});
         mul.register_builtin_decl("__mul__", op_t, Public);
         mul.register_builtin_decl("Output", Type, Public);
-        let mut div = Self::poly_trait("Div", params, Self::TOP_LEVEL);
+        let mut div = Self::poly_trait("Div", params, None, Self::TOP_LEVEL);
         div.register_superclass(poly("Output", vec![ty_tp(mono_q("R"))]), &output);
         let op_t = fn1_met(mono_q("Self"), r, mono_proj(mono_q("Self"), "Output"));
         let self_bound = subtypeof(mono_q("Self"), poly("Div", ty_params.clone()));
@@ -346,7 +351,7 @@ impl Context {
     }
 
     fn init_builtin_classes(&mut self) {
-        let mut obj = Self::mono_class("Obj", Self::TOP_LEVEL);
+        let mut obj = Self::mono_class("Obj", None, Self::TOP_LEVEL);
         let t = fn0_met(mono_q("Self"), mono_q("Self"));
         let t = quant(t, set! {subtypeof(mono_q("Self"), mono("Obj"))});
         obj.register_builtin_impl("clone", t, Const, Public);
@@ -356,20 +361,20 @@ impl Context {
         obj.register_builtin_impl("__str__", fn0_met(Obj, Str), Immutable, Public);
         obj.register_builtin_impl("__dict__", fn0_met(Obj, dict(Str, Obj)), Immutable, Public);
         obj.register_builtin_impl("__bytes__", fn0_met(Obj, mono("Bytes")), Immutable, Public);
-        let mut obj_in = Self::methods("In", Self::TOP_LEVEL);
+        let mut obj_in = Self::methods("In", None, Self::TOP_LEVEL);
         obj_in.register_builtin_impl("__in__", fn1_met(Obj, Type, Bool), Const, Public);
         obj.register_trait(Obj, poly("Eq", vec![ty_tp(Type)]), obj_in);
-        let mut obj_mutizable = Self::methods("Mutizable", Self::TOP_LEVEL);
+        let mut obj_mutizable = Self::methods("Mutizable", None, Self::TOP_LEVEL);
         obj_mutizable.register_builtin_const("MutType!", ValueObj::builtin_t(mono("Obj!")));
         obj.register_trait(Obj, mono("Mutizable"), obj_mutizable);
-        let mut float = Self::mono_class("Float", Self::TOP_LEVEL);
+        let mut float = Self::mono_class("Float", None, Self::TOP_LEVEL);
         float.register_superclass(Obj, &obj);
         // TODO: support multi platform
         float.register_builtin_const("EPSILON", ValueObj::Float(2.220446049250313e-16));
         float.register_builtin_impl("Real", Float, Const, Public);
         float.register_builtin_impl("Imag", Float, Const, Public);
         float.register_marker_trait(mono("Num"));
-        let mut float_partial_ord = Self::methods("PartialOrd", Self::TOP_LEVEL);
+        let mut float_partial_ord = Self::methods("PartialOrd", None, Self::TOP_LEVEL);
         float_partial_ord.register_builtin_impl(
             "__cmp__",
             fn1_met(Float, Float, mono("Ordering")),
@@ -383,33 +388,33 @@ impl Context {
         );
         // Float doesn't have an `Eq` implementation
         let op_t = fn1_met(Float, Float, Float);
-        let mut float_add = Self::methods("Add", Self::TOP_LEVEL);
+        let mut float_add = Self::methods("Add", None, Self::TOP_LEVEL);
         float_add.register_builtin_impl("__add__", op_t.clone(), Const, Public);
         float_add.register_builtin_const("Output", ValueObj::builtin_t(Float));
         float.register_trait(Float, poly("Add", vec![ty_tp(Float)]), float_add);
-        let mut float_sub = Self::methods("Sub", Self::TOP_LEVEL);
+        let mut float_sub = Self::methods("Sub", None, Self::TOP_LEVEL);
         float_sub.register_builtin_impl("__sub__", op_t.clone(), Const, Public);
         float_sub.register_builtin_const("Output", ValueObj::builtin_t(Float));
         float.register_trait(Float, poly("Sub", vec![ty_tp(Float)]), float_sub);
-        let mut float_mul = Self::methods("Mul", Self::TOP_LEVEL);
+        let mut float_mul = Self::methods("Mul", None, Self::TOP_LEVEL);
         float_mul.register_builtin_impl("__mul__", op_t.clone(), Const, Public);
         float_mul.register_builtin_const("Output", ValueObj::builtin_t(Float));
         float.register_trait(Float, poly("Mul", vec![ty_tp(Float)]), float_mul);
-        let mut float_div = Self::methods("Div", Self::TOP_LEVEL);
+        let mut float_div = Self::methods("Div", None, Self::TOP_LEVEL);
         float_div.register_builtin_impl("__div__", op_t, Const, Public);
         float_div.register_builtin_const("Output", ValueObj::builtin_t(Float));
         float.register_trait(Float, poly("Div", vec![ty_tp(Float)]), float_div);
-        let mut float_mutizable = Self::methods("Mutizable", Self::TOP_LEVEL);
+        let mut float_mutizable = Self::methods("Mutizable", None, Self::TOP_LEVEL);
         float_mutizable.register_builtin_const("MutType!", ValueObj::builtin_t(mono("Float!")));
         float.register_trait(Float, mono("Mutizable"), float_mutizable);
         // TODO: Int, Nat, Boolの継承元をRatioにする(今はFloat)
-        let mut ratio = Self::mono_class("Ratio", Self::TOP_LEVEL);
+        let mut ratio = Self::mono_class("Ratio", None, Self::TOP_LEVEL);
         ratio.register_superclass(Obj, &obj);
         ratio.register_builtin_impl("Real", Ratio, Const, Public);
         ratio.register_builtin_impl("Imag", Ratio, Const, Public);
         ratio.register_marker_trait(mono("Num"));
         // ratio.register_marker_trait(mono("Ord"));
-        let mut ratio_partial_ord = Self::methods("PartialOrd", Self::TOP_LEVEL);
+        let mut ratio_partial_ord = Self::methods("PartialOrd", None, Self::TOP_LEVEL);
         ratio_partial_ord.register_builtin_impl(
             "__cmp__",
             fn1_met(Ratio, Ratio, mono("Ordering")),
@@ -421,30 +426,30 @@ impl Context {
             poly("PartialOrd", vec![ty_tp(Ratio)]),
             ratio_partial_ord,
         );
-        let mut ratio_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut ratio_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         ratio_eq.register_builtin_impl("__eq__", fn1_met(Ratio, Ratio, Bool), Const, Public);
         ratio.register_trait(Ratio, poly("Eq", vec![ty_tp(Ratio)]), ratio_eq);
         let op_t = fn1_met(Ratio, Ratio, Ratio);
-        let mut ratio_add = Self::methods("Add", Self::TOP_LEVEL);
+        let mut ratio_add = Self::methods("Add", None, Self::TOP_LEVEL);
         ratio_add.register_builtin_impl("__add__", op_t.clone(), Const, Public);
         ratio_add.register_builtin_const("Output", ValueObj::builtin_t(Ratio));
         ratio.register_trait(Ratio, poly("Add", vec![ty_tp(Ratio)]), ratio_add);
-        let mut ratio_sub = Self::methods("Sub", Self::TOP_LEVEL);
+        let mut ratio_sub = Self::methods("Sub", None, Self::TOP_LEVEL);
         ratio_sub.register_builtin_impl("__sub__", op_t.clone(), Const, Public);
         ratio_sub.register_builtin_const("Output", ValueObj::builtin_t(Ratio));
         ratio.register_trait(Ratio, poly("Sub", vec![ty_tp(Ratio)]), ratio_sub);
-        let mut ratio_mul = Self::methods("Mul", Self::TOP_LEVEL);
+        let mut ratio_mul = Self::methods("Mul", None, Self::TOP_LEVEL);
         ratio_mul.register_builtin_impl("__mul__", op_t.clone(), Const, Public);
         ratio_mul.register_builtin_const("Output", ValueObj::builtin_t(Ratio));
         ratio.register_trait(Ratio, poly("Mul", vec![ty_tp(Ratio)]), ratio_mul);
-        let mut ratio_div = Self::methods("Div", Self::TOP_LEVEL);
+        let mut ratio_div = Self::methods("Div", None, Self::TOP_LEVEL);
         ratio_div.register_builtin_impl("__div__", op_t, Const, Public);
         ratio_div.register_builtin_const("Output", ValueObj::builtin_t(Ratio));
         ratio.register_trait(Ratio, poly("Div", vec![ty_tp(Ratio)]), ratio_div);
-        let mut ratio_mutizable = Self::methods("Mutizable", Self::TOP_LEVEL);
+        let mut ratio_mutizable = Self::methods("Mutizable", None, Self::TOP_LEVEL);
         ratio_mutizable.register_builtin_const("MutType!", ValueObj::builtin_t(mono("Ratio!")));
         ratio.register_trait(Ratio, mono("Mutizable"), ratio_mutizable);
-        let mut int = Self::mono_class("Int", Self::TOP_LEVEL);
+        let mut int = Self::mono_class("Int", None, Self::TOP_LEVEL);
         int.register_superclass(Float, &float); // TODO: Float -> Ratio
         int.register_superclass(Obj, &obj);
         int.register_marker_trait(mono("Num"));
@@ -453,7 +458,7 @@ impl Context {
         // class("Rational"),
         // class("Integral"),
         int.register_builtin_impl("abs", fn0_met(Int, Nat), Immutable, Public);
-        let mut int_partial_ord = Self::methods("PartialOrd", Self::TOP_LEVEL);
+        let mut int_partial_ord = Self::methods("PartialOrd", None, Self::TOP_LEVEL);
