Add a walker for ASTs

src/ast/mod.rs

@@ -16,3 +16,4 @@
 
 #[macro_use]
 pub mod tree;
+pub mod walk;

src/ast/walk.rs

@@ -0,0 +1,22 @@
+// Copyright 2018 Jeremy Wall <jeremy@marzhillstudios.com>
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+use std::error;
+
+use ast::tree;
+
+pub trait Visit {
+    fn stmt(&self, stmt: &tree::Statement) -> Result<(), Box<error::Error>>;
+    fn expr(&self, expr: &tree::Expression) -> Result<(), Box<error::Error>>;
+    fn val(&self, expr: &tree::Value) -> Result<(), Box<error::Error>>;
+}