refactored result that an overload for void doesn't need to exist

include/matador/query/expression_evaluator.hpp

@@ -19,7 +19,7 @@ public:
     void visit(const value_expression& node) override;
     void visit(const placeholder_expression& node) override;
 
-    const std::string& result() const;
+    [[nodiscard]] const std::string& result() const;
 
 private:
     const sql::dialect &dialect_;

include/matador/sql/statement.hpp

@@ -171,7 +171,8 @@ template<class Type>
 utils::result<query_result<Type>, utils::error> statement::fetch() {
   std::cout << statement_proxy_->sql() << std::endl;
   statement_proxy_->statement_->query_.result_type = typeid(Type);
-  return statement_proxy_->fetch(*bindings_).and_then([this](std::unique_ptr<query_result_impl> &&value) {
+  return statement_proxy_->fetch(*bindings_)
+  .and_then([this](std::unique_ptr<query_result_impl> &&value) -> utils::result<query_result<Type>, utils::error> {
     auto resolver = statement_proxy_->statement_->query_.resolver->object_resolver<Type>();
     const auto prototype = value->prototype();
     return utils::ok(query_result<Type>(std::forward<decltype(value)>(value), resolver, [prototype] {

include/matador/utils/result.hpp

@@ -2,7 +2,6 @@
 #define QUERY_RESULT_HPP
 
 #include <variant>
-#include <optional>
 #include <functional>
 #include <type_traits>
 
@@ -21,6 +20,7 @@ template < typename ValueType >
 class ok {
 public:
   using value_type = ValueType;
+  constexpr ok() = default;
   explicit constexpr ok(const ValueType &value) : value_(value) {}
   explicit constexpr ok(ValueType &&value) : value_(std::move(value)) {}
 
@@ -55,15 +55,43 @@ private:
   ErrorType error_;
 };
 
+namespace detail {
+template <typename ValueType, typename Func, bool IsVoidValue = std::is_void_v<ValueType>>
+struct map_result_value_type;
+
+template <typename ValueType, typename Func>
+struct map_result_value_type<ValueType, Func, false> {
+  using type = std::invoke_result_t<Func, ValueType&&>;
+};
+
+template <typename ValueType, typename Func>
+struct map_result_value_type<ValueType, Func, true> {
+  using type = std::invoke_result_t<Func>;
+};
+
+template <typename ValueType, typename Func, bool IsVoidValue = std::is_void_v<ValueType>>
+struct and_then_result_type;
+
+template <typename ValueType, typename Func>
+struct and_then_result_type<ValueType, Func, false> {
+  using type = std::invoke_result_t<Func, ValueType&&>;
+};
+
+template <typename ValueType, typename Func>
+struct and_then_result_type<ValueType, Func, true> {
+  using type = std::invoke_result_t<Func>;
+};
+}
+
 template < typename ValueType, typename ErrorType >
 class result {
 public:
   using value_type = ValueType;
   using error_type = ErrorType;
 
-  result() : result_(ValueType{}) {}
+  result() : result_(ok<value_type>{}) {}
-  result(ok<value_type> value) : result_(std::move(value.release())) {} // NOLINT(*-explicit-constructor)
+  result(ok<value_type> value) : result_(std::move(value)) {} // NOLINT(*-explicit-constructor)
-  result(failure<error_type> error) : result_(std::move(error.release())) {} // NOLINT(*-explicit-constructor)
+  result(failure<error_type> error) : result_(std::move(error)) {} // NOLINT(*-explicit-constructor)
   result(const result &x) = default;
   result& operator=(const result &x) = default;
   result(result &&x) = default;
@@ -71,31 +99,76 @@ public:
 
   operator bool() const { return is_ok(); } // NOLINT(*-explicit-constructor)
 
-  [[nodiscard]] bool is_ok() const { return std::holds_alternative<value_type>(result_); }
+  [[nodiscard]] bool is_ok() const {
-  [[nodiscard]] bool is_error() const { return std::holds_alternative<error_type>(result_); }
+    return std::holds_alternative<ok<value_type>>(result_);
-
+  }
-  ValueType&& release() { return std::move(std::get<value_type>(result_)); }
+  [[nodiscard]] bool is_error() const {
-  ErrorType&& release_error() { return std::move(std::get<error_type>(result_)); }
+    return std::holds_alternative<failure<error_type>>(result_);
-
-  const ValueType& value() const { return std::get<value_type>(result_); }
-  ValueType& value() { return std::get<value_type>(result_); }
-  const ErrorType& err() const { return std::get<error_type>(result_); }
-  ErrorType& err() { return std::get<error_type>(result_); }
-
-  constexpr const ValueType* operator->() const { return &value(); }
-  constexpr ValueType* operator->() { return &std::get<value_type>(result_); }
-
-  constexpr const ValueType& operator*() const& noexcept { return value(); }
-  constexpr ValueType& operator*() & noexcept { return value(); }
-
-  template<typename Func,
-           typename SecondValueType = std::invoke_result_t<Func, ValueType >>
-  result<SecondValueType, ErrorType> map(Func &&f) {
-    if (is_ok()) {
-      return result<SecondValueType, ErrorType>(ok(std::invoke(std::forward<Func>(f), release())));
   }
 
