diff --git a/src/main.rs b/src/main.rs
index 9276e78..552fc21 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -124,7 +124,7 @@ fn handle_module_error(input: Either<MachineErrorAt, Vec<ParseError>>) -> Error
 fn eval(
     options: &CompileOptions,
     vm: &mut Machine,
-    env: &mut Environment,
+    env: &Environment,
     value: Value,
 ) -> Option<Value> {
     let result = vm.evaluate_value(
@@ -145,7 +145,7 @@ fn eval(
 fn run_interactive(
     compile_options: &CompileOptions,
     vm: &mut Machine,
-    env: &mut Environment,
+    env: &Environment,
 ) -> Result<(), Error> {
     let mut reader = InteractiveReader::new("> ", ">> ");
 
@@ -171,7 +171,7 @@ fn run_interactive(
 fn run_module<P: AsRef<Path>>(
     compile_options: &CompileOptions,
     vm: &mut Machine,
-    env: &mut Environment,
+    env: &Environment,
     path: P,
 ) -> Result<(), Error> {
     let path = path.as_ref();
@@ -202,8 +202,8 @@ fn main() -> ExitCode {
     vm.trace_calls = args.trace.contains(&Trace::Call);
     vm.trace_returns = args.trace.contains(&Trace::Return);
     vm.trace_stack = args.trace.contains(&Trace::Stack);
-    let mut env = Environment::default();
-    prelude::load(&mut env);
+    let env = Environment::default();
+    prelude::load(&env);
     let mut arguments = vec![];
     if let Some(script) = args.module.as_ref() {
         arguments.push(format!("{}", script.display()));
@@ -214,8 +214,8 @@ fn main() -> ExitCode {
         Value::list_or_nil(arguments.into_iter().map(|arg| Value::String(arg.into()))),
     );
     let result = match args.module.as_ref() {
-        Some(module) => run_module(&compile_options, &mut vm, &mut env, module),
-        None => run_interactive(&compile_options, &mut vm, &mut env),
+        Some(module) => run_module(&compile_options, &mut vm, &env, module),
+        None => run_interactive(&compile_options, &mut vm, &env),
     };
     match result {
         Ok(()) => ExitCode::SUCCESS,
diff --git a/src/read.rs b/src/read.rs
index 62cbeb6..f30ff13 100644
--- a/src/read.rs
+++ b/src/read.rs
@@ -104,7 +104,7 @@ impl<R: BufRead> ModuleReader<R> {
     pub fn read_expression(
         &mut self,
         options: &CompileOptions,
-        env: &mut Environment,
+        env: &Environment,
     ) -> Result<Option<Rc<Expression>>, Either<MachineErrorAt, Vec<ParseError>>> {
         loop {
             let value =
@@ -127,7 +127,7 @@ impl<R: BufRead> ModuleReader<R> {
         mut self,
         module_name: Option<IdentifierValue>,
         options: &CompileOptions,
-        env: &mut Environment,
+        env: &Environment,
     ) -> Result<Rc<BytecodeFunction>, Either<MachineErrorAt, Vec<ParseError>>> {
         let mut cx = CompileContext::new(options.clone(), module_name);
         let mut body = FunctionBody {
@@ -249,7 +249,7 @@ fn read_inner<R: BufRead>(
 pub fn read<R: Reader>(
     reader: &mut R,
     vm: &mut Machine,
-    env: &mut Environment,
+    env: &Environment,
 ) -> Result<Option<Value>, MachineErrorAt> {
     let raw_value = reader.read().map_err(Into::into)?;
     let Some(raw_value) = raw_value else {
diff --git a/src/vm/env.rs b/src/vm/env.rs
index ee39f0e..25f9078 100644
--- a/src/vm/env.rs
+++ b/src/vm/env.rs
@@ -22,11 +22,11 @@ pub struct Environment {
 }
 
 impl Environment {
-    pub fn defun_native<S, D, F>(&mut self, identifier: S, docstring: D, function: F) -> Value
+    pub fn defun_native<S, D, F>(&self, identifier: S, docstring: D, function: F) -> Value
     where
         S: Into<IdentifierValue>,
         D: Into<StringValue>,
-        F: Fn(&mut Machine, &mut Environment, &[Value]) -> Result<Value, MachineError> + 'static,
+        F: Fn(&mut Machine, &Environment, &[Value]) -> Result<Value, MachineError> + 'static,
     {
         let identifier = identifier.into();
         let native = NativeFunction::new(identifier.clone(), docstring, function);
@@ -35,11 +35,11 @@ impl Environment {
         value
     }
 
