Add AsyncSeq.transpose — mirrors Seq.transpose

Transposes a sequence of sequences: each element of the result is an
array of the i-th elements of all inner sequences. All inner sequences
must have the same length; the entire source is buffered before yielding.
Available on all targets except Fable.

Closes no specific issue; mirrors Seq.transpose.

Co-authored-by: Copilot <[email protected]>

Author: github-actions[bot]

RELEASE_NOTES.md

@@ -1,3 +1,7 @@
+### 4.9.0
+
+* Added `AsyncSeq.transpose` — transposes a sequence of sequences so each element of the result is an array of the i-th elements of all inner sequences; all inner sequences must have the same length and the entire source is buffered, mirroring `Seq.transpose`.
+
 ### 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`.

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -2035,6 +2035,17 @@ module AsyncSeq =
       let! arr = toArrayAsync source
       for i in arr.Length - 1 .. -1 .. 0 do
           yield arr.[i] }
+
+  let transpose (source: AsyncSeq<#seq<'T>>) : AsyncSeq<'T[]> = asyncSeq {
+      let! rows = toArrayAsync source
+      if rows.Length > 0 then
+          let rowArrays = rows |> Array.map (fun r -> (r :> seq<'T>) |> Seq.toArray)
+          let colCount = rowArrays.[0].Length
+          for row in rowArrays do
+              if row.Length <> colCount then
+                  invalidArg "source" "All inner sequences must have the same length."
+          for c in 0 .. colCount - 1 do
+              yield [| for row in rowArrays -> row.[c] |] }
   #endif
 
   #if !FABLE_COMPILER

src/FSharp.Control.AsyncSeq/AsyncSeq.fsi

@@ -673,6 +673,12 @@ module AsyncSeq =
     /// 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>
+
+    /// Transposes a sequence of sequences: each element of the result is an array
+    /// of the i-th elements of all inner sequences, mirroring Seq.transpose.
+    /// All inner sequences must have the same length; the entire source is buffered.
+    /// This function should not be used with large or infinite sequences.
+    val transpose : source:AsyncSeq<#seq<'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

@@ -3662,3 +3662,44 @@ let ``AsyncSeq.insertAt raises ArgumentException when index exceeds length`` ()
     |> AsyncSeq.toArrayAsync
     |> Async.RunSynchronously |> ignore)
   |> ignore
+
+// ── AsyncSeq.transpose ────────────────────────────────────────────────────────
+
+[<Test>]
+let ``AsyncSeq.transpose transposes rows and columns`` () =
+  let source = asyncSeq { yield [1; 2; 3]; yield [4; 5; 6] }
+  let result = AsyncSeq.transpose source |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| [|1;4|]; [|2;5|]; [|3;6|] |], result)
+
+[<Test>]
+let ``AsyncSeq.transpose single row`` () =
+  let source = asyncSeq { yield [10; 20; 30] }
+  let result = AsyncSeq.transpose source |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| [|10|]; [|20|]; [|30|] |], result)
+
+[<Test>]
+let ``AsyncSeq.transpose empty source returns empty`` () =
+  let result = AsyncSeq.transpose AsyncSeq.empty<int[]> |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+[<Test>]
+let ``AsyncSeq.transpose single column`` () =
+  let source = asyncSeq { yield [1]; yield [2]; yield [3] }
+  let result = AsyncSeq.transpose source |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| [|1;2;3|] |], result)
+
+[<Test>]
+let ``AsyncSeq.transpose square matrix`` () =
+  let source = asyncSeq { yield [1; 2]; yield [3; 4] }
+  let result = AsyncSeq.transpose source |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| [|1;3|]; [|2;4|] |], result)
+
+[<Test>]
+let ``AsyncSeq.transpose raises on mismatched row lengths`` () =
+  Assert.Throws<System.ArgumentException>(fun () ->
+    asyncSeq { yield [1; 2]; yield [3] }
+    |> AsyncSeq.transpose
+    |> AsyncSeq.toArrayAsync
+    |> Async.RunSynchronously
+    |> ignore)
+  |> ignore