         int_partial_ord.register_builtin_impl(
             "__partial_cmp__",
             fn1_met(Int, Int, option(mono("Ordering"))),
@@ -461,29 +466,29 @@ impl Context {
             Public,
         );
         int.register_trait(Int, poly("PartialOrd", vec![ty_tp(Int)]), int_partial_ord);
-        let mut int_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut int_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         int_eq.register_builtin_impl("__eq__", fn1_met(Int, Int, Bool), Const, Public);
         int.register_trait(Int, poly("Eq", vec![ty_tp(Int)]), int_eq);
         // __div__ is not included in Int (cast to Ratio)
         let op_t = fn1_met(Int, Int, Int);
-        let mut int_add = Self::methods("Add", Self::TOP_LEVEL);
+        let mut int_add = Self::methods("Add", None, Self::TOP_LEVEL);
         int_add.register_builtin_impl("__add__", op_t.clone(), Const, Public);
         int_add.register_builtin_const("Output", ValueObj::builtin_t(Int));
         int.register_trait(Int, poly("Add", vec![ty_tp(Int)]), int_add);
-        let mut int_sub = Self::methods("Sub", Self::TOP_LEVEL);
+        let mut int_sub = Self::methods("Sub", None, Self::TOP_LEVEL);
         int_sub.register_builtin_impl("__sub__", op_t.clone(), Const, Public);
         int_sub.register_builtin_const("Output", ValueObj::builtin_t(Int));
         int.register_trait(Int, poly("Sub", vec![ty_tp(Int)]), int_sub);
-        let mut int_mul = Self::methods("Mul", Self::TOP_LEVEL);
+        let mut int_mul = Self::methods("Mul", None, Self::TOP_LEVEL);
         int_mul.register_builtin_impl("__mul__", op_t, Const, Public);
         int_mul.register_builtin_const("Output", ValueObj::builtin_t(Int));
         int.register_trait(Int, poly("Mul", vec![ty_tp(Int)]), int_mul);
-        let mut int_mutizable = Self::methods("Mutizable", Self::TOP_LEVEL);
+        let mut int_mutizable = Self::methods("Mutizable", None, Self::TOP_LEVEL);
         int_mutizable.register_builtin_const("MutType!", ValueObj::builtin_t(mono("Int!")));
         int.register_trait(Int, mono("Mutizable"), int_mutizable);
         int.register_builtin_impl("Real", Int, Const, Public);
         int.register_builtin_impl("Imag", Int, Const, Public);
-        let mut nat = Self::mono_class("Nat", Self::TOP_LEVEL);
+        let mut nat = Self::mono_class("Nat", None, Self::TOP_LEVEL);
         nat.register_superclass(Int, &int);
         nat.register_superclass(Float, &float); // TODO: Float -> Ratio
         nat.register_superclass(Obj, &obj);
@@ -503,10 +508,10 @@ impl Context {
         );
         nat.register_marker_trait(mono("Num"));
         // nat.register_marker_trait(mono("Ord"));
-        let mut nat_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut nat_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         nat_eq.register_builtin_impl("__eq__", fn1_met(Nat, Nat, Bool), Const, Public);
         nat.register_trait(Nat, poly("Eq", vec![ty_tp(Nat)]), nat_eq);
-        let mut nat_partial_ord = Self::methods("PartialOrd", Self::TOP_LEVEL);
+        let mut nat_partial_ord = Self::methods("PartialOrd", None, Self::TOP_LEVEL);
         nat_partial_ord.register_builtin_impl(
             "__cmp__",
             fn1_met(Nat, Nat, mono("Ordering")),
@@ -516,20 +521,20 @@ impl Context {
         nat.register_trait(Nat, poly("PartialOrd", vec![ty_tp(Nat)]), nat_partial_ord);
         // __sub__, __div__ is not included in Nat (cast to Int/ Ratio)
         let op_t = fn1_met(Nat, Nat, Nat);
-        let mut nat_add = Self::methods("Add", Self::TOP_LEVEL);
+        let mut nat_add = Self::methods("Add", None, Self::TOP_LEVEL);
         nat_add.register_builtin_impl("__add__", op_t.clone(), Const, Public);
         nat_add.register_builtin_const("Output", ValueObj::builtin_t(Nat));
         nat.register_trait(Nat, poly("Add", vec![ty_tp(Nat)]), nat_add);
-        let mut nat_mul = Self::methods("Mul", Self::TOP_LEVEL);
+        let mut nat_mul = Self::methods("Mul", None, Self::TOP_LEVEL);
         nat_mul.register_builtin_impl("__mul__", op_t, Const, Public);
         nat_mul.register_builtin_const("Output", ValueObj::builtin_t(Nat));
         nat.register_trait(Nat, poly("Mul", vec![ty_tp(Nat)]), nat_mul);
-        let mut nat_mutizable = Self::methods("Mutizable", Self::TOP_LEVEL);
+        let mut nat_mutizable = Self::methods("Mutizable", None, Self::TOP_LEVEL);
         nat_mutizable.register_builtin_const("MutType!", ValueObj::builtin_t(mono("Nat!")));
         nat.register_trait(Nat, mono("Mutizable"), nat_mutizable);
         nat.register_builtin_impl("Real", Nat, Const, Public);
         nat.register_builtin_impl("Imag", Nat, Const, Public);
-        let mut bool_ = Self::mono_class("Bool", Self::TOP_LEVEL);
+        let mut bool_ = Self::mono_class("Bool", None, Self::TOP_LEVEL);
         bool_.register_superclass(Nat, &nat);
         bool_.register_superclass(Int, &int);
         bool_.register_superclass(Float, &float); // TODO: Float -> Ratio
@@ -541,7 +546,7 @@ impl Context {
         bool_.register_builtin_impl("__or__", fn1_met(Bool, Bool, Bool), Const, Public);
         bool_.register_marker_trait(mono("Num"));
         // bool_.register_marker_trait(mono("Ord"));
-        let mut bool_partial_ord = Self::methods("PartialOrd", Self::TOP_LEVEL);
+        let mut bool_partial_ord = Self::methods("PartialOrd", None, Self::TOP_LEVEL);
         bool_partial_ord.register_builtin_impl(
             "__cmp__",
             fn1_met(Bool, Bool, mono("Ordering")),
@@ -553,17 +558,17 @@ impl Context {
             poly("PartialOrd", vec![ty_tp(Bool)]),
             bool_partial_ord,
         );
-        let mut bool_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut bool_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         bool_eq.register_builtin_impl("__eq__", fn1_met(Bool, Bool, Bool), Const, Public);
         bool_.register_trait(Bool, poly("Eq", vec![ty_tp(Bool)]), bool_eq);
-        let mut bool_add = Self::methods("Add", Self::TOP_LEVEL);
+        let mut bool_add = Self::methods("Add", None, Self::TOP_LEVEL);
         bool_add.register_builtin_impl("__add__", fn1_met(Bool, Bool, Int), Const, Public);
         bool_add.register_builtin_const("Output", ValueObj::builtin_t(Int));
         bool_.register_trait(Bool, poly("Add", vec![ty_tp(Bool)]), bool_add);
-        let mut bool_mutizable = Self::methods("Mutizable", Self::TOP_LEVEL);
+        let mut bool_mutizable = Self::methods("Mutizable", None, Self::TOP_LEVEL);
         bool_mutizable.register_builtin_const("MutType!", ValueObj::builtin_t(mono("Bool!")));
         bool_.register_trait(Bool, mono("Mutizable"), bool_mutizable);
-        let mut str_ = Self::mono_class("Str", Self::TOP_LEVEL);
+        let mut str_ = Self::mono_class("Str", None, Self::TOP_LEVEL);
         str_.register_superclass(Obj, &obj);
         str_.register_builtin_impl(
             "replace",
@@ -589,47 +594,48 @@ impl Context {
             Immutable,
             Public,
         );
-        let mut str_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut str_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         str_eq.register_builtin_impl("__eq__", fn1_met(Str, Str, Bool), Const, Public);
         str_.register_trait(Str, poly("Eq", vec![ty_tp(Str)]), str_eq);
-        let mut str_seq = Self::methods("Seq", Self::TOP_LEVEL);
+        let mut str_seq = Self::methods("Seq", None, Self::TOP_LEVEL);
         str_seq.register_builtin_impl("len", fn0_met(Str, Nat), Const, Public);
         str_seq.register_builtin_impl("get", fn1_met(Str, Nat, Str), Const, Public);
         str_.register_trait(Str, poly("Seq", vec![ty_tp(Str)]), str_seq);
-        let mut str_add = Self::methods("Add", Self::TOP_LEVEL);
+        let mut str_add = Self::methods("Add", None, Self::TOP_LEVEL);
         str_add.register_builtin_impl("__add__", fn1_met(Str, Str, Str), Const, Public);
         str_add.register_builtin_const("Output", ValueObj::builtin_t(Str));
         str_.register_trait(Str, poly("Add", vec![ty_tp(Str)]), str_add);
-        let mut str_mul = Self::methods("Mul", Self::TOP_LEVEL);
+        let mut str_mul = Self::methods("Mul", None, Self::TOP_LEVEL);
         str_mul.register_builtin_impl("__mul__", fn1_met(Str, Nat, Str), Const, Public);
         str_mul.register_builtin_const("Output", ValueObj::builtin_t(Str));
         str_.register_trait(Str, poly("Mul", vec![ty_tp(Nat)]), str_mul);
-        let mut str_mutizable = Self::methods("Mutizable", Self::TOP_LEVEL);
