From e3f0f054af122655c56145dc134e5a37285bc6ec Mon Sep 17 00:00:00 2001
From: Simon Brunel <simonbrunel@users.noreply.github.com>
Date: Mon, 25 Feb 2019 10:07:06 +0100
Subject: [PATCH] Implement QPromise<Sequence<T>>::reduce(reducer,
 initialValue)

Iterates over all the promise values (i.e. `Sequence<T>`) and reduces the sequence to a single value using the given `reducer` function and an optional `initialValue`. Also provide a static helper to directly reduce values (`QtPromise::reduce(values, reducer, initialValue)`).
---
 docs/.vuepress/config.js                      |   2 +
 docs/qtpromise/api-reference.md               |   2 +
 docs/qtpromise/helpers/reduce.md              |  48 ++++
 docs/qtpromise/qpromise/reduce.md             |  58 ++++
 docs/qtpromise/qtconcurrent.md                |   2 +-
 src/qtpromise/qpromise.h                      |  15 +-
 src/qtpromise/qpromise.inl                    |  20 ++
 src/qtpromise/qpromise_p.h                    |  11 +-
 src/qtpromise/qpromisehelpers.h               |  59 +++-
 tests/auto/qtpromise/helpers/helpers.pro      |   1 +
 .../auto/qtpromise/helpers/reduce/reduce.pro  |   4 +
 .../qtpromise/helpers/reduce/tst_reduce.cpp   | 271 ++++++++++++++++++
 tests/auto/qtpromise/qpromise/qpromise.pro    |   1 +
 .../auto/qtpromise/qpromise/reduce/reduce.pro |   4 +
 .../qtpromise/qpromise/reduce/tst_reduce.cpp  | 271 ++++++++++++++++++
 15 files changed, 763 insertions(+), 6 deletions(-)
 create mode 100644 docs/qtpromise/helpers/reduce.md
 create mode 100644 docs/qtpromise/qpromise/reduce.md
 create mode 100644 tests/auto/qtpromise/helpers/reduce/reduce.pro
 create mode 100644 tests/auto/qtpromise/helpers/reduce/tst_reduce.cpp
 create mode 100644 tests/auto/qtpromise/qpromise/reduce/reduce.pro
 create mode 100644 tests/auto/qtpromise/qpromise/reduce/tst_reduce.cpp

diff --git a/docs/.vuepress/config.js b/docs/.vuepress/config.js
index a6c1e31..085a4dd 100644
--- a/docs/.vuepress/config.js
+++ b/docs/.vuepress/config.js
@@ -33,6 +33,7 @@ module.exports = {
                     'qtpromise/qpromise/ispending',
                     'qtpromise/qpromise/isrejected',
                     'qtpromise/qpromise/map',
+                    'qtpromise/qpromise/reduce',
                     'qtpromise/qpromise/tap',
                     'qtpromise/qpromise/tapfail',
                     'qtpromise/qpromise/then',
@@ -51,6 +52,7 @@ module.exports = {
                     'qtpromise/helpers/each',
                     'qtpromise/helpers/filter',
                     'qtpromise/helpers/map',
+                    'qtpromise/helpers/reduce',
                     'qtpromise/helpers/resolve'
                 ]
             },
diff --git a/docs/qtpromise/api-reference.md b/docs/qtpromise/api-reference.md
index 62693ef..d8ae8f2 100644
--- a/docs/qtpromise/api-reference.md
+++ b/docs/qtpromise/api-reference.md
@@ -12,6 +12,7 @@
 * [`QPromise<T>::isPending`](qpromise/ispending.md)
 * [`QPromise<T>::isRejected`](qpromise/isrejected.md)
 * [`QPromise<T>::map`](qpromise/map.md)
+* [`QPromise<T>::reduce`](qpromise/reduce.md)
 * [`QPromise<T>::tap`](qpromise/tap.md)
 * [`QPromise<T>::tapFail`](qpromise/tapfail.md)
 * [`QPromise<T>::then`](qpromise/then.md)
@@ -31,6 +32,7 @@
 * [`QtPromise::each`](helpers/each.md)
 * [`QtPromise::filter`](helpers/filter.md)
 * [`QtPromise::map`](helpers/map.md)
+* [`QtPromise::reduce`](helpers/reduce.md)
 * [`QtPromise::resolve`](helpers/resolve.md)
 
 ## Exceptions
diff --git a/docs/qtpromise/helpers/reduce.md b/docs/qtpromise/helpers/reduce.md
new file mode 100644
index 0000000..fc81984
--- /dev/null
+++ b/docs/qtpromise/helpers/reduce.md
@@ -0,0 +1,48 @@
+---
+title: reduce
+---
+
+# QtPromise::reduce
+
+*Since: 0.5.0*
+
+```cpp
+QPromise::reduce(Sequence<T|QPromise<T>> values, Reducer reducer) -> QPromise<T>
+QPromise::reduce(Sequence<T|QPromise<T>> values, Reducer reducer, R|QPromise<R> initialValue) -> QPromise<R>
+
+// With:
+// - Sequence: STL compatible container (e.g. QVector, etc.)
