diff --git a/src/builtins/def.rs b/src/builtins/def.rs
index e9c06d6..0d4e739 100644
--- a/src/builtins/def.rs
+++ b/src/builtins/def.rs
@@ -14,27 +14,26 @@
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
-use crate::ast::Expr;
 use crate::compiler::Compiler;
-use crate::types::collections::core::{first, rest};
+use crate::types::collections::core::Seq;
 use crate::types::{ExprResult, List};
 
-pub fn def<'a>(compiler: &'a Compiler, args: &'a List) -> ExprResult<'a> {
+pub fn def<'a>(compiler: &'a Compiler, args: List) -> ExprResult<'a> {
     // TODO: We need to support docstrings for def
-    // if args.length != 3 {
-    //     // TODO: Raise a meaningful error by including the location
-    //     panic!(format!(
-    //         "`def` expects 2 parameters, '{}' given.",
-    //         args.length
-    //     ));
-    // }
+    if args.length() != 2 {
+        // TODO: Raise a meaningful error by including the location
+        panic!(format!(
+            "`def` expects 2 parameters, '{}' given.",
+            args.length()
+        ));
+    }
 
-    // //let def_ = &args.first;1
-    // let name = first(rest(args));
-    // //let value = first(rest(rest(args)));
+    //let def_ = &args.first;1
+    let name = args.first();
+    let value = args.rest().first();
 
-    // println!("<<<< {:?}", name);
-    // // TODO: make sure that `def_` is a symbol and its name is "def"
+    println!("<<<< {:?} \n {:?}", name, value);
+    // TODO: make sure that `def_` is a symbol and its name is "def"
 
     Err("Is not completed".to_string())
 }
diff --git a/src/reader.rs b/src/reader.rs
index fcc45e9..c35e8e6 100644
--- a/src/reader.rs
+++ b/src/reader.rs
@@ -126,15 +126,13 @@ impl ExprReader {
         match self.read_expr(reader) {
             Ok(value) => result.push(value),
             Err(e) => match self.get_char(reader, true) {
-                // is it an empty list ?
-                // TODO: we might want to return an actual empty list here
                 Some(')') => return Ok(Expr::list_to_cons(result)),
                 _ => return Err(e),
             },
         };
 
         loop {
-            let next = match self.get_char(reader, true) {
+            match self.get_char(reader, true) {
                 Some(')') => return Ok(Expr::list_to_cons(result)),
                 Some(e) => {
                     self.unget_char(e);
diff --git a/src/types/collections/core.rs b/src/types/collections/core.rs
index c67fe34..f95afd8 100644
--- a/src/types/collections/core.rs
+++ b/src/types/collections/core.rs
@@ -15,26 +15,25 @@
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
+/// Seq trait describes a seqable collection. Please note that `first`
+/// and `rest` return a copy of the data not the reference!
 pub trait Seq<T> {
-    fn first(&self) -> &T;
-    fn rest(&self) -> Option<&Self>;
+    type Coll;
+
+    fn first(&self) -> Option<T>;
+    fn rest(&self) -> Self::Coll;
 }
 
-pub fn first<'a, T, S: Seq<T>>(coll: impl Into<Option<&'a S>>) -> Option<&'a T>
-where
-    S: 'a,
-{
-    coll.into().and_then(first)
-    // match coll.into() {
-    //     Some(v) => Some(v.first()),
-    //     None => None,
-    // }
-}
+// pub fn first<'a, T, S: Seq<T>>(coll: impl Into<Option<&'a S>>) -> Option<&'a T>
+// where
+//     S: 'a,
+// {
+//     coll.into().and_then(first)
+// }
 
-pub fn rest<'a, T, S: Seq<T>>(coll: impl Into<Option<&'a S>>) -> Option<&'a S> {
-    coll.into().and_then(rest)
-    // match coll.into() {
-    //     Some(v) => v.rest(),
-    //     None => None,
-    // }
-}
+// pub fn rest<'a, T, S: Seq<T>>(coll: impl Into<Option<&'a S>>) -> S {
+//     match coll.into() {
+//         Some(e) => e.rest(),
+//         None =>
+//     }
+// }
diff --git a/src/types/collections/list.rs b/src/types/collections/list.rs
index 2d001e2..74138c3 100644
--- a/src/types/collections/list.rs
+++ b/src/types/collections/list.rs
@@ -40,33 +40,43 @@ impl List {
         self.elements.push(elem)
     }
 
-    // pub fn new(first: T, rest: List<T>) -> List<T> {
-    //     List::Cons(first, Box::new(rest))
-    // }
+    pub fn length(&self) -> usize {
+        self.elements.len()
+    }
 }
 
 impl Expression for List {
     fn eval() {}
     fn code_gen<'ctx>(&self, compiler: &'ctx Compiler) -> ExprResult<'ctx> {
-        // match &self.car {
-        //     Expr::Sym(s) => def(compiler, self),
-        //     _ => ,
-        // }
-        //def(compiler, self);
-        Err("Not implemented on list".to_string())
+        match self.first() {
+            Some(e) => match e {
+                Expr::Sym(s) if s.is_def() => def(compiler, self.rest()),
+                _ => Err("Not implemented on list".to_string()),
+            },
+
+            // TODO: We need to return an empty list here
+            None => Err("Can't not evaluate empty list".to_string()),
+        }
+        // def(compiler, self);
+        // Err("Not implemented on list".to_string())
     }
 }
 
-// impl Seq<Expr> for List<T> {
-//     fn first<'a>(&'a self) -> &'a Expr {
-//         &self.car
-//     }
+impl Seq<Expr> for List {
+    type Coll = List;
 
-//     fn rest<'a>(&'a self) -> Option<&'a List> {
-//         match &self.cdr {
-//             Expr::Nil => None,
-//             Expr::Cons(v) => Some(v),
-//             _ => panic!("'rest' should not match anything else!"),
-//         }
-//     }
-// }
+    fn first(&self) -> Option<Expr> {
+        match self.elements.first() {
+            Some(e) => Some(e.clone()),
+            None => None,
+        }
+    }
+
+    fn rest(&self) -> List {
+        if self.length() > 0 {
+            List::new(&self.elements[1..])
+        } else {
+            List::new_empty()
+        }
+    }
+}
diff --git a/src/types/symbol.rs b/src/types/symbol.rs
index 82a7221..b8e9557 100644
--- a/src/types/symbol.rs
+++ b/src/types/symbol.rs
@@ -20,7 +20,7 @@ use inkwell::values::AnyValueEnum;
 
 #[derive(Debug, Clone)]
 pub struct Symbol {
-    name: String,
+    pub name: String,
 }
 
 impl PartialEq for Symbol {
@@ -49,4 +49,8 @@ impl Symbol {
     pub fn new(name: String) -> Self {
         Symbol { name }
     }
+
+    pub fn is_def(&self) -> bool {
+        self.name == "def"
+    }
 }