diff --git a/src/interpreter/callable.rs b/src/interpreter/callable.rs
index bb00afc..62853e0 100644
--- a/src/interpreter/callable.rs
+++ b/src/interpreter/callable.rs
@@ -1,18 +1,15 @@
-use std::{cell::RefCell, fmt::Debug, rc::Rc};
+use std::fmt::Debug;
 
 use crate::errors::SloxResult;
 
 use super::{InterpreterState, Value};
 
 /// A callable is some object that supports being called.
-pub trait Callable: Debug + ToString {
+pub trait Callable: Debug {
     /// Return the amount of arguments supported by the callable.
     fn arity(&self) -> usize;
 
     /// Run the callable in the execution environment with the specified
     /// arguments.
-    fn call(&self, callee: &Value, environment: &mut InterpreterState, arguments: Vec<Value>) -> SloxResult<Value>;
+    fn call(&self, itpr_state: &mut InterpreterState, arguments: Vec<Value>) -> SloxResult<Value>;
 }
-
-/// A reference to a callable.
-pub type CallableRef = Rc<RefCell<dyn Callable>>;
diff --git a/src/interpreter/class.rs b/src/interpreter/class.rs
index d83b7e1..f551255 100644
--- a/src/interpreter/class.rs
+++ b/src/interpreter/class.rs
@@ -1,9 +1,28 @@
+use std::{cell::RefCell, fmt::Display, rc::Rc};
+
+use crate::errors::SloxResult;
+
+use super::{Callable, InterpreterState, Value};
+
 /// A Lox class.
 #[derive(Debug, Clone)]
 pub struct Class {
     name: String,
 }
 
+/// Classes are mostly used through references.
+pub type ClassRef = Rc<RefCell<Class>>;
+
+/// An instance of a Lox class
+#[derive(Debug, Clone)]
+pub struct Instance {
+    class: Rc<RefCell<Class>>,
+}
+
+/* -------------------- *
+ * Class implementation *
+ * -------------------- */
+
 impl Class {
     /// Create a new class, specifying its name.
     pub fn new(name: String) -> Self {
@@ -11,8 +30,40 @@ impl Class {
     }
 }
 
-impl ToString for Class {
-    fn to_string(&self) -> String {
-        self.name.clone()
+impl Display for Class {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("<class ")?;
+        f.write_str(&self.name)?;
+        f.write_str(">")
+    }
+}
+
+impl Callable for ClassRef {
+    fn arity(&self) -> usize {
+        return 0;
+    }
+
+    fn call(&self, _itpr_state: &mut InterpreterState, _arguments: Vec<Value>) -> SloxResult<Value> {
+        let instance = Instance::new(self.clone());
+        Ok(Value::from(instance))
+    }
+}
+
+/* ----------------------- *
+ * Instance implementation *
+ * ----------------------- */
+
+impl Instance {
+    fn new(class: ClassRef) -> Self {
+        Self { class }
+    }
+}
+
+impl Display for Instance {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_fmt(format_args!(
+            "<Instance of {}>",
+            self.class.borrow().to_string()
+        ))
     }
 }
diff --git a/src/interpreter/environment.rs b/src/interpreter/environment.rs
index 02ff2f0..c8dbce2 100644
--- a/src/interpreter/environment.rs
+++ b/src/interpreter/environment.rs
@@ -5,7 +5,7 @@ use crate::{
     tokens::Token,
 };
 
-use super::{native_fn, CallableRef, Value};
+use super::{Value, native_fn::{self, NativeFunction}};
 
 /// A mutable reference to an environment.
 pub(super) type EnvironmentRef = Rc<RefCell<Environment>>;
@@ -27,7 +27,7 @@ impl Default for Environment {
             enclosing: None,
             values: HashMap::new(),
         };
-        env.add_default_fun("clock", native_fn::clock());
+        env.add_default_fun(native_fn::clock());
         env
     }
 }
@@ -42,9 +42,10 @@ impl Environment {
     }
 
     /// Add a default function to the environment.
-    fn add_default_fun(&mut self, name: &str, fun: CallableRef) {
-        let value = Some(Value::Callable(fun));
-        self.values.insert(name.to_owned(), value);
+    fn add_default_fun(&mut self, fun: NativeFunction) {
+        let name = fun.name().to_owned();
+        let value = Some(Value::from(fun));
+        self.values.insert(name, value);
     }
 
