diff --git a/src/evaluator/mod.rs b/src/evaluator/mod.rs
index 5223c3c..fd28c6c 100644
--- a/src/evaluator/mod.rs
+++ b/src/evaluator/mod.rs
@@ -126,7 +126,7 @@ fn apply_function(fn_obj: Object, args: Vec<Object>) -> anyhow::Result<Object> {
             let mut extend_env = extend_function_env(fn_value.clone(), args);
             trace!("[apply_function] extend_env is {:?}", extend_env);
 
-            let evaluated = eval(fn_value.body.into(), &mut extend_env)?;
+            let evaluated = eval(fn_value.body().clone().into(), &mut extend_env)?;
             trace!("[apply_function] call function result is {}", evaluated);
 
             Ok(evaluated)
@@ -149,7 +149,7 @@ fn eval_hash_literal(node: HashLiteral, env: &mut Environment) -> anyhow::Result
 }
 
 fn extend_function_env(fn_obj: Function, args: Vec<Object>) -> Environment {
-    let mut env = Environment::new_enclosed_environment(fn_obj.env.clone());
+    let mut env = Environment::new_enclosed_environment(fn_obj.env().clone());
     for (param_idx, param) in fn_obj.parameters().iter().enumerate() {
         env.store(param.value.clone(), args[param_idx].clone()); // TODO need imporve
     }
diff --git a/src/evaluator/tests.rs b/src/evaluator/tests.rs
index 3ec7904..272d3f9 100644
--- a/src/evaluator/tests.rs
+++ b/src/evaluator/tests.rs
@@ -540,8 +540,8 @@ fn test_function_object() -> anyhow::Result<()> {
 
     let expected_body = "(x + 2);";
 
-    if format!("{}", value.body) != expected_body {
-        eprintln!("body is not {}. got = {}", expected_body, value.body);
+    if format!("{}", value.body()) != expected_body {
+        eprintln!("body is not {}. got = {}", expected_body, value.body());
     }
 
     Ok(())
diff --git a/src/object/built_in_function.rs b/src/object/built_in_function.rs
index 72818fd..3f14099 100644
--- a/src/object/built_in_function.rs
+++ b/src/object/built_in_function.rs
@@ -10,26 +10,29 @@ use std::fmt::{Display, Formatter};
 
 const BUILD_FUNC: &str = "builtin function";
 
+type BuildBoxFuncType = Box<fn(Vec<Object>) -> anyhow::Result<Object>>;
+type BuildFuncType = fn(Vec<Object>) -> anyhow::Result<Object>;
+
 #[derive(Debug, Clone, PartialOrd, PartialEq, Eq, Ord, Hash)]
 pub struct Builtin {
-    built_in_function: Box<fn(Vec<Object>) -> anyhow::Result<Object>>,
+    built_in_function: BuildBoxFuncType,
 }
 
 impl Builtin {
-    pub fn new(func: fn(Vec<Object>) -> anyhow::Result<Object>) -> Self {
+    pub fn new(func: BuildFuncType) -> Self {
         Self {
             built_in_function: Box::new(func),
         }
     }
 
-    pub fn value(&self) -> &Box<fn(Vec<Object>) -> anyhow::Result<Object>> {
+    pub fn value(&self) -> &BuildBoxFuncType {
         &self.built_in_function
     }
 }
 
 impl Display for Builtin {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
-        write!(f, "built_in_function")
+        write!(f, "{}", BUILD_FUNC)
     }
 }
 
diff --git a/src/object/function.rs b/src/object/function.rs
index 7adcd10..d9b25fc 100644
--- a/src/object/function.rs
+++ b/src/object/function.rs
@@ -10,8 +10,8 @@ use string_join::Join;
 #[derive(Debug, Clone, PartialOrd, PartialEq, Eq, Ord, Hash)]
 pub struct Function {
     parameters: Vec<Identifier>,
-    pub body: BlockStatement,
-    pub env: Environment,
+    body: BlockStatement,
+    env: Environment,
 }
 
 impl Function {
@@ -25,6 +25,14 @@ impl Function {
     pub fn parameters(&self) -> &Vec<Identifier> {
         &self.parameters
     }
+
+    pub fn body(&self) -> &BlockStatement {
+        &self.body
+    }
+
+    pub fn env(&self) -> &Environment {
+        &self.env
+    }
 }
 
 impl Display for Function {