feat(ast): add trait for Visitor pattern.

crates/fuse-ast/src/lib.rs

@@ -1,6 +1,7 @@
 mod ast;
 mod ast_factory;
 mod precedence;
+mod visit;
 
 pub use ast::*;
 pub use ast_factory::*;

crates/fuse-ast/src/visit.rs

@@ -0,0 +1,27 @@
+// based on https://rust-unofficial.github.io/patterns/patterns/behavioural/visitor.html
+
+use crate::ast::*;
+
+macro_rules! visit_list {
+    ($visitor:ident, $method:ident, $list:expr) => {
+        todo!()
+    };
+}
+
+pub trait Visitor<'ast>: Sized {
+    fn visit_chunk(&mut self, chunk: &'ast Chunk) {
+        walk_block(self, &chunk.body)
+    }
+
+    fn visit_statement(&mut self, statement: &'ast Statement) {
+        walk_statement(self, &statement)
+    }
+}
+
+pub fn walk_block<'ast, V: Visitor<'ast>>(visitor: &mut V, block: &'ast Block) {
+    visit_list!(visitor, visit_statement, block.statements)
+}
+
+pub fn walk_statement<'ast, V: Visitor<'ast>>(visitor: &mut V, statement: &'ast Statement) {
+    todo!()
+}