[CaffeinatedCoder]

Closes #128

Problem

Choose and every TraverseAsync took an IEnumerable<T>, and nothing in CodoMetis.TypeKit took an IAsyncEnumerable<T>. A paged API, a streamed import or an EF query read with AsAsyncEnumerable() had to be buffered into a list first, or unwrapped by hand with TryGetValue in an await foreach.

Change

• OptionExtensions.cs, SequenceOptionExtensions.cs: Choose() over IAsyncEnumerable<Option<T>> and Choose(selector) over IAsyncEnumerable<T>. Both are lazy and keep order, and both check their arguments at the call. The token from WithCancellation/GetAsyncEnumerator reaches the source ([EnumeratorCancellation]).
• ResultExtensions.cs: SequenceAsync(cancellationToken = default) over a stream of Result<T, TError> and of Result<TError>, plus the four TraverseAsync overloads over IAsyncEnumerable<T>, the same overloads as the sequence ones. They read the next element only after the one before it succeeded. A token they are given goes to the enumerator and is checked before each read and each operation. The enumerator is disposed on every exit. Every await is ConfigureAwait(false).
• Name check, done before the code (recorded in design/option-result.md):
  • No name binds ambiguously beside .NET 10's System.Linq.AsyncEnumerable.
  • T[] and EF Core 10.0.12's DbSet<T>, which has no IAsyncEnumerable<T> conversion, still bind to the sequence members.
  • A type implementing both IEnumerable<T> and IAsyncEnumerable<T> is CS0121 for Choose(selector) and TraverseAsync, as it already is for the BCL's Where. I left it at that rather than add an overload priority, since silently picking either reading is worse than a compile error.
• CMTK0003 already reports a dropped SequenceAsync or stream TraverseAsync by its return type: awaited or not, and converted to a plain Task. There is no analyzer change; a new test pins it.
• Docs:
  • The core README's "Chain steps" has a compiled streams sample and a paragraph.
  • The Option section has one sentence.
  • The docs/option.md and docs/result.md surface tables have rows.
  • The changelog has a line under 2.3.0.
• test/oracles: five A01-* claims for the new README sentences, held by AsyncStreamTests. Since test: read the unread documents for claims #140 removed the blanket Option.*/Result.* waivers, they are what holds Result.SequenceAsync, and they join test: read the unread documents for claims #140's claims on Choose and TraverseAsync. No waiver names a new member.

Tests

• AsyncStreamTests runs over a stream whose elements arrive at once and over one whose elements each wait. It checks:

  • order is kept;
  • default options act as None, the same as over a sequence;
  • null elements are accepted;
  • reading is lazy;
  • the token reaches the enumerator through WithCancellation, GetAsyncEnumerator and the token parameter;
  • cancellation mid-stream stops reading, including from a stream that ignores its token;
  • a cancelled token reads nothing;
  • the enumerator is disposed on the first error, on a break, and when the selector throws;
  • an uninitialized Result is refused;
  • a consumer compilation beside the async LINQ operators has no warning or ambiguity.

• NullDelegateTests, NullTaskTests, UninitializedResultTests and AsynchronousContextTests cover the new members. AsynchronousContextTests' completeness check now also takes members that return an IAsyncEnumerable.

• IgnoredOutcomeAnalyzerTests.A_dropped_SequenceAsync_or_TraverseAsync_over_a_stream_reports_as_over_a_sequence.

• Seeds: 13 defects seeded in src/ one at a time, each caught by a failing test:

  • a token not passed to the stream;
  • each of the cancellation checks dropped;
  • ConfigureAwait(false) dropped from each member;
  • the enumerator left undisposed;
  • the selector checked late.

  The stream TraverseAsync context case first missed ConfigureAwait(false), because its stream completed at once. It now reads a stream whose first element is still pending.

• Local runs:

  • full solution through build-verdict: 18,547 passed, 471 skipped;
  • dotnet pack of the core package passes package validation against the 2.2.0 baseline.

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