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Fix monadic functions for move-only types #174

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Merged
Bronek merged 1 commit into from
Feb 15, 2026
Merged

Fix monadic functions for move-only types #174

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47 changes: 37 additions & 10 deletions include/pfn/expected.hpp
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -275,11 +275,33 @@ template <class T, class E> class expected {
rhs.set_ = true;
}

constexpr T const &_value() const &
{
ASSERT(set_);
return v_;
}
constexpr T &_value() &
{
ASSERT(set_);
return v_;
}
constexpr T const &&_value() const &&
{
ASSERT(set_);
return ::std::move(v_);
}
constexpr T &&_value() &&
{
ASSERT(set_);
return ::std::move(v_);
}

template <typename Self, typename Fn>
static constexpr auto _and_then(Self &&self, Fn &&fn) //
noexcept(::std::is_nothrow_invocable_v<Fn, decltype(FWD(self).value())>
&& ::std::is_nothrow_constructible_v<E, decltype(FWD(self).error())>)
requires(::std::is_constructible_v<E, decltype(FWD(self).error())>)
requires(::std::is_invocable_v<Fn, decltype(FWD(self).value())>
&& ::std::is_constructible_v<E, decltype(FWD(self).error())>)
{
using result_t = ::std::remove_cvref_t<::std::invoke_result_t<Fn, decltype(FWD(self).value())>>;
static_assert(detail::_is_some_expected<result_t>);
Expand All @@ -293,14 +315,15 @@ template <class T, class E> class expected {
template <typename Self, typename Fn>
static constexpr auto _or_else(Self &&self, Fn &&fn) //
noexcept(::std::is_nothrow_invocable_v<Fn, decltype(FWD(self).error())>
&& ::std::is_nothrow_constructible_v<T, decltype(FWD(self).value())>)
requires(::std::is_constructible_v<T, decltype(FWD(self).value())>)
&& ::std::is_nothrow_constructible_v<T, decltype(FWD(self)._value())>)
requires(::std::is_invocable_v<Fn, decltype(FWD(self).error())>
&& ::std::is_constructible_v<T, decltype(FWD(self)._value())>)
{
using result_t = ::std::remove_cvref_t<::std::invoke_result_t<Fn, decltype(FWD(self).error())>>;
static_assert(detail::_is_some_expected<result_t>);
static_assert(::std::is_same_v<typename result_t::value_type, typename ::std::remove_cvref_t<Self>::value_type>);
if (self.has_value()) {
return result_t(::std::in_place, FWD(self).value());
return result_t(::std::in_place, FWD(self)._value());
}
return ::std::invoke(FWD(fn), FWD(self).error());
}
Expand All @@ -313,7 +336,8 @@ template <class T, class E> class expected {
|| ::std::is_nothrow_constructible_v<
::std::remove_cv_t<::std::invoke_result_t<Fn, decltype(FWD(self).value())>>,
::std::invoke_result_t<Fn, decltype(FWD(self).value())>>))
requires(::std::is_constructible_v<E, decltype(FWD(self).error())>)
requires(::std::is_invocable_v<Fn, decltype(FWD(self).value())>
&& ::std::is_constructible_v<E, decltype(FWD(self).error())>)
{
using value_t = ::std::remove_cv_t<::std::invoke_result_t<Fn, decltype(FWD(self).value())>>;
static_assert(detail::_is_valid_expected<value_t, E>);
Expand All @@ -333,11 +357,12 @@ template <class T, class E> class expected {
template <typename Self, typename Fn>
static constexpr auto _transform_error(Self &&self, Fn &&fn) //
noexcept(::std::is_nothrow_invocable_v<Fn, decltype(FWD(self).error())>
&& ::std::is_nothrow_constructible_v<T, decltype(FWD(self).value())>
&& ::std::is_nothrow_constructible_v<T, decltype(FWD(self)._value())>
&& ::std::is_nothrow_constructible_v<
::std::remove_cv_t<::std::invoke_result_t<Fn, decltype(FWD(self).error())>>,
::std::invoke_result_t<Fn, decltype(FWD(self).error())>>)
requires(::std::is_constructible_v<T, decltype(FWD(self).value())>)
requires(::std::is_invocable_v<Fn, decltype(FWD(self).error())>
&& ::std::is_constructible_v<T, decltype(FWD(self)._value())>)
{
using error_t = ::std::remove_cv_t<::std::invoke_result_t<Fn, decltype(FWD(self).error())>>;
static_assert(detail::_is_valid_unexpected<error_t>);
Expand All @@ -346,7 +371,7 @@ template <class T, class E> class expected {
if (not self.has_value()) {
return result_t(unexpect, ::std::invoke(FWD(fn), FWD(self).error()));
}
return result_t(::std::in_place, FWD(self).value());
return result_t(::std::in_place, FWD(self)._value());
}

