diff --git a/nac3core/src/codegen/expr.rs b/nac3core/src/codegen/expr.rs
index 4ad363c7..b267d330 100644
--- a/nac3core/src/codegen/expr.rs
+++ b/nac3core/src/codegen/expr.rs
@@ -7,7 +7,7 @@ use crate::{
     },
     symbol_resolver::SymbolValue,
     toplevel::{DefinitionId, TopLevelDef},
-    typecheck::typedef::{FunSignature, FuncArg, Type, TypeEnum},
+    typecheck::typedef::{FunSignature, FuncArg, Type, TypeEnum, Unifier},
 };
 use inkwell::{
     types::{BasicType, BasicTypeEnum},
@@ -21,6 +21,31 @@ use nac3parser::ast::{
 
 use super::CodeGenerator;
 
+pub fn get_subst_key(
+    unifier: &mut Unifier,
+    obj: Option<Type>,
+    fun_vars: &HashMap<u32, Type>,
+    filter: Option<&Vec<u32>>,
+) -> String {
+    let mut vars = obj
+        .map(|ty| {
+            if let TypeEnum::TObj { params, .. } = &*unifier.get_ty(ty) {
+                params.borrow().clone()
+            } else {
+                unreachable!()
+            }
+        })
+        .unwrap_or_default();
+    vars.extend(fun_vars.iter());
+    let sorted =
+        vars.keys().filter(|id| filter.map(|v| v.contains(id)).unwrap_or(true)).sorted();
+    sorted
+        .map(|id| {
+            unifier.stringify(vars[id], &mut |id| id.to_string(), &mut |id| id.to_string())
+        })
+        .join(", ")
+}
+
 impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
     pub fn build_gep_and_load(
         &mut self,
@@ -36,23 +61,7 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
         fun: &FunSignature,
         filter: Option<&Vec<u32>>,
     ) -> String {
-        let mut vars = obj
-            .map(|ty| {
-                if let TypeEnum::TObj { params, .. } = &*self.unifier.get_ty(ty) {
-                    params.borrow().clone()
-                } else {
-                    unreachable!()
-                }
-            })
-            .unwrap_or_default();
-        vars.extend(fun.vars.iter());
-        let sorted =
-            vars.keys().filter(|id| filter.map(|v| v.contains(id)).unwrap_or(true)).sorted();
-        sorted
-            .map(|id| {
-                self.unifier.stringify(vars[id], &mut |id| id.to_string(), &mut |id| id.to_string())
-            })
-            .join(", ")
+        get_subst_key(&mut self.unifier, obj, &fun.vars, filter)
     }
 
     pub fn get_attr_index(&mut self, ty: Type, attr: StrRef) -> usize {
diff --git a/nac3core/src/toplevel/composer.rs b/nac3core/src/toplevel/composer.rs
index 57ca2643..8dd5404c 100644
--- a/nac3core/src/toplevel/composer.rs
+++ b/nac3core/src/toplevel/composer.rs
@@ -6,6 +6,7 @@ use inkwell::FloatPredicate;
 use crate::{
     symbol_resolver::SymbolValue,
     typecheck::type_inferencer::{FunctionData, Inferencer},
+    codegen::expr::get_subst_key,
 };
 
 use super::*;
@@ -1835,21 +1836,13 @@ impl TopLevelComposer {
 
                         instance_to_stmt.insert(
                             // NOTE: refer to codegen/expr/get_subst_key function
-                            {
-                                let unifier = &mut self.unifier;
-                                subst
-                                    .keys()
-                                    .sorted()
-                                    .map(|id| {
-                                        let ty = subst.get(id).unwrap();
-                                        unifier.stringify(
-                                            *ty,
-                                            &mut |id| id.to_string(),
-                                            &mut |id| id.to_string(),
-                                        )
-                                    })
-                                    .join(", ")
-                            },
+                            
+                            get_subst_key(
+                                &mut self.unifier,
+                                self_type,
+                                &subst,
+                                None
+                            ),
                             FunInstance {
                                 body: Arc::new(fun_body),
                                 unifier_id: 0,