parser: Visitor trait for traversal

trunk_parser/src/lib.rs

@@ -1,5 +1,7 @@
 mod ast;
 mod parser;
+mod traverser;
 
 pub use ast::{Statement, Expression, Program, Block, Param, Identifier, Type, InfixOp, MatchArm, Catch, Case};
-pub use parser::{Parser, ParseError};
+pub use parser::{Parser, ParseError};
+pub use traverser::*;

trunk_parser/src/traverser.rs

@@ -0,0 +1,45 @@
+use crate::{Statement, Program};
+
+pub trait Visitor {
+    fn visit(&mut self, statement: &Statement);
+
+    fn traverse(&mut self, ast: Program) {
+        for statement in &ast {
+            self.visit(statement);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{Parser, Program, Visitor};
+    use trunk_lexer::Lexer;
+
+    struct CountVisitor {
+        count: usize,
+    }
+
+    impl Visitor for CountVisitor {
+        fn visit(&mut self, statement: &crate::Statement) {
+            self.count += 1;
+        }
+    }
+
+    #[test]
+    fn it_can_walk_an_ast() {
+        let ast = get_ast("<?php foo();");
+        let mut visitor = CountVisitor { count: 0 };
+        visitor.traverse(ast);
+        assert_eq!(visitor.count, 1);
+    }
+
+    fn get_ast(source: &str) -> Program {
+        let mut lexer = Lexer::new(None);
+        let tokens = lexer.tokenize(source).unwrap();
+
+        let mut parser = Parser::new(None);
+        let ast = parser.parse(tokens).unwrap();
+
+        ast
+    }
+}