What is 'ChildTask'?

[NickDarvey]

The README mentions parallelism is possible with ChildTask. What is ChildTask?

FSharp.Control.TaskSeq/README.md

Line 199 in 32c7a47

| **Parallelism** | Possible with ChildTask; support will follow | Supported explicitly |

Is the planned support (also mentioned) something like operators that let you specify the degree of paralellism?

[abelbraaksma]

@NickDarvey sorry for responding this late, I missed the notification on this thread somehow.

The issue with making IAsyncEnumerable<'T> parallel is non-trivial. The reason is that the interface is deliberately designed in such a way that you must await the MoveNext action (which returns true|false signaling whether there are more elements) before you can get to the Current property.

In other words, calling MoveNext multiple times can lead parallel execution, sure, but it can also lead to reading past the end of the stream and returning items in a random order. What's worse, is that, just like a normal seq, the moments that side effects are executed are very well defined. However, once you run this in parallel, this guarantee goes out of the window.

What is ChildTask?

This term comes from Async.StartChild and Async.StartChildAsTask, which are part of the F# Core library. They start a child task and enable parallel execution. A good definition and a discussion of the concept can be found here: https://learn.microsoft.com/en-us/dotnet/standard/parallel-programming/attached-and-detached-child-tasks

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In general, I have been wanting to add (easy) parallelism to the library, but as you may have guessed, parallelism is never easy. AsyncSeq has some support for it, as does Async.Parallel, obviously.

My current idea is to either take AsyncSeq, but then the parallel parts, and simply live with the fact that these concepts should be kept separate, or I find some way of extending the current implementation, and allow taskSeq to have parallel tasks.

On the latter, the only way I see this working is if there's a worker that tells us that we can MoveNext (i.e. that there are more items), but that it isn't finished with its task yet.

It is easy to confuse an async sequence with a sequence of asyncs. The first is supposed to be executed in order, while the other can be executed in parallel, or at will, in any order.

PS: is there a specific use case you had in mind? It may help prioritize this.

[NickDarvey]

The issue with making IAsyncEnumerable<'T> parallel is non-trivial. The reason is that the interface is deliberately designed in such a way that you must await the MoveNext action (which returns true|false signaling whether there are more elements) before you can get to the Current property.

That makes sense.

PS: is there a specific use case you had in mind? It may help prioritize this.

I wish I'd documented the use case when I asked this question—because now I can't remember.