+// - Reducer: Function(T|R accumulator, T item, int index) -> R|QPromise<R>
+```
+
+Iterates over `values` and [reduces the sequence to a single value](https://en.wikipedia.org/wiki/Fold_%28higher-order_function%29) using the given `reducer` function and an optional `initialValue`. The type returned by `reducer` determines the type of the `output` promise. If `reducer` throws, `output` is rejected with the new exception.
+
+If `reducer` returns a promise (or `QFuture`), then the result of the promise is awaited, before continuing with next iteration. If any promise in the `values` sequence is rejected or a promise returned by the `reducer` function is rejected, `output` immediately rejects with the error of the promise that rejected.
+
+```cpp
+// Concatenate the content of the given files, read asynchronously
+auto output = QtPromise::reduce(QList<QUrl>{
+    "file:f0.txt",   // contains "foo"
+    "file:f1.txt",   // contains "bar"
+    "file:f2.txt"    // contains "42"
+}, [](const QString& acc, const QString& cur, int idx) {
+    return readAsync(cur).then([=](const QString& res) {
+        return QString("%1;%2:%3").arg(acc).arg(idx).arg(res);
+    });
+}, QString("index:text"));
+
+// 'output' resolves as soon as all promises returned by
+// 'reducer' are fulfilled or at least one is rejected.
+
+// 'output' type: QPromise<QString>
+output.then([](const QString& res) {
+    // res == "index:text;0:foo;1:bar;2:42"
+    // {...}
+});
+```
+
+::: warning IMPORTANT
+The first time `reducer` is called, if no `initialValue` is provided, `accumulator` will be equal to the first value in the sequence, and `currentValue` to the second one (thus index will be `1`).
+:::
+
+See also: [`QPromise<T>::reduce`](../qpromise/reduce.md)
diff --git a/docs/qtpromise/qpromise/reduce.md b/docs/qtpromise/qpromise/reduce.md
new file mode 100644
index 0000000..0822db2
--- /dev/null
+++ b/docs/qtpromise/qpromise/reduce.md
@@ -0,0 +1,58 @@
+---
+title: .reduce
+---
+
+# QPromise::reduce
+
+*Since: 0.5.0*
+
+```cpp
+QPromise<Sequence<T>>::reduce(Reducer reducer) -> QPromise<T>
+QPromise<Sequence<T>>::reduce(Reducer reducer, R|QPromise<R> initialValue) -> QPromise<R>
+
+// With:
+// - Sequence: STL compatible container (e.g. QVector, etc.)
+// - Reducer: Function(T|R accumulator, T item, int index) -> R|QPromise<R>
+```
+
+::: warning IMPORTANT
+This method only applies to promise with sequence value.
+:::
+
+Iterates over all the promise values (i.e. `Sequence<T>`) and [reduces the sequence to a single value](https://en.wikipedia.org/wiki/Fold_%28higher-order_function%29) using the given `reducer` function and an optional `initialValue`. The type returned by `reducer` determines the type of the `output` promise.
