diff --git a/src/ch09_compiler/lexer.rs b/src/ch09_compiler/lexer.rs
new file mode 100644
index 0000000..a4b0674
--- /dev/null
+++ b/src/ch09_compiler/lexer.rs
@@ -0,0 +1,106 @@
+use std::io::Cursor;
+use std::io::Read;
+use std::iter::Peekable;
+use std::str::CharIndices;
+
+#[derive(Debug, PartialEq)]
+enum Token {
+    EOF,
+    Def,
+    Extern,
+    Identifier(String),
+    Number(f64),
+}
+
+struct Lexer<'a> {
+    buf: &'a str,
+    buf_iter: Peekable<CharIndices<'a>>,
+}
+
+impl<'a> Lexer<'a> {
+    fn new(buf: &'a str) -> Self {
+        Lexer {
+            buf: &buf,
+            buf_iter: buf.char_indices().peekable(),
+        }
+    }
+
+    fn read_identifier(&mut self, index: usize) -> Option<Token> {
+        let rest = &self.buf[index..];
+        match rest.split_whitespace().next() {
+            Some("def") => Some(Token::Def),
+            _ => None,
+        }
+    }
+}
+
+impl<'a> Iterator for Lexer<'a> {
+    type Item = Token;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let mut iter = self.buf_iter.clone();
+
+        match iter.peek() {
+            Some((i, x)) if x.is_alphabetic() => self.read_identifier(i.clone()),
+            _ => None,
+        }
+    }
+}
+
+fn get_token<R: Read>(input: &mut R) -> Token {
+    let mut chars = input.bytes().map(|b| char::from(b.unwrap()));
+
+    let last_result = chars.next();
+    if last_result.is_none() {
+        return Token::EOF;
+    }
+
+    let last_char = last_result.unwrap();
+
+    if last_char.is_alphabetic() {
+        let mut id = String::new();
+        id.push(last_char);
+
+        let tail = chars
+            .take_while(|c| c.is_alphanumeric())
+            .collect::<String>();
+        id.push_str(&tail);
+
+        return match id.as_ref() {
+            "def" => Token::Def,
+            "extern" => Token::Extern,
+            _ => Token::Identifier(id),
+        };
+    }
+
+    if last_char.is_numeric() {
+        let mut num = String::new();
+        num.push(last_char);
+
+        let tail = chars
+            .take_while(|c| c.is_numeric() || c == &'.')
+            .collect::<String>();
+        num.push_str(&tail);
+
+        return Token::Number(num.parse::<f64>().unwrap());
+    }
+
+    Token::EOF
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_new() {
+        let _lex = Lexer::new("hello");
+    }
+
+    #[test]
+    fn test_it_parses_def() {
+        let mut lex = Lexer::new("def");
+        assert_eq!(lex.next(), Some(Token::Def));
+    }
+
+}
diff --git a/src/ch09_compiler/mod.rs b/src/ch09_compiler/mod.rs
index 88eb3c4..433bff9 100644
--- a/src/ch09_compiler/mod.rs
+++ b/src/ch09_compiler/mod.rs
@@ -1 +1,2 @@
 pub mod lex;
+pub mod lexer;