diff --git a/src/language.rs b/src/language.rs
index db758d4..649ad07 100644
--- a/src/language.rs
+++ b/src/language.rs
@@ -1,3 +1,4 @@
+use std::fmt::Debug;
 
 pub struct TokenError {
     pub msg: String,
@@ -8,8 +9,8 @@ pub struct ParseError {
 }
 
 pub trait ProgrammingLanguage {
-    type Token;
-    type AST;
+    type Token: Debug;
+    type AST: Debug;
 
     fn tokenize(input: &str) -> Result<Vec<Self::Token>, TokenError>;
     fn parse(input: Vec<Self::Token>) -> Result<Self::AST, ParseError>;