+
+See [`QtPromise::reduce`](../helpers/reduce.md) for details, this method is provided for convenience and is similar to:
+
+```cpp
+promise.then([](const T& values) {
+    return QtPromise::reduce(values, reducer, initialValue);
+})
+```
+
+For example:
+
+```cpp
+auto input = QtPromise::resolve(QList<QUrl>{
+    "file:f0.txt",   // contains "foo"
+    "file:f1.txt",   // contains "bar"
+    "file:f2.txt"    // contains "42"
+});
+
+// Concatenate the content of the given files, read asynchronously
+auto output = input.reduce([](const QString& acc, const QString& cur, int idx) {
+    return readAsync(cur).then([=](const QString& res) {
+        return QString("%1;%2:%3").arg(acc).arg(idx).arg(res);
+    });
+}, QString("index:text"));
+
+// 'output' resolves as soon as all promises returned by
+// 'reducer' are fulfilled or at least one is rejected.
+
+// 'output' type: QPromise<QString>
+output.then([](const QString& res) {
+    // res == "index:text;0:foo;1:bar;2:42"
+    // {...}
+});
+```
+
+See also: [`QtPromise::reduce`](../helpers/reduce.md)
diff --git a/docs/qtpromise/qtconcurrent.md b/docs/qtpromise/qtconcurrent.md
index cef0c3d..50c802e 100644
--- a/docs/qtpromise/qtconcurrent.md
+++ b/docs/qtpromise/qtconcurrent.md
@@ -46,7 +46,7 @@ The `output` promise is resolved when the `QFuture` is [finished](https://doc.qt
 
 ## Error
 
-Exceptions thrown from a QtConcurrent thread reject the associated promise with the exception as the reason. Note that if you throw an exception that is not a subclass of `QException`, the promise with be rejected with [`QUnhandledException`](https://doc.qt.io/qt-5/qunhandledexception.html#details) (this restriction only applies to exceptions thrown from a QtConcurrent thread, [read more](https://doc.qt.io/qt-5/qexception.html#details)).
+Exceptions thrown from a QtConcurrent thread reject the associated promise with the exception as the reason. Note that if you throw an exception that is not a subclass of `QException`, the promise will be rejected with [`QUnhandledException`](https://doc.qt.io/qt-5/qunhandledexception.html#details) (this restriction only applies to exceptions thrown from a QtConcurrent thread, [read more](https://doc.qt.io/qt-5/qexception.html#details)).
 
 ```cpp
 QPromise<int> promise = ...
diff --git a/src/qtpromise/qpromise.h b/src/qtpromise/qpromise.h
index d105b42..653da62 100644
--- a/src/qtpromise/qpromise.h
+++ b/src/qtpromise/qpromise.h
@@ -76,6 +76,7 @@ public: // STATIC
 protected:
     friend struct QtPromisePrivate::PromiseFulfill<QPromise<T>>;
     friend class QtPromisePrivate::PromiseResolver<T>;
+    friend struct QtPromisePrivate::PromiseInspect;
 
     QExplicitlySharedDataPointer<QtPromisePrivate::PromiseData<T>> m_d;
 };
@@ -88,15 +89,25 @@ public:
     QPromise(F&& resolver): QPromiseBase<T>(std::forward<F>(resolver)) { }
 
     template <typename Functor>
-    inline QPromise<T> each(Functor fn);
+    inline QPromise<T>
+    each(Functor fn);
 
     template <typename Functor>
-    inline QPromise<T> filter(Functor fn);
+    inline QPromise<T>
+    filter(Functor fn);
 
     template <typename Functor>
     inline typename QtPromisePrivate::PromiseMapper<T, Functor>::PromiseType
     map(Functor fn);
 
+    template <typename Functor, typename Input>
+    inline typename QtPromisePrivate::PromiseDeduce<Input>::Type
+    reduce(Functor fn, Input initial);
+
+    template <typename Functor, typename U = T>
+    inline typename QtPromisePrivate::PromiseDeduce<typename U::value_type>::Type
+    reduce(Functor fn);
+
 public: // STATIC
 
     // DEPRECATED (remove at version 1)
