FSharp.Control.Async2

A separate asynchronous computation expression and module, renamed to Async2 and async2 so it can be developed independently without conflicting with the built-in implementation.

The current implementation uses the preview runtime-async compiler feature. Async2<'T> is a named cold carrier whose start-time CancellationToken is passed to a runtime-async Task<'T> factory. The library targets net11.0; tests use xUnit v3 with the Microsoft Testing Platform runner.

The root Directory.Build.props selects the local F# compiler at $(RuntimeAsyncBin)\fsc\Release\net11.0\fsc.dll and references $(RuntimeAsyncBin)\FSharp.Core\Release\net10.0\FSharp.Core.dll, with RuntimeAsyncBin defaulting to D:\source\fsharp\artifacts\bin. Override RuntimeAsyncBin or provide RuntimeAsyncBitsPath when building against another local F# checkout.

The focused AsyncType, AsyncModule, and AsyncModuleFunctions suites are accompanied by direct xUnit ports of older compiler-library regressions. RuntimeAsync compiler-component tests are intentionally outside this project.

AsyncSeq2

src/FSharp.Control.AsyncSeq2 remains a separate library referencing Async2, so applications using only async2 do not take on the sequence API or its packaging dependencies. It provides asyncSeq2 { ... } in Microsoft.FSharp.Control, yielding AsyncSeq2<'T> (an IAsyncEnumerable<'T>). It supports yield, yield!, for, let!/do! on tasks, value tasks, and Async2, plus synchronous or asynchronous use. The builder is adapted from FSharp.Control.TaskSeq (see src/FSharp.Control.AsyncSeq2/TASKSEQ-LICENSE.txt). Each enumeration is cold; GetAsyncEnumerator(token) passes its token to bound Async2 computations and nested asynchronous sequences. async2 can consume an AsyncSeq2<'T> directly with for. ofAsync2Seq, ofAsync2List, and ofAsync2Array turn collections of cold Async2 computations into sequences; ofSeqAsync remains an equivalent constructor for seq<Async2<'T>>.

AsyncSeq2 provides constructors, transformations, and the TaskSeq operation surface, as well as AsyncSeq-derived operations such as cycle, rev, splitInto, sortAsync, findBack, map2, allPairs, and unzip. The AsyncSeq-derived operators and the TaskSeq-compatible terminal operations use Async2 for asynchronous callbacks and cold terminal results; their cancellation token flows implicitly to the enumerator. Existing Task and ValueTask sources remain supported, but a previously started task cannot retroactively inherit an enumeration token. AsyncSeq2Src provides a broadcast source: toAsyncSeq subscribes at the current tail, and put, close, or error notify existing subscribers. See ASYNCSEQ-LICENSE.txt and TASKSEQ-LICENSE.txt for upstream attribution.

let values = asyncSeq2 {
    for i in 1..3 do
        do! Async2.Sleep 10
        yield i
}

let result = AsyncSeq2.toArray values |> Async2.RunSynchronously

FSharp.Control.AsyncSeq2.Tests contains xUnit v3/MTP builder, AsyncSeq operator, cleanup, composition, and cancellation tests. It also ports all 494 active tests from FSharp.Control.AsyncSeq's AsyncSeqTests.fs, adapting Async<'T> to Async2<'T> and accounting for the existing TaskSeq-compatible API (including strict take/skip, truncate/drop, array-valued groups, and removeManyAt range behavior). Async2.StartChild starts its child as soon as the parent creates it, so bounded channel producers and consumers can run concurrently. Disposing an enumerator from toBlockingSeq cancels its enumeration token. FSharp.Control.AsyncSeq2.TaskSeq.Tests ports all 74 non-smoke TaskSeq test files (including two upstream-skipped cases) to xUnit v3/MTP. Transformation tests call AsyncSeq2 directly with native async2 callbacks; xUnit test boundaries and explicit Task and ValueTask interoperability tests retain those carriers. The old F# Async<'T> sources and conversion helpers have been replaced by Async2<'T>; use Async2 computations for cold, cancellable operations.