diff --git a/lang/ast/iterate.c b/lang/ast/iterate.c
index e3895c7..6036a95 100644
--- a/lang/ast/iterate.c
+++ b/lang/ast/iterate.c
@@ -6,6 +6,7 @@ enum ivy_status iterate_regular(
 	struct ivy_ast_node *node, struct ivy_ast_node_iterator *it,
 	ivy_ast_node_iteration_callback callback)
 {
+#if 0
 	b_queue_push_back(&it->it_queue, &node->n_it.it_entry);
 	node->n_it.it_depth = 0;
 
@@ -33,6 +34,7 @@ enum ivy_status iterate_regular(
 
 		b_queue_delete(&it->it_queue, entry);
 	}
+#endif
 
 	return IVY_OK;
 }
@@ -96,7 +98,7 @@ enum ivy_status iterate_postorder(
 			return IVY_ERR_INTERNAL_FAILURE;
 		}
 
-		callback(node, it);
+		// callback(node, it);
 	}
 
 	return IVY_OK;
@@ -104,18 +106,60 @@ enum ivy_status iterate_postorder(
 
 enum ivy_status ivy_ast_node_iterate(
 	struct ivy_ast_node *node, struct ivy_ast_node_iterator *it,
-	enum ivy_ast_iterator_mode mode, ivy_ast_node_iteration_callback callback)
+	ivy_ast_node_iteration_callback callback, void *arg)
 {
-	memset(it, 0x0, sizeof *it);
+	b_queue_push_back(&it->it_queue, &node->n_it.it_entry);
+	node->n_it.it_depth = 0;
 
-	switch (mode) {
-	case IVY_AST_ITERATE_REGULAR:
-		return iterate_regular(node, it, callback);
-	case IVY_AST_ITERATE_POSTORDER:
-		return iterate_postorder(node, it, callback);
-	default:
-		return IVY_ERR_INVALID_VALUE;
+	b_queue_iterator q_it;
+	b_queue_iterator_begin(&it->it_queue, &q_it);
+
+	while (b_queue_iterator_is_valid(&q_it)) {
+		struct ivy_ast_node_iterator_entry *it_entry = b_unbox(
+			struct ivy_ast_node_iterator_entry, q_it.entry, it_entry);
+		node = b_unbox(struct ivy_ast_node, it_entry, n_it);
+
+		if (!node) {
+			/* this should never happen. */
+			return IVY_ERR_INTERNAL_FAILURE;
+		}
+
+		enum ivy_status status
+			= callback(node, IVY_AST_ITERATION_PRE, it, arg);
+		if (status != IVY_OK) {
+			return status;
+		}
+
+		const struct ast_node_type *type = get_ast_node_type(node->n_type);
+		if (type->n_collect_children) {
+			it->it_insert_after = q_it.entry;
+			type->n_collect_children(node, it);
+		}
+
+		b_queue_iterator_next(&q_it);
 	}
+
+	while (!b_queue_empty(&it->it_queue)) {
+		b_queue_entry *entry = b_queue_pop_back(&it->it_queue);
+		if (!entry) {
+			break;
+		}
+
+		node = b_unbox(struct ivy_ast_node, entry, n_it);
+
+		if (!node) {
+			/* this should never happen. */
+			return IVY_ERR_INTERNAL_FAILURE;
+		}
+
+		enum ivy_status status
+			= callback(node, IVY_AST_ITERATION_POST, it, arg);
+		if (status != IVY_OK) {
+			return status;
+		}
+	}
+
+	return IVY_OK;
 }
 
 void ast_node_iterator_enqueue_node(
diff --git a/lang/ast/iterate.h b/lang/ast/iterate.h
index 5d5f70f..d098493 100644
--- a/lang/ast/iterate.h
+++ b/lang/ast/iterate.h
@@ -1,6 +1,9 @@
 #ifndef _AST_ITERATE_H_
 #define _AST_ITERATE_H_
 
+struct ivy_ast_node_iterator;
+struct ivy_ast_node;
+
 extern void ast_node_iterator_enqueue_node(
 	struct ivy_ast_node_iterator *it, struct ivy_ast_node *parent,
 	struct ivy_ast_node *node);
diff --git a/lang/include/ivy/lang/ast.h b/lang/include/ivy/lang/ast.h
index e273c27..de68c2b 100644
--- a/lang/include/ivy/lang/ast.h
+++ b/lang/include/ivy/lang/ast.h
@@ -54,9 +54,9 @@ enum ivy_ast_node_type {
 	IVY_AST_TYPE_COUNT,
 };
 
-enum ivy_ast_iterator_mode {
-	IVY_AST_ITERATE_REGULAR,
-	IVY_AST_ITERATE_POSTORDER,
+enum ivy_ast_iteration_type {
+	IVY_AST_ITERATION_PRE,
+	IVY_AST_ITERATION_POST,
 };
 
 struct ivy_ast_node_iterator {
@@ -317,7 +317,8 @@ struct ivy_ast_block_node {
 };
 
 typedef enum ivy_status (*ivy_ast_node_iteration_callback)(
-	struct ivy_ast_node *, struct ivy_ast_node_iterator *);
+	struct ivy_ast_node *, enum ivy_ast_iteration_type,
+	struct ivy_ast_node_iterator *, void *);
 
 IVY_API enum ivy_status ivy_parser_create(struct ivy_parser **parser);
 IVY_API void ivy_parser_destroy(struct ivy_parser *parser);
@@ -332,7 +333,7 @@ IVY_API enum ivy_status ivy_parser_push_token(
 
 IVY_API enum ivy_status ivy_ast_node_iterate(
 	struct ivy_ast_node *node, struct ivy_ast_node_iterator *it,
-	enum ivy_ast_iterator_mode mode, ivy_ast_node_iteration_callback callback);
+	ivy_ast_node_iteration_callback callback, void *arg);
 IVY_API void ivy_ast_node_to_string(struct ivy_ast_node *node, struct b_string *out);
 IVY_API void ivy_ast_node_destroy(struct ivy_ast_node *node);