diff --git a/src/qtpromise/qpromise.inl b/src/qtpromise/qpromise.inl
index 3ac7c64..fc1f485 100644
--- a/src/qtpromise/qpromise.inl
+++ b/src/qtpromise/qpromise.inl
@@ -197,6 +197,26 @@ QPromise<T>::map(Functor fn)
     });
 }
 
+template <typename T>
+template <typename Functor, typename Input>
+inline typename QtPromisePrivate::PromiseDeduce<Input>::Type
+QPromise<T>::reduce(Functor fn, Input initial)
+{
+    return this->then([=](const T& values) {
+        return QtPromise::reduce(values, fn, initial);
+    });
+}
+
+template <typename T>
+template <typename Functor, typename U>
+inline typename QtPromisePrivate::PromiseDeduce<typename U::value_type>::Type
+QPromise<T>::reduce(Functor fn)
+{
+    return this->then([=](const T& values) {
+        return QtPromise::reduce(values, fn);
+    });
+}
+
 template <typename T>
 template <template <typename, typename...> class Sequence, typename ...Args>
 inline QPromise<QVector<T>> QPromise<T>::all(const Sequence<QPromise<T>, Args...>& promises)
diff --git a/src/qtpromise/qpromise_p.h b/src/qtpromise/qpromise_p.h
index f8d8556..1231cd4 100644
--- a/src/qtpromise/qpromise_p.h
+++ b/src/qtpromise/qpromise_p.h
@@ -558,6 +558,15 @@ protected:
     }
 };
 
-} // namespace QtPromise
+struct PromiseInspect
+{
+    template <typename T>
+    static inline PromiseData<T>* get(const QtPromise::QPromise<T>& p)
+    {
+        return p.m_d.data();
+    }
+};
+
+} // namespace QtPromisePrivate
 
 #endif // QTPROMISE_QPROMISE_H
diff --git a/src/qtpromise/qpromisehelpers.h b/src/qtpromise/qpromisehelpers.h
index 8b9a0c4..0e2d404 100644
--- a/src/qtpromise/qpromisehelpers.h
+++ b/src/qtpromise/qpromisehelpers.h
@@ -169,7 +169,8 @@ connect(const Sender* sender, FSignal fsignal, RSignal rsignal)
 }
 
 template <typename Sequence, typename Functor>
-static inline QPromise<Sequence> each(const Sequence& values, Functor&& fn)
+static inline QPromise<Sequence>
+each(const Sequence& values, Functor&& fn)
 {
     return QPromise<Sequence>::resolve(values).each(std::forward<Functor>(fn));
 }
@@ -200,7 +201,8 @@ map(const Sequence& values, Functor fn)
 }
 
 template <typename Sequence, typename Functor>
-static inline QPromise<Sequence> filter(const Sequence& values, Functor fn)
+static inline QPromise<Sequence>
+filter(const Sequence& values, Functor fn)
 {
     return QtPromise::map(values, fn)
         .then([=](const QVector<bool>& filters) {
@@ -219,6 +221,59 @@ static inline QPromise<Sequence> filter(const Sequence& values, Functor fn)
         });
 }
 
+template <typename T, template <typename...> class Sequence = QVector, typename Reducer, typename Input, typename ...Args>
+static inline typename QtPromisePrivate::PromiseDeduce<Input>::Type
+reduce(const Sequence<T, Args...>& values, Reducer fn, Input initial)
+{
+    using namespace QtPromisePrivate;
+    using PromiseType = typename PromiseDeduce<T>::Type;
+    using ValueType = typename PromiseType::Type;
+
+    int idx = 0;
+
+    auto promise = QtPromise::resolve(std::move(initial));
+    for (const auto& value : values) {
+        auto input = QtPromise::resolve(value);
+        promise = promise.then([=]() {
+            return input.then([=](const ValueType& cur) {
+                return fn(PromiseInspect::get(promise)->value().data(), cur, idx);
+            });
+        });
+
+        idx++;
+    }
+
+    return promise;
+}
+
+template <typename T, template <typename...> class Sequence = QVector, typename Reducer, typename ...Args>
+static inline typename QtPromisePrivate::PromiseDeduce<T>::Type
+reduce(const Sequence<T, Args...>& values, Reducer fn)
