Add default constructor to handle

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

furc/src/ast.cpp

@@ -0,0 +1,45 @@
+#include "furc/ast/declaration.hpp"
+#include "furc/ast/literal.hpp"
+#include "furc/ast/node.hpp"
+#include "furc/ast/program.hpp"
+#include "furc/ast/statement.hpp"
+
+#include <ostream>
+
+namespace furc::ast {
+
+std::ostream& integer_literal_node::print(std::ostream& os) const {
+    if (m_value.has_error()) return os << m_value.error();
+    return os << "integer literal (" << *m_value << ")";
+}
+
+std::ostream& function_declarartion_node::print(std::ostream& os) const {
+    return os << "function " << m_name->string << " declaration";
+}
+
+std::ostream& function_definition_node::print(std::ostream& os) const {
+    function_declarartion_node::print(os);
+    os << ':';
+    if (m_body.present()) {
+        for (const auto& entry : m_body->statements)
+            os << '\n' << entry;
+        return os << '\n' << m_body->end << ": " << m_name->string << " end";
+    }
+    return os << m_body.error(); // error
+}
+
+std::ostream& return_statement_node::print(std::ostream& os) const {
+    os << "return statement";
+    if (m_value.present()) return os << " (" << *m_value << ')';
+    return os;
+}
+
+std::ostream& program_node::print(std::ostream& os) const {
+    os << "program:";
+    for (const auto& handle : m_declarations) {
+        os << '\n' << handle;
+    }
+    return os;
+}
+
+} // namespace furc::ast