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) {
+    // {...}
+});
+```