[Repo Assist] Add AsyncSeq.mapFold, mapFoldAsync, allPairs, rev (#266)

* Add AsyncSeq.mapFold, mapFoldAsync, allPairs, rev

Four new Seq-mirroring combinators:
- mapFoldAsync: maps elements with an async folder that threads state,
  returns (results array, final state); mirrors Seq.mapFold
- mapFold: synchronous variant of mapFoldAsync
- allPairs: cartesian product of two async sequences; buffers second
  source before iteration; mirrors Seq.allPairs
- rev: reverses an async sequence; buffers entire source; mirrors Seq.rev

All 297 tests pass (10 new tests added).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* ci: trigger checks

* Update RELEASE_NOTES for version 4.8.0

Updated release notes for version 4.8.0, adding new features and improvements.

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>

Author: 3 people

RELEASE_NOTES.md

@@ -1,3 +1,10 @@
+### 4.8.0
+
+* Added `AsyncSeq.mapFoldAsync` — maps each element using an asynchronous folder that also threads an accumulator state, returning both the array of results and the final state; mirrors `Seq.mapFold`.
+* Added `AsyncSeq.mapFold` — synchronous variant of `AsyncSeq.mapFoldAsync`, mirroring `Seq.mapFold`.
+* Added `AsyncSeq.allPairs` — returns an async sequence of all pairs from two input sequences (cartesian product); the second source is fully buffered before iteration, mirroring `Seq.allPairs`.
+* Added `AsyncSeq.rev` — returns a new async sequence with all elements in reverse order; the entire source sequence is buffered before yielding, mirroring `Seq.rev`.
+
 ### 4.7.0
 
 * Added `AsyncSeq.splitAt` — splits a sequence at the given index, returning the first `count` elements as an array and the remaining elements as a new `AsyncSeq`. Mirrors `Seq.splitAt`. The source is enumerated once.

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -1295,6 +1295,23 @@ module AsyncSeq =
   let reduce (f: 'T -> 'T -> 'T) (source: AsyncSeq<'T>) : Async<'T> =
       reduceAsync (fun a b -> f a b |> async.Return) source
 
+  let mapFoldAsync (folder: 'State -> 'T -> Async<'Result * 'State>) (state: 'State) (source: AsyncSeq<'T>) : Async<'Result array * 'State> = async {
+      let results = ResizeArray<'Result>()
+      let mutable st = state
+      use ie = source.GetEnumerator()
+      let! move = ie.MoveNext()
+      let b = ref move
+      while b.Value.IsSome do
+          let! (r, st') = folder st b.Value.Value
+          results.Add(r)
+          st <- st'
+          let! next = ie.MoveNext()
+          b := next
+      return (results.ToArray(), st) }
+
+  let mapFold (folder: 'State -> 'T -> 'Result * 'State) (state: 'State) (source: AsyncSeq<'T>) : Async<'Result array * 'State> =
+      mapFoldAsync (fun st x -> folder st x |> async.Return) state source
+
   let length (source : AsyncSeq<'T>) =
     fold (fun st _ -> st + 1L) 0L source
 
@@ -1702,6 +1719,25 @@ module AsyncSeq =
   let zipWith3 (f:'T1 -> 'T2 -> 'T3 -> 'U) (source1:AsyncSeq<'T1>) (source2:AsyncSeq<'T2>) (source3:AsyncSeq<'T3>) : AsyncSeq<'U> =
       zipWithAsync3 (fun a b c -> f a b c |> async.Return) source1 source2 source3
 
