# QtPromise::reduce

Since: 0.5.0

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 (opens new window) 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.

// 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"
    // {...}
});

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).