From ce3ed72dd402e28037f0331ed3862dc37d00e34f Mon Sep 17 00:00:00 2001
From: Simon Brunel <simonbrunel@users.noreply.github.com>
Date: Sat, 20 May 2017 09:40:42 +0200
Subject: [PATCH] Promise creation from callback only (resolver)

Make sure that the promise state can only be changed by the promise producer (and not consumers) by removing the `fulfill` and `reject` methods from the instance members and introducing a new constructor accepting a resolver lambda. That also means that a promise can't anymore be default constructed.

Add the static `QPromise<T>::resolve` and `QPromise<T>::reject` methods to create synchronously fulfilled or rejected promises, and fix the `qPromise` helper to handle deduced promises (e.g. `qPromise(QFuture<int>()) -> QPromise<int>`).
---
 src/qtpromise/qpromise.h                     |  60 ++--
 src/qtpromise/qpromise.inl                   | 307 +++++++++++++------
 src/qtpromise/qpromise_p.h                   | 230 +++++++-------
 src/qtpromise/qpromise_qfuture.inl           |  77 ++---
 src/qtpromise/qpromiseerror.h                |  42 +++
 src/qtpromise/qpromiseglobal.h               |  47 +--
 src/qtpromise/qtpromise.pri                  |   1 +
 tests/auto/qpromise/tst_qpromise.cpp         | 137 ++++-----
 tests/auto/requirements/tst_requirements.cpp | 265 +++++++++-------
 9 files changed, 677 insertions(+), 489 deletions(-)
 create mode 100644 src/qtpromise/qpromiseerror.h

diff --git a/src/qtpromise/qpromise.h b/src/qtpromise/qpromise.h
index fb80f15..ccdd055 100644
--- a/src/qtpromise/qpromise.h
+++ b/src/qtpromise/qpromise.h
@@ -1,25 +1,21 @@
 #ifndef _QTPROMISE_QPROMISE_H
 #define _QTPROMISE_QPROMISE_H
 
-// QPromise
+// QtPromise
 #include "qpromise_p.h"
 #include "qpromiseglobal.h"
 
 // Qt
 #include <QExplicitlySharedDataPointer>
-#include <QVariant>
 
 namespace QtPromise {
 
-using namespace QtPromisePrivate;
-
 template <typename T>
 class QPromiseBase
 {
 public:
     using Type = T;
 
-    QPromiseBase() : m_d(new QPromiseData<T>()) { }
     virtual ~QPromiseBase() { }
 
     bool isFulfilled() const { return m_d->resolved && !m_d->rejected; }
@@ -27,22 +23,34 @@ public:
     bool isPending() const { return !m_d->resolved; }
 
     template <typename TFulfilled, typename TRejected = std::nullptr_t>
-    inline auto then(TFulfilled fulfilled, TRejected rejected = nullptr)
-        -> typename QPromiseHandler<T, TFulfilled>::Promise;
+    inline typename QtPromisePrivate::PromiseHandler<T, TFulfilled>::Promise
+    then(TFulfilled fulfilled, TRejected rejected = nullptr);
 
     template <typename TRejected>
-    inline auto fail(TRejected rejected)
-        -> typename QPromiseHandler<T, std::nullptr_t>::Promise;
-
-    template <typename TError>
-    inline QPromise<T> reject(const TError& error);
+    inline typename QtPromisePrivate::PromiseHandler<T, std::nullptr_t>::Promise
+    fail(TRejected rejected);
 
     inline QPromise<T> wait() const;
 
-protected:
-    QExplicitlySharedDataPointer<QPromiseData<T> > m_d;
+public: // STATIC
+    template <typename E>
+    inline static QPromise<T> reject(const E& error);
 
-    virtual void notify(const typename QPromiseData<T>::HandlerList& handlers) const = 0;
+protected:
+    friend class QPromiseResolve<T>;
+    friend class QPromiseReject<T>;
+
+    QExplicitlySharedDataPointer<QtPromisePrivate::PromiseData<T> > m_d;
+
+    inline QPromiseBase();
+
+    template <typename F, typename std::enable_if<QtPromisePrivate::ArgsOf<F>::count == 1, int>::type = 0>
+    inline QPromiseBase(F resolver);
+
+    template <typename F, typename std::enable_if<QtPromisePrivate::ArgsOf<F>::count != 1, int>::type = 0>
+    inline QPromiseBase(F resolver);
+
+    virtual void notify(const typename QtPromisePrivate::PromiseData<T>::HandlerList& handlers) const = 0;
 
     inline void dispatch();
 };
@@ -51,42 +59,41 @@ template <typename T>
 class QPromise: public QPromiseBase<T>
 {
 public:
-    QPromise() : QPromiseBase<T>() {}
+    template <typename F>
+    QPromise(F resolver): QPromiseBase<T>(resolver) { }
 
     template <typename THandler>
     inline QPromise<T> finally(THandler handler);
 
-    inline QPromise<T> fulfill(const T& value);
-
 public: // STATIC
     inline static QPromise<QVector<T> > all(const QVector<QPromise<T> >& promises);
+    inline static QPromise<T> resolve(const T& value);
 
 protected:
-    inline void notify(const typename QPromiseData<T>::HandlerList& handlers) const Q_DECL_OVERRIDE;
+    inline void notify(const typename QtPromisePrivate::PromiseData<T>::HandlerList& handlers) const Q_DECL_OVERRIDE;
 
 private:
     friend class QPromiseBase<T>;
 
     QPromise(const QPromiseBase<T>& p) : QPromiseBase<T>(p) { }
-
 };
 
 template <>
 class QPromise<void>: public QPromiseBase<void>
 {
 public:
-    QPromise(): QPromiseBase<void>() {}
+    template <typename F>
+    QPromise(F resolver): QPromiseBase<void>(resolver) { }
 
     template <typename THandler>
     inline QPromise<void> finally(THandler handler);
 
-    inline QPromise<void> fulfill();
-
 public: // STATIC
     inline static QPromise<void> all(const QVector<QPromise<void> >& promises);
+    inline static QPromise<void> resolve();
 
 protected:
-    inline void notify(const typename QPromiseData<void>::HandlerList& handlers) const Q_DECL_OVERRIDE;
+    inline void notify(const typename QtPromisePrivate::PromiseData<void>::HandlerList& handlers) const Q_DECL_OVERRIDE;
 
 private:
     friend class QPromiseBase<void>;
@@ -94,13 +101,8 @@ private:
     QPromise(const QPromiseBase<void>& p) : QPromiseBase<void>(p) { }
 };
 
-using QVariantPromise = QtPromise::QPromise<QVariant>;
-
 } // namespace QtPromise
 
-Q_DECLARE_TYPEINFO(QtPromise::QVariantPromise, Q_MOVABLE_TYPE);
-Q_DECLARE_METATYPE(QtPromise::QVariantPromise)
-
 #include "qpromise.inl"
 #include "qpromise_qfuture.inl"
 
diff --git a/src/qtpromise/qpromise.inl b/src/qtpromise/qpromise.inl
index 3736c7b..42b5264 100644
--- a/src/qtpromise/qpromise.inl
+++ b/src/qtpromise/qpromise.inl
@@ -4,15 +4,91 @@
 
 namespace QtPromise {
 
+template <class T = void>
+class QPromiseResolve
+{
+public:
+    QPromiseResolve(const QPromise<T>& p)
+        : m_promise(p)
+    { }
+
+    void operator()(const T& value) const
+    {
+        auto p = m_promise;
+        if (p.isPending()) {
+            p.m_d->rejected = false;
+            p.m_d->resolved = true;
+            p.m_d->value = value;
+            p.dispatch();
+        }
+    }
+
+private:
+    QPromise<T> m_promise;
+};
+
+template <>
+class QPromiseResolve<void>
+{
+public:
+    QPromiseResolve(const QPromise<void>& p)
+        : m_promise(p)
+    { }
+
+    void operator()() const
+    {
+        auto p = m_promise;
+        if (p.isPending()) {
+            p.m_d->rejected = false;
+            p.m_d->resolved = true;
+            p.dispatch();
+        }
+    }
+
+private:
+    QPromise<void> m_promise;
+};
+
+template <class T = void>
+class QPromiseReject
+{
+public:
+    QPromiseReject()
+    { }
+
+    QPromiseReject(const QPromise<T>& p)
+        : m_promise(p)
+    { }
+
+    void operator()(const QPromiseError& error) const
+    {
+        auto p = m_promise;
+        if (p.isPending()) {
+            p.m_d->rejected = true;
+            p.m_d->resolved = true;
+            p.m_d->error = error;
+            p.dispatch();
+        }
+    }
+
+private:
+    QPromise<T> m_promise;
+};
+
 template <typename T>
 template <typename TFulfilled, typename TRejected>
-inline auto QPromiseBase<T>::then(TFulfilled fulfilled, TRejected rejected)
-    -> typename QPromiseHandler<T, TFulfilled>::Promise
+inline typename QtPromisePrivate::PromiseHandler<T, TFulfilled>::Promise
+QPromiseBase<T>::then(TFulfilled fulfilled, TRejected rejected)
 {
-    typename QPromiseHandler<T, TFulfilled>::Promise next;
+    using namespace QtPromisePrivate;
+    using PromiseType = typename PromiseHandler<T, TFulfilled>::Promise;
 