-    return result<SecondValueType, ErrorType>(failure(release_error()));
+  template <typename T = ValueType>
+  std::enable_if_t<!std::is_void_v<T>, T&&> release() {
+    return std::move(std::get<ok<value_type>>(result_).release());
+  }
+  ErrorType&& release_error() {
+    return std::move(std::get<failure<error_type>>(result_).release());
+  }
+
+  template <typename T = ValueType>
+  std::enable_if_t<!std::is_void_v<T>, const T&> value() const {
+    return std::get<ok<value_type>>(result_).value();
+  }
+  template <typename T = ValueType>
+  std::enable_if_t<!std::is_void_v<T>, T&> value() {
+    return std::get<ok<value_type>>(result_).value();
+  }
+  const ErrorType& err() const {
+    return std::get<failure<error_type>>(result_).value();
+  }
+  ErrorType& err() {
+    return std::get<failure<error_type>>(result_).value();
+  }
+
+  template <typename T = ValueType>
+  constexpr std::enable_if_t<!std::is_void_v<T>, const T*> operator->() const {
+    return &value();
+  }
+  template <typename T = ValueType>
+  constexpr std::enable_if_t<!std::is_void_v<T>, T*> operator->() {
+    return &value();
+  }
+
+  template <typename T = ValueType>
+  constexpr std::enable_if_t<!std::is_void_v<T>, const T&> operator*() const& noexcept {
+    return value();
+  }
+  template <typename T = ValueType>
+  constexpr std::enable_if_t<!std::is_void_v<T>, T&> operator*() & noexcept {
+    return value();
+  }
+
+  template<typename Func,
+           typename SecondValueType = typename detail::map_result_value_type<ValueType, Func>::type>
+  result<SecondValueType, ErrorType> map(Func &&f) {
+    if (is_error()) {
+      return failure<ErrorType>(release_error());
+    }
+
+    if constexpr (std::is_void_v<ValueType>) {
+      if constexpr (std::is_void_v<SecondValueType>) {
+        std::invoke(std::forward<Func>(f));
+        return ok<void>{};
+      } else {
+        return ok<SecondValueType>(std::invoke(std::forward<Func>(f)));
+      }
+    } else {
+      if constexpr (std::is_void_v<SecondValueType>) {
+        std::invoke(std::forward<Func>(f), release());
+        return ok<void>{};
+      } else {
+        return ok<SecondValueType>(std::invoke(std::forward<Func>(f), release()));
+      }
+    }
   }
 
   template<typename Func,
@@ -105,21 +178,30 @@ public:
       return failure<SecondErrorType>{std::invoke(std::forward<Func>(f), release_error())};
     }
 
+    if constexpr (std::is_void_v<ValueType>) {
+      return ok<void>{};
+    } else {
       return ok<ValueType>(release());
     }
+  }
+
+  template <typename Func,
+            typename ReturnResult = typename detail::and_then_result_type<ValueType, Func>::type>
+  ReturnResult and_then(Func &&f) {
+    static_assert(is_result<ReturnResult>::value, "and_then() callback must return matador::utils::result");
 
-  template<typename Func,
-           typename SecondValueType = typename std::invoke_result_t<Func, ValueType>::value_type>
-  result<SecondValueType, ErrorType> and_then(Func &&f) {
-    using ReturnResult = std::invoke_result_t<Func, ValueType>;
     if (is_ok()) {
+      if constexpr (std::is_void_v<ValueType>) {
+        return std::invoke(std::forward<Func>(f));
+      } else {
         return std::invoke(std::forward<Func>(f), release());
       }
-
-    return ReturnResult(failure(release_error()));
     }
 
