From 90aa957ca9b7c217af7569009d1675e0f3ff8e9b Mon Sep 17 00:00:00 2001
From: Lexi Winter <lexi@le-fay.org>
Date: Thu, 26 Jun 2025 20:47:45 +0100
Subject: ucl, config: use monadic error handling more

---
 nihil.config/CMakeLists.txt   |  1 -
 nihil.config/error.ccm        | 23 -----------
 nihil.config/nihil.config.ccm |  1 -
 nihil.config/option.cc        | 70 ++++++++++++++++++++++----------
 nihil.config/option.ccm       | 61 +++++++++++++++++++++-------
 nihil.config/read.cc          | 35 ++++++----------
 nihil.config/read.ccm         |  6 +--
 nihil.config/store.cc         | 92 ++++++++++++++++++++++---------------------
 nihil.config/store.ccm        | 41 +++++++++----------
 nihil.config/string.cc        | 42 +++++++++++---------
 nihil.config/string.ccm       | 34 +++++++++++++---
 nihil.config/tests/string.cc  | 22 ++++++-----
 nihil.config/write.cc         | 27 +++++--------
 nihil.config/write.ccm        |  6 +--
 14 files changed, 251 insertions(+), 210 deletions(-)
 delete mode 100644 nihil.config/error.ccm

(limited to 'nihil.config')

diff --git a/nihil.config/CMakeLists.txt b/nihil.config/CMakeLists.txt
index 2aa0dae..fb2c1db 100644
--- a/nihil.config/CMakeLists.txt
+++ b/nihil.config/CMakeLists.txt
@@ -5,7 +5,6 @@ target_link_libraries(nihil.config PUBLIC nihil nihil.ucl)
 target_sources(nihil.config
 	PUBLIC FILE_SET modules TYPE CXX_MODULES FILES
 	nihil.config.ccm
-	error.ccm
 	read.ccm
 	store.ccm
 	write.ccm
diff --git a/nihil.config/error.ccm b/nihil.config/error.ccm
deleted file mode 100644
index 4e7131a..0000000
--- a/nihil.config/error.ccm
+++ /dev/null
@@ -1,23 +0,0 @@
-/*
- * This source code is released into the public domain.
- */
-
-module;
-
-#include <format>
-#include <utility>
-
-export module nihil.config:error;
-
-import nihil;
-
-namespace nihil::config {
-
-/*
- * Exception thrown when an issue occurs with the configuration.
- */
-export struct error : generic_error {
-	error(std::string what) : generic_error(std::move(what)) {}
-};
-
-} // namespace nihil::config
diff --git a/nihil.config/nihil.config.ccm b/nihil.config/nihil.config.ccm
index d20c3fe..8957305 100644
--- a/nihil.config/nihil.config.ccm
+++ b/nihil.config/nihil.config.ccm
@@ -6,7 +6,6 @@ module;
 
 export module nihil.config;
 
-export import :error;
 export import :option;
 export import :read;
 export import :store;
diff --git a/nihil.config/option.cc b/nihil.config/option.cc
index 9bf77c9..588a48f 100644
--- a/nihil.config/option.cc
+++ b/nihil.config/option.cc
@@ -4,58 +4,86 @@
 
 module;
 
+#include <coroutine>
+#include <expected>
 #include <iostream>
 #include <string>
 
 module nihil.config;
 
+import nihil;
+import nihil.ucl;
+
 namespace nihil::config {
 
+option::option(std::string_view name, std::string_view description)
+	: m_name(name)
+	, m_description(description)
+{
+	auto okay = store::get().register_option(this);
+	if (okay)
+		return;
+
+	std::print(std::cerr,
+		   "INTERNAL ERROR: failed to register "
+		   "configuration option '{}': {}",
+		   m_name, okay.error());
+	std::exit(1);
+}
+
+option::~option()
+{
+	std::ignore = store::get().unregister_option(this);
+}
+
 auto option::name(this option const &self) noexcept
--> std::string_view
+	-> std::string_view
 {
-	return self._name;
+	return self.m_name;
 }
 
-// Human-readable description of this option.
 auto option::description(this option const &self) noexcept
--> std::string_view
+	-> std::string_view
 {
-	return self._description;
+	return self.m_description;
 }
 