-    m_d->handlers << QPromiseHandler<T, TFulfilled>::create(next, fulfilled);
-    m_d->catchers << QPromiseCatcher<T, TRejected>::create(next, rejected);
+    PromiseType next([=](
+        const QPromiseResolve<typename PromiseType::Type>& resolve,
+        const QPromiseReject<typename PromiseType::Type>& reject) {
+        m_d->handlers << PromiseHandler<T, TFulfilled>::create(fulfilled, resolve, reject);
+        m_d->catchers << PromiseCatcher<T, TRejected>::create(rejected, resolve, reject);
+    });
 
     if (m_d->resolved) {
         dispatch();
@@ -23,26 +99,12 @@ inline auto QPromiseBase<T>::then(TFulfilled fulfilled, TRejected rejected)
 
 template <typename T>
 template <typename TRejected>
-inline auto QPromiseBase<T>::fail(TRejected rejected)
-    -> typename QPromiseHandler<T, std::nullptr_t>::Promise
+inline typename QtPromisePrivate::PromiseHandler<T, std::nullptr_t>::Promise
+QPromiseBase<T>::fail(TRejected rejected)
 {
     return then(nullptr, rejected);
 }
 
-template <typename T>
-template <typename TError>
-inline QPromise<T> QPromiseBase<T>::reject(const TError& error)
-{
-    if (!m_d->resolved) {
-        m_d->error = QtPromisePrivate::to_exception_ptr(error);
-        m_d->rejected = true;
-        m_d->resolved = true;
-        dispatch();
-    }
-
-    return *this;
-}
-
 template <typename T>
 inline QPromise<T> QPromiseBase<T>::wait() const
 {
@@ -55,20 +117,67 @@ inline QPromise<T> QPromiseBase<T>::wait() const
     return *this;
 }
 
+template <typename T>
+template <typename E>
+inline QPromise<T> QPromiseBase<T>::reject(const E& error)
+{
+    return QPromise<T>([=](const QPromiseResolve<T>&) {
+        throw error;
+    });
+}
+
+template <typename T>
+inline QPromiseBase<T>::QPromiseBase()
+    : m_d(new QtPromisePrivate::PromiseData<T>())
+{
+}
+
+template <typename T>
+template <typename F, typename std::enable_if<QtPromisePrivate::ArgsOf<F>::count == 1, int>::type>
+inline QPromiseBase<T>::QPromiseBase(F resolver)
+    : m_d(new QtPromisePrivate::PromiseData<T>())
+{
+    auto resolve = QPromiseResolve<T>(*this);
+    auto reject = QPromiseReject<T>(*this);
+
+    try {
+        resolver(resolve);
+    } catch(...) {
+        reject(std::current_exception());
+    }
+}
+
+template <typename T>
+template <typename F, typename std::enable_if<QtPromisePrivate::ArgsOf<F>::count != 1, int>::type>
+inline QPromiseBase<T>::QPromiseBase(F resolver)
+    : m_d(new QtPromisePrivate::PromiseData<T>())
+{
+    auto resolve = QPromiseResolve<T>(*this);
+    auto reject = QPromiseReject<T>(*this);
+
+    try {
+        resolver(resolve, reject);
+    } catch(...) {
+        reject(std::current_exception());
+    }
+}
+
 template <typename T>
 inline void QPromiseBase<T>::dispatch()
 {
+    using namespace QtPromisePrivate;
+
     Q_ASSERT(m_d->resolved);
 
-    typename QPromiseData<T>::HandlerList handlers;
-    typename QPromiseData<T>::CatcherList catchers;
+    typename PromiseData<T>::HandlerList handlers;
+    typename PromiseData<T>::CatcherList catchers;
 
     handlers.swap(m_d->handlers);
     catchers.swap(m_d->catchers);
 
     if (m_d->rejected) {
-        const std::exception_ptr error = m_d->error;
-        qtpromise_defer([catchers, error]() {
+        const QPromiseError error = m_d->error;
+        qtpromise_defer([=]() {
             for (auto catcher: catchers) {
                 catcher(error);
             }
@@ -83,67 +192,62 @@ template <typename THandler>
 inline QPromise<T> QPromise<T>::finally(THandler handler)
 {
     return this->then([=](const T& res) {
-        return QPromise<void>().fulfill().then(handler).then([=](){
+        return QPromise<void>::resolve().then(handler).then([=](){
             return res;
         });
     }, [=]() {
         const auto exception = std::current_exception();
-        return QPromise<void>().fulfill().then(handler).then([=](){
+        return QPromise<void>::resolve().then(handler).then([=](){
             std::rethrow_exception(exception);
             return T();
         });
     });
 }
 
-template <typename T>
-inline QPromise<T> QPromise<T>::fulfill(const T& value)
-{
-    if (!this->m_d->resolved) {
-        this->m_d->rejected = false;
-        this->m_d->resolved = true;
-        this->m_d->value = value;
-        this->dispatch();
-    }
-
-    return *this;
-}
-
 template <typename T>
 inline QPromise<QVector<T> > QPromise<T>::all(const QVector<QPromise<T> >& promises)
 {
-    QPromise<QVector<T> > next;
-
     const int count = promises.size();
     if (count == 0) {
-        return next.fulfill({});
+        return QPromise<QVector<T> >::resolve({});
     }
 
-    QSharedPointer<int> remaining(new int(count));
-    QSharedPointer<QVector<T> > results(new QVector<T>(count));
+    return QPromise<QVector<T> >([=](
+        const QPromiseResolve<QVector<T> >& resolve,
+        const QPromiseReject<QVector<T> >& reject) {
 
-    for (int i=0; i<count; ++i) {
-        QPromise<T>(promises[i]).then([=](const T& res) mutable {
-            if (next.isPending()) {
+        QSharedPointer<int> remaining(new int(count));
+        QSharedPointer<QVector<T> > results(new QVector<T>(count));
+
+        for (int i=0; i<count; ++i) {
+            QPromise<T>(promises[i]).then([=](const T& res) mutable {
                 (*results)[i] = res;
                 if (--(*remaining) == 0) {
-                    next.fulfill(*results);
+                    resolve(*results);
                 }
-            }
-        }, [=]() mutable {
-            if (next.isPending()) {
-                next.reject(std::current_exception());
-            }
-        });
-    }
-
-    return next;
+            }, [=]() mutable {
+                if (*remaining != -1) {
+                    *remaining = -1;
+                    reject(std::current_exception());
+                }
+            });
+        }
+    });
 }
 
 template <typename T>
-inline void QPromise<T>::notify(const typename QPromiseData<T>::HandlerList& handlers) const
+inline QPromise<T> QPromise<T>::resolve(const T& value)
+{
+    return QPromise<T>([=](const QPromiseResolve<T>& resolve) {
+        resolve(value);
+    });
+}
+
+template <typename T>
+inline void QPromise<T>::notify(const typename QtPromisePrivate::PromiseData<T>::HandlerList& handlers) const
 {
     const T value = this->m_d->value;
-    qtpromise_defer([handlers, value]() {
+    QtPromisePrivate::qtpromise_defer([handlers, value]() {
         for (auto handler: handlers) {
             handler(value);
         }
@@ -154,52 +258,53 @@ template <typename THandler>
 inline QPromise<void> QPromise<void>::finally(THandler handler)
 {
     return this->then([=]() {
-        return QPromise<void>().fulfill().then(handler).then([](){});
+        return QPromise<void>::resolve().then(handler).then([](){});
     }, [=]() {
         const auto exception = std::current_exception();
-        return QPromise<void>().fulfill().then(handler).then([=](){
+        return QPromise<void>::resolve().then(handler).then([=](){
             std::rethrow_exception(exception);
         });
     });
 }
 