+        let mut str_mutizable = Self::methods("Mutizable", None, Self::TOP_LEVEL);
         str_mutizable.register_builtin_const("MutType!", ValueObj::builtin_t(mono("Str!")));
         str_.register_trait(Str, mono("Mutizable"), str_mutizable);
-        let mut type_ = Self::mono_class("Type", Self::TOP_LEVEL);
+        let mut type_ = Self::mono_class("Type", None, Self::TOP_LEVEL);
         type_.register_superclass(Obj, &obj);
         type_.register_builtin_impl("mro", array(Type, TyParam::erased(Nat)), Immutable, Public);
         type_.register_marker_trait(mono("Named"));
-        let mut type_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut type_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         type_eq.register_builtin_impl("__eq__", fn1_met(Type, Type, Bool), Const, Public);
         type_.register_trait(Type, poly("Eq", vec![ty_tp(Type)]), type_eq);
-        let mut class_type = Self::mono_class("ClassType", Self::TOP_LEVEL);
+        let mut class_type = Self::mono_class("ClassType", None, Self::TOP_LEVEL);
         class_type.register_superclass(Type, &type_);
         class_type.register_superclass(Obj, &obj);
         class_type.register_marker_trait(mono("Named"));
-        let mut class_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut class_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         class_eq.register_builtin_impl("__eq__", fn1_met(Class, Class, Bool), Const, Public);
         class_type.register_trait(Class, poly("Eq", vec![ty_tp(Class)]), class_eq);
-        let mut module = Self::mono_class("Module", Self::TOP_LEVEL);
+        let mut module = Self::mono_class("Module", None, Self::TOP_LEVEL);
         module.register_superclass(Obj, &obj);
         module.register_marker_trait(mono("Named"));
-        let mut module_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut module_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         module_eq.register_builtin_impl("__eq__", fn1_met(Module, Module, Bool), Const, Public);
         module.register_trait(Module, poly("Eq", vec![ty_tp(Module)]), module_eq);
         let mut array_ = Self::poly_class(
             "Array",
             vec![PS::t_nd("T"), PS::named_nd("N", Nat)],
+            None,
             Self::TOP_LEVEL,
         );
         array_.register_superclass(Obj, &obj);
@@ -655,7 +661,7 @@ impl Context {
         ));
         // [T; N].MutType! = [T; !N] (neither [T!; N] nor [T; N]!)
         array_.register_builtin_const("MutType!", mut_type);
-        let mut array_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut array_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         array_eq.register_builtin_impl(
             "__eq__",
             fn1_met(array_t.clone(), array_t.clone(), Bool),
@@ -666,9 +672,9 @@ impl Context {
         array_.register_marker_trait(mono("Mutizable"));
         array_.register_marker_trait(poly("Seq", vec![ty_tp(mono_q("T"))]));
         // TODO: make Tuple6, Tuple7, ... etc.
-        let mut tuple_ = Self::mono_class("Tuple", Self::TOP_LEVEL);
+        let mut tuple_ = Self::mono_class("Tuple", None, Self::TOP_LEVEL);
         tuple_.register_superclass(Obj, &obj);
-        let mut tuple_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut tuple_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         tuple_eq.register_builtin_impl(
             "__eq__",
             fn1_met(mono("Tuple"), mono("Tuple"), Bool),
@@ -680,10 +686,10 @@ impl Context {
             poly("Eq", vec![ty_tp(mono("Tuple"))]),
             tuple_eq,
         );
-        let mut tuple1 = Self::poly_class("Tuple1", vec![PS::t_nd("A")], Self::TOP_LEVEL);
+        let mut tuple1 = Self::poly_class("Tuple1", vec![PS::t_nd("A")], None, Self::TOP_LEVEL);
         tuple1.register_superclass(mono("Tuple"), &tuple_);
         tuple1.register_superclass(Obj, &obj);
-        let mut tuple1_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut tuple1_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         tuple1_eq.register_builtin_impl(
             "__eq__",
             fn1_met(
@@ -702,11 +708,12 @@ impl Context {
         let mut tuple2 = Self::poly_class(
             "Tuple2",
             vec![PS::t_nd("A"), PS::t_nd("B")],
+            None,
             Self::TOP_LEVEL,
         );
         tuple2.register_superclass(mono("Tuple"), &tuple_);
         tuple2.register_superclass(Obj, &obj);
-        let mut tuple2_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut tuple2_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         tuple2_eq.register_builtin_impl(
             "__eq__",
             fn1_met(
@@ -731,11 +738,12 @@ impl Context {
         let mut tuple3 = Self::poly_class(
             "Tuple3",
             vec![PS::t_nd("A"), PS::t_nd("B"), PS::t_nd("C")],
+            None,
             Self::TOP_LEVEL,
         );
         tuple3.register_superclass(mono("Tuple"), &tuple_);
         tuple3.register_superclass(Obj, &obj);
-        let mut tuple3_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut tuple3_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         tuple3_eq.register_builtin_impl(
             "__eq__",
             fn1_met(
@@ -769,11 +777,12 @@ impl Context {
         let mut tuple4 = Self::poly_class(
             "Tuple4",
             vec![PS::t_nd("A"), PS::t_nd("B"), PS::t_nd("C"), PS::t_nd("D")],
+            None,
             Self::TOP_LEVEL,
         );
         tuple4.register_superclass(mono("Tuple"), &tuple_);
         tuple4.register_superclass(Obj, &obj);
-        let mut tuple4_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut tuple4_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         tuple4_eq.register_builtin_impl(
             "__eq__",
             fn1_met(
@@ -833,11 +842,12 @@ impl Context {
                 PS::t_nd("D"),
                 PS::t_nd("E"),
             ],
+            None,
             Self::TOP_LEVEL,
         );
         tuple5.register_superclass(mono("Tuple"), &tuple_);
         tuple5.register_superclass(Obj, &obj);
-        let mut tuple5_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut tuple5_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         tuple5_eq.register_builtin_impl(
             "__eq__",
             fn1_met(
@@ -902,11 +912,12 @@ impl Context {
                 PS::t_nd("E"),
                 PS::t_nd("F"),
             ],
+            None,
             Self::TOP_LEVEL,
         );
         tuple6.register_superclass(mono("Tuple"), &tuple_);
         tuple6.register_superclass(Obj, &obj);
-        let mut tuple6_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut tuple6_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         tuple6_eq.register_builtin_impl(
             "__eq__",
             fn1_met(
@@ -976,11 +987,12 @@ impl Context {
                 PS::t_nd("F"),
                 PS::t_nd("G"),
             ],
+            None,
             Self::TOP_LEVEL,
         );
         tuple7.register_superclass(mono("Tuple"), &tuple_);
         tuple7.register_superclass(Obj, &obj);
-        let mut tuple7_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut tuple7_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         tuple7_eq.register_builtin_impl(
             "__eq__",
             fn1_met(
@@ -1055,11 +1067,12 @@ impl Context {
                 PS::t_nd("G"),
                 PS::t_nd("H"),
             ],
+            None,
             Self::TOP_LEVEL,
         );
         tuple8.register_superclass(mono("Tuple"), &tuple_);
         tuple8.register_superclass(Obj, &obj);
-        let mut tuple8_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut tuple8_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         tuple8_eq.register_builtin_impl(
             "__eq__",
             fn1_met(
@@ -1126,16 +1139,16 @@ impl Context {
             ),
             tuple8_eq,
         );
-        let mut record = Self::mono_class("Record", Self::TOP_LEVEL);
+        let mut record = Self::mono_class("Record", None, Self::TOP_LEVEL);
         record.register_superclass(Obj, &obj);
-        let mut record_type = Self::mono_class("RecordType", Self::TOP_LEVEL);
+        let mut record_type = Self::mono_class("RecordType", None, Self::TOP_LEVEL);
         record_type.register_superclass(mono("Record"), &record);
         record_type.register_superclass(mono("Type"), &type_);
         record_type.register_superclass(Obj, &obj);
-        let mut float_mut = Self::mono_class("Float!", Self::TOP_LEVEL);