-    pub fn defmacro_native<S, D, F>(&mut self, identifier: S, docstring: D, function: F)
+    pub fn defmacro_native<S, D, F>(&self, identifier: S, docstring: D, function: F)
     where
         S: Into<IdentifierValue>,
         D: Into<StringValue>,
-        F: Fn(&mut Machine, &mut Environment, &[Value]) -> Result<Value, MachineError> + 'static,
+        F: Fn(&mut Machine, &Environment, &[Value]) -> Result<Value, MachineError> + 'static,
     {
         let identifier = identifier.into();
         let native = NativeFunction::new(identifier.clone(), docstring, function);
@@ -49,7 +49,7 @@ impl Environment {
     }
 
     pub fn defmacro_bytecode<S: Into<IdentifierValue>>(
-        &mut self,
+        &self,
         identifier: S,
         value: Rc<BytecodeFunction>,
     ) {
@@ -73,7 +73,7 @@ impl Environment {
     }
 
     pub fn set_global_value<S: Into<IdentifierValue>, V: Into<Value>>(
-        &mut self,
+        &self,
         identifier: S,
         value: V,
     ) {
diff --git a/src/vm/machine.rs b/src/vm/machine.rs
index c885615..93638ff 100644
--- a/src/vm/machine.rs
+++ b/src/vm/machine.rs
@@ -150,7 +150,7 @@ impl Machine {
         Ok(())
     }
 
-    fn execute_get_global(&mut self, env: &mut Environment) -> Result<(), MachineError> {
+    fn execute_get_global(&mut self, env: &Environment) -> Result<(), MachineError> {
         let identifier = self.pop()?;
         let Value::Identifier(identifier) = identifier else {
             return Err(MachineError::InvalidInstructionArgument(
@@ -168,7 +168,7 @@ impl Machine {
         Ok(())
     }
 
-    fn execute_set_global(&mut self, env: &mut Environment) -> Result<(), MachineError> {
+    fn execute_set_global(&mut self, env: &Environment) -> Result<(), MachineError> {
         let identifier = self.pop()?;
         let Value::Identifier(identifier) = identifier else {
             return Err(MachineError::InvalidInstructionArgument(
@@ -186,7 +186,7 @@ impl Machine {
 
     fn execute_call(
         &mut self,
-        env: &mut Environment,
+        env: &Environment,
         argument_count: usize,
     ) -> Result<(), MachineError> {
         let base_pointer = self
@@ -380,7 +380,7 @@ impl Machine {
         Ok(())
     }
 
-    fn execute_declare_macro(&mut self, env: &mut Environment) -> Result<(), MachineError> {
+    fn execute_declare_macro(&mut self, env: &Environment) -> Result<(), MachineError> {
         let identifier = self.pop()?;
         let function = self.pop()?;
         let Value::Identifier(identifier) = identifier else {
@@ -448,7 +448,7 @@ impl Machine {
         frame.closure.function.disassemble(frame.ip, 0, 0, false);
     }
 
-    fn execute_next(&mut self, env: &mut Environment) -> Result<(), MachineError> {
+    fn execute_next(&mut self, env: &Environment) -> Result<(), MachineError> {
         if self.trace_instructions {
             if self.trace_stack {
                 self.trace_stack();
@@ -571,7 +571,7 @@ impl Machine {
 
     pub fn evaluate_closure(
         &mut self,
-        env: &mut Environment,
+        env: &Environment,
         closure: ClosureValue,
         argument_count: usize,
     ) -> Result<Value, MachineErrorAt> {
@@ -593,7 +593,7 @@ impl Machine {
 
