config/Param: add method With()

src/config/Data.hxx

@@ -54,6 +54,14 @@ struct ConfigData {
 		return list.empty() ? nullptr : &list.front();
 	}
 
+	template<typename F>
+	auto With(ConfigOption option, F &&f) const {
+		const auto *param = GetParam(option);
+		return param != nullptr
+			? param->With(std::forward<F>(f))
+			: f(nullptr);
+	}
+
 	gcc_pure
 	const char *GetString(ConfigOption option,
 			      const char *default_value=nullptr) const noexcept;

src/config/Param.hxx

@@ -66,6 +66,19 @@ struct ConfigParam {
 	 */
 	[[noreturn]]
 	void ThrowWithNested() const;
+
+	/**
+	 * Invoke a function with the configured value; if the
+	 * function throws, call ThrowWithNested().
+	 */
+	template<typename F>
+	auto With(F &&f) const {
+		try {
+			return f(value.c_str());
+		} catch (...) {
+			ThrowWithNested();
+		}
+	}
 };
 
 #endif