Merge pull request #308 from fsprojects/repo-assist/improve-oflist-ofarray-cycle-20260413-aff071fc4272ec05

[Repo Assist] Add AsyncSeq.ofList, AsyncSeq.ofArray, and AsyncSeq.cycle

Author: dsyme

RELEASE_NOTES.md

@@ -1,3 +1,9 @@
+### 4.13.0
+
+* Added `AsyncSeq.ofList` — creates an async sequence from an F# list with an optimised direct-enumerator implementation (avoids `IEnumerator<T>` boxing).
+* Added `AsyncSeq.ofArray` — creates an async sequence from an array with an optimised index-based enumerator (avoids `IEnumerator<T>` boxing).
+* Added `AsyncSeq.cycle` — infinitely cycles through all elements of a source async sequence; returns empty if the source is empty.
+
 ### 4.12.0
 
 * Tests: Added tests for `mapiAsync`, `tryPickAsync`, `pickAsync`, and `groupByAsync` — these four async functions previously had no test coverage.

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -790,6 +790,35 @@ module AsyncSeq =
                                 dispose e
                             | _ -> () }) :> AsyncSeq<'T>
 
+  let ofList (source: 'T list) : AsyncSeq<'T> =
+    AsyncSeqImpl(fun () ->
+        let mutable remaining = source
+        { new IAsyncSeqEnumerator<'T> with
+              member _.MoveNext() =
+                  async {
+                      match remaining with
+                      | [] -> return None
+                      | h :: t ->
+                          remaining <- t
+                          return Some h
+                  }
+              member _.Dispose() = () }) :> AsyncSeq<'T>
+
+  let ofArray (source: 'T []) : AsyncSeq<'T> =
+    AsyncSeqImpl(fun () ->
+        let mutable i = 0
+        { new IAsyncSeqEnumerator<'T> with
+              member _.MoveNext() =
+                  async {
+                      if i < source.Length then
+                          let v = source.[i]
+                          i <- i + 1
+                          return Some v
+                      else
+                          return None
+                  }
+              member _.Dispose() = () }) :> AsyncSeq<'T>
+
   let appendSeq (seq2: seq<'T>) (source: AsyncSeq<'T>) : AsyncSeq<'T> =
     append source (ofSeq seq2)
 
@@ -2160,6 +2189,15 @@ module AsyncSeq =
   let toArraySynchronously (source:AsyncSeq<'T>) = toArrayAsync source |> Async.RunSynchronously
   #endif
 
+  let cycle (source: AsyncSeq<'T>) : AsyncSeq<'T> =
+    asyncSeq {
+        let! arr = source |> toArrayAsync
+        if arr.Length > 0 then
+            while true do
+                for x in arr do
+                    yield x
+    }
+
   let partitionAsync (predicate: 'T -> Async<bool>) (source: AsyncSeq<'T>) : Async<'T[] * 'T[]> = async {
     let trues = ResizeArray<'T>()
     let falses = ResizeArray<'T>()

src/FSharp.Control.AsyncSeq/AsyncSeq.fsi

@@ -60,6 +60,11 @@ module AsyncSeq =
     /// Creates an async sequence given by evaluating the specified async computation until it returns None.
     val replicateUntilNoneAsync : Async<'T option> -> AsyncSeq<'T>
 
+    /// Returns an async sequence which infinitely cycles through all elements of the source sequence.
+    /// The source is materialised into an array on first enumeration. Returns an empty sequence if
+    /// the source is empty.
+    val cycle : source:AsyncSeq<'T> -> AsyncSeq<'T>
+
     /// Returns an async sequence which emits an element on a specified period.
     val intervalMs : periodMs:int -> AsyncSeq<DateTime>
 
@@ -488,6 +493,14 @@ module AsyncSeq =
     /// input synchronous sequence and returns them one-by-one.
     val ofSeq : source:seq<'T> -> AsyncSeq<'T>
 
+    /// Creates an asynchronous sequence that lazily takes elements from an
+    /// F# list and returns them one-by-one.
+    val ofList : source:'T list -> AsyncSeq<'T>
+
+    /// Creates an asynchronous sequence that lazily takes elements from an
+    /// array and returns them one-by-one.
+    val ofArray : source:'T [] -> AsyncSeq<'T>
+
     /// Creates an asynchronous sequence that lazily takes element from an
     /// input synchronous sequence of asynchronous computation and returns them one-by-one.
     val ofSeqAsync : seq<Async<'T>> -> AsyncSeq<'T>

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

@@ -4436,3 +4436,69 @@ let ``AsyncSeq.groupByAsync with all-same key produces single group`` () =
     |> AsyncSeq.toArrayAsync
     |> Async.RunSynchronously
   Assert.AreEqual([| ("same", [|1;2;3|]) |], result)
+
+// ===== ofList =====
+
+[<Test>]
+let ``AsyncSeq.ofList returns elements in order`` () =
+  let result = AsyncSeq.ofList [1; 2; 3; 4; 5] |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| 1; 2; 3; 4; 5 |], result)
+
+[<Test>]
+let ``AsyncSeq.ofList on empty list returns empty`` () =
+  let result = AsyncSeq.ofList ([] : int list) |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+[<Test>]
+let ``AsyncSeq.ofList produces same result as ofSeq`` () =
+  let xs = [10; 20; 30]
+  let fromList = AsyncSeq.ofList xs |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  let fromSeq  = AsyncSeq.ofSeq xs  |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual(fromSeq, fromList)
+
+// ===== ofArray =====
+
+[<Test>]
+let ``AsyncSeq.ofArray returns elements in order`` () =
+  let result = AsyncSeq.ofArray [| 10; 20; 30 |] |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([| 10; 20; 30 |], result)
+
+[<Test>]
+let ``AsyncSeq.ofArray on empty array returns empty`` () =
+  let result = AsyncSeq.ofArray ([||] : int []) |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+[<Test>]
+let ``AsyncSeq.ofArray produces same result as ofSeq`` () =
+  let xs = [| 1; 2; 3; 4 |]
+  let fromArray = AsyncSeq.ofArray xs |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  let fromSeq   = AsyncSeq.ofSeq   xs |> AsyncSeq.toArrayAsync |> Async.RunSynchronously
+  Assert.AreEqual(fromSeq, fromArray)
+
+// ===== cycle =====
+
+[<Test>]
+let ``AsyncSeq.cycle repeats elements indefinitely`` () =
+  let result =
+    AsyncSeq.cycle (AsyncSeq.ofList [1; 2; 3])
+    |> AsyncSeq.take 7
+    |> AsyncSeq.toArrayAsync
+    |> Async.RunSynchronously
+  Assert.AreEqual([| 1; 2; 3; 1; 2; 3; 1 |], result)
+
+[<Test>]
+let ``AsyncSeq.cycle on empty sequence returns empty`` () =
+  let result =
+    AsyncSeq.cycle AsyncSeq.empty<int>
+    |> AsyncSeq.toArrayAsync
+    |> Async.RunSynchronously
+  Assert.AreEqual([||], result)
+
+[<Test>]
+let ``AsyncSeq.cycle on singleton repeats single element`` () =
+  let result =
+    AsyncSeq.cycle (AsyncSeq.singleton 42)
+    |> AsyncSeq.take 5
+    |> AsyncSeq.toArrayAsync
+    |> Async.RunSynchronously
+  Assert.AreEqual([| 42; 42; 42; 42; 42 |], result)