diff --git a/src/eval.rs b/src/eval.rs
new file mode 100644
index 0000000..48ca0a8
--- /dev/null
+++ b/src/eval.rs
@@ -0,0 +1,6 @@
+pub trait Evaluator {
+	type Expr;
+	type Error;
+
+	fn eval(&mut self, input: Self::Expr) -> Result<Self::Expr, Self::Error>;
+}
diff --git a/src/lib.rs b/src/lib.rs
index d115a78..5fe3b8e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -5,3 +5,4 @@ pub mod binding;
 pub mod lex;
 pub mod parse;
 pub mod print;
+pub mod eval;