-inline QPromise<void> QPromise<void>::fulfill()
-{
-    if (!m_d->resolved) {
-        m_d->rejected = false;
-        m_d->resolved = true;
-        dispatch();
-    }
-
-    return *this;
-}
-
 inline QPromise<void> QPromise<void>::all(const QVector<QPromise<void> >& promises)
 {
-    QPromise<void> next;
-
-    QSharedPointer<int> remaining(new int(promises.size()));
-
-    for (const auto& promise: promises) {
-        QPromise<void>(promise).then([=]() mutable {
-            if (next.isPending()) {
-                if (--(*remaining) == 0) {
-                    next.fulfill();
-                }
-            }
-        }, [=]() mutable {
-            if (next.isPending()) {
-                next.reject(std::current_exception());
-            }
-        });
+    const int count = promises.size();
+    if (count == 0) {
+        return QPromise<void>::resolve();
     }
 
-    return next;
+    return QPromise<void>([=](
+        const QPromiseResolve<void>& resolve,
+        const QPromiseReject<void>& reject) {
+
+        QSharedPointer<int> remaining(new int(promises.size()));
+
+        for (const auto& promise: promises) {
+            QPromise<void>(promise).then([=]() {
+                if (--(*remaining) == 0) {
+                    resolve();
+                }
+            }, [=]() {
+                if (*remaining != -1) {
+                    *remaining = -1;
+                    reject(std::current_exception());
+                }
+            });
+        }
+    });
 }
 
-inline void QPromise<void>::notify(const typename QPromiseData<void>::HandlerList& handlers) const
+inline QPromise<void> QPromise<void>::resolve()
 {
-    qtpromise_defer([handlers]() {
+    return QPromise<void>([](const QPromiseResolve<void>& resolve) {
+        resolve();
+    });
+}
+
+inline void QPromise<void>::notify(const typename QtPromisePrivate::PromiseData<void>::HandlerList& handlers) const
+{
+    QtPromisePrivate::qtpromise_defer([handlers]() {
         for (const auto& handler: handlers) {
             handler();
         }
@@ -209,9 +314,23 @@ inline void QPromise<void>::notify(const typename QPromiseData<void>::HandlerLis
 // Helpers
 
 template <typename T>
-QPromise<T> qPromise(const T& value)
+typename QtPromisePrivate::PromiseDeduce<T>::Type qPromise(const T& value)
 {
-    return QPromise<T>().fulfill(value);
+    using namespace QtPromisePrivate;
+    using Promise = typename PromiseDeduce<T>::Type;
+    return Promise([=](
+        const QPromiseResolve<typename Promise::Type>& resolve,
+        const QPromiseReject<typename Promise::Type>& reject) {
+        PromiseFulfill<T>::call(value, resolve, reject);
+    });
+}
+
+QPromise<void> qPromise()
+{
+    return QPromise<void>([](
+        const QPromiseResolve<void>& resolve) {
+        resolve();
+    });
 }
 
 template <typename T>
diff --git a/src/qtpromise/qpromise_p.h b/src/qtpromise/qpromise_p.h
index d8e49b7..eddc2aa 100644
--- a/src/qtpromise/qpromise_p.h
+++ b/src/qtpromise/qpromise_p.h
@@ -1,7 +1,8 @@
 #ifndef _QTPROMISE_QPROMISE_P_H
 #define _QTPROMISE_QPROMISE_P_H
 
-// QPromise
+// QtPromise
+#include "qpromiseerror.h"
 #include "qpromiseglobal.h"
 
 // Qt
@@ -10,18 +11,21 @@
 #include <QSharedData>
 #include <QVector>
 
-// STL
-#include <functional>
-
 namespace QtPromise {
-template <typename T = void>
+
+template <typename T>
 class QPromise;
-}
+
+template <typename T>
+class QPromiseResolve;
+
+template <typename T>
+class QPromiseReject;
+
+} // namespace QtPromise
 
 namespace QtPromisePrivate {
 
-using namespace QtPromise;
-
 template <typename F>
 inline void qtpromise_defer(F f)
 {
@@ -29,265 +33,269 @@ inline void qtpromise_defer(F f)
 }
 
 template <typename T>
-struct QPromiseDeduce
+struct PromiseDeduce
 {
-    using Type = QPromise<Unqualified<T> >;
+    using Type = QtPromise::QPromise<Unqualified<T> >;
 };
 
 template <typename T>
-struct QPromiseDeduce<QPromise<T> >
-    : public QPromiseDeduce<T>
+struct PromiseDeduce<QtPromise::QPromise<T> >
+    : public PromiseDeduce<T>
 { };
 
-template <typename T, typename TPromise = typename QPromiseDeduce<T>::Type>
-struct QPromiseFulfill
+template <typename T>
+struct PromiseFulfill
 {
-    static void call(TPromise next, const T& value)
+    static void call(
+        const T& value,
+        const QtPromise::QPromiseResolve<T>& resolve,
+        const QtPromise::QPromiseReject<T>&)
     {
-        next.fulfill(value);
+        resolve(value);
     }
 };
 
 template <typename T>
-struct QPromiseFulfill<QPromise<T>, QPromise<T> >
+struct PromiseFulfill<QtPromise::QPromise<T> >
 {
-    static void call(QPromise<T> next, QPromise<T> promise)
+    static void call(
+        QtPromise::QPromise<T> promise,
+        const QtPromise::QPromiseResolve<T>& resolve,
+        const QtPromise::QPromiseReject<T>& reject)
     {
         promise.then(
-            [=](const T& value) mutable {
-                next.fulfill(value);
+            [=](const T& value) {
+                resolve(value);
             },
-            [=]() mutable {
-                next.reject(std::current_exception());
+            [=]() { // catch all
+                reject(std::current_exception());
             });
     }
 };
 
 template <>
-struct QPromiseFulfill<QPromise<void>, QPromise<void> >
+struct PromiseFulfill<QtPromise::QPromise<void> >
 {
-    template <typename TPromise>
-    static void call(TPromise next, TPromise promise)
+    template <typename TPromise, typename TResolve, typename TReject>
+    static void call(TPromise promise, TResolve resolve, TReject reject)
     {
         promise.then(
-            [=]() mutable {
-                next.fulfill();
+            [=]() {
+                resolve();
             },
-            [=]() mutable {
-                next.reject(std::current_exception());
+            [=]() { // catch all
+                reject(std::current_exception());
             });
     }
 };
 
 template <typename T, typename TRes>
-struct QPromiseDispatch
+struct PromiseDispatch
 {
-    using Promise = typename QPromiseDeduce<TRes>::Type;
+    using Promise = typename PromiseDeduce<TRes>::Type;
     using ResType = Unqualified<TRes>;
 
-    template <typename TPromise, typename THandler>
-    static void call(TPromise next, THandler handler, const T& value)
+    template <typename THandler, typename TResolve, typename TReject>
+    static void call(const T& value, THandler handler, TResolve resolve, TReject reject)
     {
-        ResType res;
         try {
-            res = handler(value);
+            const ResType res = handler(value);
+            PromiseFulfill<ResType>::call(res, resolve, reject);
         } catch (...) {
-            next.reject(std::current_exception());
-            return;
+            reject(std::current_exception());
         }
-        QPromiseFulfill<ResType>::call(next, res);
     }
 };
 
 template <typename T>
-struct QPromiseDispatch<T, void>
+struct PromiseDispatch<T, void>
 {
-    using Promise = QPromise<void>;
+    using Promise = QtPromise::QPromise<void>;
 