     pub fn evaluate_closure_args(
         &mut self,
-        env: &mut Environment,
+        env: &Environment,
         closure: ClosureValue,
         args: &[Value],
     ) -> Result<Value, MachineErrorAt> {
@@ -635,7 +635,7 @@ impl Machine {
         &mut self,
         compile_options: CompileOptions,
         chunk_name: Option<IdentifierValue>,
-        env: &mut Environment,
+        env: &Environment,
         value: Value,
     ) -> Result<Value, MachineErrorAt> {
         let value = value.macro_expand(self, env, false)?;
@@ -669,7 +669,7 @@ mod tests {
     };
 
     fn try_eval(
-        env: &mut Environment,
+        env: &Environment,
         mut instructions: Vec<u8>,
         constants: Vec<Value>,
     ) -> (Machine, Result<Value, MachineErrorAt>) {
@@ -691,11 +691,7 @@ mod tests {
         (machine, result)
     }
 
-    fn eval0(
-        env: &mut Environment,
-        instructions: Vec<u8>,
-        constants: Vec<Value>,
-    ) -> (Machine, Value) {
+    fn eval0(env: &Environment, instructions: Vec<u8>, constants: Vec<Value>) -> (Machine, Value) {
         let (machine, value) = try_eval(env, instructions, constants);
         let value = value.expect("evaluation failed");
         (machine, value)
diff --git a/src/vm/macros.rs b/src/vm/macros.rs
index 5841648..9bdecb7 100644
--- a/src/vm/macros.rs
+++ b/src/vm/macros.rs
@@ -13,7 +13,7 @@ pub trait MacroExpand: Sized {
     fn macro_expand(
         &self,
         vm: &mut Machine,
-        env: &mut Environment,
+        env: &Environment,
         tail: bool,
     ) -> Result<Self, MachineErrorAt>;
 }
@@ -22,7 +22,7 @@ impl MacroExpand for Value {
     fn macro_expand(
         &self,
         vm: &mut Machine,
-        env: &mut Environment,
+        env: &Environment,
         tail: bool,
     ) -> Result<Self, MachineErrorAt> {
         match self {
diff --git a/src/vm/prelude/collections.rs b/src/vm/prelude/collections.rs
index 885929a..27752b2 100644
--- a/src/vm/prelude/collections.rs
+++ b/src/vm/prelude/collections.rs
@@ -9,7 +9,7 @@ use crate::{
     },
 };
 
-pub fn load(env: &mut Environment) {
+pub fn load(env: &Environment) {
     // // vectors
     // env.defun_native("getv", |vm, _, args| {
     //     let [vec, index] = args else {
diff --git a/src/vm/prelude/convert.rs b/src/vm/prelude/convert.rs
index 0b771c7..8ed729d 100644
--- a/src/vm/prelude/convert.rs
+++ b/src/vm/prelude/convert.rs
@@ -7,7 +7,7 @@ use crate::{
     },
 };
 
-pub fn load(env: &mut Environment) {
+pub fn load(env: &Environment) {
     env.defun_native(
         "->string",
         "Converts a value to string representation",
diff --git a/src/vm/prelude/debug.rs b/src/vm/prelude/debug.rs
index 46c1db8..da9024f 100644
--- a/src/vm/prelude/debug.rs
+++ b/src/vm/prelude/debug.rs
@@ -3,7 +3,7 @@ use crate::{
     vm::{Value, env::Environment, value::Keyword},
 };
 
-pub fn load(env: &mut Environment) {
+pub fn load(env: &Environment) {
     env.defun_native(
         "explain",
         "Provides an explanation for a given value",
diff --git a/src/vm/prelude/eval.rs b/src/vm/prelude/eval.rs
index 3f3ad78..6404472 100644
--- a/src/vm/prelude/eval.rs
+++ b/src/vm/prelude/eval.rs
@@ -4,7 +4,7 @@ use crate::{
     vm::{Value, env::Environment},
 };
 
-pub fn load(env: &mut Environment) {
+pub fn load(env: &Environment) {
     env.defun_native("eval", "Evaluates the given expression", |vm, env, args| {
         // TODO eval in env
         let [value] = args else {
diff --git a/src/vm/prelude/functional.rs b/src/vm/prelude/functional.rs
index e558dba..1ae1311 100644
--- a/src/vm/prelude/functional.rs
+++ b/src/vm/prelude/functional.rs
@@ -6,7 +6,7 @@ use crate::{
     },
 };
 