+        let mut float_mut = Self::mono_class("Float!", None, Self::TOP_LEVEL);
         float_mut.register_superclass(Float, &float);
         float_mut.register_superclass(Obj, &obj);
-        let mut float_mut_mutable = Self::methods("Mutable", Self::TOP_LEVEL);
+        let mut float_mut_mutable = Self::methods("Mutable", None, Self::TOP_LEVEL);
         float_mut_mutable.register_builtin_const("ImmutType", ValueObj::builtin_t(Float));
         let f_t = param_t("f", func(vec![param_t("old", Float)], None, vec![], Float));
         let t = pr_met(
@@ -1147,10 +1160,10 @@ impl Context {
         );
         float_mut_mutable.register_builtin_impl("update!", t, Immutable, Public);
         float_mut.register_trait(mono("Float!"), mono("Mutable"), float_mut_mutable);
-        let mut ratio_mut = Self::mono_class("Ratio!", Self::TOP_LEVEL);
+        let mut ratio_mut = Self::mono_class("Ratio!", None, Self::TOP_LEVEL);
         ratio_mut.register_superclass(Ratio, &ratio);
         ratio_mut.register_superclass(Obj, &obj);
-        let mut ratio_mut_mutable = Self::methods("Mutable", Self::TOP_LEVEL);
+        let mut ratio_mut_mutable = Self::methods("Mutable", None, Self::TOP_LEVEL);
         ratio_mut_mutable.register_builtin_const("ImmutType", ValueObj::builtin_t(Ratio));
         let f_t = param_t(
             "f",
@@ -1170,11 +1183,11 @@ impl Context {
         );
         ratio_mut_mutable.register_builtin_impl("update!", t, Immutable, Public);
         ratio_mut.register_trait(mono("Ratio!"), mono("Mutable"), ratio_mut_mutable);
-        let mut int_mut = Self::mono_class("Int!", Self::TOP_LEVEL);
+        let mut int_mut = Self::mono_class("Int!", None, Self::TOP_LEVEL);
         int_mut.register_superclass(Int, &int);
         int_mut.register_superclass(mono("Float!"), &float_mut);
         int_mut.register_superclass(Obj, &obj);
-        let mut int_mut_mutable = Self::methods("Mutable", Self::TOP_LEVEL);
+        let mut int_mut_mutable = Self::methods("Mutable", None, Self::TOP_LEVEL);
         int_mut_mutable.register_builtin_const("ImmutType", ValueObj::builtin_t(Int));
         let f_t = param_t("f", func(vec![param_t("old", Int)], None, vec![], Int));
         let t = pr_met(
@@ -1186,12 +1199,12 @@ impl Context {
         );
         int_mut_mutable.register_builtin_impl("update!", t, Immutable, Public);
         int_mut.register_trait(mono("Int!"), mono("Mutable"), int_mut_mutable);
-        let mut nat_mut = Self::mono_class("Nat!", Self::TOP_LEVEL);
+        let mut nat_mut = Self::mono_class("Nat!", None, Self::TOP_LEVEL);
         nat_mut.register_superclass(Nat, &nat);
         nat_mut.register_superclass(mono("Int!"), &int_mut);
         nat_mut.register_superclass(mono("Float!"), &float_mut);
         nat_mut.register_superclass(Obj, &obj);
-        let mut nat_mut_mutable = Self::methods("Mutable", Self::TOP_LEVEL);
+        let mut nat_mut_mutable = Self::methods("Mutable", None, Self::TOP_LEVEL);
         nat_mut_mutable.register_builtin_const("ImmutType", ValueObj::builtin_t(Nat));
         let f_t = param_t("f", func(vec![param_t("old", Nat)], None, vec![], Nat));
         let t = pr_met(
@@ -1203,13 +1216,13 @@ impl Context {
         );
         nat_mut_mutable.register_builtin_impl("update!", t, Immutable, Public);
         nat_mut.register_trait(mono("Nat!"), mono("Mutable"), nat_mut_mutable);
-        let mut bool_mut = Self::mono_class("Bool!", Self::TOP_LEVEL);
+        let mut bool_mut = Self::mono_class("Bool!", None, Self::TOP_LEVEL);
         bool_mut.register_superclass(Bool, &bool_);
         bool_mut.register_superclass(mono("Nat!"), &nat_mut);
         bool_mut.register_superclass(mono("Int!"), &int_mut);
         bool_mut.register_superclass(mono("Float!"), &float_mut);
         bool_mut.register_superclass(Obj, &obj);
-        let mut bool_mut_mutable = Self::methods("Mutable", Self::TOP_LEVEL);
+        let mut bool_mut_mutable = Self::methods("Mutable", None, Self::TOP_LEVEL);
         bool_mut_mutable.register_builtin_const("ImmutType", ValueObj::builtin_t(Bool));
         let f_t = param_t("f", func(vec![param_t("old", Bool)], None, vec![], Bool));
         let t = pr_met(
@@ -1221,10 +1234,10 @@ impl Context {
         );
         bool_mut_mutable.register_builtin_impl("update!", t, Immutable, Public);
         bool_mut.register_trait(mono("Bool!"), mono("Mutable"), bool_mut_mutable);
-        let mut str_mut = Self::mono_class("Str!", Self::TOP_LEVEL);
+        let mut str_mut = Self::mono_class("Str!", None, Self::TOP_LEVEL);
         str_mut.register_superclass(Str, &str_);
         str_mut.register_superclass(Obj, &obj);
-        let mut str_mut_mutable = Self::methods("Mutable", Self::TOP_LEVEL);
+        let mut str_mut_mutable = Self::methods("Mutable", None, Self::TOP_LEVEL);
         str_mut_mutable.register_builtin_const("ImmutType", ValueObj::builtin_t(Str));
         let f_t = param_t("f", func(vec![param_t("old", Str)], None, vec![], Str));
         let t = pr_met(
@@ -1241,6 +1254,7 @@ impl Context {
         let mut array_mut_ = Self::poly_class(
             "Array!",
             vec![PS::t_nd("T"), PS::named_nd("N", mono("Nat!"))],
+            None,
             Self::TOP_LEVEL,
         );
         array_mut_.register_superclass(array_t.clone(), &array_);
@@ -1294,14 +1308,14 @@ impl Context {
             vec![],
             NoneType,
         );
-        let mut array_mut_mutable = Self::methods("Mutable", Self::TOP_LEVEL);
+        let mut array_mut_mutable = Self::methods("Mutable", None, Self::TOP_LEVEL);
         array_mut_mutable.register_builtin_impl("update!", t, Immutable, Public);
         array_mut_.register_trait(array_mut_t.clone(), mono("Mutable"), array_mut_mutable);
         let range_t = poly("Range", vec![TyParam::t(mono_q("T"))]);
-        let mut range = Self::poly_class("Range", vec![PS::t_nd("T")], Self::TOP_LEVEL);
+        let mut range = Self::poly_class("Range", vec![PS::t_nd("T")], None, Self::TOP_LEVEL);
         range.register_superclass(Obj, &obj);
         range.register_marker_trait(poly("Output", vec![ty_tp(mono_q("T"))]));
-        let mut range_eq = Self::methods("Eq", Self::TOP_LEVEL);
+        let mut range_eq = Self::methods("Eq", None, Self::TOP_LEVEL);
         range_eq.register_builtin_impl(
             "__eq__",
             fn1_met(range_t.clone(), range_t.clone(), Bool),
@@ -1313,16 +1327,16 @@ impl Context {
             poly("Eq", vec![ty_tp(range_t.clone())]),
             range_eq,
         );
-        let mut proc = Self::mono_class("Procedure", Self::TOP_LEVEL);
+        let mut proc = Self::mono_class("Procedure", None, Self::TOP_LEVEL);
         proc.register_superclass(Obj, &obj);
         // TODO: lambda
         proc.register_marker_trait(mono("Named"));
-        let mut func = Self::mono_class("Func", Self::TOP_LEVEL);
+        let mut func = Self::mono_class("Func", None, Self::TOP_LEVEL);
         func.register_superclass(mono("Proc"), &proc);
         func.register_superclass(Obj, &obj);
         // TODO: lambda
         func.register_marker_trait(mono("Named"));
-        let mut qfunc = Self::mono_class("QuantifiedFunc", Self::TOP_LEVEL);
+        let mut qfunc = Self::mono_class("QuantifiedFunc", None, Self::TOP_LEVEL);
         qfunc.register_superclass(mono("Func"), &func);
         qfunc.register_superclass(Obj, &obj);
         self.register_builtin_type(Obj, obj, Const);
@@ -1668,6 +1682,7 @@ impl Context {
             "Interval",
             params,
             // super: vec![Type::from(&m..=&n)],
+            None,
             Self::TOP_LEVEL,
         );
         let op_t = fn1_met(
@@ -1675,7 +1690,7 @@ impl Context {
             Type::from(&o..=&p),
             Type::from(m.clone() + o.clone()..=n.clone() + p.clone()),
         );
-        let mut interval_add = Self::methods("Add", Self::TOP_LEVEL);
+        let mut interval_add = Self::methods("Add", None, Self::TOP_LEVEL);
         interval_add.register_builtin_impl("__add__", op_t, Const, Public);
         interval_add.register_builtin_const(
             "Output",
@@ -1686,7 +1701,7 @@ impl Context {
             poly("Add", vec![TyParam::from(&o..=&p)]),
             interval_add,
         );
-        let mut interval_sub = Self::methods("Sub", Self::TOP_LEVEL);
+        let mut interval_sub = Self::methods("Sub", None, Self::TOP_LEVEL);
         let op_t = fn1_met(
             Type::from(&m..=&n),
             Type::from(&o..=&p),
@@ -1711,7 +1726,7 @@ impl Context {
 