-// If true, this option is set to its default value.
 auto option::is_default(this option const &self) noexcept
--> bool
+	-> bool
+{
+	return self.m_is_default;
+}
+
+auto option::is_default(this option  &self, bool b) -> void
 {
-	return self._is_default;
+	self.m_is_default = b;
 }
 
-// Get or set this option as a string.
-auto option::string(this option const &self)
--> std::string
+auto option::string(this option const &self) -> std::string
 {
 	return self.get_string();
 }
 
 auto option::string(this option &self, std::string_view value)
--> void
+	-> std::expected<void, error>
 {
-	self.set_string(value);
-	self._is_default = false;
+	co_await self.set_string(value);
+	self.is_default(false);
+	co_return {};
 }
 
-option::option(std::string_view name,
-       std::string_view description)
-	: _name(name)
-	, _description(description)
+auto option::ucl(this option const &self)
+	-> std::expected<nihil::ucl::object, error>
 {
+	return self.get_ucl();
 }
 
-auto option::is_default(bool b)
--> void
+auto option::ucl(this option &self, nihil::ucl::object const &value)
+	-> std::expected<void, error>
 {
-	_is_default = b;
+	co_await self.set_ucl(value);
+	self.is_default(false);
+	co_return {};
 }
 
 auto operator<<(std::ostream &strm, option const &opt)
diff --git a/nihil.config/option.ccm b/nihil.config/option.ccm
index c6a8329..c095d34 100644
--- a/nihil.config/option.ccm
+++ b/nihil.config/option.ccm
@@ -4,46 +4,67 @@
 
 module;
 
+#include <expected>
 #include <iosfwd>
 #include <string>
 
 export module nihil.config:option;
 
+import nihil;
 import nihil.ucl;
-import :error;
 
 namespace nihil::config {
 
 /*
  * Base class for options; this is what config_store interacts with.
+ *
+ * Base classes should override the four protected functions:
+ *
+ * 	get_string()
+ * 	set_string()
+ * 	get_ucl()
+ * 	set_ucl()
+ *
+ * Overriding any other members is not permitted.
  */
 
 export struct option
 {
+	virtual ~option();
+
 	// Short name of this option.
-	auto name(this option const &self) noexcept -> std::string_view;
+	[[nodiscard]] auto name(this option const &) noexcept
+		-> std::string_view;
 
 	// Human-readable description of this option.
-	auto description(this option const &self) noexcept -> std::string_view;
+	[[nodiscard]] auto description(this option const &) noexcept
+		-> std::string_view;
 
 	// If true, this option is set to its default value.
-	auto is_default(this option const &self) noexcept -> bool;
+	[[nodiscard]] auto is_default(this option const &) noexcept
+		-> bool;
 
-	// Get or set this option as a string.
-	auto string(this option const &self) -> std::string;
-	auto string(this option &self, std::string_view value) -> void;
+	/*
+	 * Get or set this option as a string.  The specific conversion
+	 * method depends on the derived option type.
+	 */
+	[[nodiscard]] auto string(this option const &) -> std::string;
+	[[nodiscard]] auto string(this option &, std::string_view value)
+		-> std::expected<void, error>;
 
 	/*
 	 * Return this object as a UCL object.  This is used when writing the
 	 * configuration file.
 	 */
-	virtual auto to_ucl() const -> ucl::object = 0;
+	[[nodiscard]] auto ucl(this option const &)
+		-> std::expected<ucl::object, error>;
 
 	/*
 	 * Set this object from a UCL object.  This is used when reading the
 	 * configuration file.
 	 */
-	virtual auto from_ucl(ucl::object const &) -> void = 0;
+	[[nodiscard]] auto ucl(this option &, ucl::object const &)
+		-> std::expected<void, error>;
 
 	// Not copyable or movable.
 	option(option const &) = delete;
@@ -52,18 +73,28 @@ export struct option
 protected:
 	option(std::string_view name, std::string_view description);
 
-	auto is_default(bool b) -> void;
+	auto is_default(this option &, bool) -> void;
 
 	/*
 	 * Get or set this option as a string.
 	 */
-	virtual auto get_string() const -> std::string = 0;
-	virtual auto set_string(std::string_view) -> void = 0;
+	virtual auto get_string() const
+		-> std::string = 0;
+	virtual auto set_string(std::string_view)
+		-> std::expected<void, error> = 0;
+
+	/*
+	 * Get or set this option as a UCL object.
+	 */
+	virtual auto get_ucl() const
+		-> std::expected<ucl::object, error> = 0;
+	virtual auto set_ucl(ucl::object const &)
+		-> std::expected<void, error> = 0;
 
 private:
-	std::string _name;
-	std::string _description;
-	bool _is_default = true;
+	std::string	m_name;
+	std::string	m_description;
+	bool		m_is_default = true;
 };
 