-    template <typename TPromise, typename THandler>
-    static void call(TPromise next, THandler handler, const T& value)
+    template <typename THandler, typename TResolve, typename TReject>
+    static void call(const T& value, THandler handler, TResolve resolve, TReject reject)
     {
         try {
             handler(value);
+            resolve();
         } catch (...) {
-            next.reject(std::current_exception());
-            return;
+            reject(std::current_exception());
         }
-        next.fulfill();
     }
 };
 
 template <typename TRes>
-struct QPromiseDispatch<void, TRes>
+struct PromiseDispatch<void, TRes>
 {
-    using Promise = typename QPromiseDeduce<TRes>::Type;
+    using Promise = typename PromiseDeduce<TRes>::Type;
     using ResType = Unqualified<TRes>;
 
-    template <typename TPromise, typename THandler>
-    static void call(TPromise next, THandler handler)
+    template <typename THandler, typename TResolve, typename TReject>
+    static void call(THandler handler, TResolve resolve, TReject reject)
     {
-        ResType res;
         try {
-            res = handler();
+            const ResType res = handler();
+            PromiseFulfill<ResType>::call(res, resolve, reject);
         } catch (...) {
-            next.reject(std::current_exception());
-            return;
+            reject(std::current_exception());
         }
-        QPromiseFulfill<ResType>::call(next, res);
     }
 };
 
 template <>
-struct QPromiseDispatch<void, void>
+struct PromiseDispatch<void, void>
 {
-    using Promise = QPromise<void>;
+    using Promise = QtPromise::QPromise<void>;
 
-    template <typename TPromise, typename THandler>
-    static void call(TPromise next, THandler handler)
+    template <typename THandler, typename TResolve, typename TReject>
+    static void call(THandler handler, TResolve resolve, TReject reject)
     {
         try {
             handler();
+            resolve();
         } catch (...) {
-            next.reject(std::current_exception());
-            return;
+            reject(std::current_exception());
         }
-        next.fulfill();
     }
 };
 
 template <typename T, typename THandler, typename TArg = typename ArgsOf<THandler>::first>
-struct QPromiseHandler
+struct PromiseHandler
 {
     using ResType = typename std::result_of<THandler(T)>::type;
-    using Promise = typename QPromiseDispatch<T, ResType>::Promise;
+    using Promise = typename PromiseDispatch<T, ResType>::Promise;
 
-    static std::function<void(T)> create(const Promise& next, THandler handler)
+    template <typename TResolve, typename TReject>
+    static std::function<void(T)> create(THandler handler, TResolve resolve, TReject reject)
     {
-        return [=](const T& value) mutable {
-            QPromiseDispatch<T, ResType>::call(next, handler, value);
+        return [=](const T& value) {
+            PromiseDispatch<T, ResType>::call(value, handler, resolve, reject);
         };
     }
 };
 
 template <typename T, typename THandler>
-struct QPromiseHandler<T, THandler, void>
+struct PromiseHandler<T, THandler, void>
 {
     using ResType = typename std::result_of<THandler()>::type;
-    using Promise = typename QPromiseDispatch<T, ResType>::Promise;
+    using Promise = typename PromiseDispatch<T, ResType>::Promise;
 
-    static std::function<void(T)> create(const Promise& next, THandler handler)
+    template <typename TResolve, typename TReject>
+    static std::function<void(T)> create(THandler handler, TResolve resolve, TReject reject)
     {
-        return [=](const T&) mutable {
-            QPromiseDispatch<void, ResType>::call(next, handler);
+        return [=](const T&) {
+            PromiseDispatch<void, ResType>::call(handler, resolve, reject);
         };
     }
 };
 
 template <typename THandler>
-struct QPromiseHandler<void, THandler, void>
+struct PromiseHandler<void, THandler, void>
 {
     using ResType = typename std::result_of<THandler()>::type;
-    using Promise = typename QPromiseDispatch<void, ResType>::Promise;
+    using Promise = typename PromiseDispatch<void, ResType>::Promise;
 
-    static std::function<void()> create(const Promise& next, THandler handler)
+    template <typename TResolve, typename TReject>
+    static std::function<void()> create(THandler handler, TResolve resolve, TReject reject)
     {
         return [=]() {
-            QPromiseDispatch<void, ResType>::call(next, handler);
+            PromiseDispatch<void, ResType>::call(handler, resolve, reject);
         };
     }
 };
 
 template <typename T>
-struct QPromiseHandler<T, std::nullptr_t, void>
+struct PromiseHandler<T, std::nullptr_t, void>
 {
-    using Promise = QPromise<T>;
+    using Promise = QtPromise::QPromise<T>;
 
-    static std::function<void(T)> create(const Promise& next, std::nullptr_t)
+    template <typename TResolve, typename TReject>
+    static std::function<void(T)> create(std::nullptr_t, TResolve resolve, TReject reject)
     {
-        return [next](const T& value) mutable {
+        return [=](const T& value) {
             // 2.2.7.3. If onFulfilled is not a function and promise1 is fulfilled,
             // promise2 must be fulfilled with the same value as promise1.
-            QPromiseFulfill<T>::call(next, value);
+            PromiseFulfill<T>::call(value, resolve, reject);
         };
     }
 };
 
 template <>
-struct QPromiseHandler<void, std::nullptr_t, void>
+struct PromiseHandler<void, std::nullptr_t, void>
 {
-    using Promise = QPromise<void>;
+    using Promise = QtPromise::QPromise<void>;
 
-    template <typename TPromise>
-    static std::function<void()> create(const TPromise& next, std::nullptr_t)
+    template <typename TResolve, typename TReject>
+    static std::function<void()> create(std::nullptr_t, TResolve resolve, TReject)
     {
-        return [=]() mutable {
+        return [=]() {
             // 2.2.7.3. If onFulfilled is not a function and promise1 is fulfilled,
             // promise2 must be fulfilled with the same value as promise1.
-            TPromise(next).fulfill();
+            resolve();
         };
     }
 };
 
 template <typename T, typename THandler, typename TArg = typename ArgsOf<THandler>::first>
-struct QPromiseCatcher
+struct PromiseCatcher
 {
-    using Type = std::function<void(std::exception_ptr)>;
+    using Functor = std::function<void(QtPromise::QPromiseError)>;
     using ResType = typename std::result_of<THandler(TArg)>::type;
 
-    template <typename TPromise>
-    static Type create(const TPromise& next, THandler handler)
+    template <typename TResolve, typename TReject>
+    static Functor create(THandler handler, TResolve resolve, TReject reject)
     {
-        return [=](const std::exception_ptr& eptr) mutable {
+        return [=](const QtPromise::QPromiseError& error) {
             try {
-                std::rethrow_exception(eptr);
+                error.rethrow();
             } catch (const TArg& error) {
-                QPromiseDispatch<TArg, ResType>::call(next, handler, error);
+                PromiseDispatch<TArg, ResType>::call(error, handler, resolve, reject);
             } catch(...) {
-                TPromise(next).reject(std::current_exception());
+                reject(std::current_exception());
             }
         };
     }
 };
 
 template <typename T, typename THandler>
-struct QPromiseCatcher<T, THandler, void>
+struct PromiseCatcher<T, THandler, void>
 {
-    using Type = std::function<void(std::exception_ptr)>;
+    using Functor = std::function<void(QtPromise::QPromiseError)>;
     using ResType = typename std::result_of<THandler()>::type;
 
-    template <typename TPromise>
-    static Type create(const TPromise& next, THandler handler)
+    template <typename TResolve, typename TReject>
+    static Functor create(THandler handler, TResolve resolve, TReject reject)
     {
-        return [=](const std::exception_ptr& eptr) mutable {
+        return [=](const QtPromise::QPromiseError& error) {
             try {
-                std::rethrow_exception(eptr);
+                error.rethrow();
             } catch (...) {
-                QPromiseDispatch<void, ResType>::call(next, handler);
+                PromiseDispatch<void, ResType>::call(handler, resolve, reject);
             }
         };
     }
 };
 
 template <typename T>
-struct QPromiseCatcher<T, std::nullptr_t, void>
+struct PromiseCatcher<T, std::nullptr_t, void>
 {
-    using Type = std::function<void(std::exception_ptr)>;
+    using Functor = std::function<void(QtPromise::QPromiseError)>;
 