     pub(crate) fn init_builtins() -> Self {
         // TODO: capacityを正確に把握する
-        let mut ctx = Context::module("<builtins>".into(), 40);
+        let mut ctx = Context::module("<builtins>".into(), None, 40);
         ctx.init_builtin_funcs();
         ctx.init_builtin_const_funcs();
         ctx.init_builtin_procs();
@@ -1728,6 +1743,7 @@ impl Context {
             ContextKind::Module,
             vec![],
             Some(Context::init_builtins()),
+            Some(SharedModuleCache::new()),
             Context::TOP_LEVEL,
         )
     }
diff --git a/compiler/erg_compiler/context/initialize/py_mods/importlib.rs b/compiler/erg_compiler/context/initialize/py_mods/importlib.rs
index d23f8804..d4309d75 100644
--- a/compiler/erg_compiler/context/initialize/py_mods/importlib.rs
+++ b/compiler/erg_compiler/context/initialize/py_mods/importlib.rs
@@ -11,7 +11,7 @@ use Visibility::*;
 
 impl Context {
     pub(crate) fn init_py_importlib_mod() -> Self {
-        let mut importlib = Context::module("importlib".into(), 15);
+        let mut importlib = Context::module("importlib".into(), None, 15);
         importlib.register_builtin_impl("reload!", proc1(Module, NoneType), Immutable, Public);
         importlib
     }
diff --git a/compiler/erg_compiler/context/initialize/py_mods/io.rs b/compiler/erg_compiler/context/initialize/py_mods/io.rs
index 66ca5634..8769a3db 100644
--- a/compiler/erg_compiler/context/initialize/py_mods/io.rs
+++ b/compiler/erg_compiler/context/initialize/py_mods/io.rs
@@ -12,8 +12,8 @@ use Visibility::*;
 
