diff --git a/src/environment.rs b/src/environment.rs
index ed6a132..2cb3d76 100644
--- a/src/environment.rs
+++ b/src/environment.rs
@@ -1,21 +1,23 @@
 use crate::parser::make_symbol;
 use crate::parser::Datum;
+use std::cell::RefCell;
 use std::mem;
+use std::rc::Rc;
 
 type Mapping = (Datum, Datum);
 
 #[derive(Debug)]
-pub struct Frame<'a> {
+pub struct Frame {
     mappings: Vec<Mapping>,
-    outer: Option<&'a Frame<'a>>,
+    outer: Option<Rc<RefCell<Frame>>>,
 }
 
-pub type Env<'a> = Frame<'a>;
+pub type Env = Rc<RefCell<Frame>>;
 
-pub fn get_global_environment<'a>() -> Env<'a> {
+pub fn get_global_environment() -> Env {
     use crate::parser::Datum::Primitive;
     use crate::primitives::*;
-    Frame {
+    let f = Frame {
         mappings: vec![
             (make_symbol("+"), Primitive(add)),
             (make_symbol("*"), Primitive(mul)),
@@ -34,68 +36,67 @@ pub fn get_global_environment<'a>() -> Env<'a> {
             (make_symbol("display"), Primitive(display)),
         ],
         outer: None,
-    }
+    };
+    Rc::new(RefCell::new(f))
 }
 
 pub fn lookup_variable_value(var: &Datum, env: &Env) -> Datum {
-    if let Datum::Symbol(symbol_name) = var {
-        for mapping in &env.mappings {
-            if let (Datum::Symbol(current_name), datum) = mapping {
-                if symbol_name == current_name {
-                    return datum.clone();
-                }
+    let symbol_name = Datum::as_str(var).expect("LOOKUP-VARIABLE-VALUE -- not a symbol");
+    let frame = env.borrow();
+
+    for mapping in &frame.mappings {
+        if let (Datum::Symbol(current_name), datum) = mapping {
+            if symbol_name == *current_name {
+                return datum.clone();
             }
         }
-        match env.outer {
-            Some(outer_env) => lookup_variable_value(var, outer_env),
-            None => panic!("LOOKUP-VARIABLE-VALUE -- unbound-variable-error {:?}", var),
-        }
-    } else {
-        panic!("LOOKUP-VARIABLE-VALUE -- not-a-symbol {:?}", var)
+    }
+
+    match &frame.outer {
+        Some(outer_env) => lookup_variable_value(var, &outer_env),
+        _ => panic!("LOOKUP-VARIABLE-VALUE -- does not exist {}", var),
     }
 }
 
 pub fn define_variable(var: &Datum, val: Datum, env: &mut Env) -> Datum {
-    if let Datum::Symbol(symbol_name) = var {
-        // Assign to variable if it already exits
-        for mapping in &mut env.mappings {
-            if let (Datum::Symbol(current_name), _) = mapping {
-                if symbol_name == current_name {
-                    mapping.1 = val;
-                    return var.clone();
-                }
+    let symbol_name = Datum::as_str(var).expect("DEFINE-VARIABLE -- not a symbol");
+    let mut frame = env.borrow_mut();
+
+    // Assign to variable if it already exits
+    for mapping in &mut frame.mappings {
+        if let (Datum::Symbol(current_name), _) = mapping {
+            if symbol_name == *current_name {
+                mapping.1 = val;
+                return var.clone();
             }
         }
-        // Create variable because it does not exist yet
-        let pair = (var.clone(), val);
-        env.mappings.push(pair);
-    } else {
-        panic!("DEFINE-VARIABLE -- not-a-symbol {:?}", var)
     }
+
+    // Create variable because it does not exist yet
+    let pair = (var.clone(), val);
+    frame.mappings.push(pair);
     var.clone()
 }
 