+  let allPairs (source1: AsyncSeq<'T1>) (source2: AsyncSeq<'T2>) : AsyncSeq<'T1 * 'T2> = asyncSeq {
+      let buf = System.Collections.Generic.List<'T2>()
+      use ie2 = source2.GetEnumerator()
+      let! move2 = ie2.MoveNext()
+      let b2 = ref move2
+      while b2.Value.IsSome do
+          buf.Add(b2.Value.Value)
+          let! next2 = ie2.MoveNext()
+          b2 := next2
+      use ie1 = source1.GetEnumerator()
+      let! move1 = ie1.MoveNext()
+      let b1 = ref move1
+      while b1.Value.IsSome do
+          let x = b1.Value.Value
+          for y in buf do
+              yield (x, y)
+          let! next1 = ie1.MoveNext()
+          b1 := next1 }
+
   let zappAsync (fs:AsyncSeq<'T -> Async<'U>>) (s:AsyncSeq<'T>) : AsyncSeq<'U> =
       zipWithAsync (|>) s fs
 
@@ -1994,6 +2030,11 @@ module AsyncSeq =
 
   let sortWith (comparer:'T -> 'T -> int) (source:AsyncSeq<'T>) : array<'T> =
     toSortedSeq (Array.sortWith comparer) source
+
+  let rev (source: AsyncSeq<'T>) : AsyncSeq<'T> = asyncSeq {
+      let! arr = toArrayAsync source
+      for i in arr.Length - 1 .. -1 .. 0 do
+          yield arr.[i] }
   #endif
 
   #if !FABLE_COMPILER

src/FSharp.Control.AsyncSeq/AsyncSeq.fsi

@@ -258,6 +258,16 @@ module AsyncSeq =
     /// specified 'reduction' function. Raises InvalidOperationException if the sequence is empty.
     val reduce : reduction:('T -> 'T -> 'T) -> source:AsyncSeq<'T> -> Async<'T>
 
+    /// Asynchronously maps each element of the async sequence with an asynchronous folder function that
+    /// also threads an accumulator state through the computation. Returns the array of results and the
+    /// final state, mirroring Seq.mapFold.
+    val mapFoldAsync : folder:('State -> 'T -> Async<'Result * 'State>) -> state:'State -> source:AsyncSeq<'T> -> Async<'Result array * 'State>
+
+    /// Maps each element of the async sequence with a folder function that also threads an accumulator
+    /// state through the computation. Returns the array of results and the final state,
+    /// mirroring Seq.mapFold.
+    val mapFold : folder:('State -> 'T -> 'Result * 'State) -> state:'State -> source:AsyncSeq<'T> -> Async<'Result array * 'State>
+
     /// Asynchronously sum the elements of the input asynchronous sequence using the specified function.
     val inline sum : source:AsyncSeq< ^T > -> Async< ^T>
                                 when ^T : (static member ( + ) : ^T * ^T -> ^T)
@@ -496,6 +506,10 @@ module AsyncSeq =
     /// The resulting sequence stops when any of the argument sequences stop.
     val zipWith3 : mapping:('T1 -> 'T2 -> 'T3 -> 'U) -> source1:AsyncSeq<'T1> -> source2:AsyncSeq<'T2> -> source3:AsyncSeq<'T3> -> AsyncSeq<'U>
 
+    /// Returns an async sequence of all pairs of elements from the two input sequences.
+    /// The second sequence is fully buffered before iteration begins, mirroring Seq.allPairs.
+    val allPairs : source1:AsyncSeq<'T1> -> source2:AsyncSeq<'T2> -> AsyncSeq<'T1 * 'T2>
+
     /// Builds a new asynchronous sequence whose elements are generated by
     /// applying the specified function to all elements of the input sequence.
     ///
@@ -654,6 +668,11 @@ module AsyncSeq =
     /// that sequence is iterated. As a result this function should not be used with
     /// large or infinite sequences.
     val sortWith : comparer:('T -> 'T -> int) -> source:AsyncSeq<'T> -> array<'T>
+
+    /// Returns a new async sequence with the elements in reverse order. The entire source
+    /// sequence is buffered before yielding any elements, mirroring Seq.rev.
+    /// This function should not be used with large or infinite sequences.
+    val rev : source:AsyncSeq<'T> -> AsyncSeq<'T>
     #endif
 
     /// Interleaves two async sequences of the same type into a resulting sequence. The provided

