Add AsyncSeq.head and AsyncSeq.tail

[Caleb9]

Hello!

Regular Seq contains simple convenience functions head and tail. I implemented those for AsyncSeq, maybe they could be added?

Thanks.

[abelbraaksma]

Isn't Async.head the same as Async.first, which already exists? Well, firstOrDefault exists, which is close enough, no? https://fsprojects.github.io/FSharp.Control.AsyncSeq/reference/fsharp-control-asyncseq.html#firstOrDefault

Though there's no tail. There should probably be a tail and tryTail pair, and yes, maybe it does make sense to have a head and tryHead for parity with Seq.

[Caleb9]

It sure is, it's just a slight inconsistency with non-async collections API (i.e. Seq has head and tail). It's no biggie, the PR can be closed if it's not desirable to have these two APIs aligned in this way :).

[abelbraaksma]

FWIW, TaskSeq, which uses a resumable state machine and task from F# 6 under the hood, and implements IAsyncEnumerable<_> similar to async foreach in C#, has those functions: https://github.com/fsprojects/FSharp.Control.TaskSeq

[abelbraaksma]

hi @Caleb9, sorry for leaving this PR hanging (I wasn't a maintainer yet, at the time). We'll need a few tests, but otherwise this looks good to go in.

I'd understand if you're not around anymore for editing your PR, in which case I can take over and finish it.

[dsyme]

@abelbraaksma Should we close this old one out?