Improve AST nodes

Signed-off-by: CHatingPython <chatingpython@gmail.com>

furc/include/furc/ast/expression.hpp

@@ -0,0 +1,26 @@
+#ifndef FURC_AST_EXPRESSION_HPP
+#define FURC_AST_EXPRESSION_HPP
+
+#include "furc/ast/node.hpp"
+#include "furc/ast/statement.hpp"
+
+namespace furc {
+namespace ast {
+
+enum class expression_node_t {
+    Literal
+};
+
+class expression_node : public statement_node {
+public:
+    node_t category() const override { return node_t::Expression; }
+
+    statement_node_t statement_type() const override { return statement_node_t::Expression; }
+
+    virtual expression_node_t expression_type() const = 0;
+};
+
+} // namespace ast
+} // namespace furc
+
+#endif // FURC_AST_EXPRESSION_HPP