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

@@ -3409,6 +3409,84 @@ let ``AsyncSeq.sortWith returns empty array for empty sequence`` () =
   let result = AsyncSeq.sortWith compare AsyncSeq.empty<int>
   Assert.AreEqual([||], result)
 
+// ── AsyncSeq.mapFold ──────────────────────────────────────────────────────────
+
+[<Test>]
+let ``AsyncSeq.mapFold maps elements and accumulates state`` () =
+  let source = asyncSeq { yield 1; yield 2; yield 3 }
+  let results, finalState =
+    AsyncSeq.mapFold (fun acc x -> (x * 2, acc + x)) 0 source |> Async.RunSynchronously
+  Assert.AreEqual([| 2; 4; 6 |], results)
+  Assert.AreEqual(6, finalState)
+
+[<Test>]
+let ``AsyncSeq.mapFold returns empty array and initial state for empty sequence`` () =
+  let results, finalState =
+    AsyncSeq.mapFold (fun acc x -> (x, acc + x)) 99 AsyncSeq.empty<int> |> Async.RunSynchronously
+  Assert.AreEqual([||], results)
+  Assert.AreEqual(99, finalState)
+
+[<Test>]
+let ``AsyncSeq.mapFoldAsync maps elements asynchronously and accumulates state`` () =
+  let source = asyncSeq { yield 10; yield 20; yield 30 }
+  let results, finalState =
+    AsyncSeq.mapFoldAsync (fun acc x -> async { return (x + 1, acc + x) }) 0 source
+    |> Async.RunSynchronously
+  Assert.AreEqual([| 11; 21; 31 |], results)
+  Assert.AreEqual(60, finalState)
+
+[<Test>]
+let ``AsyncSeq.mapFoldAsync returns empty array and initial state for empty sequence`` () =
+  let results, finalState =
+    AsyncSeq.mapFoldAsync (fun acc x -> async { return (x, acc + 1) }) 5 AsyncSeq.empty<int>
+    |> Async.RunSynchronously
+  Assert.AreEqual([||], results)
+  Assert.AreEqual(5, finalState)
+
+// ── AsyncSeq.allPairs ────────────────────────────────────────────────────────
+
+[<Test>]
+let ``AsyncSeq.allPairs returns cartesian product`` () =
+  let s1 = asyncSeq { yield 1; yield 2 }
+  let s2 = asyncSeq { yield 'a'; yield 'b'; yield 'c' }
+  let result =
+    AsyncSeq.allPairs s1 s2 |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual(
+    [| (1,'a'); (1,'b'); (1,'c'); (2,'a'); (2,'b'); (2,'c') |],
+    result)
+
+[<Test>]
+let ``AsyncSeq.allPairs returns empty when first source is empty`` () =
+  let result =
+    AsyncSeq.allPairs AsyncSeq.empty<int> (asyncSeq { yield 1; yield 2 })
+    |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+[<Test>]
+let ``AsyncSeq.allPairs returns empty when second source is empty`` () =
+  let result =
+    AsyncSeq.allPairs (asyncSeq { yield 1; yield 2 }) AsyncSeq.empty<int>
+    |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+// ── AsyncSeq.rev ─────────────────────────────────────────────────────────────
+
+[<Test>]
+let ``AsyncSeq.rev reverses a sequence`` () =
+  let source = asyncSeq { yield 1; yield 2; yield 3; yield 4; yield 5 }
+  let result = AsyncSeq.rev source |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| 5; 4; 3; 2; 1 |], result)
+
+[<Test>]
+let ``AsyncSeq.rev returns empty sequence for empty input`` () =
+  let result = AsyncSeq.rev AsyncSeq.empty<int> |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+[<Test>]
+let ``AsyncSeq.rev returns singleton for single element`` () =
+  let result = AsyncSeq.rev (asyncSeq { yield 42 }) |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| 42 |], result)
+
 [<Test>]
 let ``AsyncSeq.splitAt splits a sequence at the given index`` () =
   let source = asyncSeq { yield 1; yield 2; yield 3; yield 4; yield 5 }