Add AsyncSeq.min, max, minBy, maxBy, minByAsync, maxByAsync (#243)

Six new aggregation combinators mirroring Seq.min/max/minBy/maxBy:
- minByAsync / maxByAsync: find element with min/max async-projected key
- minBy / maxBy: synchronous projection variants
- min / max: compare elements directly (require 'T : comparison)

All raise InvalidOperationException on empty sequences, matching Seq behaviour.
Includes 8 tests covering correctness, async projections, and empty-sequence errors.

Fixes shadowing of Operators.max in bufferByTime by qualifying the call.

Co-authored-by: Repo Assist <repo-assist@github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>

Author: 4 people

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -1185,6 +1185,46 @@ module AsyncSeq =
   let inline sum (source : AsyncSeq<'T>) : Async<'T> =
     (LanguagePrimitives.GenericZero, source) ||> fold (+)
 
+  let minByAsync (projection: 'T -> Async<'Key>) (source: AsyncSeq<'T>) : Async<'T> =
+    async {
+      let! result =
+        source |> foldAsync (fun (acc: ('T * 'Key) option) v ->
+          async {
+            let! k = projection v
+            match acc with
+            | None -> return Some (v, k)
+            | Some (_, ak) -> return if k < ak then Some (v, k) else acc
+          }) None
+      match result with
+      | None -> return raise (System.InvalidOperationException("The input sequence was empty."))
+      | Some (v, _) -> return v }
+
+  let minBy (projection: 'T -> 'Key) (source: AsyncSeq<'T>) : Async<'T> =
+    minByAsync (projection >> async.Return) source
+
+  let maxByAsync (projection: 'T -> Async<'Key>) (source: AsyncSeq<'T>) : Async<'T> =
+    async {
+      let! result =
+        source |> foldAsync (fun (acc: ('T * 'Key) option) v ->
+          async {
+            let! k = projection v
+            match acc with
+            | None -> return Some (v, k)
+            | Some (_, ak) -> return if k > ak then Some (v, k) else acc
+          }) None
+      match result with
+      | None -> return raise (System.InvalidOperationException("The input sequence was empty."))
+      | Some (v, _) -> return v }
+
+  let maxBy (projection: 'T -> 'Key) (source: AsyncSeq<'T>) : Async<'T> =
+    maxByAsync (projection >> async.Return) source
+
+  let min (source: AsyncSeq<'T>) : Async<'T> =
+    minBy id source
+
+  let max (source: AsyncSeq<'T>) : Async<'T> =
+    maxBy id source
+
   let inline sumBy (projection : 'T -> ^U) (source : AsyncSeq<'T>) : Async<^U> =
     fold (fun s x -> s + projection x) LanguagePrimitives.GenericZero source
 
@@ -1747,7 +1787,7 @@ module AsyncSeq =
           | Some rem -> async.Return rem
           | None -> Async.StartChildAsTask(ie.MoveNext())
         let t = Stopwatch.GetTimestamp()
-        let! time = Async.StartChildAsTask(Async.Sleep (max 0 rt))
+        let! time = Async.StartChildAsTask(Async.Sleep (Operators.max 0 rt))
         let! moveOr = Async.chooseTasks move time
         let delta = int ((Stopwatch.GetTimestamp() - t) * 1000L / Stopwatch.Frequency)
         match moveOr with

src/FSharp.Control.AsyncSeq/AsyncSeq.fsi

@@ -227,6 +227,24 @@ module AsyncSeq =
                                 when ^T : (static member ( + ) : ^T * ^T -> ^T)
                                 and  ^T : (static member Zero : ^T)
 