+{
+    using namespace QtPromisePrivate;
+    using PromiseType = typename PromiseDeduce<T>::Type;
+    using ValueType = typename PromiseType::Type;
+
+    Q_ASSERT(values.size());
+
+    int idx = 1;
+
+    auto it = values.begin();
+    auto promise = QtPromise::resolve(*it);
+    for (++it; it != values.end(); ++it) {
+        auto input = QtPromise::resolve(*it);
+        promise = promise.then([=]() {
+            return input.then([=](const ValueType& cur) {
+                return fn(PromiseInspect::get(promise)->value().data(), cur, idx);
+            });
+        });
+
+        idx++;
+    }
+
+    return promise;
+}
+
 // DEPRECATIONS (remove at version 1)
 
 template <typename... Args>
diff --git a/tests/auto/qtpromise/helpers/helpers.pro b/tests/auto/qtpromise/helpers/helpers.pro
index e7a7bac..7c46dd7 100644
--- a/tests/auto/qtpromise/helpers/helpers.pro
+++ b/tests/auto/qtpromise/helpers/helpers.pro
@@ -6,5 +6,6 @@ SUBDIRS += \
     each \
     filter \
     map \
+    reduce \
     reject \
     resolve
diff --git a/tests/auto/qtpromise/helpers/reduce/reduce.pro b/tests/auto/qtpromise/helpers/reduce/reduce.pro
new file mode 100644
index 0000000..ac34f12
--- /dev/null
+++ b/tests/auto/qtpromise/helpers/reduce/reduce.pro
@@ -0,0 +1,4 @@
+TARGET = tst_helpers_reduce
+SOURCES += $$PWD/tst_reduce.cpp
+
+include(../../qtpromise.pri)
diff --git a/tests/auto/qtpromise/helpers/reduce/tst_reduce.cpp b/tests/auto/qtpromise/helpers/reduce/tst_reduce.cpp
new file mode 100644
index 0000000..3c2dc7f
--- /dev/null
+++ b/tests/auto/qtpromise/helpers/reduce/tst_reduce.cpp
@@ -0,0 +1,271 @@
+#include "../../shared/data.h"
+#include "../../shared/utils.h"
+
+// QtPromise
+#include <QtPromise>
+
+// Qt
+#include <QtTest>
+
+using namespace QtPromise;
+
+class tst_helpers_reduce : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void emptySequence();
+    void regularValues();
+    void promiseValues();
+    void convertResultType();
+    void delayedInitialValue();
+    void delayedFulfilled();
+    void delayedRejected();
+    void functorThrows();
+    void sequenceTypes();
+};
+
+QTEST_MAIN(tst_helpers_reduce)
+#include "tst_reduce.moc"
+
+namespace {
+
+template <class Sequence>
+struct SequenceTester
+{
+    static void exec()
+    {
+        Sequence inputs{
+            QtPromise::resolve(4).delay(400),
+            QtPromise::resolve(6).delay(300),
+            QtPromise::resolve(8).delay(200)
+        };
+        QVector<int> v0;
+        QVector<int> v1;
+
+        auto p0 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+            v0 << acc << cur << idx;
+            return acc + cur + idx;
+        });
+        auto p1 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+            v1 << acc << cur << idx;
+            return acc + cur + idx;
+        }, QtPromise::resolve(2).delay(100));
+
+        Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+        Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+        QCOMPARE(p0.isPending(), true);
+        QCOMPARE(p1.isPending(), true);
+        QCOMPARE(waitForValue(p0, -1), 21);
+        QCOMPARE(waitForValue(p1, -1), 23);
+        QCOMPARE(v0, QVector<int>({4, 6, 1, 11, 8, 2}));
+        QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+    }
+};
+
+} // anonymous namespace
+
+void tst_helpers_reduce::emptySequence()
+{
+    bool called = false;
+
+    auto p = QtPromise::reduce(QVector<int>{}, [&](...) {
+        called = true;
+        return 43;
+    }, 42);
+
+    // NOTE(SB): reduce() on an empty sequence without an initial value is an error!