-    template <typename TPromise>
-    static Type create(const TPromise& next, std::nullptr_t)
+    template <typename TResolve, typename TReject>
+    static Functor create(std::nullptr_t, TResolve, TReject reject)
     {
-        return [=](const std::exception_ptr& eptr) mutable {
+        return [=](const QtPromise::QPromiseError& error) {
             // 2.2.7.4. If onRejected is not a function and promise1 is rejected,
             // promise2 must be rejected with the same reason as promise1
-            TPromise(next).reject(eptr);
+            reject(error);
         };
     }
 };
 
 template <class T>
-struct QPromiseDataBase: public QSharedData
+struct PromiseDataBase: public QSharedData
 {
-    using ErrorType = std::exception_ptr;
+    using ErrorType = QtPromise::QPromiseError;
     using CatcherList = QVector<std::function<void(ErrorType)> >;
 
     bool resolved;
@@ -297,7 +305,7 @@ struct QPromiseDataBase: public QSharedData
 };
 
 template <typename T>
-struct QPromiseData: QPromiseDataBase<T>
+struct PromiseData: PromiseDataBase<T>
 {
     using HandlerList = QVector<std::function<void(T)> >;
 
@@ -306,7 +314,7 @@ struct QPromiseData: QPromiseDataBase<T>
 };
 
 template <>
-struct QPromiseData<void>: QPromiseDataBase<void>
+struct PromiseData<void>: PromiseDataBase<void>
 {
     using HandlerList = QVector<std::function<void()> >;
 
diff --git a/src/qtpromise/qpromise_qfuture.inl b/src/qtpromise/qpromise_qfuture.inl
index cc29194..eec8726 100644
--- a/src/qtpromise/qpromise_qfuture.inl
+++ b/src/qtpromise/qpromise_qfuture.inl
@@ -4,76 +4,61 @@
 namespace QtPromisePrivate {
 
 template <typename T>
-struct QPromiseDeduce<QFuture<T> >
-    : public QPromiseDeduce<T>
+struct PromiseDeduce<QFuture<T> >
+    : public PromiseDeduce<T>
 { };
 
 template <typename T>
-struct QPromiseFulfill<QFuture<T>, QPromise<T> >
+struct PromiseFulfill<QFuture<T> >
 {
-    static void call(QPromise<T> next, const QFuture<T>& future)
+    static void call(
+        const QFuture<T>& future,
+        const QtPromise::QPromiseResolve<T>& resolve,
+        const QtPromise::QPromiseReject<T>& reject)
     {
         using Watcher = QFutureWatcher<T>;
 
         Watcher* watcher = new Watcher();
-        QObject::connect(
-            watcher, &Watcher::finished,
-            [next, watcher]() mutable {
-                T res;
-                try {
-                    res = watcher->result();
-                } catch(...) {
-                    next.reject(std::current_exception());
-                }
+        QObject::connect(watcher, &Watcher::finished, [=]() mutable {
+            try {
+                T res = watcher->result();
+                PromiseFulfill<T>::call(res, resolve, reject);
+            } catch(...) {
+                reject(std::current_exception());
+            }
 
-                watcher->deleteLater();
-                if (next.isPending()) {
-                    QPromiseFulfill<T>::call(next, res);
-                }
-            });
+            watcher->deleteLater();
+        });
 
         watcher->setFuture(future);
     }
 };
 
 template <>
-struct QPromiseFulfill<QFuture<void>, QPromise<void> >
+struct PromiseFulfill<QFuture<void> >
 {
-    static void call(QPromise<void> next, const QFuture<void>& future)
+    static void call(
+        const QFuture<void>& future,
+        const QtPromise::QPromiseResolve<void>& resolve,
+        const QtPromise::QPromiseReject<void>& reject)
     {
         using Watcher = QFutureWatcher<void>;
 
         Watcher* watcher = new Watcher();
-        QObject::connect(
-            watcher, &Watcher::finished,
-            [next, watcher]() mutable {
-                try {
-                    // let's rethrown possibe exception
-                    watcher->waitForFinished();
-                } catch(...) {
-                    next.reject(std::current_exception());
-                }
+        QObject::connect(watcher, &Watcher::finished, [=]() mutable {
+            try {
+                // let's rethrown possibe exception
+                watcher->waitForFinished();
+                resolve();
+            } catch(...) {
+                reject(std::current_exception());
+            }
 
-                watcher->deleteLater();
-                if (next.isPending()) {
-                    next.fulfill();
-                }
-            });
+            watcher->deleteLater();
+        });
 
         watcher->setFuture(future);
     }
 };
 
-} // namespace QtPromisePrivate
-
-namespace QtPromise {
-
-template <typename T>
-QPromise<T> qPromise(const QFuture<T>& future)
-{
-    QPromise<T> next;
-    QPromiseFulfill<QFuture<T> >::call(next, future);
-    return next;
-}
-
 } // namespace QtPromise
diff --git a/src/qtpromise/qpromiseerror.h b/src/qtpromise/qpromiseerror.h
new file mode 100644
index 0000000..e21a148
--- /dev/null
+++ b/src/qtpromise/qpromiseerror.h
@@ -0,0 +1,42 @@
+#ifndef _QTPROMISE_QPROMISEERROR_H
+#define _QTPROMISE_QPROMISEERROR_H
+
+// QtPromise
+#include "qpromiseglobal.h"
+
+namespace QtPromise {
+
+class QPromiseError
+{
+public:
+    QPromiseError()
+        : exception(nullptr)
+    { }
+
+    template <typename T>
+    QPromiseError(const T& value)
+        : exception(nullptr)
+    {
+        try {
+            throw value;
+        } catch(...) {
+            exception = std::current_exception();
+        }
+    }
+
+    QPromiseError(const std::exception_ptr& exception)
+        : exception(exception)
+    { }
+
+    void rethrow() const
+    {
+        std::rethrow_exception(exception);
+    }
+
+private:
+    std::exception_ptr exception;
+};
+
+} // namespace QtPromise
+
+#endif // _QTPROMISE_QPROMISEERROR_H
diff --git a/src/qtpromise/qpromiseglobal.h b/src/qtpromise/qpromiseglobal.h
index bfd1e46..94bea65 100644
--- a/src/qtpromise/qpromiseglobal.h
+++ b/src/qtpromise/qpromiseglobal.h
@@ -6,6 +6,7 @@
 
 // STL
 #include <functional>
+#include <array>
 
 namespace QtPromisePrivate
 {
@@ -13,6 +14,22 @@ namespace QtPromisePrivate
 template <typename T>
 using Unqualified = typename std::remove_cv<typename std::remove_reference<T>::type>::type;
 
+/*!
+ * \struct HasCallOperator
+ * http://stackoverflow.com/a/5839442
+ */
+template <typename T>
+struct HasCallOperator
+{
+    template <class U>
+    static auto check(const U* u)
+        -> decltype(&U::operator(), char(0));
+
+    static std::array<char, 2> check(...);
+
+    static const bool value = (sizeof(check((T*)0)) == 1);
+};
+
 /*!
  * \struct ArgsOf
  * http://stackoverflow.com/a/7943765
@@ -23,6 +40,7 @@ struct ArgsTraits
 {
     using types = std::tuple<Args...>;
     using first = typename std::tuple_element<0, types>::type;
+    static const size_t count = std::tuple_size<types>::value;
 };
 
 template <>
@@ -30,14 +48,16 @@ struct ArgsTraits<>
 {
     using types = std::tuple<>;
     using first = void;
+    static const size_t count = 0;
 };
 
-template <typename T>
-struct ArgsOf : public ArgsOf<decltype(&T::operator())>
+template <typename T, typename Enable = void>
+struct ArgsOf : public ArgsTraits<>
 { };
 
-template <>
-struct ArgsOf<std::nullptr_t> : public ArgsTraits<>
+template <typename T>
+struct ArgsOf<T, typename std::enable_if<HasCallOperator<T>::value>::type>
+    : public ArgsOf<decltype(&T::operator())>
 { };
 
 template <typename TReturn, typename... Args>
@@ -100,25 +120,6 @@ template <typename T>
 struct ArgsOf<const volatile T&&> : public ArgsOf<T>
 { };
 
