Add an example of error handling of QFuture without using exceptions

Also fixed a few unrelated typos in docs.

Task-number: QTBUG-83236
Change-Id: I776cda8f0ef4de6c4a93e94092dc19e94d1884c8
Reviewed-by: Vitaly Fanaskov <vitaly.fanaskov@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/corelib/doc/snippets/code/src_corelib_thread_qfuture.cpp

@@ -72,3 +72,30 @@ i.toBack();
 while (i.hasPrevious())
     qDebug() << i.previous();
 //! [2]
+
+//! [3]
+using NetworkReply = std::variant<QByteArray, QNetworkReply::NetworkError>;
+
+enum class IOError { FailedToRead, FailedToWrite };
+using IOResult = std::variant<QString, IOError>;
+//! [3]
+
+//! [4]
+QFuture<IOResult> future = QtConcurrent::run([url] {
+        ...
+        return NetworkReply(QNetworkReply::TimeoutError);
+}).then([](NetworkReply reply) {
+    if (auto error = std::get_if<QNetworkReply::NetworkError>(&reply))
+        return IOResult(IOError::FailedToRead);
+
+    auto data = std::get_if<QByteArray>(&reply);
+    // try to write *data and return IOError::FailedToWrite on failure
+    ...
+});
+
+auto result = future.result();
+if (auto filePath = std::get_if<QString>(&result)) {
+    // do something with *filePath
+else
+    // process the error
+//! [4]

src/corelib/thread/qfuture.qdoc

@@ -51,9 +51,31 @@
 
     QFuture provides a \l{Java-style iterators}{Java-style iterator}
     (QFutureIterator) and an \l{STL-style iterators}{STL-style iterator}
-    (QFuture::const_iterator). Using these iterators is  another way to access
+    (QFuture::const_iterator). Using these iterators is another way to access
     results in the future.
 
+    If the result of one asynchronous computation needs to be passed
+    to another, QFuture provides a convenient way of chaining multiple
+    sequential computations using then(). Additionally, onFailed() can be used
+    to handle any failures that occurred in the chain. Note that QFuture relies
+    on exceptions for the error handling. If using exceptions is not an option,
+    you can still indicate the error state of QFuture, by making the error type
+    part of the QFuture type. For example, you can use std::variant, std::any or
+    similar for keeping the result or failure or make your custom type.
+
+    The example below demonstrates how the error handling can be done without
+    using exceptions. Let's say we want to send a network request to obtain a large
+    file from a network location. Then we want to write it to the file system and
+    return its location in case of a success. Both of these operations may fail
+    with different errors. So, we use std::variant to keep the result
+    or error:
+
+    \snippet code/src_corelib_thread_qfuture.cpp 3
+
+    And we combine the two operations using then():
+
+    \snippet code/src_corelib_thread_qfuture.cpp 4
+
     QFuture also offers ways to interact with a runnning computation. For
     instance, the computation can be canceled with the cancel() function. To
     pause the computation, use the setPaused() function or one of the pause(),
@@ -347,7 +369,7 @@
     \sa takeResult(), result(), resultAt(), results(), resultCount(), isValid()
 */
 
-/*  \fn template <typename T> std::vector<T> QFuture<T>::takeResult()
+/*! \fn template <typename T> std::vector<T> QFuture<T>::takeResult()
 
     Call this function only if isValid() returns \c true, otherwise
     the behavior is undefined. This function takes the first result from
@@ -369,12 +391,12 @@
     \sa takeResults(), result(), results(), resultAt(), isValid()
 */
 
-/* \fn template <typename T> std::vector<T> QFuture<T>::isValid() const
+/*! \fn template <typename T> bool QFuture<T>::isValid() const
 
-   Returns true if a result or results can be accessed or taken from this
-   QFuture object. Returns false after the result was taken from the future.
+    Returns \c true if a result or results can be accessed or taken from this
+    QFuture object. Returns false after the result was taken from the future.
 
-   \sa takeResults(), takeResult(), result(), results(), resultAt()
+    \sa takeResults(), takeResult(), result(), results(), resultAt()
 */
 
 /*! \fn template <typename T> QFuture<T>::const_iterator QFuture<T>::begin() const