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<int>>::value));
+
+    QCOMPARE(waitForValue(p, -1), 42);
+    QCOMPARE(called, false);
+}
+
+void tst_helpers_reduce::regularValues()
+{
+    QVector<int> inputs{4, 6, 8};
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        return acc + cur + idx;
+    });
+    auto p1 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        return acc + cur + idx;
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForValue(p0, -1), 21);
+    QCOMPARE(waitForValue(p1, -1), 23);
+    QCOMPARE(v0, QVector<int>({4, 6, 1, 11, 8, 2}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+}
+
+void tst_helpers_reduce::promiseValues()
+{
+    QVector<QPromise<int>> inputs{
+        QtPromise::resolve(4).delay(400),
+        QtPromise::resolve(6).delay(300),
+        QtPromise::resolve(8).delay(200)
+    };
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        return acc + cur + idx;
+    });
+    auto p1 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        return acc + cur + idx;
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForValue(p0, -1), 21);
+    QCOMPARE(waitForValue(p1, -1), 23);
+    QCOMPARE(v0, QVector<int>({4, 6, 1, 11, 8, 2}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+}
+
+void tst_helpers_reduce::convertResultType()
+{
+    QVector<int> inputs{4, 6, 8};
+
+    auto p = QtPromise::reduce(inputs, [&](const QString& acc, int cur, int idx) {
+        return QString("%1:%2:%3").arg(acc).arg(cur).arg(idx);
+    }, QString("foo"));
+
+    // NOTE(SB): when no initial value is given, the result type is the sequence type.
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<QString>>::value));
+
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForValue(p, QString()), QString("foo:4:0:6:1:8:2"));
+}
+
+void tst_helpers_reduce::delayedInitialValue()
+{
+    QVector<int> values;
+
+    auto p = QtPromise::reduce(QVector<int>{4, 6, 8}, [&](int acc, int cur, int idx) {
+        values << acc << cur << idx;
+        return acc + cur + idx;
+    }, QtPromise::resolve(2).delay(100));
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<int>>::value));
+
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForValue(p, -1), 23);
+    QCOMPARE(values, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+}
+
+void tst_helpers_reduce::delayedFulfilled()
+{
+    QVector<int> inputs{4, 6, 8};
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        return QtPromise::resolve(acc + cur + idx).delay(100);
+    });
+    auto p1 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        return QtPromise::resolve(acc + cur + idx).delay(100);
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForValue(p0, -1), 21);
+    QCOMPARE(waitForValue(p1, -1), 23);
+    QCOMPARE(v0, QVector<int>({4, 6, 1, 11, 8, 2}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+}
+
+void tst_helpers_reduce::delayedRejected()
+{
+    QVector<int> inputs{4, 6, 8};
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        if (cur == 6) {
+            return QPromise<int>::reject(QString("foo"));
+        }
+        return QtPromise::resolve(acc + cur + idx);
+    });
+    auto p1 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        if (cur == 6) {
+            return QPromise<int>::reject(QString("bar"));
+        }
+        return QtPromise::resolve(acc + cur + idx);
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForError(p0, QString()), QString("foo"));
+    QCOMPARE(waitForError(p1, QString()), QString("bar"));
+    QCOMPARE(v0, QVector<int>({4, 6, 1}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1}));
+}
+
+void tst_helpers_reduce::functorThrows()
+{
+    QVector<int> inputs{4, 6, 8};
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        if (cur == 6) {
+            throw QString("foo");
+        }
+        return acc + cur + idx;
+    });
+    auto p1 = QtPromise::reduce(inputs, [&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        if (cur == 6) {
+            throw QString("bar");
+        }
+        return acc + cur + idx;
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForError(p0, QString()), QString("foo"));
+    QCOMPARE(waitForError(p1, QString()), QString("bar"));
+    QCOMPARE(v0, QVector<int>({4, 6, 1}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1}));
+}
+
+void tst_helpers_reduce::sequenceTypes()
+{
+    SequenceTester<QLinkedList<QPromise<int>>>::exec();
+    SequenceTester<QList<QPromise<int>>>::exec();
+    SequenceTester<QVector<QPromise<int>>>::exec();
+    SequenceTester<std::list<QPromise<int>>>::exec();
