README.md

Runtime-async sample builders

These computation-expression builders are built on the compiler's runtime-async intrinsics (see docs/runtime-async.md). They are test fixtures and examples, not FSharp.Core APIs.

runtimeTask (RuntimeTaskBuilder.fs)

Produces hot Task<'T> and ValueTask<'T> values, like task { }. Source overloads turn tasks, custom awaitables and Async<'T> into started delegates, so Bind and MergeSources need no per-type overloads. and! starts every source before awaiting any of them.

async2 (Async2Builder.fs)

Produces cold, cancellable Async2<'T> computations. The cancellation token is threaded through the computation, checked between steps, and passed to cold sources such as Async<'T> and other async2 computations. and! starts the right source before awaiting the left.

asyncSeq2 (AsyncSeq2Builder.fs)

Produces IAsyncEnumerable<'T> using __runtimeAsyncSequence.

• let! and do! accept tasks, value tasks, custom awaitables, async2 and Async computations, and CancellationToken -> Task factories. Cold sources receive the enumeration's cancellation token.
• for and yield! accept both seq<'T> and IAsyncEnumerable<'T>.
• One driver enumerator runs nested yield! sources in a loop, so deep recursion does not grow the call stack.
• A yield! in tail position (YieldFromFinal) hands off to the new source and releases the enumerator it came from. A yield! that isn't in tail position keeps its parent enumerator until the nested source finishes.
• A second MoveNextAsync, or a DisposeAsync, while a move is pending throws InvalidOperationException.

Tests

• RuntimeTasks.fs: the FSharp.Core task { } tests, ported to runtimeTask.
• EnumerableTests.fs and RuntimeAsyncEnumerableTests.fs: asyncSeq2 behavior, including try/with, cancellation and recursion.
• RuntimeAsyncSequence.fs: a small asyncSeq2 usage and lifecycle example.