 impl Context {
     pub(crate) fn init_py_io_mod() -> Self {
-        let mut io = Context::module("io".into(), 15);
-        let mut string_io = Context::mono_class(Str::ever("StringIO!"), 0);
+        let mut io = Context::module("io".into(), None, 15);
+        let mut string_io = Context::mono_class(Str::ever("StringIO!"), None, 0);
         // FIXME: include Obj (pass main_ctx as a param)
         // string_io.register_superclass(Obj, obj);
         string_io.register_builtin_impl(
diff --git a/compiler/erg_compiler/context/initialize/py_mods/math.rs b/compiler/erg_compiler/context/initialize/py_mods/math.rs
index 0bd63129..c909c79c 100644
--- a/compiler/erg_compiler/context/initialize/py_mods/math.rs
+++ b/compiler/erg_compiler/context/initialize/py_mods/math.rs
@@ -11,7 +11,7 @@ use Visibility::*;
 
 impl Context {
     pub(crate) fn init_py_math_mod() -> Self {
-        let mut math = Context::module("math".into(), 10);
+        let mut math = Context::module("math".into(), None, 10);
         math.register_builtin_impl("pi", Float, Immutable, Public);
         math.register_builtin_impl("tau", Float, Immutable, Public);
         math.register_builtin_impl("e", Float, Immutable, Public);
diff --git a/compiler/erg_compiler/context/initialize/py_mods/random.rs b/compiler/erg_compiler/context/initialize/py_mods/random.rs
index 09f02e1a..7f1b1a9d 100644
--- a/compiler/erg_compiler/context/initialize/py_mods/random.rs
+++ b/compiler/erg_compiler/context/initialize/py_mods/random.rs
@@ -14,7 +14,7 @@ use Visibility::*;
 
 impl Context {
     pub(crate) fn init_py_random_mod() -> Self {
-        let mut random = Context::module("random".into(), 10);
+        let mut random = Context::module("random".into(), None, 10);
         random.register_builtin_impl(
             "seed!",
             proc(
diff --git a/compiler/erg_compiler/context/initialize/py_mods/socket.rs b/compiler/erg_compiler/context/initialize/py_mods/socket.rs
index 84088f94..fedf52c3 100644
--- a/compiler/erg_compiler/context/initialize/py_mods/socket.rs
+++ b/compiler/erg_compiler/context/initialize/py_mods/socket.rs
@@ -12,8 +12,8 @@ use Visibility::*;
 
 impl Context {
     pub(crate) fn init_py_socket_mod() -> Self {
-        let mut socket = Context::module("socket".into(), 15);
-        let mut sock = Context::mono_class(Str::ever("Socket!"), 0);
+        let mut socket = Context::module("socket".into(), None, 15);
+        let mut sock = Context::mono_class(Str::ever("Socket!"), None, 0);
         // FIXME: include Obj (pass main_ctx as a param)
         // sock.register_superclass(Obj, obj);
         sock.register_builtin_impl(
diff --git a/compiler/erg_compiler/context/initialize/py_mods/sys.rs b/compiler/erg_compiler/context/initialize/py_mods/sys.rs
index ae36ea30..ab392fad 100644
--- a/compiler/erg_compiler/context/initialize/py_mods/sys.rs
+++ b/compiler/erg_compiler/context/initialize/py_mods/sys.rs
@@ -12,7 +12,7 @@ use Visibility::*;
 
 impl Context {
     pub(crate) fn init_py_sys_mod() -> Self {
-        let mut sys = Context::module("sys".into(), 15);
+        let mut sys = Context::module("sys".into(), None, 15);
         sys.register_builtin_impl("argv", array(Str, TyParam::erased(Nat)), Immutable, Public);
         sys.register_builtin_impl("byteorder", Str, Immutable, Public);
         sys.register_builtin_impl(
diff --git a/compiler/erg_compiler/context/initialize/py_mods/time.rs b/compiler/erg_compiler/context/initialize/py_mods/time.rs
index 7b91ef9b..c4c374ae 100644
--- a/compiler/erg_compiler/context/initialize/py_mods/time.rs
+++ b/compiler/erg_compiler/context/initialize/py_mods/time.rs
@@ -11,7 +11,7 @@ use Visibility::*;
 
 impl Context {
     pub(crate) fn init_py_time_mod() -> Self {
-        let mut time = Context::module("time".into(), 15);
+        let mut time = Context::module("time".into(), None, 15);
         time.register_builtin_impl("sleep!", proc1(Float, NoneType), Immutable, Public);
         time.register_builtin_impl("time!", proc0(Float), Immutable, Public);
         time
diff --git a/compiler/erg_compiler/context/inquire.rs b/compiler/erg_compiler/context/inquire.rs
index c43fc344..96d01ced 100644
--- a/compiler/erg_compiler/context/inquire.rs
+++ b/compiler/erg_compiler/context/inquire.rs
@@ -1223,7 +1223,11 @@ impl Context {
     }
 
     fn rec_get_mod(&self, name: &str) -> Option<&Context> {
-        if let Some(mod_) = self.mods.get(name) {
+        if let Some(mod_) = self
+            .mod_cache
+            .as_ref()
+            .and_then(|cache| cache.ref_ctx(name))
+        {
             Some(mod_)
         } else if let Some(outer) = &self.outer {
             outer.rec_get_mod(name)
diff --git a/compiler/erg_compiler/context/mod.rs b/compiler/erg_compiler/context/mod.rs
index 155acdd9..a6c4323a 100644
--- a/compiler/erg_compiler/context/mod.rs
+++ b/compiler/erg_compiler/context/mod.rs
@@ -37,6 +37,7 @@ use erg_parser::token::Token;
 
 use crate::context::instantiate::ConstTemplate;
 use crate::error::{TyCheckError, TyCheckErrors, TyCheckResult};
+use crate::mod_cache::SharedModuleCache;
 use crate::varinfo::{Mutability, ParamIdx, VarInfo, VarKind};
 use Mutability::*;
 use Visibility::*;
@@ -310,7 +311,7 @@ pub struct Context {
     // patches can be accessed like normal records
     // but when used as a fallback to a type, values are traversed instead of accessing by keys
     pub(crate) patches: Dict<VarName, Context>,
-    pub(crate) mods: Dict<VarName, Context>,
+    pub(crate) mod_cache: Option<SharedModuleCache>,
     pub(crate) level: usize,
 }
 
@@ -322,6 +323,7 @@ impl Default for Context {
             ContextKind::Dummy,
             vec![],
             None,
+            None,
             Self::TOP_LEVEL,
         )
     }
@@ -340,7 +342,7 @@ impl fmt::Display for Context {
             .field("mono_types", &self.mono_types)
             .field("poly_types", &self.poly_types)
             .field("patches", &self.patches)
-            .field("mods", &self.mods)
+            // .field("mod_cache", &self.mod_cache)
             .finish()
     }
 }
@@ -352,9 +354,10 @@ impl Context {
         kind: ContextKind,
         params: Vec<ParamSpec>,
         outer: Option<Context>,
+        mod_cache: Option<SharedModuleCache>,
         level: usize,
     ) -> Self {
-        Self::with_capacity(name, kind, params, outer, 0, level)
+        Self::with_capacity(name, kind, params, outer, 0, mod_cache, level)
     }
 
     #[allow(clippy::too_many_arguments)]
@@ -364,6 +367,7 @@ impl Context {
         params: Vec<ParamSpec>,
         outer: Option<Context>,
         capacity: usize,
+        mod_cache: Option<SharedModuleCache>,
         level: usize,
     ) -> Self {
         let mut params_ = Vec::new();
@@ -400,15 +404,21 @@ impl Context {
             consts: Dict::default(),
             mono_types: Dict::default(),
             poly_types: Dict::default(),
-            mods: Dict::default(),
+            mod_cache,
             patches: Dict::default(),
             level,
         }
     }
 
     #[inline]
-    pub fn mono(name: Str, kind: ContextKind, outer: Option<Context>, level: usize) -> Self {
-        Self::with_capacity(name, kind, vec![], outer, 0, level)
+    pub fn mono(
+        name: Str,
+        kind: ContextKind,
+        outer: Option<Context>,
+        mod_cache: Option<SharedModuleCache>,
+        level: usize,
+    ) -> Self {
+        Self::with_capacity(name, kind, vec![], outer, 0, mod_cache, level)
     }
 
     #[inline]
@@ -417,61 +427,111 @@ impl Context {
         kind: ContextKind,
         params: Vec<ParamSpec>,
         outer: Option<Context>,
+        mod_cache: Option<SharedModuleCache>,
         level: usize,
     ) -> Self {
-        Self::with_capacity(name, kind, params, outer, 0, level)
+        Self::with_capacity(name, kind, params, outer, 0, mod_cache, level)
     }
 
-    pub fn poly_trait<S: Into<Str>>(name: S, params: Vec<ParamSpec>, level: usize) -> Self {
+    pub fn poly_trait<S: Into<Str>>(
+        name: S,
+        params: Vec<ParamSpec>,
+        mod_cache: Option<SharedModuleCache>,
+        level: usize,
+    ) -> Self {
         let name = name.into();
-        Self::poly(name, ContextKind::Trait, params, None, level)
+        Self::poly(name, ContextKind::Trait, params, None, mod_cache, level)
     }
 