+    SequenceTester<std::vector<QPromise<int>>>::exec();
+}
diff --git a/tests/auto/qtpromise/qpromise/qpromise.pro b/tests/auto/qtpromise/qpromise/qpromise.pro
index a43dc53..aeff700 100644
--- a/tests/auto/qtpromise/qpromise/qpromise.pro
+++ b/tests/auto/qtpromise/qpromise/qpromise.pro
@@ -8,6 +8,7 @@ SUBDIRS += \
     finally \
     map \
     operators \
+    reduce \
     resolve \
     tap \
     tapfail \
diff --git a/tests/auto/qtpromise/qpromise/reduce/reduce.pro b/tests/auto/qtpromise/qpromise/reduce/reduce.pro
new file mode 100644
index 0000000..259eb95
--- /dev/null
+++ b/tests/auto/qtpromise/qpromise/reduce/reduce.pro
@@ -0,0 +1,4 @@
+TARGET = tst_qpromise_reduce
+SOURCES += $$PWD/tst_reduce.cpp
+
+include(../../qtpromise.pri)
diff --git a/tests/auto/qtpromise/qpromise/reduce/tst_reduce.cpp b/tests/auto/qtpromise/qpromise/reduce/tst_reduce.cpp
new file mode 100644
index 0000000..5b7047a
--- /dev/null
+++ b/tests/auto/qtpromise/qpromise/reduce/tst_reduce.cpp
@@ -0,0 +1,271 @@
+#include "../../shared/data.h"
+#include "../../shared/utils.h"
+
+// QtPromise
+#include <QtPromise>
+
+// Qt
+#include <QtTest>
+
+using namespace QtPromise;
+
+class tst_qpromise_reduce : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void emptySequence();
+    void regularValues();
+    void promiseValues();
+    void convertResultType();
+    void delayedInitialValue();
+    void delayedFulfilled();
+    void delayedRejected();
+    void functorThrows();
+    void sequenceTypes();
+};
+
+QTEST_MAIN(tst_qpromise_reduce)
+#include "tst_reduce.moc"
+
+namespace {
+
+template <class Sequence>
+struct SequenceTester
+{
+    static void exec()
+    {
+        Sequence inputs{
+            QtPromise::resolve(4).delay(400),
+            QtPromise::resolve(6).delay(300),
+            QtPromise::resolve(8).delay(200)
+        };
+        QVector<int> v0;
+        QVector<int> v1;
+
+        auto p0 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+            v0 << acc << cur << idx;
+            return acc + cur + idx;
+        });
+        auto p1 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+            v1 << acc << cur << idx;
+            return acc + cur + idx;
+        }, QtPromise::resolve(2).delay(100));
+
+        Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+        Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+        QCOMPARE(p0.isPending(), true);
+        QCOMPARE(p1.isPending(), true);
+        QCOMPARE(waitForValue(p0, -1), 21);
+        QCOMPARE(waitForValue(p1, -1), 23);
+        QCOMPARE(v0, QVector<int>({4, 6, 1, 11, 8, 2}));
+        QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+    }
+};
+
+} // anonymous namespace
+
+void tst_qpromise_reduce::emptySequence()
+{
+    bool called = false;
+
+    auto p = QtPromise::resolve(QVector<int>{}).reduce([&](...) {
+        called = true;
+        return 43;
+    }, 42);
+
+    // NOTE(SB): reduce() on an empty sequence without an initial value is an error!
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<int>>::value));
+
+    QCOMPARE(waitForValue(p, -1), 42);
+    QCOMPARE(called, false);
+}
+
+void tst_qpromise_reduce::regularValues()
+{
+    QVector<int> inputs{4, 6, 8};
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        return acc + cur + idx;
+    });
+    auto p1 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        return acc + cur + idx;
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForValue(p0, -1), 21);
+    QCOMPARE(waitForValue(p1, -1), 23);
+    QCOMPARE(v0, QVector<int>({4, 6, 1, 11, 8, 2}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+}
+
+void tst_qpromise_reduce::promiseValues()
+{
+    QVector<QPromise<int>> inputs{
+        QtPromise::resolve(4).delay(400),
+        QtPromise::resolve(6).delay(300),
+        QtPromise::resolve(8).delay(200)
+    };
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        return acc + cur + idx;
+    });
+    auto p1 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        return acc + cur + idx;
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForValue(p0, -1), 21);
+    QCOMPARE(waitForValue(p1, -1), 23);
+    QCOMPARE(v0, QVector<int>({4, 6, 1, 11, 8, 2}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+}
+
+void tst_qpromise_reduce::convertResultType()
+{
+    QVector<int> inputs{4, 6, 8};
+
+    auto p = QtPromise::resolve(inputs).reduce([&](const QString& acc, int cur, int idx) {
+        return QString("%1:%2:%3").arg(acc).arg(cur).arg(idx);
+    }, QString("foo"));
+
+    // NOTE(SB): when no initial value is given, the result type is the sequence type.