     /// Define a new variable.
diff --git a/src/interpreter/functions.rs b/src/interpreter/functions.rs
index d45c654..cac76bd 100644
--- a/src/interpreter/functions.rs
+++ b/src/interpreter/functions.rs
@@ -1,4 +1,4 @@
-use std::{cell::RefCell, rc::Rc};
+use std::fmt::Display;
 
 use itertools::izip;
 
@@ -9,8 +9,8 @@ use super::{
 use crate::{ast, errors::SloxResult, tokens::Token};
 
 /// A function implemented in the Lox-ish language.
-#[derive(Debug)]
-pub(super) struct Function {
+#[derive(Debug, Clone)]
+pub struct Function {
     name: Option<Token>,
     params: Vec<Token>,
     body: Vec<ast::StmtNode>,
@@ -23,14 +23,13 @@ impl Function {
         params: &[Token],
         body: &[ast::StmtNode],
         environment: EnvironmentRef,
-    ) -> Rc<RefCell<Self>> {
-        let fun = Self {
+    ) -> Self {
+        Self {
             name: name.cloned(),
             params: params.to_owned(),
             body: body.to_owned(),
             env: environment,
-        };
-        Rc::new(RefCell::new(fun))
+        }
     }
 }
 
@@ -39,17 +38,12 @@ impl Callable for Function {
         self.params.len()
     }
 
-    fn call(
-        &self,
-        _callee: &Value,
-        es: &mut InterpreterState,
-        arguments: Vec<Value>,
-    ) -> SloxResult<Value> {
+    fn call(&self, itpr_state: &mut InterpreterState, arguments: Vec<Value>) -> SloxResult<Value> {
         assert_eq!(arguments.len(), self.arity());
         let param_env = InterpreterState {
             environment: Environment::create_child(&self.env),
-            globals: es.globals.clone(),
-            locals: es.locals,
+            globals: itpr_state.globals.clone(),
+            locals: itpr_state.locals,
         };
         for (arg, value) in izip!(self.params.iter(), arguments.into_iter()) {
             param_env
@@ -72,11 +66,15 @@ impl Callable for Function {
     }
 }
 
-impl ToString for Function {
-    fn to_string(&self) -> String {
+impl Display for Function {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match &self.name {
-            None => "<lambda>".to_owned(),
-            Some(token) => format!("<fun {}>", token.lexeme),
+            None => f.write_str("<lambda>"),
+            Some(token) => {
+                f.write_str("<fun ")?;
+                f.write_str(&token.lexeme)?;
+                f.write_str(">")
+            }
         }
     }
 }
diff --git a/src/interpreter/interpretable.rs b/src/interpreter/interpretable.rs
index c938077..d69f969 100644
--- a/src/interpreter/interpretable.rs
+++ b/src/interpreter/interpretable.rs
@@ -200,7 +200,7 @@ impl StmtNode {
         let class = Class::new(decl.name.lexeme.clone());
         es.environment
             .borrow_mut()
-            .assign(&decl.name, Value::Class(class))?;
+            .assign(&decl.name, class.into())?;
         Ok(InterpreterFlowControl::default())
     }
 
@@ -214,7 +214,7 @@ impl StmtNode {
         );
         es.environment
             .borrow_mut()
-            .define(&decl.name, Some(Value::Callable(fun)))?;
+            .define(&decl.name, Some(fun.into()))?;
         Ok(InterpreterFlowControl::default())
     }
 
@@ -338,10 +338,8 @@ impl Interpretable for ExprNode {
                 arguments,
             } => self.on_call(es, callee, right_paren, arguments),
             ExprNode::Lambda { params, body } => {
-                Ok(
-                    Value::Callable(Function::new(None, params, body, es.environment.clone()))
-                        .into(),
-                )
+                let lambda = Function::new(None, params, body, es.environment.clone());
+                Ok(Value::from(lambda).into())
             }
         }
     }
@@ -492,23 +490,23 @@ impl ExprNode {
             }
             v
         };
-        if let Value::Callable(callable_ref) = &callee {
-            let callable = callable_ref.borrow();
-            if callable.arity() != arg_values.len() {
-                Err(SloxError::with_token(
-                    ErrorKind::Runtime,
-                    right_paren,
-                    format!(
-                        "expected {} arguments, found {}",
-                        arg_values.len(),
-                        callable.arity()
-                    ),
-                ))
-            } else {
-                Ok(callable.call(&callee, es, arg_values)?.into())
-            }
-        } else {
-            error(right_paren, "can only call functions and classes")
-        }
+        callee.with_callable(
+            |callable| {
+                if callable.arity() != arg_values.len() {
+                    Err(SloxError::with_token(
+                        ErrorKind::Runtime,
+                        right_paren,
+                        format!(
+                            "expected {} arguments, found {}",
+                            arg_values.len(),
+                            callable.arity()
+                        ),
+                    ))
+                } else {
+                    Ok(callable.call(es, arg_values)?.into())
+                }
+            },
+            || error(right_paren, "expression result is not callable")
+        )
     }
 }
diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index 9483801..e3e7c87 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -6,7 +6,7 @@ mod interpretable;
 mod native_fn;
 mod value;
 