 /*
diff --git a/nihil.config/read.cc b/nihil.config/read.cc
index 0a5fcad..e7def91 100644
--- a/nihil.config/read.cc
+++ b/nihil.config/read.cc
@@ -11,15 +11,15 @@ module;
 #include <iterator>
 #include <string>
 
+module nihil.config;
+
 import nihil;
 import nihil.ucl;
 
-module nihil.config;
-
 namespace nihil::config {
 
 auto read_from(std::filesystem::path const &filename)
-	-> std::expected<void, nihil::error>
+	-> std::expected<void, error>
 {
 	// TODO: nihil.ucl should have a way to load UCL from a filename.
 
@@ -29,33 +29,20 @@ auto read_from(std::filesystem::path const &filename)
 		// Ignore ENOENT, it simply means we haven't created the
 		// config file yet, so default values will be used.
 		if (err.error().root_cause() == std::errc::no_such_file_or_directory)
-			return {};
+			co_return {};
 		auto errstr = std::format("cannot read {}", filename.string());
-		return std::unexpected(nihil::error(errstr, err.error()));
+		co_return std::unexpected(error(errstr, err.error()));
 	}
 
 	// Parse the UCL.
-	try {
-		auto uclconfig = ucl::parse(config_text);
-
-		for (auto &&[key, value] : uclconfig) {
-			auto &opt = store::get().fetch(key);
-			opt.from_ucl(value);
-		}
-	} catch (unknown_option const &) {
-		// This is probably an old option which has been removed;
-		// ignore it, and we'll remove the bad option next time
-		// we write the config.
-	} catch (error const &err) {
-		// Include the filename in any other config errors.
-		throw error(std::format("{}: {}", filename.string(),
-					err.what()));
-	} catch (ucl::error const &err) {
-		throw error(std::format("{}: {}", filename.string(),
-					err.what()));
+	auto uclconfig = co_await ucl::parse(config_text);
+
+	for (auto &&[key, value] : uclconfig) {
+		auto opt = co_await store::get().fetch(key);
+		co_await opt->ucl(value);
 	}
 
-	return {};
+	co_return {};
 }
 
 } // namespace nihil::config
diff --git a/nihil.config/read.ccm b/nihil.config/read.ccm
index 18f7213..fca78eb 100644
--- a/nihil.config/read.ccm
+++ b/nihil.config/read.ccm
@@ -7,16 +7,16 @@ module;
 #include <expected>
 #include <filesystem>
 
-import nihil;
-
 export module nihil.config:read;
 
+import nihil;
+
 namespace nihil::config {
 
 /*
  * Load the configuration from a file.
  */
 export [[nodiscard]] auto read_from(std::filesystem::path const &filename)
-	-> std::expected<void, nihil::error>;
+	-> std::expected<void, error>;
 
 } // namespace nihil::config
diff --git a/nihil.config/store.cc b/nihil.config/store.cc
index 2ec8ade..6f93677 100644
--- a/nihil.config/store.cc
+++ b/nihil.config/store.cc
@@ -5,6 +5,7 @@
 module;
 
 #include <coroutine>
+#include <expected>
 #include <filesystem>
 #include <format>
 #include <map>
@@ -15,75 +16,78 @@ import nihil;
 
 namespace nihil::config {
 
-unknown_option::unknown_option(std::string_view option_name)
-	: error(std::format("unknown configuration variable '{}'",
-			    option_name))
-	, _option_name(option_name)
-{
-}
+store::store() = default;
 
-auto unknown_option::option_name(this unknown_option const &self)
--> std::string_view
+auto store::get() -> store &
 {
-	return self._option_name;
-}
-
-auto store::get()
--> store&
-{
-	if (instance == nullptr)
-		instance = new store;
-
-	return *instance;
+	static auto instance = store();
+	return instance;
 }
 
 
 auto store::register_option(this store &self, option *object)
--> void
+	-> std::expected<void, error>
 {
-	auto [it, okay] = self.options.insert(
+	auto [it, okay] = self.m_options.insert(
 				std::pair{object->name(), object});
 