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<QString>>::value));
+
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForValue(p, QString()), QString("foo:4:0:6:1:8:2"));
+}
+
+void tst_qpromise_reduce::delayedInitialValue()
+{
+    QVector<int> values;
+
+    auto p = QtPromise::resolve(QVector<int>{4, 6, 8}).reduce([&](int acc, int cur, int idx) {
+        values << acc << cur << idx;
+        return acc + cur + idx;
+    }, QtPromise::resolve(2).delay(100));
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<int>>::value));
+
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForValue(p, -1), 23);
+    QCOMPARE(values, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+}
+
+void tst_qpromise_reduce::delayedFulfilled()
+{
+    QVector<int> inputs{4, 6, 8};
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        return QtPromise::resolve(acc + cur + idx).delay(100);
+    });
+    auto p1 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        return QtPromise::resolve(acc + cur + idx).delay(100);
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForValue(p0, -1), 21);
+    QCOMPARE(waitForValue(p1, -1), 23);
+    QCOMPARE(v0, QVector<int>({4, 6, 1, 11, 8, 2}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1, 13, 8, 2}));
+}
+
+void tst_qpromise_reduce::delayedRejected()
+{
+    QVector<int> inputs{4, 6, 8};
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        if (cur == 6) {
+            return QPromise<int>::reject(QString("foo"));
+        }
+        return QtPromise::resolve(acc + cur + idx);
+    });
+    auto p1 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        if (cur == 6) {
+            return QPromise<int>::reject(QString("bar"));
+        }
+        return QtPromise::resolve(acc + cur + idx);
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForError(p0, QString()), QString("foo"));
+    QCOMPARE(waitForError(p1, QString()), QString("bar"));
+    QCOMPARE(v0, QVector<int>({4, 6, 1}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1}));
+}
+
+void tst_qpromise_reduce::functorThrows()
+{
+    QVector<int> inputs{4, 6, 8};
+    QVector<int> v0;
+    QVector<int> v1;
+
+    auto p0 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v0 << acc << cur << idx;
+        if (cur == 6) {
+            throw QString("foo");
+        }
+        return acc + cur + idx;
+    });
+    auto p1 = QtPromise::resolve(inputs).reduce([&](int acc, int cur, int idx) {
+        v1 << acc << cur << idx;
+        if (cur == 6) {
+            throw QString("bar");
+        }
+        return acc + cur + idx;
+    }, 2);
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p0), QPromise<int>>::value));
+    Q_STATIC_ASSERT((std::is_same<decltype(p1), QPromise<int>>::value));
+
+    QCOMPARE(p0.isPending(), true);
+    QCOMPARE(p1.isPending(), true);
+    QCOMPARE(waitForError(p0, QString()), QString("foo"));
+    QCOMPARE(waitForError(p1, QString()), QString("bar"));
+    QCOMPARE(v0, QVector<int>({4, 6, 1}));
+    QCOMPARE(v1, QVector<int>({2, 4, 0, 6, 6, 1}));
+}
+
+void tst_qpromise_reduce::sequenceTypes()
+{
+    SequenceTester<QLinkedList<QPromise<int>>>::exec();
+    SequenceTester<QList<QPromise<int>>>::exec();
+    SequenceTester<QVector<QPromise<int>>>::exec();
+    SequenceTester<std::list<QPromise<int>>>::exec();
+    SequenceTester<std::vector<QPromise<int>>>::exec();
+}