+    /// Asynchronously find the element with the minimum projected value. Raises InvalidOperationException if the sequence is empty.
+    val minByAsync : projection:('T -> Async<'Key>) -> source:AsyncSeq<'T> -> Async<'T> when 'Key : comparison
+
+    /// Asynchronously find the element with the minimum projected value. Raises InvalidOperationException if the sequence is empty.
+    val minBy : projection:('T -> 'Key) -> source:AsyncSeq<'T> -> Async<'T> when 'Key : comparison
+
+    /// Asynchronously find the element with the maximum projected value. Raises InvalidOperationException if the sequence is empty.
+    val maxByAsync : projection:('T -> Async<'Key>) -> source:AsyncSeq<'T> -> Async<'T> when 'Key : comparison
+
+    /// Asynchronously find the element with the maximum projected value. Raises InvalidOperationException if the sequence is empty.
+    val maxBy : projection:('T -> 'Key) -> source:AsyncSeq<'T> -> Async<'T> when 'Key : comparison
+
+    /// Asynchronously find the minimum element. Raises InvalidOperationException if the sequence is empty.
+    val min : source:AsyncSeq<'T> -> Async<'T> when 'T : comparison
+
+    /// Asynchronously find the maximum element. Raises InvalidOperationException if the sequence is empty.
+    val max : source:AsyncSeq<'T> -> Async<'T> when 'T : comparison
+
     /// Asynchronously sum the mapped elements of an asynchronous sequence using a synchronous projection.
     val inline sumBy : projection:('T -> ^U) -> source:AsyncSeq<'T> -> Async< ^U>
                                 when ^U : (static member ( + ) : ^U * ^U -> ^U)

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

@@ -211,6 +211,54 @@ let ``AsyncSeq.sum works``() =
       let expected = ls |> List.sum
       Assert.True((expected = actual))
 
+[<Test>]
+let ``AsyncSeq.min returns minimum element``() =
+  for i in 1 .. 10 do
+      let ls = [ 1 .. i ] |> List.rev
+      let actual = AsyncSeq.ofSeq ls |> AsyncSeq.min |> Async.RunSynchronously
+      Assert.AreEqual(1, actual)
+
+[<Test>]
+let ``AsyncSeq.min raises on empty sequence``() =
+  Assert.Throws<System.InvalidOperationException>(fun () ->
+      AsyncSeq.empty<int> |> AsyncSeq.min |> Async.RunSynchronously |> ignore) |> ignore
+
+[<Test>]
+let ``AsyncSeq.max returns maximum element``() =
+  for i in 1 .. 10 do
+      let ls = [ 1 .. i ]
+      let actual = AsyncSeq.ofSeq ls |> AsyncSeq.max |> Async.RunSynchronously
+      Assert.AreEqual(i, actual)
+
+[<Test>]
+let ``AsyncSeq.max raises on empty sequence``() =
+  Assert.Throws<System.InvalidOperationException>(fun () ->
+      AsyncSeq.empty<int> |> AsyncSeq.max |> Async.RunSynchronously |> ignore) |> ignore
+
+[<Test>]
+let ``AsyncSeq.minBy returns element with minimum projected value``() =
+  let ls = [ ("b", 2); ("a", 1); ("c", 3) ]
+  let actual = AsyncSeq.ofSeq ls |> AsyncSeq.minBy snd |> Async.RunSynchronously
+  Assert.AreEqual(("a", 1), actual)
+
+[<Test>]
+let ``AsyncSeq.maxBy returns element with maximum projected value``() =
+  let ls = [ ("b", 2); ("a", 1); ("c", 3) ]
+  let actual = AsyncSeq.ofSeq ls |> AsyncSeq.maxBy snd |> Async.RunSynchronously
+  Assert.AreEqual(("c", 3), actual)
+
+[<Test>]
+let ``AsyncSeq.minByAsync uses async projection``() =
+  let ls = [ 3; 1; 4; 1; 5; 9 ]
+  let actual = AsyncSeq.ofSeq ls |> AsyncSeq.minByAsync (fun x -> async.Return x) |> Async.RunSynchronously
+  Assert.AreEqual(1, actual)
+
+[<Test>]
+let ``AsyncSeq.maxByAsync uses async projection``() =
+  let ls = [ 3; 1; 4; 1; 5; 9 ]
+  let actual = AsyncSeq.ofSeq ls |> AsyncSeq.maxByAsync (fun x -> async.Return x) |> Async.RunSynchronously
+  Assert.AreEqual(9, actual)
+
 [<Test>]
 let ``AsyncSeq.sumBy works``() =
   for i in 0 .. 10 do