Merge pull request #325 from fsprojects/repo-assist/improve-findIndexBack-20260505-0babe087728a3488

[Repo Assist] Add AsyncSeq.tryFindIndexBack, findIndexBack, tryFindIndexBackAsync, findIndexBackAsync

Author: dsyme

RELEASE_NOTES.md

@@ -2,6 +2,7 @@
 
 * Performance: Optimised `AsyncSeq.pairwise` to use a `hasPrev` flag and a direct `mutable` field instead of wrapping the previous element in `Some`. Previously, each iteration allocated a new `'T option` object on the heap; the new implementation eliminates that allocation entirely, reducing GC pressure for long sequences.
 * Bug fix: `AsyncSeq.splitAt` and `AsyncSeq.tryTail` now correctly dispose the underlying enumerator when an exception or cancellation occurs during the initial `MoveNext` call. Previously the enumerator could leak if the source sequence threw during the first few steps.
+* Added `AsyncSeq.tryFindIndexBack` and `AsyncSeq.findIndexBack` — return the index of the last element satisfying a predicate. Mirrors `Array.tryFindIndexBack` / `Array.findIndexBack`. Async-predicate variants `tryFindIndexBackAsync` and `findIndexBackAsync` are also included.
 
 ### 4.16.0
 

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -1460,6 +1460,45 @@ module AsyncSeq =
     | None -> return raise (System.Collections.Generic.KeyNotFoundException("An element satisfying the predicate was not found in the collection."))
     | Some i -> return i }
 
+  let tryFindIndexBack f (source : AsyncSeq<'T>) = async {
+    use ie = source.GetEnumerator()
+    let! v = ie.MoveNext()
+    let mutable b = v
+    let mutable i = 0
+    let mutable res = None
+    while b.IsSome do
+        if f b.Value then res <- Some i
+        let! next = ie.MoveNext()
+        b <- next
+        i <- i + 1
+    return res }
+
+  let tryFindIndexBackAsync f (source : AsyncSeq<'T>) = async {
+    use ie = source.GetEnumerator()
+    let! v = ie.MoveNext()
+    let mutable b = v
+    let mutable i = 0
+    let mutable res = None
+    while b.IsSome do
+        let! matches = f b.Value
+        if matches then res <- Some i
+        let! next = ie.MoveNext()
+        b <- next
+        i <- i + 1
+    return res }
+
+  let findIndexBack f (source : AsyncSeq<'T>) = async {
+    let! result = tryFindIndexBack f source
+    match result with
+    | None -> return raise (System.Collections.Generic.KeyNotFoundException("An element satisfying the predicate was not found in the collection."))
+    | Some i -> return i }
+
+  let findIndexBackAsync f (source : AsyncSeq<'T>) = async {
+    let! result = tryFindIndexBackAsync f source
+    match result with
+    | None -> return raise (System.Collections.Generic.KeyNotFoundException("An element satisfying the predicate was not found in the collection."))
+    | Some i -> return i }
+
   let exists f (source : AsyncSeq<'T>) =
     source |> tryFind f |> Async.map Option.isSome
 

src/FSharp.Control.AsyncSeq/AsyncSeq.fsi

@@ -414,6 +414,22 @@ module AsyncSeq =
     /// Raises KeyNotFoundException if no matching element is found.
     val findIndexAsync : predicate:('T -> Async<bool>) -> source:AsyncSeq<'T> -> Async<int>
 
+    /// Asynchronously find the index of the last value in a sequence for which the predicate returns true.
+    /// Returns None if no matching element is found.
+    val tryFindIndexBack : predicate:('T -> bool) -> source:AsyncSeq<'T> -> Async<int option>
+
+    /// Asynchronously find the index of the last value in a sequence for which the async predicate returns true.
+    /// Returns None if no matching element is found.
+    val tryFindIndexBackAsync : predicate:('T -> Async<bool>) -> source:AsyncSeq<'T> -> Async<int option>
+
+    /// Asynchronously find the index of the last value in a sequence for which the predicate returns true.
+    /// Raises KeyNotFoundException if no matching element is found.
+    val findIndexBack : predicate:('T -> bool) -> source:AsyncSeq<'T> -> Async<int>
+
+    /// Asynchronously find the index of the last value in a sequence for which the async predicate returns true.
+    /// Raises KeyNotFoundException if no matching element is found.
+    val findIndexBackAsync : predicate:('T -> Async<bool>) -> source:AsyncSeq<'T> -> Async<int>
+
     /// Asynchronously determine if there is a value in the sequence for which the predicate returns true
     val exists : predicate:('T -> bool) -> source:AsyncSeq<'T> -> Async<bool>
 

tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs

@@ -3491,6 +3491,71 @@ let ``AsyncSeq.tryFindIndexAsync returns None when not found`` () =
     |> Async.RunSynchronously
   Assert.AreEqual(None, result)
 
