Implement QtPromise::attempt(functor, args...)

Add a new helper that calls functor immediately and returns a promise fulfilled with the value returned by functor. Any synchronous exceptions will be turned into rejections on the returned promise. This is a convenient method that can be used instead of handling both synchronous and asynchronous exception flows.

Also simplify PromiseDispatch which now calls the functor with a variable number of arguments (including none).

docs/SUMMARY.md

@@ -22,5 +22,6 @@
   * [::resolve (static)](qtpromise/qpromise/resolve.md)
   * [qPromise](qtpromise/helpers/qpromise.md)
   * [qPromiseAll](qtpromise/helpers/qpromiseall.md)
+  * [QtPromise::attempt](qtpromise/helpers/attempt.md)
   * [QtPromise::filter](qtpromise/helpers/filter.md)
   * [QtPromise::map](qtpromise/helpers/map.md)

docs/qtpromise/api-reference.md

@@ -27,5 +27,6 @@
 
 * [`qPromise`](helpers/qpromise.md)
 * [`qPromiseAll`](helpers/qpromiseall.md)
+* [`QtPromise::attempt`](helpers/attempt.md)
 * [`QtPromise::filter`](helpers/filter.md)
 * [`QtPromise::map`](helpers/map.md)

docs/qtpromise/helpers/attempt.md

@@ -0,0 +1,41 @@
+## `QtPromise::attempt`
+
+```cpp
+QtPromise::attempt(Functor functor, Args...) -> QPromise<R>
+
+// With:
+// - Functor: Function(Args...) -> R | QPromise<R>
+```
+
+Calls `functor` immediately and returns a promise fulfilled with the value returned by
+`functor`. Any synchronous exceptions will be turned into rejections on the returned
+promise. This is a convenient method that can be used instead of handling both synchronous
+and asynchronous exception flows.
+
+The type `R` of the `output` promise depends on the type returned by the `functor` function.
+If `functor` returns a promise (or `QFuture`), the `output` promise is delayed and will be
+resolved by the returned promise.
+
+```cpp
+QPromise<QByteArray> download(const QUrl& url);
+
+QPromise<QByteArray> process(const QUrl& url)
+{
+    return QtPromise::attempt([&]() {
+        if (!url.isValid()) {
+            throw InvalidUrlException();
+        }
+
+        return download(url);
+    }
+}
+
+auto output = process(url);
+
+// 'output' type: QPromise<QByteArray>
+output.then([](const QByteArray& res) {
+    // {...}
+}).fail([](const InvalidUrlException& err) {
+    // {...}
+});
+```

src/qtpromise/qpromise_p.h

@@ -119,6 +119,13 @@ struct PromiseDeduce<QtPromise::QPromise<T>>
     : public PromiseDeduce<T>
 { };
 
+template <typename Functor, typename... Args>
+struct PromiseFunctor
+{
+    using ResultType = typename std::result_of<Functor(Args...)>::type;
+    using PromiseType = typename PromiseDeduce<ResultType>::Type;
+};
+
 template <typename T>
 struct PromiseFulfill
 {
@@ -176,51 +183,17 @@ struct PromiseFulfill<QtPromise::QPromise<void>>
     }
 };
 
-template <typename T, typename TRes>
+template <typename Result>
 struct PromiseDispatch
 {
-    using Promise = typename PromiseDeduce<TRes>::Type;
-    using ResType = Unqualified<TRes>;
-
-    template <typename THandler, typename TResolve, typename TReject>
-    static void call(const T& value, THandler handler, const TResolve& resolve, const TReject& reject)
+    template <typename Resolve, typename Reject, typename Functor, typename... Args>
+    static void call(const Resolve& resolve, const Reject& reject, Functor fn, Args&&... args)
     {
         try {
-            PromiseFulfill<ResType>::call(handler(value), resolve, reject);
-        } catch (...) {
-            reject(std::current_exception());
-        }
-    }
-};
-
-template <typename T>
-struct PromiseDispatch<T, void>
-{
-    using Promise = QtPromise::QPromise<void>;
-
-    template <typename THandler, typename TResolve, typename TReject>
-    static void call(const T& value, THandler handler, const TResolve& resolve, const TReject& reject)
-    {
-        try {
-            handler(value);
-            resolve();
-        } catch (...) {
-            reject(std::current_exception());
-        }
-    }
-};
-
-template <typename TRes>
-struct PromiseDispatch<void, TRes>
-{
-    using Promise = typename PromiseDeduce<TRes>::Type;
-    using ResType = Unqualified<TRes>;
-
-    template <typename THandler, typename TResolve, typename TReject>
-    static void call(THandler handler, const TResolve& resolve, const TReject& reject)
-    {
-        try {
-            PromiseFulfill<ResType>::call(handler(), resolve, reject);
+            PromiseFulfill<Unqualified<Result>>::call(
+                fn(std::forward<Args>(args)...),
+                resolve,
+                reject);
         } catch (...) {
             reject(std::current_exception());
         }
@@ -228,15 +201,13 @@ struct PromiseDispatch<void, TRes>
 };
 