-    pub fn poly_class<S: Into<Str>>(name: S, params: Vec<ParamSpec>, level: usize) -> Self {
+    pub fn poly_class<S: Into<Str>>(
+        name: S,
+        params: Vec<ParamSpec>,
+        mod_cache: Option<SharedModuleCache>,
+        level: usize,
+    ) -> Self {
         let name = name.into();
-        Self::poly(name, ContextKind::Class, params, None, level)
+        Self::poly(name, ContextKind::Class, params, None, mod_cache, level)
     }
 
     #[inline]
-    pub fn mono_trait<S: Into<Str>>(name: S, level: usize) -> Self {
-        Self::poly_trait(name, vec![], level)
+    pub fn mono_trait<S: Into<Str>>(
+        name: S,
+        mod_cache: Option<SharedModuleCache>,
+        level: usize,
+    ) -> Self {
+        Self::poly_trait(name, vec![], mod_cache, level)
     }
 
     #[inline]
-    pub fn mono_class<S: Into<Str>>(name: S, level: usize) -> Self {
-        Self::poly_class(name, vec![], level)
+    pub fn mono_class<S: Into<Str>>(
+        name: S,
+        mod_cache: Option<SharedModuleCache>,
+        level: usize,
+    ) -> Self {
+        Self::poly_class(name, vec![], mod_cache, level)
     }
 
     #[inline]
-    pub fn methods<S: Into<Str>>(name: S, level: usize) -> Self {
-        Self::with_capacity(name.into(), ContextKind::MethodDefs, vec![], None, 2, level)
+    pub fn methods<S: Into<Str>>(
+        name: S,
+        mod_cache: Option<SharedModuleCache>,
+        level: usize,
+    ) -> Self {
+        Self::with_capacity(
+            name.into(),
+            ContextKind::MethodDefs,
+            vec![],
+            None,
+            2,
+            mod_cache,
+            level,
+        )
     }
 
     #[inline]
-    pub fn poly_patch<S: Into<Str>>(name: S, params: Vec<ParamSpec>, level: usize) -> Self {
-        Self::poly(name.into(), ContextKind::Trait, params, None, level)
+    pub fn poly_patch<S: Into<Str>>(
+        name: S,
+        params: Vec<ParamSpec>,
+        mod_cache: Option<SharedModuleCache>,
+        level: usize,
+    ) -> Self {
+        Self::poly(
+            name.into(),
+            ContextKind::Trait,
+            params,
+            None,
+            mod_cache,
+            level,
+        )
     }
 
     #[inline]
-    pub fn module(name: Str, capacity: usize) -> Self {
+    pub fn module(name: Str, mod_cache: Option<SharedModuleCache>, capacity: usize) -> Self {
         Self::with_capacity(
             name,
             ContextKind::Module,
             vec![],
             None,
             capacity,
+            mod_cache,
             Self::TOP_LEVEL,
         )
     }
 
     #[inline]
-    pub fn instant(name: Str, capacity: usize, outer: Context) -> Self {
+    pub fn instant(
+        name: Str,
+        capacity: usize,
+        mod_cache: Option<SharedModuleCache>,
+        outer: Context,
+    ) -> Self {
         Self::with_capacity(
             name,
             ContextKind::Instant,
             vec![],
             Some(outer),
             capacity,
+            mod_cache,
             Self::TOP_LEVEL,
         )
     }
diff --git a/compiler/erg_compiler/context/register.rs b/compiler/erg_compiler/context/register.rs
index 56d70658..49a026ec 100644
--- a/compiler/erg_compiler/context/register.rs
+++ b/compiler/erg_compiler/context/register.rs
@@ -18,6 +18,7 @@ use crate::context::{ClassDefType, Context, DefaultInfo, RegistrationMode, Trait
 use crate::error::readable_name;
 use crate::error::{TyCheckError, TyCheckResult};
 use crate::hir;
+use crate::mod_cache::ModuleEntry;
 use crate::varinfo::{Mutability, ParamIdx, VarInfo, VarKind};
 use Mutability::*;
 use RegistrationMode::*;
@@ -593,8 +594,10 @@ impl Context {
             TypeKind::Class => {
                 if gen.t.is_monomorphic() {
                     // let super_traits = gen.impls.iter().map(|to| to.typ().clone()).collect();
-                    let mut ctx = Self::mono_class(gen.t.name(), self.level);
-                    let mut methods = Self::methods(gen.t.name(), self.level);
+                    let mut ctx =
+                        Self::mono_class(gen.t.name(), self.mod_cache.clone(), self.level);
+                    let mut methods =
+                        Self::methods(gen.t.name(), self.mod_cache.clone(), self.level);
                     let require = gen.require_or_sup.typ().clone();
                     let new_t = func1(require, gen.t.clone());
                     methods.register_fixed_auto_impl("__new__", new_t.clone(), Immutable, Private);
@@ -611,12 +614,14 @@ impl Context {
                 if gen.t.is_monomorphic() {
                     let super_classes = vec![gen.require_or_sup.typ().clone()];
                     // let super_traits = gen.impls.iter().map(|to| to.typ().clone()).collect();
-                    let mut ctx = Self::mono_class(gen.t.name(), self.level);
+                    let mut ctx =
+                        Self::mono_class(gen.t.name(), self.mod_cache.clone(), self.level);
                     for sup in super_classes.into_iter() {
                         let (_, sup_ctx) = self.get_nominal_type_ctx(&sup).unwrap();
                         ctx.register_superclass(sup, sup_ctx);
                     }
-                    let mut methods = Self::methods(gen.t.name(), self.level);
+                    let mut methods =
+                        Self::methods(gen.t.name(), self.mod_cache.clone(), self.level);
                     if let Some(sup) = self.rec_get_const_obj(&gen.require_or_sup.typ().name()) {
                         let sup = enum_unwrap!(sup, ValueObj::Type);
                         let param_t = match sup {
@@ -700,32 +705,55 @@ impl Context {
             hir::Expr::Lit(lit) => {
                 if self.subtype_of(&lit.value.class(), &Str) {
                     let name = enum_unwrap!(lit.value.clone(), ValueObj::Str);
-                    match &name[..] {
-                        "importlib" => {
-                            self.mods
-                                .insert(var_name.clone(), Self::init_py_importlib_mod());
+                    if let Some(mod_cache) = self.mod_cache.as_mut() {
+                        match &name[..] {
+                            "importlib" => {
+                                mod_cache.register(
+                                    var_name.clone(),
+                                    ModuleEntry::builtin(Self::init_py_importlib_mod()),
+                                );
+                            }
+                            "io" => {
+                                mod_cache.register(
+                                    var_name.clone(),
+                                    ModuleEntry::builtin(Self::init_py_io_mod()),
+                                );
+                            }
+                            "math" => {
+                                mod_cache.register(
+                                    var_name.clone(),
+                                    ModuleEntry::builtin(Self::init_py_math_mod()),
+                                );
+                            }
+                            "random" => {
+                                mod_cache.register(
+                                    var_name.clone(),
+                                    ModuleEntry::builtin(Self::init_py_random_mod()),
+                                );
+                            }
+                            "socket" => {
+                                mod_cache.register(
+                                    var_name.clone(),
+                                    ModuleEntry::builtin(Self::init_py_socket_mod()),
+                                );
+                            }
+                            "sys" => {
+                                mod_cache.register(
+                                    var_name.clone(),
+                                    ModuleEntry::builtin(Self::init_py_sys_mod()),
+                                );
+                            }
+                            "time" => {
+                                mod_cache.register(
+                                    var_name.clone(),
+                                    ModuleEntry::builtin(Self::init_py_time_mod()),
+                                );
+                            }
+                            other => todo!("importing {other}"),
                         }
-                        "io" => {
-                            self.mods.insert(var_name.clone(), Self::init_py_io_mod());
-                        }
-                        "math" => {
-                            self.mods.insert(var_name.clone(), Self::init_py_math_mod());
-                        }
-                        "random" => {
-                            self.mods
-                                .insert(var_name.clone(), Self::init_py_random_mod());
-                        }
-                        "socket" => {
-                            self.mods
-                                .insert(var_name.clone(), Self::init_py_socket_mod());
-                        }
-                        "sys" => {
-                            self.mods.insert(var_name.clone(), Self::init_py_sys_mod());
-                        }
-                        "time" => {
-                            self.mods.insert(var_name.clone(), Self::init_py_time_mod());
-                        }
-                        other => todo!("importing {other}"),
+                    } else {
+                        // maybe unreachable
+                        todo!("importing {name} in the builtin module")
                     }
                 } else {
                     return Err(TyCheckError::type_mismatch_error(
diff --git a/compiler/erg_compiler/lib.rs b/compiler/erg_compiler/lib.rs
index 950c2dd4..392a1fbc 100644
--- a/compiler/erg_compiler/lib.rs
+++ b/compiler/erg_compiler/lib.rs
@@ -11,6 +11,7 @@ pub mod effectcheck;
 pub mod error;
 pub mod hir;
 pub mod lower;
+pub mod mod_cache;
 pub mod optimize;
 pub mod ownercheck;
 pub mod reorder;
diff --git a/compiler/erg_compiler/mod_cache.rs b/compiler/erg_compiler/mod_cache.rs
new file mode 100644
index 00000000..be423fbe
--- /dev/null
+++ b/compiler/erg_compiler/mod_cache.rs
@@ -0,0 +1,115 @@
+use std::borrow::Borrow;
+use std::hash::Hash;
+use std::rc::Rc;
+
+use erg_common::dict::Dict;
+use erg_common::shared::Shared;
+
+use erg_parser::ast::VarName;
+
+use crate::context::Context;
+use crate::hir::HIR;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct ModId(usize);
+
+impl ModId {
+    pub const fn new(id: usize) -> Self {
+        Self(id)
+    }
+    pub const fn builtin() -> Self {
+        Self(0)
+    }
+    pub const fn main() -> Self {
+        Self(1)
+    }
+}
+
+#[derive(Debug)]
+pub struct ModuleEntry {
+    id: ModId, // builtin == 0, __main__ == 1
+    hir: Option<HIR>,
+    ctx: Rc<Context>,
+}
+
+impl ModuleEntry {
+    pub fn new(id: ModId, hir: Option<HIR>, ctx: Context) -> Self {
+        Self {
+            id,
+            hir,
+            ctx: Rc::new(ctx),
+        }
+    }
+
+    pub fn builtin(ctx: Context) -> Self {
+        Self {
+            id: ModId::builtin(),
+            hir: None,
+            ctx: Rc::new(ctx),
+        }
+    }
+}
+
+#[derive(Debug, Default)]
+pub struct ModuleCache {
+    cache: Dict<VarName, ModuleEntry>,
+}
+
+impl ModuleCache {
+    pub fn new() -> Self {
+        Self { cache: Dict::new() }
+    }
+
+    pub fn get<Q: Eq + Hash + ?Sized>(&self, name: &Q) -> Option<&ModuleEntry>
+    where
+        VarName: Borrow<Q>,
+    {
+        self.cache.get(name)
+    }
+
+    pub fn register(&mut self, name: VarName, entry: ModuleEntry) {
+        self.cache.insert(name, entry);
+    }
+
+    pub fn remove<Q: Eq + Hash + ?Sized>(&mut self, name: &Q) -> Option<ModuleEntry>
+    where
+        VarName: Borrow<Q>,
+    {
+        self.cache.remove(name)
+    }
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct SharedModuleCache(Shared<ModuleCache>);
+
+impl SharedModuleCache {
+    pub fn new() -> Self {
+        Self(Shared::new(ModuleCache::new()))
+    }
+
+    pub fn get_ctx<Q: Eq + Hash + ?Sized>(&self, name: &Q) -> Option<Rc<Context>>
+    where
+        VarName: Borrow<Q>,
+    {
+        self.0.borrow().get(name).map(|entry| entry.ctx.clone())
+    }
+
+    pub fn ref_ctx<Q: Eq + Hash + ?Sized>(&self, name: &Q) -> Option<&Context>
+    where
+        VarName: Borrow<Q>,
+    {
+        let ref_ = unsafe { self.0.as_ptr().as_ref().unwrap() };
+        ref_.get(name).map(|entry| entry.ctx.as_ref())
+    }
+
+    pub fn register(&self, name: VarName, entry: ModuleEntry) {
+        self.0.borrow_mut().register(name, entry);
+    }
+
+    pub fn remove<Q: Eq + Hash + ?Sized>(&mut self, name: &Q) -> Option<ModuleEntry>
+    where
+        VarName: Borrow<Q>,
+    {
+        self.0.borrow_mut().remove(name)
+    }
+}
diff --git a/compiler/erg_type/free.rs b/compiler/erg_type/free.rs
index 6c42efc7..d5477352 100644
--- a/compiler/erg_type/free.rs
+++ b/compiler/erg_type/free.rs
@@ -3,7 +3,7 @@ use std::fmt;
 use std::mem;
 