public:
Expand Down Expand Up @@ -962,7 +987,7 @@ class expected<T, E> {
template <typename Self, typename Fn>
static constexpr auto _and_then(Self &&self, Fn &&fn) //
noexcept(::std::is_nothrow_invocable_v<Fn> && ::std::is_nothrow_constructible_v<E, decltype(FWD(self).error())>)
requires(::std::is_constructible_v<E, decltype(FWD(self).error())>)
requires(::std::is_invocable_v<Fn> && ::std::is_constructible_v<E, decltype(FWD(self).error())>)
{
using result_t = ::std::remove_cvref_t<::std::invoke_result_t<Fn>>;
static_assert(detail::_is_some_expected<result_t>);
Expand All @@ -976,6 +1001,7 @@ class expected<T, E> {
template <typename Self, typename Fn>
static constexpr auto _or_else(Self &&self, Fn &&fn) //
noexcept(::std::is_nothrow_invocable_v<Fn, decltype(FWD(self).error())>)
requires(::std::is_invocable_v<Fn, decltype(FWD(self).error())>)
{
using result_t = ::std::remove_cvref_t<::std::invoke_result_t<Fn, decltype(FWD(self).error())>>;
static_assert(detail::_is_some_expected<result_t>);
Expand All @@ -992,7 +1018,7 @@ class expected<T, E> {
&& (::std::is_void_v<::std::invoke_result_t<Fn>>
|| ::std::is_nothrow_constructible_v<::std::remove_cv_t<::std::invoke_result_t<Fn>>,
::std::invoke_result_t<Fn>>))
requires(::std::is_constructible_v<E, decltype(FWD(self).error())>)
requires(::std::is_invocable_v<Fn> && ::std::is_constructible_v<E, decltype(FWD(self).error())>)
{
using value_t = ::std::remove_cv_t<::std::invoke_result_t<Fn>>;
static_assert(detail::_is_valid_expected<value_t, E>);
Expand All @@ -1015,6 +1041,7 @@ class expected<T, E> {
&& ::std::is_nothrow_constructible_v<
::std::remove_cv_t<::std::invoke_result_t<Fn, decltype(FWD(self).error())>>,
::std::invoke_result_t<Fn, decltype(FWD(self).error())>>)
requires(::std::is_invocable_v<Fn, decltype(FWD(self).error())>)
{
using error_t = ::std::remove_cv_t<::std::invoke_result_t<Fn, decltype(FWD(self).error())>>;
static_assert(detail::_is_valid_unexpected<error_t>);
Expand Down
50 changes: 50 additions & 0 deletions tests/pfn/expected.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,7 @@ using std::unexpected;
#include <catch2/catch_all.hpp>

#include <initializer_list>
#include <memory>
#include <stdexcept>
#include <string>
#include <type_traits>
Expand Down Expand Up @@ -2509,6 +2510,15 @@ TEST_CASE("expected non void", "[expected][polyfill]")
SUCCEED();
}
}

SECTION("move-only type")
{
using T = std::unique_ptr<int>;
expected<T, Error> e(std::in_place, std::make_unique<int>(42));
static_assert(std::is_same_v<decltype(std::move(e)), expected<T, Error> &&>);
auto res = std::move(e).and_then([](T p) -> expected<int, Error> { return *p + 1; });
CHECK(res.value() == 43);
}
}

SECTION("or_else")
Expand Down Expand Up @@ -2569,6 +2579,14 @@ TEST_CASE("expected non void", "[expected][polyfill]")
SUCCEED();
}
}

SECTION("move-only type")
{
using T = std::unique_ptr<int>;
expected<int, T> e(unexpect, std::make_unique<int>(42));
auto res = std::move(e).or_else([](T p) -> expected<int, int> { return unexpected(*p + 1); });
CHECK(res.error() == 43);
}
}

SECTION("transform")
Expand Down Expand Up @@ -2628,6 +2646,14 @@ TEST_CASE("expected non void", "[expected][polyfill]")
SUCCEED();
}
}

SECTION("move-only type")
{
using T = std::unique_ptr<int>;
expected<T, Error> e(std::in_place, std::make_unique<int>(42));
auto res = std::move(e).transform([](T p) { return *p + 1; });
CHECK(res.value() == 43);
}
}

SECTION("transform_error")
Expand Down Expand Up @@ -2685,6 +2711,14 @@ TEST_CASE("expected non void", "[expected][polyfill]")
SUCCEED();
}
}

SECTION("move-only type")
{
using T = std::unique_ptr<int>;
expected<int, T> e(unexpect, std::make_unique<int>(42));
auto res = std::move(e).transform_error([](T p) { return *p + 1; });
CHECK(res.error() == 43);
}
}
}

Expand Down Expand Up @@ -4223,6 +4257,14 @@ TEST_CASE("expected void", "[expected_void][polyfill]")
SUCCEED();
}
}

SECTION("move-only type")
{
using T = std::unique_ptr<int>;
expected<void, T> e(unexpect, std::make_unique<int>(42));
auto res = std::move(e).or_else([](T p) -> expected<void, int> { return unexpected(*p + 1); });
CHECK(res.error() == 43);
}
}

SECTION("transform")
Expand Down Expand Up @@ -4332,6 +4374,14 @@ TEST_CASE("expected void", "[expected_void][polyfill]")
SUCCEED();
}
}

SECTION("move-only type")
{
using T = std::unique_ptr<int>;
expected<void, T> e(unexpect, std::make_unique<int>(42));
auto res = std::move(e).transform_error([](T p) { return *p + 1; });
CHECK(res.error() == 43);
}
}
}

Expand Down