-/*!
- * \fn to_exception_ptr
- */
-template <typename T>
-std::exception_ptr to_exception_ptr(const T& value)
-{
-    try {
-        throw value;
-    } catch(...) {
-        return std::current_exception();
-    }
-}
-
-template <>
-std::exception_ptr to_exception_ptr(const std::exception_ptr& value)
-{
-    return value;
-}
-
 } // namespace QtPromisePrivate
 
 #endif // ifndef _QTPROMISE_QPROMISEGLOBAL_H
diff --git a/src/qtpromise/qtpromise.pri b/src/qtpromise/qtpromise.pri
index ed263e4..a941102 100644
--- a/src/qtpromise/qtpromise.pri
+++ b/src/qtpromise/qtpromise.pri
@@ -3,4 +3,5 @@ HEADERS += \
     $$PWD/qpromise.inl \
     $$PWD/qpromise_p.h \
     $$PWD/qpromise_qfuture.inl \
+    $$PWD/qpromiseerror.h \
     $$PWD/qpromiseglobal.h
diff --git a/tests/auto/qpromise/tst_qpromise.cpp b/tests/auto/qpromise/tst_qpromise.cpp
index 9efbeee..231a986 100644
--- a/tests/auto/qpromise/tst_qpromise.cpp
+++ b/tests/auto/qpromise/tst_qpromise.cpp
@@ -5,15 +5,13 @@
 #include <QtTest>
 
 using namespace QtPromise;
-
-static const int ASYNC_DELAY = 256;
+using namespace QtPromisePrivate;
 
 class tst_qpromise: public QObject
 {
     Q_OBJECT
 
 private Q_SLOTS:
-
     void finallyReturns();
     void finallyThrows();
     void finallyDelayedFulfilled();
@@ -26,75 +24,68 @@ QTEST_MAIN(tst_qpromise)
 
 void tst_qpromise::finallyReturns()
 {
-    {
-        QPromise<int> p;
+    {   // fulfilled
         QVector<int> values;
-        auto next = p.finally([&values]() {
+        auto next = QPromise<int>::resolve(42).finally([&]() {
             values << 8;
-            return 16;
+            return 16; // ignored!
         });
 
-        p.fulfill(42);
-        next.then([&values](int r) {
+        next.then([&](int r) {
             values << r;
         }).wait();
 
-        QVERIFY(p.isFulfilled());
         QVERIFY(next.isFulfilled());
         QCOMPARE(values, QVector<int>({8, 42}));
     }
-    {
-        QPromise<int> p;
-        QVector<int> values;
-        auto next = p.finally([&values]() {
-            values << 8;
-            return 16;
+    {   // rejected
+        QString error;
+        int value = -1;
+        auto next = QPromise<int>([](const QPromiseResolve<int>) {
+            throw QString("foo");
+        }).finally([&]() {
+            value = 8;
+            return 16; // ignored!
         });
 
-        p.reject(QString("foo"));
-        next.then([&values](int r) {
-            values << r;
+        next.fail([&](const QString& err) {
+            error = err;
+            return 42;
         }).wait();
 
-        QVERIFY(p.isRejected());
         QVERIFY(next.isRejected());
-        QCOMPARE(values, QVector<int>({8}));
+        QCOMPARE(error, QString("foo"));
+        QCOMPARE(value, 8);
     }
 }
 
 void tst_qpromise::finallyThrows()
 {
-    {
-        QPromise<int> p;
+    {   // fulfilled
         QString error;
-        auto next = p.finally([]() {
+        auto next = QPromise<int>::resolve(42).finally([&]() {
             throw QString("bar");
         });
 
-        p.fulfill(42);
-        next.fail([&error](const QString& err) {
+        next.fail([&](const QString& err) {
             error = err;
             return 0;
         }).wait();
 
-        QVERIFY(p.isFulfilled());
         QVERIFY(next.isRejected());
         QCOMPARE(error, QString("bar"));
     }
-    {
-        QPromise<int> p;
+    {   // rejected
         QString error;
-        auto next = p.finally([]() {
+        auto next = QPromise<int>::reject(QString("foo")).finally([&]() {
             throw QString("bar");
         });
 
-        p.reject(QString("foo"));
-        next.fail([&error](const QString& err) {
+        next.fail([&](const QString& err) {
             error = err;
             return 0;
         }).wait();
 
-        QVERIFY(p.isRejected());
         QVERIFY(next.isRejected());
         QCOMPARE(error, QString("bar"));
     }
@@ -102,97 +93,89 @@ void tst_qpromise::finallyThrows()
 
 void tst_qpromise::finallyDelayedFulfilled()
 {
-    {
-        QPromise<int> p0;
+    {   // fulfilled
         QVector<int> values;
-        auto next = p0.finally([&values]() {
-            QPromise<int> p1;
-            QTimer::singleShot(ASYNC_DELAY, [p1, &values]() mutable {
-                values << 64;
-                p1.fulfill(16);
+        auto next = QPromise<int>::resolve(42).finally([&]() {
+            QPromise<int> p([&](const QPromiseResolve<int>& resolve) {
+                qtpromise_defer([=, &values]() {
+                    values << 64;
+                    resolve(16); // ignored!
+                });
             });
 
             values << 8;
-            return p1;
+            return p;
         });
 
-        p0.fulfill(42);
-        next.then([&values](int r) {
+        next.then([&](int r) {
             values << r;
         }).wait();
 
-        QVERIFY(p0.isFulfilled());
         QVERIFY(next.isFulfilled());
         QCOMPARE(values, QVector<int>({8, 64, 42}));
     }
-    {
-        QPromise<int> p0;
+    {   // rejected
+        QString error;
         QVector<int> values;
-        auto next = p0.finally([&values]() {
-            QPromise<int> p1;
-            QTimer::singleShot(ASYNC_DELAY, [p1, &values]() mutable {
-                values << 64;
-                p1.fulfill(16);
+        auto next = QPromise<int>::reject(QString("foo")).finally([&]() {
+            QPromise<int> p([&](const QPromiseResolve<int>& resolve) {
+                qtpromise_defer([=, &values]() {
+                    values << 64;
+                    resolve(16); // ignored!
+                });
             });
 
             values << 8;
-            return p1;
+            return p;
         });
 
-        p0.reject(QString("foo"));
-        next.then([&values](int r) {
+        next.then([&](int r) {
             values << r;
+        }, [&](const QString& err) {
+            error = err;
         }).wait();
 
-        QVERIFY(p0.isRejected());
         QVERIFY(next.isRejected());
+        QCOMPARE(error, QString("foo"));
         QCOMPARE(values, QVector<int>({8, 64}));
     }
 }
 
 void tst_qpromise::finallyDelayedRejected()
 {
-    {
-        QPromise<int> p0;
+    {   // fulfilled
         QString error;
-        auto next = p0.finally([]() {
-            QPromise<int> p1;
-            QTimer::singleShot(ASYNC_DELAY, [p1]() mutable {
-                p1.reject(QString("bar"));
+        auto next = QPromise<int>::resolve(42).finally([]() {
+            return QPromise<int>([](const QPromiseResolve<int>&, const QPromiseReject<int>& reject) {
+                qtpromise_defer([=]() {
+                    reject(QString("bar"));
+                });
             });
-
-            return p1;
         });
 
-        p0.fulfill(42);
-        next.fail([&error](const QString& err) {
+        next.fail([&](const QString& err) {
             error = err;
             return 0;
         }).wait();
 
-        QVERIFY(p0.isFulfilled());
         QVERIFY(next.isRejected());
         QCOMPARE(error, QString("bar"));
     }
-    {
-        QPromise<int> p0;
+    {   // rejected
         QString error;
-        auto next = p0.finally([]() {
-            QPromise<int> p1;
-            QTimer::singleShot(ASYNC_DELAY, [p1]() mutable {
-                p1.reject(QString("bar"));
+        auto next = QPromise<int>::reject(QString("foo")).finally([]() {
+            return QPromise<int>([](const QPromiseResolve<int>&, const QPromiseReject<int>& reject) {
+                qtpromise_defer([=]() {
+                    reject(QString("bar"));
+                });
             });
-
-            return p1;
         });
 