+// ===== tryFindIndexBack / findIndexBack / tryFindIndexBackAsync / findIndexBackAsync =====
+
+[<Test>]
+let ``AsyncSeq.tryFindIndexBack returns index of last matching element`` () =
+  let result = AsyncSeq.ofSeq [ 1; 2; 3; 2; 1 ] |> AsyncSeq.tryFindIndexBack (fun x -> x = 2) |> Async.RunSynchronously
+  Assert.AreEqual(Some 3, result)
+
+[<Test>]
+let ``AsyncSeq.tryFindIndexBack returns None when no match`` () =
+  let result = AsyncSeq.ofSeq [ 1; 2; 3 ] |> AsyncSeq.tryFindIndexBack (fun x -> x = 99) |> Async.RunSynchronously
+  Assert.AreEqual(None, result)
+
+[<Test>]
+let ``AsyncSeq.tryFindIndexBack returns None for empty sequence`` () =
+  let result = AsyncSeq.empty<int> |> AsyncSeq.tryFindIndexBack (fun _ -> true) |> Async.RunSynchronously
+  Assert.AreEqual(None, result)
+
+[<Test>]
+let ``AsyncSeq.tryFindIndexBack returns index of last element when all match`` () =
+  let result = AsyncSeq.ofSeq [ 1; 2; 3 ] |> AsyncSeq.tryFindIndexBack (fun _ -> true) |> Async.RunSynchronously
+  Assert.AreEqual(Some 2, result)
+
+[<Test>]
+let ``AsyncSeq.findIndexBack returns index of last matching element`` () =
+  let result = AsyncSeq.ofSeq [ 10; 20; 30; 20; 10 ] |> AsyncSeq.findIndexBack (fun x -> x = 20) |> Async.RunSynchronously
+  Assert.AreEqual(3, result)
+
+[<Test>]
+let ``AsyncSeq.findIndexBack raises KeyNotFoundException when no match`` () =
+  Assert.Throws<System.Collections.Generic.KeyNotFoundException>(fun () ->
+    AsyncSeq.ofSeq [ 1; 2; 3 ] |> AsyncSeq.findIndexBack (fun x -> x = 99) |> Async.RunSynchronously |> ignore)
+  |> ignore
+
+[<Test>]
+let ``AsyncSeq.tryFindIndexBackAsync returns index of last matching element`` () =
+  let result =
+    AsyncSeq.ofSeq [ 1; 2; 3; 2; 1 ]
+    |> AsyncSeq.tryFindIndexBackAsync (fun x -> async { return x = 2 })
+    |> Async.RunSynchronously
+  Assert.AreEqual(Some 3, result)
+
+[<Test>]
+let ``AsyncSeq.tryFindIndexBackAsync returns None when no match`` () =
+  let result =
+    AsyncSeq.ofSeq [ 1; 2; 3 ]
+    |> AsyncSeq.tryFindIndexBackAsync (fun x -> async { return x = 99 })
+    |> Async.RunSynchronously
+  Assert.AreEqual(None, result)
+
+[<Test>]
+let ``AsyncSeq.findIndexBackAsync returns index of last matching element`` () =
+  let result =
+    AsyncSeq.ofSeq [ 5; 4; 3; 4; 5 ]
+    |> AsyncSeq.findIndexBackAsync (fun x -> async { return x = 4 })
+    |> Async.RunSynchronously
+  Assert.AreEqual(3, result)
+
+[<Test>]
+let ``AsyncSeq.findIndexBackAsync raises KeyNotFoundException when no match`` () =
+  Assert.Throws<System.Collections.Generic.KeyNotFoundException>(fun () ->
+    AsyncSeq.ofSeq [ 1; 2; 3 ]
+    |> AsyncSeq.findIndexBackAsync (fun x -> async { return x = 99 })
+    |> Async.RunSynchronously |> ignore)
+  |> ignore
+
 // ===== tryFindBack / findBack / tryFindBackAsync / findBackAsync =====
 
 [<Test>]