FoldWhile/FoldUntil with an async predicate

[fjmorel]

I have a a list of strings. I want to iterate it to get an EitherAsync<ErrorModel, DataModel> for each string, but stop at the first entry that returns an ErrorModel and return that error instead of continuing.

Initially, I had this which returns what I want (first error or full list if successful) but still iterates over the entire list when there's an error:

public EitherAsync<ErrorModel, Seq<DataModel>> GetManyAsync(Seq<string> guids)
{
    EitherAsync<ErrorModel, Seq<DataModel>> seed = new Seq<DataModel>();
    return guids
        .Aggregate(seed, (either, guid) => either
            .Bind(results => GetDataAsync(guid)
                .Map(result => results.Add(result))));
}
public EitherAsync<ServiceError, DataModel> GetDataAsync(string guid) { ... }

I then tried to use this library's FoldWhile/FoldUntil to stop when the Either goes Left, but I end up having to use Task.Result which I definitely do not want to do.

public EitherAsync<ErrorModel, Seq<DataModel>> GetManyAsync(Seq<string> guids)
{
    EitherAsync<ErrorModel, Seq<DataModel>> state = new Seq<DataModel>();
    return guids
        .FoldWhile(state, (either, guid) => either
            .Bind(results => GetDataAsync(guid)
                .Map(result => results.Add(result))), x => x.IsRight.Result);
}
public EitherAsync<ServiceError, DataModel> GetDataAsync(string guid) { ... }

Is there some other way to do this that I am missing? The original Aggregate isn't a huge problem because the passed in list is going to be at most a few hundred strings.

[louthy]

You can use SequenceSerial and TraverseSerial to turn a sequence of asynchronous actions into a asynchronous action of sequence.

I haven't got a debugger to hand, but try this:

    guids.SequenceSerial(GetDataAsync);

It will only return a sequence if all items succeed, otherwise you get the first error. There's also SequenceParallel and TraverseParallel. The rules are the same, but the error you get back might not be the first sequentially.

[fjmorel]

Thank you! That was exactly what I was looking for.