-	if (!okay)
-		throw error(std::format(
-			"INTERNAL ERROR: attempt to register "
-			"duplicate config value '{0}'",
-			object->name()));
+	if (okay)
+		return {};
+
+	return std::unexpected(error(std::format(
+			"attempt to register duplicate "
+			"configuration option '{0}'",
+			object->name())));
 }
 
 auto store::unregister_option(this store &self, option *object)
--> void
+	-> std::expected<void, error>
 {
-	auto it = self.options.find(object->name());
-	if (it == self.options.end())
-		throw error(std::format(
-			"INTERNAL ERROR: attempt to unregister "
-			"non-existent config value '{}'",
-			object->name()));
-
-	self.options.erase(it);
+	auto it = self.m_options.find(object->name());
+	if (it == self.m_options.end())
+		return std::unexpected(error(std::format(
+			"attempt to unregister non-existent "
+			"configuration option '{}'",
+			object->name())));
+
+	self.m_options.erase(it);
+	return {};
 }
 
 auto store::fetch(this store const &self, std::string_view name)
--> option &
+	-> std::expected<option const *, error>
 {
-	if (auto it = self.options.find(name); it != self.options.end())
-		return *it->second;
+	if (auto it = self.m_options.find(name); it != self.m_options.end())
+		return it->second;
 
-	throw unknown_option(name);
+	return std::unexpected(error(std::format(
+		"unknown configuration option '{}'",
+		name)));
+}
+
+auto store::fetch(this store &self, std::string_view name)
+	-> std::expected<option *, error>
+{
+	auto opt = co_await static_cast<store const &>(self).fetch(name);
+	co_return const_cast<option *>(opt);
+}
+
+auto store::all(this store const &self) -> nihil::generator<option const *>
+{
+	for (auto &&it : self.m_options)
+		co_yield it.second;
 }
 
-auto store::all(this store const &self)
--> nihil::generator<option const &>
+auto store::all(this store &self) -> nihil::generator<option *>
 {
-	for (auto &&it : self.options)
-		co_yield *it.second;
+	for (auto &&it : self.m_options)
+		co_yield it.second;
 }
 
 auto get_option(std::string_view option_name)
--> option &
+	-> std::expected<option *, error>
 {
-	return store::get().fetch(option_name);
+	co_return co_await store::get().fetch(option_name);
 }
 
 } // namespace nihil::config
diff --git a/nihil.config/store.ccm b/nihil.config/store.ccm
index 77b44b5..03d09d3 100644
--- a/nihil.config/store.ccm
+++ b/nihil.config/store.ccm
@@ -9,51 +9,50 @@ module;
  */
 
 #include <coroutine>
+#include <expected>
 #include <string>
 #include <map>
 
 export module nihil.config:store;
 
 import nihil;
-import :error;
 import :option;
 
 namespace nihil::config {
 
-// Exception thrown on an attempt to fetch an undefined option.
-export struct unknown_option final : error {
-	unknown_option(std::string_view option_name);
-	auto option_name(this unknown_option const &self) -> std::string_view;
-
-private:
-	std::string _option_name;
-};
-
 struct store final {
 	/*
 	 * Get the global config store.
 	 */
-	static auto get() -> store &;
+	[[nodiscard]] static auto get() -> store &;
 
 	/*
 	 * Register a new value with the config store.
 	 */
-	auto register_option(this store &self, option *object) -> void;
+	[[nodiscard]] auto register_option(this store &, option *object)
+		-> std::expected<void, error>;
 
 	/*
 	 * Remove a value from the config store.
 	 */
-	auto unregister_option(this store &self, option *object) -> void;
+	[[nodiscard]] auto unregister_option(this store &, option *object)
+		-> std::expected<void, error>;
 
 	/*
 	 * Fetch an existing value in the config store.
 	 */
-	auto fetch(this store const &self, std::string_view name) -> option &;
+	[[nodiscard]] auto fetch(this store const &, std::string_view name)
+		-> std::expected<option const *, error>;
+	[[nodiscard]] auto fetch(this store &, std::string_view name)
+		-> std::expected<option *, error>;
 
