Tail recursion for TaskSeq

@dsyme

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.toAsyncEnum

As 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.

Yes, this should be rewritten with a loop

@abelbraaksma

@abelbraaksma BTW I think I am going to prioritise an F# 8 addition to allow builders to specify ReturnFromTailcall or YieldFromTailcall methods.

@dsyme

@dsyme that would certainly simplify an addition like this, and would also allow task to support tail calls. Awesome! We can do some early-adoption testing here through TaskSeq.

See fsharp/fslang-suggestions#1006 instead.

This is now implemented, yield! desugars to YieldFromFinal in tail call positions: dotnet/fsharp#18804
Needs RFC, though.

@majocha

@majocha, thanks for letting me know! I only found out myself with the release of F# 10. Definitely going to give this a go.

/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)

Pull request created: #310

Generated by Repo Assist

Added the F# bug about YieldFromFinal here: dotnet/fsharp#19402

We can't make progress on this until that's fixed

Hello,

Now that the upstream fix is in, is it possible that we reintroduce tail recursion support?

Thanks

🤖 This is an automated response from Repo Assist.

Thanks for the ping, @jberzy! You're right — the upstream F# fix (dotnet/fsharp#19402, implementing YieldFromFinal) has landed in F# 10.

However, actually enabling tail-call transfer in taskSeq is non-trivial. The challenge (as analysed in PR #380 which implemented the taskSeqDynamic resumable path) is that the YieldFromFinal overload is invoked by the compiler in all tail-call positions of yield!, 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 YieldFromFinal overload stubs to the builder so F# 10 compiles without warning, delegating to YieldFrom for 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 the YieldFromFinal/YieldFromFinalAsync stubs to the TaskSeqBuilder, so F# 10 users at least get clean compilation? I can do that on request.

Generated by 🌈 Repo Assist, see workflow run. Learn more.
_{Comment /repo-assist to run again}

_{Add this agentic workflow to your repo}

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@42c2ab5b4e4c9273534c39259b2e0df7f20f07e9

Navigation Menu

Repository navigation

Tail recursion for TaskSeq #62

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Tail recursion for TaskSeq #62

Description

Activity

dsyme commented on Nov 1, 2022

dsyme commented on Nov 5, 2022

abelbraaksma commented on Nov 6, 2022

abelbraaksma commented on Oct 29, 2023

majocha commented on Aug 1, 2025

abelbraaksma commented on Nov 14, 2025

dsyme commented on Mar 8, 2026

github-actions commented on Mar 8, 2026

dsyme commented on Mar 8, 2026

github-actions commented on Mar 14, 2026

jberzy commented on Jul 16, 2026

github-actions commented on Jul 31, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions