# QPromise::reduce

Since: 0.5.0

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>

IMPORTANT

This method only applies to promise with sequence value.

Iterates over all the promise values (i.e. Sequence<T>) and reduces the sequence (opens new window) to a single value using the given reducer function and an optional initialValue. The type returned by reducer determines the type of the output promise.

See QtPromise::reduce for details, this method is provided for convenience and is similar to:

promise.then([](const T& values) {
    return QtPromise::reduce(values, reducer, initialValue);
})

For example:

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"
    // {...}
});