-pub(self) use callable::{Callable, CallableRef};
+pub(self) use callable::Callable;
 pub(self) use environment::*;
 pub use interpretable::*;
 pub use value::*;
diff --git a/src/interpreter/native_fn.rs b/src/interpreter/native_fn.rs
index b868bd1..c29ad2d 100644
--- a/src/interpreter/native_fn.rs
+++ b/src/interpreter/native_fn.rs
@@ -1,45 +1,94 @@
 use std::{
-    cell::RefCell,
-    rc::Rc,
+    fmt::{Debug, Display},
     time::{SystemTime, UNIX_EPOCH},
 };
 
 use crate::errors::SloxResult;
 
-use super::{Callable, CallableRef, InterpreterState, Value};
+use super::{Callable, InterpreterState, Value};
+
+/* ----------------------- *
+ * Native function support *
+ * ----------------------- */
+
+/// A function pointer to the implementation of some native function.
+type NativeFunctionHandler = fn(&mut InterpreterState, Vec<Value>) -> SloxResult<Value>;
+
+/// A native function.
+#[derive(Clone)]
+pub struct NativeFunction {
+    /// Name of the function.
+    name: String,
+    /// Arity of the function.
+    arity: usize,
+    /// Rust function that actually implements the function.
+    handler: NativeFunctionHandler,
+}
+
+impl NativeFunction {
+    /// Initialize a native function's record.
+    fn new(name: &str, arity: usize, handler: NativeFunctionHandler) -> Self {
+        Self {
+            name: name.to_owned(),
+            arity,
+            handler,
+        }
+    }
+
+    /// Access the native function's name
+    pub fn name(&self) -> &str {
+        &self.name
+    }
+}
+
+impl PartialEq for NativeFunction {
+    fn eq(&self, other: &Self) -> bool {
+        self.name == other.name
+    }
+}
+
+impl Display for NativeFunction {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("<native function ")?;
+        f.write_str(&self.name)?;
+        f.write_str(">")
+    }
+}
+
+impl Debug for NativeFunction {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_fmt(format_args!(
+            "NativeFunction {{ name: {:?}, arity: {} }}",
+            self.name, self.arity
+        ))
+    }
+}
+
+impl Callable for NativeFunction {
+    fn arity(&self) -> usize {
+        self.arity
+    }
+
+    fn call(&self, itpr_state: &mut InterpreterState, arguments: Vec<Value>) -> SloxResult<Value> {
+        (self.handler)(itpr_state, arguments)
+    }
+}
 
 /* ----------- *
  *   clock()   *
  * ----------- */
 
-#[derive(Debug)]
-struct Clock;
-
-impl Callable for Clock {
-    fn arity(&self) -> usize {
-        0
-    }
-
-    fn call(
-        &self,
-        _callee: &Value,
-        _environment: &mut InterpreterState,
-        _arguments: Vec<Value>,
-    ) -> SloxResult<Value> {
-        let now = SystemTime::now();
-        let since_epoch = now
-            .duration_since(UNIX_EPOCH)
-            .expect("looks like it's 2038 already");
-        Ok(Value::Number((since_epoch.as_millis() as f64) / 1000.0))
-    }
+fn _clock_implementation(
+    _environment: &mut InterpreterState,
+    _arguments: Vec<Value>,
+) -> SloxResult<Value> {
+    let now = SystemTime::now();
+    let since_epoch = now
+        .duration_since(UNIX_EPOCH)
+        .expect("looks like it's 2038 already");
+    Ok(Value::Number((since_epoch.as_millis() as f64) / 1000.0))
 }
 