-pub fn load(env: &mut Environment) {
+pub fn load(env: &Environment) {
     // env.defun_native("map", |vm, env, args| {
     //     let [f, xs] = args else {
     //         return Err(vm.error_at_ip(MachineErrorKind::InvalidArgument));
diff --git a/src/vm/prelude/io.rs b/src/vm/prelude/io.rs
index fe2596a..a09e673 100644
--- a/src/vm/prelude/io.rs
+++ b/src/vm/prelude/io.rs
@@ -76,7 +76,7 @@ impl Stream {
     }
 }
 
-pub fn load(env: &mut Environment) {
+pub fn load(env: &Environment) {
     let stdin: Rc<dyn NativeObject> = Rc::new(Stream::Stdin);
     let stdout: Rc<dyn NativeObject> = Rc::new(Stream::Stdout);
 
diff --git a/src/vm/prelude/math.rs b/src/vm/prelude/math.rs
index 72990bc..3e123d0 100644
--- a/src/vm/prelude/math.rs
+++ b/src/vm/prelude/math.rs
@@ -12,7 +12,7 @@ use crate::{
 
 pub(crate) fn dispatch_arithmetic(
     instruction: Instruction,
-) -> fn(&mut Machine, &mut Environment, &[Value]) -> Result<Value, MachineError> {
+) -> fn(&mut Machine, &Environment, &[Value]) -> Result<Value, MachineError> {
     match instruction {
         Instruction::Add => builtin_add,
         Instruction::Sub => builtin_sub,
@@ -30,7 +30,7 @@ pub(crate) fn dispatch_arithmetic(
     }
 }
 
-pub(super) fn load(env: &mut Environment) {
+pub(super) fn load(env: &Environment) {
     env.defun_native("+", "Adds values", builtin_add);
     env.defun_native("-", "Subtracts values", builtin_sub);
     let mul = env.defun_native("*", "Multiplies values", builtin_mul);
@@ -137,14 +137,14 @@ where
 
 pub(crate) fn builtin_add(
     _vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     builtin_fold(value_add, Value::Number(0.into()), args)
 }
 pub(crate) fn builtin_sub(
     _vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     match args {
@@ -165,14 +165,14 @@ pub(crate) fn builtin_sub(
 }
 pub(crate) fn builtin_mul(
     _vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     builtin_fold_t(Mul::mul, NumberValue::from(1), args)
 }
 pub(crate) fn builtin_mod(
     _vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     let [x, y] = args else {
@@ -184,7 +184,7 @@ pub(crate) fn builtin_mod(
 }
 pub(crate) fn builtin_div(
     _vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     match args {
@@ -206,35 +206,35 @@ pub(crate) fn builtin_div(
 
 // pub(crate) fn builtin_and(
 //     vm: &mut Machine,
-//     _env: &mut Environment,
+//     _env: &Environment,
 //     args: &[Value],
 // ) -> Result<Value, MachineError> {
 //     builtin_fold_t(BitAnd::bitand, true, args)
 // }
 // pub(crate) fn builtin_or(
 //     vm: &mut Machine,
-//     _env: &mut Environment,
+//     _env: &Environment,
 //     args: &[Value],
 // ) -> Result<Value, MachineError> {
 //     builtin_fold_t(BitOr::bitor, false, args)
 // }
 // pub(crate) fn builtin_bitwise_and(
 //     vm: &mut Machine,
-//     _env: &mut Environment,
+//     _env: &Environment,
 //     args: &[Value],
 // ) -> Result<Value, MachineError> {
 //     builtin_fold_t(BitAnd::bitand, 1, args)
 // }
 // pub(crate) fn builtin_bitwise_or(
 //     vm: &mut Machine,
-//     _env: &mut Environment,
+//     _env: &Environment,
 //     args: &[Value],
 // ) -> Result<Value, MachineError> {
 //     builtin_fold_t(BitOr::bitor, 0, args)
 // }
 // pub(crate) fn builtin_bitwise_xor(
 //     vm: &mut Machine,
-//     _env: &mut Environment,
+//     _env: &Environment,
 //     args: &[Value],
 // ) -> Result<Value, MachineError> {
 //     builtin_fold_t(BitXor::bitxor, 0, args)
@@ -296,42 +296,42 @@ pub(crate) fn builtin_cmp(
 }
 pub(crate) fn builtin_cmp_eq(
     vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     builtin_cmp(vm, args, CompareOperation::Eq)
 }
 pub(crate) fn builtin_cmp_ne(
     vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     builtin_cmp(vm, args, CompareOperation::Ne)
 }
 pub(crate) fn builtin_cmp_gt(
     vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     builtin_cmp(vm, args, CompareOperation::Gt)
 }
 pub(crate) fn builtin_cmp_lt(
     vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     builtin_cmp(vm, args, CompareOperation::Lt)
 }
 pub(crate) fn builtin_cmp_ge(
     vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     builtin_cmp(vm, args, CompareOperation::Ge)
 }
 pub(crate) fn builtin_cmp_le(
     vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     builtin_cmp(vm, args, CompareOperation::Le)
@@ -339,7 +339,7 @@ pub(crate) fn builtin_cmp_le(
 