 	/*
 	 * Fetch all values in the configuration store.
 	 */
-	auto all(this store const &self) -> nihil::generator<option const &>;
+	[[nodiscard]] auto all(this store const &self)
+		-> nihil::generator<option const *>;
+	[[nodiscard]] auto all(this store &self)
+		-> nihil::generator<option *>;
 
 	// Not movable or copyable.
 	store(store const &) = delete;
@@ -62,19 +61,15 @@ struct store final {
 	store& operator=(store &&) = delete;
 
 private:
-	/*
-	 * The global configuration store, created by init() and accessed via
-	 * get().
-	 */
-	inline static store *instance = nullptr;
+	store();
 
-	std::map<std::string_view, option *> options;
-	store() = default;
+	std::map<std::string_view, option *> m_options;
 };
 
 /*
  * The public API.
  */
-export auto get_option(std::string_view option_name) -> option &;
+export auto get_option(std::string_view option_name)
+	-> std::expected<option *, error>;
 
 } // namespace nihil::config
diff --git a/nihil.config/string.cc b/nihil.config/string.cc
index 6b201ae..7d0c038 100644
--- a/nihil.config/string.cc
+++ b/nihil.config/string.cc
@@ -4,11 +4,14 @@
 
 module;
 
+#include <coroutine>
+#include <expected>
 #include <format>
 #include <string>
 
 module nihil.config;
 
+import nihil;
 import nihil.ucl;
 
 namespace nihil::config {
@@ -18,40 +21,41 @@ string::string(
 	std::string_view name,
 	std::string_view description) noexcept
 	: option(name, description)
-	, _storage(storage)
+	, m_storage(storage)
 {
-	store::get().register_option(this);
 }
 
-string::~string()
-{
-	store::get().unregister_option(this);
-}
+string::~string() = default;
 
 auto string::get_string() const -> std::string
 {
-	return _storage;
+	return m_storage;
 }
 
-auto string::set_string(std::string_view new_value) -> void
+auto string::set_string(std::string_view new_value)
+	-> std::expected<void, error>
 {
-	_storage = new_value;
+	m_storage = new_value;
+	return {};
 }
 
-auto string::to_ucl() const -> ucl::object
+auto string::get_ucl() const -> std::expected<ucl::object, error>
 {
-	return ucl::string(_storage);
+	return ucl::string(m_storage);
 }
 
-auto string::from_ucl(ucl::object const &uclobj) -> void
+auto string::set_ucl(ucl::object const &uclobj) -> std::expected<void, error>
 {
-	try {
-		_storage = object_cast<ucl::string>(uclobj).value();
-		is_default(false);
-	} catch (ucl::type_mismatch const &exc) {
-		throw error(std::format("'{}': expected string, not {}",
-			    name(), str(exc.actual_type())));
-	}
+	auto obj = co_await object_cast<ucl::string>(uclobj)
+		.transform_error([&] (ucl::type_mismatch const &m) {
+			return error(std::format(
+				"'{}': expected string, not {}",
+				name(), str(m.actual_type())));
+		});
+
+	m_storage = obj.value();
+	is_default(false);
+	co_return {};
 }
 
 } // namespace nihil::config
diff --git a/nihil.config/string.ccm b/nihil.config/string.ccm
index ae5efb9..b6a7e05 100644
--- a/nihil.config/string.ccm
+++ b/nihil.config/string.ccm
@@ -4,16 +4,21 @@
 
 module;
 
+#include <expected>
 #include <format>
 #include <string>
 
 export module nihil.config:string;
 
+import nihil;
 import nihil.ucl;
 import :option;
 
 namespace nihil::config {
 
+/*
+ * A string option.  The backing type is std::string.
+ */
 export struct string final : option
 {
 	string(std::string &storage,
@@ -22,14 +27,31 @@ export struct string final : option
 
 	~string();
 
-	auto get_string() const -> std::string override;
-	auto set_string(std::string_view new_value) -> void override;
-
-	auto to_ucl() const -> ucl::object override;
-	auto from_ucl(ucl::object const &uclobj) -> void override;
+	/*
+	 * Get this option as a string; simply returns the storage.
+	 */
+	[[nodiscard]] auto get_string() const -> std::string override;
+
+	/*
+	 * Set this option to a string value; assigns to the storage.
+	 */
+	[[nodiscard]] auto set_string(std::string_view new_value)
+		-> std::expected<void, error> override;
+
+	/*
+	 * Convert this option to a UCL object.
+	 */
+	[[nodiscard]] auto get_ucl() const
+		-> std::expected<ucl::object, error> override;
+
+	/*
+	 * Set this option from a UCL object.
+	 */
+	[[nodiscard]] auto set_ucl(ucl::object const &uclobj)
+		-> std::expected<void, error> override;
 
 private:
-	std::string &_storage;
+	std::string &m_storage;
 };
 
