diff --git a/src/com/annimon/everlastingsummer/ast/Expression.java b/src/com/annimon/everlastingsummer/ast/Expression.java
new file mode 100644
index 0000000..043b030
--- /dev/null
+++ b/src/com/annimon/everlastingsummer/ast/Expression.java
@@ -0,0 +1,9 @@
+package com.annimon.everlastingsummer.ast;
+
+/**
+ * @author aNNiMON
+ */
+public interface Expression {
+    
+    double eval();
+}