diff --git a/src/builtin.rs b/src/builtin.rs
index 797a9ec5182caa49226a81387cce3b1fb6602bb1..4d54dd12e687fa9213ba2fadac47345e63fdb53d 100644
--- a/src/builtin.rs
+++ b/src/builtin.rs
@@ -2,7 +2,7 @@ use std::io::{stdout, Write};
 use std::env;
 use std::process;
 
-use super::{Shell, Variables};
+use super::Shell;
 use super::input_editor::readln;
 
 pub fn cd(args: &[String]) {
diff --git a/src/expansion.rs b/src/expansion.rs
index 74e7f6c0a6a493727562ffc2bc312146ff57697b..ae1fbca6ab60b31681327fa36e78cd299648ef4f 100644
--- a/src/expansion.rs
+++ b/src/expansion.rs
@@ -1,25 +1,32 @@
 use super::peg::Job;
-use super::Variables;
+use super::Shell;
 
-pub fn expand_variables(jobs: &mut [Job], variables: &Variables) {
-    for mut job in &mut jobs[..] {
-        job.command = expand_string(&job.command, variables).to_string();
-        job.args = job.args
-                      .iter()
-                      .map(|original: &String| expand_string(&original, variables).to_string())
-                      .collect();
-    }
+pub trait Expand {
+    fn expand_variables(&self, jobs: &mut [Job]);
+    fn expand_string<'a>(&'a self, original: &'a str) -> &'a str;
 }
 
-#[inline]
-fn expand_string<'a>(original: &'a str, variables: &'a Variables) -> &'a str {
-    if original.starts_with("$") {
-        if let Some(value) = variables.get(&original[1..]) {
-            &value
+impl Expand for Shell {
+    fn expand_variables(&self, jobs: &mut [Job]) {
+        for mut job in &mut jobs[..] {
+            job.command = self.expand_string(&job.command).to_string();
+            job.args = job.args
+                .iter()
+                .map(|original: &String| self.expand_string(&original).to_string())
+                .collect();
+        }
+    }
+
+    #[inline]
+    fn expand_string<'a>(&'a self, original: &'a str) -> &'a str {
+        if original.starts_with("$") {
+            if let Some(value) = self.variables.get(&original[1..]) {
+                &value
+            } else {
+                ""
+            }
         } else {
-            ""
+            original
         }
-    } else {
-        original
     }
 }
diff --git a/src/main.rs b/src/main.rs
index bce7745eda3fdb3a40cbbf51a907160ff7d2a4d5..e7ba51c7e0f1b4de6258aa6f83fac21a01c6bf6f 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -12,7 +12,7 @@ use self::to_num::ToNum;
 use self::directory_stack::DirectoryStack;
 use self::input_editor::readln;
 use self::peg::parse;
-use self::expansion::expand_variables;
+use self::expansion::Expand;
 
 pub mod builtin;
 pub mod directory_stack;
@@ -21,12 +21,14 @@ pub mod input_editor;
 pub mod peg;
 pub mod expansion;
 
-pub type Variables = BTreeMap<String, String>;
+pub struct Mode {
+    value: bool,
+}
 
 /// This struct will contain all of the data structures related to this
 /// instance of the shell.
 pub struct Shell {
-    variables: Variables,
+    variables: BTreeMap<String, String>,
     modes: Vec<Mode>,
     directory_stack: DirectoryStack,
     history: VecDeque<String>,
@@ -79,7 +81,7 @@ impl Shell {
         }
 
         let mut jobs = parse(command_string);
-        expand_variables(&mut jobs, &self.variables);
+        self.expand_variables(&mut jobs);
 
         // Execute commands
         for job in jobs.iter() {
@@ -346,11 +348,6 @@ impl Command {
     }
 }
 
-pub struct Mode {
-    value: bool,
-}
-
-
 fn main() {
     let commands = Command::map();
     let mut shell = Shell::new();