 template <>
-struct PromiseDispatch<void, void>
+struct PromiseDispatch<void>
 {
-    using Promise = QtPromise::QPromise<void>;
-
-    template <typename THandler, typename TResolve, typename TReject>
-    static void call(THandler handler, const TResolve& resolve, const TReject& reject)
+    template <typename Resolve, typename Reject, typename Functor, typename... Args>
+    static void call(const Resolve& resolve, const Reject& reject, Functor fn, Args&&... args)
     {
         try {
-            handler();
+            fn(std::forward<Args>(args)...);
             resolve();
         } catch (...) {
             reject(std::current_exception());
@@ -248,7 +219,7 @@ template <typename T, typename THandler, typename TArg = typename ArgsOf<THandle
 struct PromiseHandler
 {
     using ResType = typename std::result_of<THandler(T)>::type;
-    using Promise = typename PromiseDispatch<T, ResType>::Promise;
+    using Promise = typename PromiseDeduce<ResType>::Type;
 
     template <typename TResolve, typename TReject>
     static std::function<void(const T&)> create(
@@ -257,7 +228,7 @@ struct PromiseHandler
         const TReject& reject)
     {
         return [=](const T& value) {
-            PromiseDispatch<T, ResType>::call(value, std::move(handler), resolve, reject);
+            PromiseDispatch<ResType>::call(resolve, reject, handler, value);
         };
     }
 };
@@ -266,7 +237,7 @@ template <typename T, typename THandler>
 struct PromiseHandler<T, THandler, void>
 {
     using ResType = typename std::result_of<THandler()>::type;
-    using Promise = typename PromiseDispatch<T, ResType>::Promise;
+    using Promise = typename PromiseDeduce<ResType>::Type;
 
     template <typename TResolve, typename TReject>
     static std::function<void(const T&)> create(
@@ -275,7 +246,7 @@ struct PromiseHandler<T, THandler, void>
         const TReject& reject)
     {
         return [=](const T&) {
-            PromiseDispatch<void, ResType>::call(handler, resolve, reject);
+            PromiseDispatch<ResType>::call(resolve, reject, handler);
         };
     }
 };
@@ -284,7 +255,7 @@ template <typename THandler>
 struct PromiseHandler<void, THandler, void>
 {
     using ResType = typename std::result_of<THandler()>::type;
-    using Promise = typename PromiseDispatch<void, ResType>::Promise;
+    using Promise = typename PromiseDeduce<ResType>::Type;
 
     template <typename TResolve, typename TReject>
     static std::function<void()> create(
@@ -293,7 +264,7 @@ struct PromiseHandler<void, THandler, void>
         const TReject& reject)
     {
         return [=]() {
-            PromiseDispatch<void, ResType>::call(handler, resolve, reject);
+            PromiseDispatch<ResType>::call(resolve, reject, handler);
         };
     }
 };
@@ -351,7 +322,7 @@ struct PromiseCatcher
             try {
                 error.rethrow();
             } catch (const TArg& error) {
-                PromiseDispatch<TArg, ResType>::call(error, handler, resolve, reject);
+                PromiseDispatch<ResType>::call(resolve, reject, handler, error);
             } catch (...) {
                 reject(std::current_exception());
             }
@@ -374,7 +345,7 @@ struct PromiseCatcher<T, THandler, void>
             try {
                 error.rethrow();
             } catch (...) {
-                PromiseDispatch<void, ResType>::call(handler, resolve, reject);
+                PromiseDispatch<ResType>::call(resolve, reject, handler);
             }
         };
     }

src/qtpromise/qpromisehelpers.h

@@ -38,6 +38,32 @@ static inline QPromise<void> qPromiseAll(const Sequence<QPromise<void>, Args...>
     return QPromise<void>::all(promises);
 }
 