-impl ToString for Clock {
-    fn to_string(&self) -> String {
-        "<native function clock()>".to_owned()
-    }
-}
-
-pub(super) fn clock() -> CallableRef {
-    Rc::new(RefCell::new(Clock {}))
+pub(super) fn clock() -> NativeFunction {
+    NativeFunction::new("clock", 0, _clock_implementation)
 }
diff --git a/src/interpreter/value.rs b/src/interpreter/value.rs
index 47f28f5..0680d1a 100644
--- a/src/interpreter/value.rs
+++ b/src/interpreter/value.rs
@@ -1,6 +1,11 @@
 use std::{cell::RefCell, fmt::Display, rc::Rc};
 
-use super::{Callable, class::Class};
+use super::{
+    class::{Class, ClassRef, Instance},
+    functions::Function,
+    native_fn::NativeFunction,
+    Callable,
+};
 
 /// A value being handled by the interpreter.
 #[derive(Debug, Clone)]
@@ -9,8 +14,50 @@ pub enum Value {
     Boolean(bool),
     String(String),
     Number(f64),
-    Callable(Rc<RefCell<dyn Callable>>),
-    Class(Class),
+    Object(Rc<RefCell<Object>>),
+}
+
+#[derive(Debug, Clone)]
+pub enum Object {
+    NativeFunction(NativeFunction),
+    LoxFunction(Function),
+    Class(ClassRef),
+    Instance(Instance),
+}
+
+/* -------------------- *
+ * Value implementation *
+ * -------------------- */
+
+impl Value {
+    /// Check whether a value is truthy or not.
+    pub fn is_truthy(&self) -> bool {
+        match self {
+            Self::Nil => false,
+            Self::Boolean(b) => *b,
+            _ => true,
+        }
+    }
+
+    /// Run some code using the callable object wrapped inside a value.
+    /// If the value is not callable, an error function will be called
+    /// instead.
+    pub fn with_callable<Fok, Ferr, Rt>(&self, fok: Fok, ferr: Ferr) -> Rt
+    where
+        Fok: FnOnce(&dyn Callable) -> Rt,
+        Ferr: FnOnce() -> Rt,
+    {
+        let obj = match self {
+            Value::Object(obj_ref) => obj_ref.borrow(),
+            _ => return ferr(),
+        };
+        match &*obj {
+            Object::NativeFunction(func) => fok(func),
+            Object::LoxFunction(func) => fok(func),
+            Object::Class(class) => fok(class),
+            Object::Instance(_) => ferr(),
+        }
+    }
 }
 
 impl PartialEq for Value {
@@ -32,19 +79,52 @@ impl Display for Value {
             Value::Boolean(b) => b.fmt(f),
             Value::String(s) => s.fmt(f),
             Value::Number(n) => n.fmt(f),
-            Value::Callable(c) => f.write_str(&c.borrow().to_string()),
-            Value::Class(c) => f.write_str(&c.to_string()),
+            Value::Object(obj) => obj.borrow().fmt(f),
         }
     }
 }
 
-impl Value {
-    /// Check whether a value is truthy or not.
-    pub fn is_truthy(&self) -> bool {
+impl From<NativeFunction> for Value {
+    fn from(value: NativeFunction) -> Self {
+        Value::Object(Rc::new(RefCell::new(Object::NativeFunction(value))))
+    }
+}
+
+impl From<Function> for Value {
+    fn from(value: Function) -> Self {
+        Value::Object(Rc::new(RefCell::new(Object::LoxFunction(value))))
+    }
+}
+
+impl From<Class> for Value {
+    fn from(value: Class) -> Self {
+        Value::from(Rc::new(RefCell::new(value)))
+    }
+}
+
+impl From<ClassRef> for Value {
+    fn from(value: ClassRef) -> Self {
+        Value::Object(Rc::new(RefCell::new(Object::Class(value))))
+    }
+}
+
+impl From<Instance> for Value {
+    fn from(value: Instance) -> Self {
+        Value::Object(Rc::new(RefCell::new(Object::Instance(value))))
+    }
+}
+
+/* --------------------- *
+ * Object implementation *
+ * --------------------- */
+
+impl Display for Object {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            Self::Nil => false,
-            Self::Boolean(b) => *b,
-            _ => true,
+            Object::NativeFunction(func) => func.fmt(f),
+            Object::LoxFunction(func) => func.fmt(f),
+            Object::Class(cls) => cls.borrow().fmt(f),
+            Object::Instance(inst) => inst.fmt(f),
         }
     }
 }