-        p0.reject(QString("foo"));
-        next.fail([&error](const QString& err) {
+        next.fail([&](const QString& err) {
             error = err;
             return 0;
         }).wait();
 
-        QVERIFY(p0.isRejected());
         QVERIFY(next.isRejected());
         QCOMPARE(error, QString("bar"));
     }
diff --git a/tests/auto/requirements/tst_requirements.cpp b/tests/auto/requirements/tst_requirements.cpp
index 8832b5e..f6d0110 100644
--- a/tests/auto/requirements/tst_requirements.cpp
+++ b/tests/auto/requirements/tst_requirements.cpp
@@ -5,6 +5,7 @@
 #include <QtTest>
 
 using namespace QtPromise;
+using namespace QtPromisePrivate;
 
 // https://promisesaplus.com/#requirements
 class tst_requirements: public QObject
@@ -37,115 +38,151 @@ QTEST_MAIN(tst_requirements)
 void tst_requirements::statePending()
 {
     // 2.1.1. When pending, a promise:
+    // 2.1.1.1. may transition to either the fulfilled state
     {
-        QPromise<> p;
+        QPromise<int> p([&](const QPromiseResolve<int>& resolve) {
+            qtpromise_defer([=]() { resolve(42); });
+        });
+
         QVERIFY(p.isPending());
         QVERIFY(!p.isFulfilled());
         QVERIFY(!p.isRejected());
-    }
 
-    // 2.1.1.1. may transition to either the fulfilled state
-    {
-        QPromise<> p;
-        p.fulfill();
+        p.wait();
+
         QVERIFY(!p.isPending());
         QVERIFY(p.isFulfilled());
+        QVERIFY(!p.isRejected());
     }
 
     // 2.1.1.1. ... or the rejected state
-    {
-        QPromise<> p;
-        p.reject("foo");
+   {
+        QPromise<int> p([&](const QPromiseResolve<int>&, const QPromiseReject<int>& reject) {
+            qtpromise_defer([=]() { reject(QString("foo")); });
+        });
+
+        QVERIFY(p.isPending());
+        QVERIFY(!p.isFulfilled());
+        QVERIFY(!p.isRejected());
+
+        p.wait();
+
         QVERIFY(!p.isPending());
+        QVERIFY(!p.isFulfilled());
         QVERIFY(p.isRejected());
     }
 }
 
 void tst_requirements::stateFulfilled()
 {
-    QVector<int> values;
-    auto log_value = [&values](int res) { values << res; };
-
-    QPromise<int> p;
-    QVERIFY(p.isPending());
+    QString error;
+    int value = -1;
 
     // 2.1.2. When fulfilled, a promise:
-    p.fulfill(42).then(log_value).wait();
+    QPromise<int> p([](
+        const QPromiseResolve<int>& resolve,
+        const QPromiseReject<int>& reject) {
+        qtpromise_defer([=]() {
+            // 2.1.2.2. must have a value, which must not change.
+            resolve(42);
+            resolve(43);
+
+            // 2.1.2.1. must not transition to any other state.
+            reject(QString("foo"));
+        });
+    });
+
+    QVERIFY(p.isPending());
+
+    p.then([&](int res) {
+        value = res;
+    }, [&](const QString& err) {
+        error = err;
+    }).wait();
+
     QVERIFY(p.isFulfilled());
     QVERIFY(!p.isRejected());
-
-    // 2.1.2.1. must not transition to any other state.
-    p.reject("foo").then(log_value).wait();
-    QVERIFY(p.isFulfilled());
-    QVERIFY(!p.isRejected());
-
-    // 2.1.2.2. must have a value, which must not change.
-    p.fulfill(51).then(log_value).wait();
-    QVERIFY(p.isFulfilled());
-    QVERIFY(!p.isRejected());
-
-    QCOMPARE(values, QVector<int>({42, 42, 42}));
+    QVERIFY(error.isEmpty());
+    QCOMPARE(value, 42);
 }
 
 void tst_requirements::stateRejected()
 {
-    QStringList errors;
-    auto log_error = [&errors](const QString& err) { errors << err; return -1; };
-
-    QPromise<int> p;
-    QVERIFY(p.isPending());
+    QString error;
+    int value = -1;
 
     // 2.1.3 When rejected, a promise:
-    p.reject(QString("foo")).then(nullptr, log_error).wait();
+    QPromise<int> p([](
+        const QPromiseResolve<int>& resolve,
+        const QPromiseReject<int>& reject) {
+        qtpromise_defer([=]() {
+            // 2.1.3.2. must have a reason, which must not change.
+            reject(QString("foo"));
+            reject(QString("bar"));
+
+            // 2.1.3.1. must not transition to any other state.
+            resolve(42);
+        });
+    });
+
+    QVERIFY(p.isPending());
+
+    p.then([&](int res) {
+        value = res;
+    }, [&](const QString& err) {
+        error = err;
+    }).wait();
+
     QVERIFY(!p.isFulfilled());
     QVERIFY(p.isRejected());
-
-    // 2.1.3.1. must not transition to any other state.
-    p.fulfill(51).then(nullptr, log_error).wait();
-    QVERIFY(!p.isFulfilled());
-    QVERIFY(p.isRejected());
-
-    // 2.1.3.2. must have a reason, which must not change.
-    p.reject(QString("bar")).then(nullptr, log_error).wait();
-    QVERIFY(!p.isFulfilled());
-    QVERIFY(p.isRejected());
-
-    QCOMPARE(errors, QStringList({"foo", "foo", "foo"}));
+    QCOMPARE(error, QString("foo"));
+    QCOMPARE(value, -1);
 }
 
 void tst_requirements::thenArguments()
 {
-    // Both onFulfilled and onRejected are given
+    // 2.2.1. Both onFulfilled and onRejected are given
     {
-        int value = 0;
-        QPromise<int> p;
-        p.fulfill(42).then([&value](int res) { value = res; }, [](const QString&){}).wait();
-        QVERIFY(p.isFulfilled());
+        QString error;
+        int value = -1;
+        QPromise<int>::resolve(42).then(
+            [&](int res) { value = res; },
+            [&](const QString& err) { error = err; }
+        ).wait();
+
+        QVERIFY(error.isEmpty());
         QCOMPARE(value, 42);
     }
     {
         QString error;
-        QPromise<int> p;
-        p.reject(QString("foo")).then([](int) {}, [&error](const QString& err){ error = err; }).wait();
-        QVERIFY(p.isRejected());
+        int value = -1;
+        QPromise<int>::reject(QString("foo")).then(
+            [&](int res) { value = res; },
+            [&](const QString& err){ error = err; }
+        ).wait();
+
         QCOMPARE(error, QString("foo"));
+        QCOMPARE(value, -1);
     }
 
     // 2.2.1. onFulfilled is an optional arguments:
     {
         QString error;
-        QPromise<int> p;
-        p.reject(QString("foo")).then(nullptr, [&error](const QString& err){ error = err; return 42; }).wait();
-        QVERIFY(p.isRejected());
+        QPromise<int>::reject(QString("foo")).then(
+            nullptr,
+            [&](const QString& err){ error = err; return 42; }
+        ).wait();
+
         QCOMPARE(error, QString("foo"));
     }
 
     // 2.2.1. onRejected is an optional arguments:
     {
-        int value = 0;
-        QPromise<int> p;
-        p.fulfill(42).then([&value](int res) { value = res; }).wait();
-        QVERIFY(p.isFulfilled());
+        int value = -1;
+        QPromise<int>::resolve(42).then(
+            [&value](int res) { value = res; }
+        ).wait();
+
         QCOMPARE(value, 42);
     }
 
@@ -157,55 +194,57 @@ void tst_requirements::thenArguments()
 void tst_requirements::thenOnFulfilled()
 {
     // 2.2.2. If onFulfilled is a function:
-
-    QPromise<int> p0;
     QVector<int> values;
-    auto p1 = p0.then([&values](int res) { values << res; }); // @TODO .wait(10)
+    QPromise<int> p0([](const QPromiseResolve<int>& resolve) {
+        qtpromise_defer([=]() {
+            // 2.2.2.3. it must not be called more than once
+            resolve(42);
+            resolve(43);
+        });
+    });
+
+    auto p1 = p0.then([&](int res) { values << res; });
 
