add Result::transform

Author: liam-teale

include/common/result.hpp

@@ -228,6 +228,24 @@ class Result<T, Errs...> {
         }, std::forward<Self>(self).error);
     }
 
+    /**
+     * @brief If this Result instance does not contain an error, apply a callable to the expected
+     * value
+     *
+     * @tparam Self deduced self type
+     * @tparam F the type of the callable
+     * @param self the current Result instance
+     * @param f the callable
+     */
+    template<typename Self, typename F>
+        requires std::invocable<F, Self> && std::convertible_to<std::invoke_result_t<F, Self>, Self>
+    constexpr Self transform(this Self&& self, F&& f) {
+        if (self.has_error()) {
+            return std::forward<Self>(self);
+        }
+        return std::invoke(f, std::forward<Self>(self));
+    }
+
     constexpr operator T&() & {
         return value;
     }