 use erg_common::addr_eq;
-use erg_common::rccell::RcCell;
+use erg_common::shared::Shared;
 use erg_common::traits::LimitedDisplay;
 
 use crate::typaram::TyParam;
@@ -14,8 +14,8 @@ pub type Level = usize;
 pub type Id = usize;
 
 thread_local! {
-    static UNBOUND_ID: RcCell<usize> = RcCell::new(0);
-    static REFINEMENT_VAR_ID: RcCell<usize> = RcCell::new(0);
+    static UNBOUND_ID: Shared<usize> = Shared::new(0);
+    static REFINEMENT_VAR_ID: Shared<usize> = Shared::new(0);
 }
 
 pub fn fresh_varname() -> String {
@@ -340,7 +340,7 @@ impl<T> FreeKind<T> {
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct Free<T>(RcCell<FreeKind<T>>);
+pub struct Free<T>(Shared<FreeKind<T>>);
 
 impl<T: LimitedDisplay> fmt::Display for Free<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -390,13 +390,13 @@ impl<T: Clone> Free<T> {
 
 impl<T: Clone + HasLevel> Free<T> {
     pub fn new(f: FreeKind<T>) -> Self {
-        Self(RcCell::new(f))
+        Self(Shared::new(f))
     }
 
     pub fn new_unbound(level: Level, constraint: Constraint) -> Self {
         UNBOUND_ID.with(|id| {
             *id.borrow_mut() += 1;
-            Self(RcCell::new(FreeKind::unbound(
+            Self(Shared::new(FreeKind::unbound(
                 *id.borrow(),
                 level,
                 constraint,
@@ -405,13 +405,13 @@ impl<T: Clone + HasLevel> Free<T> {
     }
 
     pub fn new_named_unbound(name: Str, level: Level, constraint: Constraint) -> Self {
-        Self(RcCell::new(FreeKind::named_unbound(
+        Self(Shared::new(FreeKind::named_unbound(
             name, level, constraint,
         )))
     }
 
     pub fn new_linked(t: T) -> Self {
-        Self(RcCell::new(FreeKind::Linked(t)))
+        Self(Shared::new(FreeKind::Linked(t)))
     }
 
     pub fn link(&self, to: &T) {
diff --git a/compiler/erg_type/value.rs b/compiler/erg_type/value.rs
index 9bb7fe1c..b74e1cf4 100644
--- a/compiler/erg_type/value.rs
+++ b/compiler/erg_type/value.rs
@@ -9,9 +9,9 @@ use std::ops::Neg;
 use std::rc::Rc;
 
 use erg_common::dict::Dict;
-use erg_common::rccell::RcCell;
 use erg_common::serialize::*;
 use erg_common::set;
+use erg_common::shared::Shared;
 use erg_common::vis::Field;
 use erg_common::{fmt_iter, impl_display_from_debug, switch_lang};
 use erg_common::{RcArray, Str};
@@ -124,7 +124,7 @@ pub enum ValueObj {
     NotImplemented,
     NegInf,
     Inf,
-    Mut(RcCell<ValueObj>),
+    Mut(Shared<ValueObj>),
     #[default]
     Illegal, // to avoid conversions with TryFrom
 }