+template <typename Functor, typename... Args>
+static inline typename QtPromisePrivate::PromiseFunctor<Functor, Args...>::PromiseType
+attempt(Functor&& fn, Args&&... args)
+{
+    using namespace QtPromisePrivate;
+    using FunctorType = PromiseFunctor<Functor, Args...>;
+    using PromiseType = typename FunctorType::PromiseType;
+    using ValueType = typename PromiseType::Type;
+
+    // NOTE: std::forward<T<U>>: MSVC 2013 fails when forwarding
+    // template type (error: "expects 4 arguments - 0 provided").
+    // However it succeeds with type alias.
+    // TODO: should we expose QPromise::ResolveType & RejectType?
+    using ResolveType = QPromiseResolve<ValueType>;
+    using RejectType = QPromiseReject<ValueType>;
+
+    return PromiseType(
+        [&](ResolveType&& resolve, RejectType&& reject) {
+            PromiseDispatch<typename FunctorType::ResultType>::call(
+                std::forward<ResolveType>(resolve),
+                std::forward<RejectType>(reject),
+                std::forward<Functor>(fn),
+                std::forward<Args>(args)...);
+        });
+}
+
 template <typename Sequence, typename Functor>
 static inline typename QtPromisePrivate::PromiseMapper<Sequence, Functor>::PromiseType
 map(const Sequence& values, Functor fn)

tests/auto/qtpromise/helpers/attempt/attempt.pro

@@ -0,0 +1,5 @@
+QT += concurrent
+TARGET = tst_helpers_attempt
+SOURCES += $$PWD/tst_attempt.cpp
+
+include(../../qtpromise.pri)

tests/auto/qtpromise/helpers/attempt/tst_attempt.cpp

@@ -0,0 +1,99 @@
+// Tests
+#include "../../shared/utils.h"
+
+// QtPromise
+#include <QtPromise>
+
+// Qt
+#include <QtConcurrent>
+#include <QtTest>
+
+// STL
+#include <memory>
+
+using namespace QtPromise;
+
+class tst_helpers_attempt : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void voidResult();
+    void typedResult();
+    void futureResult();
+    void promiseResult();
+    void functorThrows();
+    void callWithParams();
+};
+
+QTEST_MAIN(tst_helpers_attempt)
+#include "tst_attempt.moc"
+
+void tst_helpers_attempt::voidResult()
+{
+    auto p = QtPromise::attempt([]() {});
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<void>>::value));
+    QCOMPARE(p.isFulfilled(), true);
+    QCOMPARE(waitForValue(p, -1, 42), 42);
+}
+
+void tst_helpers_attempt::typedResult()
+{
+    auto p = QtPromise::attempt([]() {
+        return QString("foo");
+    });
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<QString>>::value));
+    QCOMPARE(p.isFulfilled(), true);
+    QCOMPARE(waitForValue(p, QString()), QString("foo"));
+}
+
+void tst_helpers_attempt::futureResult()
+{
+    auto p = QtPromise::attempt([]() {
+        return QtConcurrent::run([]() {
+            return QString("foo");
+        });
+    });
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<QString>>::value));
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForValue(p, QString()), QString("foo"));
+}
+
+void tst_helpers_attempt::promiseResult()
+{
+    auto p = QtPromise::attempt([]() {
+        return QtPromise::qPromise(42).delay(200);
+    });
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<int>>::value));
+    QCOMPARE(p.isPending(), true);
+    QCOMPARE(waitForValue(p, -1), 42);
+}
+
+void tst_helpers_attempt::functorThrows()
+{
+    auto p = QtPromise::attempt([]() {
+        if (true) {
+            throw QString("bar");
+        }
+        return 42;
+    });
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<int>>::value));
+    QCOMPARE(p.isRejected(), true);
+    QCOMPARE(waitForError(p, QString()), QString("bar"));
+}
+
+void tst_helpers_attempt::callWithParams()
+{
+    auto p = QtPromise::attempt([&](int i, const QString& s) {
+        return QString("%1:%2").arg(i).arg(s);
+    }, 42, "foo");
+
+    Q_STATIC_ASSERT((std::is_same<decltype(p), QPromise<QString>>::value));
+    QCOMPARE(p.isFulfilled(), true);
+    QCOMPARE(waitForValue(p, QString()), QString("42:foo"));
+}

tests/auto/qtpromise/helpers/helpers.pro

@@ -1,6 +1,7 @@
 TEMPLATE = subdirs
 SUBDIRS += \
     all \
+    attempt \
     filter \
     map \
     reject \