cli: improve command dispatch a bit

1 files changed, 51 insertions, 0 deletions

diff --git a/nihil.cli/command.cc b/nihil.cli/command.cc
new file mode 100644
index 0000000..475cad0
--- /dev/null
+++ b/nihil.cli/command.cc
@@ -0,0 +1,51 @@
+/*
+ * This source code is released into the public domain.
+ */
+
+module;
+
+#include <expected>
+#include <functional>
+#include <iostream>
+#include <print>
+#include <string>
+
+// For EX_USAGE.  While <sysexits.h> is deprecated, there's no other standard
+// exit code for 'usage error'; some programs use 2 (common on Linux), but
+// 2 is also used for many other exit codes.
+#include <sysexits.h>
+
+module nihil.cli;
+
+import nihil.error;
+import :registry;
+
+namespace nihil {
+
+command::command(std::string_view path, std::string_view usage,
+		 command_function_t handler)
+	: command_node(path)
+	, m_usage(usage)
+	, m_handler(std::move(handler))
+{
+	register_command(this);
+}
+
+auto command::usage(this command const &self) noexcept -> std::string_view
+{
+	return self.m_usage;
+}
+
+auto command::invoke(int argc, char **argv) const
+	-> std::expected<int, error>
+{
+	try {
+		return std::invoke(m_handler, argc, argv);
+	} catch (usage_error const &err) {
+		std::print(std::cerr, "{}\n", err.what());
+		std::print(std::cerr, "usage: {} {}", path(), usage());
+		return EX_USAGE;
+	}
+}
+
+} // namespace nihil