 pub(crate) fn builtin_not(
     _vm: &mut Machine,
-    _env: &mut Environment,
+    _env: &Environment,
     args: &[Value],
 ) -> Result<Value, MachineError> {
     let [x] = args else {
diff --git a/src/vm/prelude/mod.rs b/src/vm/prelude/mod.rs
index 7692d92..af9264f 100644
--- a/src/vm/prelude/mod.rs
+++ b/src/vm/prelude/mod.rs
@@ -10,7 +10,7 @@ mod math;
 
 pub(crate) use math::*;
 
-pub fn load(env: &mut Environment) {
+pub fn load(env: &Environment) {
     math::load(env);
     eval::load(env);
     functional::load(env);
diff --git a/src/vm/value/convert.rs b/src/vm/value/convert.rs
index 7324517..651d2ba 100644
--- a/src/vm/value/convert.rs
+++ b/src/vm/value/convert.rs
@@ -144,7 +144,7 @@ impl AnyFunction {
     pub fn invoke(
         &self,
         vm: &mut Machine,
-        env: &mut Environment,
+        env: &Environment,
         args: &[Value],
     ) -> Result<Value, MachineError> {
         match self {
diff --git a/src/vm/value/native.rs b/src/vm/value/native.rs
index 996b85b..c88d527 100644
--- a/src/vm/value/native.rs
+++ b/src/vm/value/native.rs
@@ -22,7 +22,7 @@ pub trait NativeObject: Any + fmt::Debug + fmt::Display {
 pub struct NativeValue(Rc<dyn NativeObject>);
 
 pub type NativeFunctionImpl =
-    Rc<dyn Fn(&mut Machine, &mut Environment, &[Value]) -> Result<Value, MachineError> + 'static>;
+    Rc<dyn Fn(&mut Machine, &Environment, &[Value]) -> Result<Value, MachineError> + 'static>;
 
 #[derive(Clone)]
 pub struct NativeFunction {
@@ -36,7 +36,7 @@ impl NativeFunction {
     where
         S: Into<IdentifierValue>,
         D: Into<StringValue>,
-        F: Fn(&mut Machine, &mut Environment, &[Value]) -> Result<Value, MachineError> + 'static,
+        F: Fn(&mut Machine, &Environment, &[Value]) -> Result<Value, MachineError> + 'static,
     {
         Self {
             name: name.into(),
@@ -56,7 +56,7 @@ impl NativeFunction {
     pub fn invoke(
         &self,
         machine: &mut Machine,
-        environment: &mut Environment,
+        environment: &Environment,
         arguments: &[Value],
     ) -> Result<Value, MachineError> {
         (self.inner)(machine, environment, arguments)
diff --git a/tests/integration.rs b/tests/integration.rs
index 2837120..0c39706 100644
--- a/tests/integration.rs
+++ b/tests/integration.rs
@@ -18,7 +18,7 @@ impl Read for SliceReader<'_> {
 }
 
 #[track_caller]
-fn eval_str_in(code: &str, env: &mut Environment) -> Result<Value, MachineErrorAt> {
+fn eval_str_in(code: &str, env: &Environment) -> Result<Value, MachineErrorAt> {
     let mut machine = Machine::default();
     let reader = BufReader::new(SliceReader(code.as_bytes()));
     let mut reader = FileReader::new(reader);