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Tail recursion for TaskSeq #62
Description
We've removed the tail recursion because it was hard to consolidate it into yield! (it was instead done with return!, which has no place in taskSeq).
However, today I helped someone with some code that he considered for taskSeq which had the following approach:
let getPoliciesAsync policyid =
asyncSeq{
use connection = new NpgsqlConnection(npgsqlConnectionStringBuilder.ConnectionString)
use command = new NpgsqlCommand($"SELECT policy_data FROM policy.tbl_policy where policy_id = {Sql.uuid policyid};", connection)
let! reader = command.ExecuteReaderAsync() |> Async.AwaitTask
let rec someRec() = asyncSeq{
let! rowExists = reader.ReadAsync() |> Async.AwaitTask
if rowExists then
yield {| dt1 = reader.GetString(0) |}
yield! someRec()
}
yield! someRec()
} |> AsyncSeq.toAsyncEnumAs you can see, it uses asyncSeq, but also: it is recursive. The same approach with taskSeq would likely be more performant, however, if there are a lot of rows, this becomes problematic. This code can be rewritten with a loop, though.
@dsyme, sharing this with you in case we want to revisit this at some point.
Reacted by Natalie Perret
Activity
dsyme commentedon Nov 1, 2022
Yes, this should be rewritten with a loop
dsyme commentedon Nov 5, 2022
@abelbraaksma BTW I think I am going to prioritise an F# 8 addition to allow builders to specify
ReturnFromTailcallorYieldFromTailcallmethods.abelbraaksma commentedon Nov 6, 2022
@dsyme that would certainly simplify an addition like this, and would also allow
taskto support tail calls. Awesome! We can do some early-adoption testing here throughTaskSeq.abelbraaksma commentedon Oct 29, 2023
See fsharp/fslang-suggestions#1006 instead.
majocha commentedon Aug 1, 2025
This is now implemented,
yield!desugars toYieldFromFinalin tail call positions: dotnet/fsharp#18804Needs RFC, though.
abelbraaksma commentedon Nov 14, 2025
@majocha, thanks for letting me know! I only found out myself with the release of F# 10. Definitely going to give this a go.
dsyme commentedon Mar 8, 2026
/repo-assist let's make use of this, make a PR to add YieldFromFinal and implement a tailall transfer in the state machine driver. If you can work that out I'll be impressed
Add YieldFromFinal overloads for F# 10 compatibility (#62)
github-actions commentedon Mar 8, 2026
Pull request created: #310
dsyme commentedon Mar 8, 2026
Added the F# bug about YieldFromFinal here: dotnet/fsharp#19402
We can't make progress on this until that's fixed
github-actions commentedon Mar 14, 2026
[-]Consider tail recursion[/-][+]Tail recursion for TaskSeq[/+]jberzy commentedon Jul 16, 2026
Hello,
Now that the upstream fix is in, is it possible that we reintroduce tail recursion support?
Thanks
github-actions commentedon Jul 31, 2026
🤖 This is an automated response from Repo Assist.
Thanks for the ping,
@jberzy! You're right — the upstream F# fix (dotnet/fsharp#19402, implementingYieldFromFinal) has landed in F# 10.However, actually enabling tail-call transfer in
taskSeqis non-trivial. The challenge (as analysed in PR #380 which implemented thetaskSeqDynamicresumable path) is that theYieldFromFinaloverload is invoked by the compiler in all tail-call positions ofyield!, including calls inside loop bodies (e.g.for c in source do yield! f c). At that call site there is no way to distinguish a genuine "final" call (where we could hand off the enumerator permanently) from a within-loop tail call (where we must not, or we'd lose all remaining iterations).The safe incremental step would be to add the
YieldFromFinaloverload stubs to the builder so F# 10 compiles without warning, delegating toYieldFromfor now. The actual optimization can come later once a reliable way to discriminate call sites is available (perhaps via source-level annotation or a future compiler hint).@abelbraaksma— would you like me to open a draft PR that adds theYieldFromFinal/YieldFromFinalAsyncstubs to theTaskSeqBuilder, so F# 10 users at least get clean compilation? I can do that on request.Add this agentic workflow to your repo
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