[Repo Assist] Add AsyncSeq.tryFindBack, findBack, tryFindBackAsync, findBackAsync

[github-actions]

🤖 This PR was created by Repo Assist, an automated AI assistant.

Summary

Adds four backward-search functions that mirror the F# standard library Seq module:

Function	Signature	Description
tryFindBack	('T -> bool) -> AsyncSeq<'T> -> Async<'T option>	Returns the last element matching a sync predicate, or None
findBack	('T -> bool) -> AsyncSeq<'T> -> Async<'T>	Like above, but raises KeyNotFoundException on no match
tryFindBackAsync	('T -> Async(bool)) -> AsyncSeq<'T> -> Async<'T option>	Async-predicate variant of tryFindBack
findBackAsync	('T -> Async(bool)) -> AsyncSeq<'T> -> Async<'T>	Async-predicate variant of findBack

Motivation

AsyncSeq already has tryFind / find (return the first match). The complementary backward variants — present in Seq.tryFindBack / Seq.findBack since F# 4.0 — were missing. These are useful for scanning time-ordered sequences or event logs where the most recent matching record is needed.

Implementation

tryFindBackAsync is the primitive: it iterates the entire sequence, overwriting result whenever the predicate holds, and returns the final value. The other three functions are simple wrappers.

let tryFindBackAsync (predicate: 'T -> Async(bool)) (source: AsyncSeq<'T>) : Async<'T option> = async {
    use ie = source.GetEnumerator()
    let! move = ie.MoveNext()
    let mutable b = move
    let mutable result = None
    while b.IsSome do
      let! ok = predicate b.Value
      if ok then result <- b
      let! next = ie.MoveNext()
      b <- next
    return result }

Changes

• src/FSharp.Control.AsyncSeq/AsyncSeq.fs — 4 new function implementations
• src/FSharp.Control.AsyncSeq/AsyncSeq.fsi — 4 new public signatures with XML-doc
• tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs — 10 new tests
• RELEASE_NOTES.md — added to 4.11.0 entry

New Tests

Test	What it checks
tryFindBack returns last matching element	Returns Some 6 from [1;3;5;2;4;6;7] with even predicate
tryFindBack returns None when no element matches	All-odd sequence → None
tryFindBack returns None for empty sequence	Empty → None
tryFindBack returns last element when all match	Always-true predicate → last element
findBack returns last matching element	Returns last even from mixed sequence
findBack raises KeyNotFoundException when no match	No even in all-odd → exception
tryFindBackAsync returns last element satisfying async predicate	x < 4 → Some 3
tryFindBackAsync returns None when nothing matches	x > 99 → None
findBackAsync returns last element satisfying async predicate	x % 2 = 0 → last even
findBackAsync raises KeyNotFoundException when no match	x > 99 → exception

Test Status

✅ Build succeeded (pre-existing OOM on netstandard2.1 full-solution build — infrastructure limitation, not caused by this change).

✅ All 358/358 tests pass (dotnet test tests/FSharp.Control.AsyncSeq.Tests/...).

Generated by Repo Assist · ◷

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@4957663821dbb3260348084fa2f1659701950fef