diff --git a/eval.ml b/eval.ml
index 50694c4..ae42076 100644
--- a/eval.ml
+++ b/eval.ml
@@ -15,7 +15,7 @@ type value =
 and expr = Ast.t
 
 (* environment for eval *)
-and env = (string * value) list
+and env = Env of (string * value) list
 
 exception No_operation
 exception Too_many_arguments
@@ -79,16 +79,16 @@ end
 module Env = struct
   type t = env
 
-  let empty = []
+  let empty = Env []
 
-  let get_opt e name =
+  let get_opt (Env e) name =
     List.assoc_opt name e
 
-  let bind v e =
-    v::e
+  let bind v (Env e) =
+    Env (v::e)
 
-  let bind_seq seq e =
-    List.of_seq seq @ e
+  let bind_seq seq (Env e) =
+    Env (List.of_seq seq @ e)
 end
 
 (* operators *)