-pub fn set_variable(var: &Datum, val: Datum, env: &mut Env) -> Datum {
-    if let Datum::Symbol(symbol_name) = var {
-        for mapping in &mut env.mappings {
-            if let (Datum::Symbol(current_name), _) = mapping {
-                if symbol_name == current_name {
-                    mapping.1 = val;
-                    return var.clone();
-                }
+pub fn set_variable(var: &Datum, val: Datum, env: &Env) -> Datum {
+    let symbol_name = Datum::as_str(var).expect("SET-VARIABLE -- not a symbol");
+    let mut frame = env.borrow_mut();
+    for mapping in &mut frame.mappings {
+        if let (Datum::Symbol(current_name), _) = mapping {
+            if symbol_name == *current_name {
+                mapping.1 = val;
+                return var.clone();
             }
         }
-        match env.outer {
-            // XXX: search outer environments here
-            // Some(outer_env) => set_variable(var, val, outer_env),
-            _ => panic!("SET-VARIABLE -- unbound-var {:?}", var),
-        }
-    } else {
-        panic!("DEFINE-VARIABLE -- not-a-symbol {:?}", var)
+    }
+
+    match &frame.outer {
+        Some(outer_env) => set_variable(var, val, &outer_env),
+        _ => panic!("LOOKUP-VARIABLE-VALUE -- does not exist {}", var),
     }
 }
 
-pub fn extend<'a>(vars: Datum, vals: Datum, env: &'a Env) -> Env<'a> {
+pub fn extend(vars: Datum, vals: Datum, env: &mut Env) -> Env {
     let mut mappings = vec![];
     if let (Datum::List(mut vars), Datum::List(mut vals)) = (vars, vals) {
         if vars.len() != vals.len() {
@@ -108,8 +109,10 @@ pub fn extend<'a>(vars: Datum, vals: Datum, env: &'a Env) -> Env<'a> {
     } else {
         panic!("EXTEND-ENVIRONMENT -- vars or vals not lists")
     }
-    Frame {
-        mappings: mappings,
-        outer: Some(env),
-    }
+
+    let f = Frame {
+        mappings,
+        outer: Some(env.clone()),
+    };
+    Rc::new(RefCell::new(f))
 }
diff --git a/src/interpreter.rs b/src/interpreter.rs
index fe86ece..94f1b5c 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -72,11 +72,11 @@ fn interpret_define(args: &Vec<Datum>, env: &mut Env) -> Datum {
                 }
             }
 
-            // XXX: a procedure includes the current environment
             let body = args[2..].to_vec();
             let proc = Procedure {
                 parameters: parameters,
                 body: body,
+                env: env.clone(),
             };
             environment::define_variable(definition_var, proc, env)
         }
@@ -156,15 +156,30 @@ fn interpret_application(vec: &Vec<Datum>, env: &mut Env) -> Datum {
 
     match &args[0] {
         Primitive(proc) => proc(args),
-        Lambda { parameters, body } | Procedure { parameters, body } => {
-            let lambda_args = args[1..].to_vec();
-            let mut new_env =
-                environment::extend(List(parameters.to_vec()), List(lambda_args), env);
+        Lambda { parameters, body } => {
+            let lambda_args = List(args[1..].to_vec());
+            let lambda_params = List(parameters.to_vec());
+            let mut new_env = environment::extend(lambda_params, lambda_args, env);
             for i in 0..(body.len() - 1) {
                 interpret(&body[i], &mut new_env);
             }
             interpret(&body[body.len() - 1], &mut new_env)
         }
+        Procedure {
+            parameters,
+            body,
+            env,
+        } => {
+            let lambda_args = List(args[1..].to_vec());
+            let lambda_params = List(parameters.to_vec());
+            let mut proc_env = env.clone();
+            let mut new_env = environment::extend(lambda_params, lambda_args, &mut proc_env);
+            for i in 0..(body.len() - 1) {
+                interpret(&body[i], &mut new_env);
+            }
+            // This is my attempt to implement tail-call optimiation.
+            interpret(&body[body.len() - 1], &mut new_env)
+        }
         _ => panic!("INTERPRET-APPLICATION -- not-aplicable {:?}", args[0]),
     }
 }
diff --git a/src/parser.rs b/src/parser.rs
index 06bae0c..c489621 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -1,3 +1,4 @@
+use crate::environment::Env;
 use crate::lexer::Token;
 use crate::lexer::Tokens;
 use std::cmp::Ordering;
@@ -19,6 +20,7 @@ pub enum Datum {
     Procedure {
         parameters: Vec<Datum>,
         body: Vec<Datum>,
+        env: Env,
     },
     Unspecified,
 }
@@ -80,6 +82,15 @@ impl PartialOrd for Datum {
     }
 }
 
+impl Datum {
+    pub fn as_str(&self) -> Option<std::string::String> {
+        match self {
+            Datum::Symbol(s) => Some(s.to_string()),
+            _ => None,
+        }
+    }
+}
+
 pub fn parse(tokens: Tokens) -> Datum {
     let (datum, _) = parse_datum(&tokens, 0);
     datum