[Repo Assist] Add AsyncSeq.mapAsyncUnorderedParallelThrottled

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -925,6 +925,34 @@ module AsyncSeq =
         | Choice1Of2 value -> return value
         | Choice2Of2 ex -> return raise ex })
   }
+
+  let mapAsyncUnorderedParallelThrottled (parallelism:int) (f:'a -> Async<'b>) (s:AsyncSeq<'a>) : AsyncSeq<'b> = asyncSeq {
+    use mb = MailboxProcessor.Start (fun _ -> async.Return())
+    use sm = new SemaphoreSlim(parallelism)
+    let! err =
+      s
+      |> iterAsync (fun a -> async {
+        do! sm.WaitAsync () |> Async.awaitTaskUnitCancellationAsError
+        let! b = Async.StartChild (async {
+          try
+            let! result = f a
+            sm.Release() |> ignore
+            return Choice1Of2 result
+          with ex ->
+            sm.Release() |> ignore
+            return Choice2Of2 ex
+        })
+        mb.Post (Some b) })
+      |> Async.map (fun _ -> mb.Post None)
+      |> Async.StartChildAsTask
+    yield!
+      replicateUntilNoneAsync (Task.chooseTask (err |> Task.taskFault) (async.Delay mb.Receive))
+      |> mapAsync (fun childAsync -> async {
+        let! result = childAsync
+        match result with
+        | Choice1Of2 value -> return value
+        | Choice2Of2 ex -> return raise ex })
+  }
   #endif
 
   let chooseAsync f (source:AsyncSeq<'T>) =

src/FSharp.Control.AsyncSeq/AsyncSeq.fsi

@@ -543,6 +543,14 @@ module AsyncSeq =
     /// Parallelism is bound by the ThreadPool.
     val mapAsyncUnorderedParallel : mapping:('T -> Async<'U>) -> s:AsyncSeq<'T> -> AsyncSeq<'U>
 
+    /// Builds a new asynchronous sequence whose elements are generated by
+    /// applying the specified function to all elements of the input sequence,
+    /// with at most <c>parallelism</c> mapping operations running concurrently.
+    ///
+    /// The function is applied to elements in parallel (throttled), and results are emitted
+    /// in the order they complete (unordered), without preserving the original order.
+    val mapAsyncUnorderedParallelThrottled : parallelism:int -> mapping:('T -> Async<'U>) -> s:AsyncSeq<'T> -> AsyncSeq<'U>
+
     /// Applies a key-generating function to each element and returns an async sequence containing unique keys
     /// and async sequences containing elements corresponding to the key.
     ///

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

@@ -1717,6 +1717,59 @@ let ``AsyncSeq.mapAsyncUnorderedParallel should not preserve order`` () =
   Assert.IsTrue(allPresent, "All input elements should be present in results")
 
 
+[<Test>]
+let ``AsyncSeq.mapAsyncUnorderedParallelThrottled should produce all results`` () =
+  let input = [1; 2; 3; 4; 5]
+  let expected = [2; 4; 6; 8; 10] |> Set.ofList
+
+  let actual =
+    input
+    |> AsyncSeq.ofSeq
+    |> AsyncSeq.mapAsyncUnorderedParallelThrottled 3 (fun x -> async {
+      do! Async.Sleep(10)
+      return x * 2
+    })
+    |> AsyncSeq.toListAsync
+    |> runTest
+    |> Set.ofList
+
+  Assert.AreEqual(expected, actual)
+
+[<Test>]
+let ``AsyncSeq.mapAsyncUnorderedParallelThrottled should propagate handler exception`` () =
+  let res =
+    AsyncSeq.init 100L id
+    |> AsyncSeq.mapAsyncUnorderedParallelThrottled 10 (fun i -> async {
+      if i = 50L then return failwith "oh no"
+      else return i * 2L
+    })
+    |> AsyncSeq.toListAsync
+    |> Async.Catch
+    |> (fun x -> Async.RunSynchronously (x, timeout = 10000))
+
+  match res with
+  | Choice2Of2 _ -> ()
+  | Choice1Of2 _ -> Assert.Fail ("error expected")
+
+[<Test>]
+let ``AsyncSeq.mapAsyncUnorderedParallelThrottled should throttle`` () =
+  let count = ref 0
+  let parallelism = 5
+
+  let result =
+    AsyncSeq.init 50L id
+    |> AsyncSeq.mapAsyncUnorderedParallelThrottled parallelism (fun i -> async {
+      let c = Interlocked.Increment count
+      if c > parallelism then
+        return failwith (sprintf "concurrency exceeded: %d > %d" c parallelism)
+      do! Async.Sleep 5
+      Interlocked.Decrement count |> ignore
+      return i * 2L })
+    |> AsyncSeq.toListAsync
+    |> Async.RunSynchronously
+
+  Assert.AreEqual(50, result.Length)
+
 //[<Test>]
 //let ``AsyncSeq.mapParallelAsyncBounded should maintain order`` () =
 //  let ls = List.init 500 id