-  template<typename Func,
+    return ReturnResult(failure<ErrorType>(release_error()));
+  }
+
+  template <typename Func,
             typename FailureType = std::invoke_result_t<Func, ErrorType&&>,
             typename SecondErrorType = typename FailureType::value_type>
   result<ValueType, SecondErrorType> or_else(Func &&f) {
@@ -129,69 +211,16 @@ public:
       );
     }
 
-    return result<ValueType, SecondErrorType>(ok<ValueType>(release()));
+    if constexpr (std::is_void_v<ValueType>) {
+      return ok<void>{};
+    } else {
+      return ok<ValueType>(release());
+    }
   }
 
 private:
-  std::variant<value_type, error_type> result_;
+  std::variant<ok<value_type>, failure<error_type>> result_;
 };
-
-template < typename ErrorType >
-class result<void, ErrorType>
-{
-public:
-  using value_type = void;
-  using error_type = ErrorType;
-
-  result() = default;
-  result(ok<void> /*value*/) {}
-  result(failure<error_type> error) : result_(std::move(error.release())) {} // NOLINT(*-explicit-constructor)
-  result(const result &x) = default;
-  result& operator=(const result &x) = default;
-  result(result &&x) = default;
-  result& operator=(result &&x) = default;
-
-  operator bool() const { return is_ok(); } // NOLINT(*-explicit-constructor)
-
-  [[nodiscard]] bool is_ok() const { return !result_.has_value(); }
-  [[nodiscard]] bool is_error() const { return result_.has_value(); }
-
-  ErrorType&& release_error() { return std::move(*result_); }
-
-  const ErrorType& err() const { return result_.value(); }
-  ErrorType err() { return result_.value(); }
-
-  template<typename Func, typename SecondValueType = std::invoke_result_t<Func>>
-  result<SecondValueType, ErrorType> map(Func &&f) {
-    if (is_ok()) {
-      return result<SecondValueType, ErrorType>(ok(f()));
-    }
-
-    return result<SecondValueType, ErrorType>(failure(release_error()));
-  }
-
-  template<typename Func>
-  result and_then(Func &&f) {
-    if (is_ok()) {
-      return f();
-    }
-
-    return result(failure(release_error()));
-  }
-
-  template<typename Func, typename SecondErrorType = typename std::invoke_result_t<Func, ErrorType&& >::value_type>
-  result<void, SecondErrorType> or_else(Func &&f) {
-    if (is_error()) {
-      return f(release_error());
-    }
-
-    return result<void, SecondErrorType>(ok<void>());
-  }
-
-private:
-  std::optional<error_type> result_;
-};
-
 }
 
 #endif //QUERY_RESULT_HPP

include/matador/utils/value.hpp

@@ -56,7 +56,7 @@ public:
     if (!res.is_ok()) {
       return std::nullopt;
     }
-    return *res;
+    return res.value();
   }
 
   template<class Type>

source/orm/query/intermediates/fetchable_query.cpp

@@ -30,7 +30,7 @@ utils::result<sql::query_result<sql::record>, utils::error> fetchable_query::fet
   auto ctx = compiler.build(*context_, exec.dialect(), std::nullopt);
   ctx.resolver = exec.resolver();
   return exec.fetch(ctx)
-    .and_then([](auto &&res) {
+    .and_then([](auto &&res) -> utils::result<sql::query_result<sql::record>, utils::error> {
       const auto prototype = res->prototype();
       return utils::ok(sql::query_result<sql::record>(std::forward<decltype(res)>(res), prototype));
     });

test/backends/QueryFixture.cpp

@@ -37,7 +37,8 @@ void QueryFixture::check_table_not_exists(const std::string &table_name) const {
 }
 
 void QueryFixture::drop_table_if_exists(const std::string &table_name) const {
-  const auto result = db.exists(table_name).and_then([&table_name, this](const bool exists) {
+  const auto result = db.exists(table_name)
+  .and_then([&table_name, this](const bool exists) -> utils::result<bool, utils::error> {
     if (exists) {
       auto res = query::drop()
         .table(table_name)

test/backends/SequenceFixture.cpp

@@ -35,7 +35,8 @@ void SequenceFixture::check_sequence_not_exists(const std::string& sequence_name
 }
 
 void SequenceFixture::drop_sequence_if_exists(const std::string& sequence_name) const {
-  const auto result = db.sequence_exists(sequence_name).and_then([&sequence_name, this](const bool exists) {
+  const auto result = db.sequence_exists(sequence_name).
+  and_then([&sequence_name, this](const bool exists) -> utils::result<bool, utils::error> {
     if (exists) {
       auto res = query::drop()
         .sequence(sequence_name)