Add rsl::maybe_error

.codespell_words

@@ -0,0 +1 @@
+unexpect

include/rsl/monad.hpp

@@ -123,6 +123,28 @@ template <typename T, typename E>
     return exp.has_value();
 }
 
+/**
+ * @brief Tests if any of the expected args passed in has an error.
+ *
+ * @param args tl::expected<T, E> parameter pack
+ *
+ * @tparam E    The error type
+ * @tparam Args The value types for the tl::expected<T, E> args
+ *
+ * @return The first error found or nothing
+ */
+template <typename E, typename... Args>
+[[nodiscard]] constexpr auto maybe_error(tl::expected<Args, E>... args) {
+    auto maybe = std::optional<E>();
+    (
+        [&](auto& exp) {
+            if (maybe.has_value()) return;
+            if (has_error(exp)) maybe = exp.error();
+        }(args),
+        ...);
+    return maybe;
+}
+
 template <typename>
 constexpr inline bool is_optional_impl = false;
 template <typename T>

tests/monad.cpp

@@ -228,3 +228,26 @@ TEST_CASE("operator|") {
         CHECK(val == Catch::Approx(4.61538));
     }
 }
+
+TEST_CASE("rsl::maybe_error") {
+    SECTION("No error") {
+        CHECK(rsl::maybe_error(tl::expected<int, std::string>(42)) == std::nullopt);
+        CHECK(rsl::maybe_error(tl::expected<int, std::string>(1), tl::expected<int, std::string>(2),
+                               tl::expected<int, std::string>(3), tl::expected<int, std::string>(4),
+                               tl::expected<int, std::string>(5)) == std::nullopt);
+    }
+
+    SECTION("Errors") {
+        CHECK(rsl::maybe_error<std::string, int>(tl::unexpected<std::string>("oops")) ==
+              std::optional<std::string>("oops"));
+        CHECK(rsl::maybe_error(tl::expected<int, std::string>(tl::unexpect, "oops1"),
+                               tl::expected<int, std::string>(tl::unexpect, "oops2"),
+                               tl::expected<int, std::string>(tl::unexpect, "oops3")) ==
+              std::optional<std::string>("oops1"));
+        CHECK(rsl::maybe_error(tl::expected<int, std::string>(tl::unexpect, "oops1"),
+                               tl::expected<int, std::string>(1337),
+                               tl::expected<int, std::string>(tl::unexpect, "oops2"),
+                               tl::expected<int, std::string>(tl::unexpect, "oops3")) ==
+              std::optional<std::string>("oops1"));
+    }
+}