 } // namespace nihil::config
diff --git a/nihil.config/tests/string.cc b/nihil.config/tests/string.cc
index 7e95190..aeb1ef8 100644
--- a/nihil.config/tests/string.cc
+++ b/nihil.config/tests/string.cc
@@ -12,23 +12,25 @@ TEST_CASE("nihil.config: string option", "[nihil][nihil.config]")
 {
 	std::string storage;
 
-	REQUIRE_THROWS_AS(nihil::config::get_option("test_option"),
-			  nihil::config::unknown_option);
+	auto opt = nihil::config::get_option("test_option");
+	REQUIRE(!opt);
 
 	{
 		auto string_option = nihil::config::string(
 			storage, "test_option", "This is a test option");
 
-		auto &opt = nihil::config::get_option("test_option");
-		REQUIRE(opt.name() == "test_option");
-		REQUIRE(opt.description() == "This is a test option");
-		REQUIRE(opt.is_default() == true);
-		REQUIRE(opt.string() == "");
+		auto opt = nihil::config::get_option("test_option");
+		REQUIRE(opt);
 
-		opt.string("testing");
+		REQUIRE((*opt)->name() == "test_option");
+		REQUIRE((*opt)->description() == "This is a test option");
+		REQUIRE((*opt)->is_default() == true);
+		REQUIRE((*opt)->string() == "");
+
+		REQUIRE((*opt)->string("testing"));
 		REQUIRE(storage == "testing");
 	}
 
-	REQUIRE_THROWS_AS(nihil::config::get_option("test_option"),
-			  nihil::config::unknown_option);
+	opt = nihil::config::get_option("test_option");
+	REQUIRE(!opt);
 }
diff --git a/nihil.config/write.cc b/nihil.config/write.cc
index 2b451bd..8c02c43 100644
--- a/nihil.config/write.cc
+++ b/nihil.config/write.cc
@@ -4,43 +4,36 @@
 
 module;
 
+#include <coroutine>
 #include <expected>
 #include <filesystem>
 #include <format>
 #include <utility>
 
+module nihil.config;
+
 import nihil;
 import nihil.ucl;
 
-module nihil.config;
-
 namespace nihil::config {
 
 auto write_to(std::filesystem::path const &filename)
-	-> std::expected<void, nihil::error>
-try {
+	-> std::expected<void, error>
+{
 	auto uclconfig = ucl::map<ucl::object>();
 
 	// Add all the options to the UCL object.
 	for (auto const &option : store::get().all()) {
-		if (option.is_default())
+		if (option->is_default())
 			continue;
 
-		uclconfig.insert({option.name(), option.to_ucl()});
+		auto uobj = co_await option->ucl();
+		uclconfig.insert({option->name(), uobj});
 	}
 
 	auto ucl_text = std::format("{:c}", uclconfig);
-	auto ret = safe_write_file(filename, ucl_text);
-	if (!ret)
-		return std::unexpected(nihil::error(
-			std::format("cannot write {}", filename.string()),
-			ret.error()));
-
-	return {};
-} catch (ucl::error const &exc) {
-	return std::unexpected(nihil::error(
-			"failed to serialize configuration",
-			nihil::error(exc.what())));
+	co_await safe_write_file(filename, ucl_text);
+	co_return {};
 }
 
 };
diff --git a/nihil.config/write.ccm b/nihil.config/write.ccm
index 1a07dd7..cb35d76 100644
--- a/nihil.config/write.ccm
+++ b/nihil.config/write.ccm
@@ -7,16 +7,16 @@ module;
 #include <expected>
 #include <filesystem>
 
-import nihil;
-
 export module nihil.config:write;
 
+import nihil;
+
 export namespace nihil::config {
 
 /*
  * Write all config values (except defaults) to disk.
  */
 auto write_to(std::filesystem::path const &filename) ->
-	std::expected<void, nihil::error>;
+	std::expected<void, error>;
 
 };
-- 
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