     // 2.2.2.2. it must not be called before promise is fulfilled.
     QVERIFY(p0.isPending());
     QVERIFY(p1.isPending());
     QVERIFY(values.isEmpty());
 
+    p1.wait();
+
     // 2.2.2.1. it must be called after promise is fulfilled,
     // with promise’s value as its first argument.
-    p0.fulfill(42);
-    p1.wait();
     QVERIFY(p0.isFulfilled());
     QVERIFY(p1.isFulfilled());
     QCOMPARE(values, QVector<int>({42}));
-
-    // 2.2.2.3. it must not be called more than once
-    p0.fulfill(12);
-    p1.wait();
-    QCOMPARE(values, QVector<int>({42}));
 }
 
 void tst_requirements::thenOnRejected()
 {
     // 2.2.3. If onRejected is a function:
-
-    QPromise<> p0;
     QStringList errors;
-    auto p1 = p0.then(nullptr, [&errors](const QString& err) { errors << err; }); // @TODO .wait(10)
+    QPromise<void> p0([](const QPromiseResolve<void>&, const QPromiseReject<void>& reject) {
+        qtpromise_defer([=]() {
+            // 2.2.3.3. it must not be called more than once.
+            reject(QString("foo"));
+            reject(QString("bar"));
+        });
+    });
+
+    auto p1 = p0.then(nullptr, [&](const QString& err) { errors << err; });
 
     // 2.2.3.2. it must not be called before promise is rejected.
     QVERIFY(p0.isPending());
     QVERIFY(p1.isPending());
     QVERIFY(errors.isEmpty());
 
+    p1.wait();
+
     // 2.2.3.1. it must be called after promise is rejected,
     // with promise’s reason as its first argument.
-    p0.reject(QString("foo"));
-    p1.wait();
     QVERIFY(p0.isRejected());
     QVERIFY(p1.isFulfilled());
     QCOMPARE(errors, QStringList({"foo"}));
-
-    // 2.2.2.3. it must not be called more than once.
-    p0.reject(12);
-    p1.wait();
-    QCOMPARE(errors, QStringList({"foo"}));
 }
 
 void tst_requirements::thenAsynchronous()
@@ -214,14 +253,13 @@ void tst_requirements::thenAsynchronous()
     // stack contains only platform code (ie. executed asynchronously, after the event
     // loop turn in which then is called, and with a fresh stack).
 
-    int value = 0;
-    QPromise<int> p0;
-    p0.fulfill(42);
+    int value = -1;
+    auto p0 = QPromise<int>::resolve(42);
     QVERIFY(p0.isFulfilled());
 
-    auto p1 = p0.then([&value](int res){ value = res; });
+    auto p1 = p0.then([&](int res){ value = res; });
     QVERIFY(p1.isPending());
-    QCOMPARE(value, 0);
+    QCOMPARE(value, -1);
 
     p1.wait();
     QVERIFY(p1.isFulfilled());
@@ -235,32 +273,38 @@ void tst_requirements::thenMultipleCalls()
     // 2.2.6.1. If/when promise is fulfilled, all respective onFulfilled callbacks
     // must execute in the order of their originating calls to then:
     {
-        QPromise<int> p;
         QVector<int> values;
-        auto all = qPromiseAll(QVector<QPromise<void> >{
-            p.then([&values](int r) { values << r + 1; }),
-            p.then([&values](int r) { values << r + 2; }),
-            p.then([&values](int r) { values << r + 3; })
+        QPromise<int> p([](const QPromiseResolve<int>& resolve) {
+            qtpromise_defer([=]() {
+                resolve(42);
+            });
         });
 
-        p.fulfill(42);
-        all.wait();
+        qPromiseAll(QVector<QPromise<void> >{
+            p.then([&](int r) { values << r + 1; }),
+            p.then([&](int r) { values << r + 2; }),
+            p.then([&](int r) { values << r + 3; })
+        }).wait();
+
         QCOMPARE(values, QVector<int>({43, 44, 45}));
     }
 
     // 2.2.6.2. If/when promise is rejected, all respective onRejected callbacks
     // must execute in the order of their originating calls to then:
     {
-        QPromise<int> p;
         QVector<int> values;
-        auto all = qPromiseAll(QVector<QPromise<int> >{
-            p.then(nullptr, [&values](int r) { values << r + 1; return r + 1; }),
-            p.then(nullptr, [&values](int r) { values << r + 2; return r + 2; }),
-            p.then(nullptr, [&values](int r) { values << r + 3; return r + 3; })
+        QPromise<int> p([](const QPromiseResolve<int>&, const QPromiseReject<int>& reject) {
+            qtpromise_defer([=]() {
+                reject(8);
+            });
         });
 
-        p.reject(8);
-        all.wait();
+        qPromiseAll(QVector<QPromise<int> >{
+            p.then(nullptr, [&](int r) { values << r + 1; return r + 1; }),
+            p.then(nullptr, [&](int r) { values << r + 2; return r + 2; }),
+            p.then(nullptr, [&](int r) { values << r + 3; return r + 3; })
+        }).wait();
+
         QCOMPARE(values, QVector<int>({9, 10, 11}));
     }
 }
@@ -269,8 +313,8 @@ void tst_requirements::thenHandlers()
 {
     // 2.2.7. then must return a promise: p2 = p1.then(onFulfilled, onRejected);
     {
-        QPromise<int> p1;
         auto handler = [](){ return 42; };
+        auto p1 = QPromise<int>::resolve(42);
         Q_STATIC_ASSERT((std::is_same<decltype(p1.then(handler, nullptr)), QPromise<int> >::value));
         Q_STATIC_ASSERT((std::is_same<decltype(p1.then(nullptr, handler)), QPromise<int> >::value));
         Q_STATIC_ASSERT((std::is_same<decltype(p1.then(handler, handler)), QPromise<int> >::value));
@@ -283,19 +327,21 @@ void tst_requirements::thenHandlers()
     // p2 must be rejected with e as the reason.
     {
         QString reason;
-        QPromise<int> p1;
-        auto p2 = p1.fulfill(42).then([](){ throw QString("foo"); });
-        p2.then(nullptr, [&reason](const QString& e) { reason = e; }).wait();
+        auto p1 = QPromise<int>::resolve(42);
+        auto p2 = p1.then([](){ throw QString("foo"); });
+        p2.then(nullptr, [&](const QString& e) { reason = e; }).wait();
 
+        QVERIFY(p1.isFulfilled());
         QVERIFY(p2.isRejected());
         QCOMPARE(reason, QString("foo"));
     }
     {
         QString reason;
-        QPromise<int> p1;
-        auto p2 = p1.reject(QString("foo")).then(nullptr, [](){ throw QString("bar"); return 42; });
-        p2.then(nullptr, [&reason](const QString& e) { reason = e; return 0; }).wait();
+        auto p1 = QPromise<int>::reject(QString("foo"));
+        auto p2 = p1.then(nullptr, [](){ throw QString("bar"); return 42; });
+        p2.then(nullptr, [&](const QString& e) { reason = e; return 0; }).wait();
 
+        QVERIFY(p1.isRejected());
         QVERIFY(p2.isRejected());
         QCOMPARE(reason, QString("bar"));
     }
@@ -304,11 +350,12 @@ void tst_requirements::thenHandlers()
     // p2 must be fulfilled with the same value as promise1
     {
         QString value;
-        QPromise<QString> p1;
-        auto p2 = p1.fulfill("42").then(nullptr, [](){ return QString(); });
+        auto p1 = QPromise<QString>::resolve("42");
+        auto p2 = p1.then(nullptr, [](){ return QString(); });
         Q_STATIC_ASSERT((std::is_same<decltype(p2), QPromise<QString> >::value));
-        p2.then([&value](const QString& e) { value = e; }).wait();
+        p2.then([&](const QString& e) { value = e; }).wait();
 
+        QVERIFY(p1.isFulfilled());
         QVERIFY(p2.isFulfilled());
         QCOMPARE(value, QString("42"));
     }