Umbrella: Funcky 4.0.0, The Closed Hierarchy Update #1035

Umbrella for the 4.0.0 milestone, groomed on 29 Oct 2026. Funcky 4.0.0 redefines the monads on the .NET 11 type system (C# unions and closed hierarchies) and ships the breaking changes that 3.8.0 prepared with obsoletions and analyzer code fixes.

Release plan

Target date 10 November 2026, the .NET 11 release day; the first time Funcky ships close to a .NET release. 4.0.0 is not released before .NET 11 is available, and global.json moves from the release candidate to the GA SDK in the last PR before the tag. Packages: Funcky 4.0.0 | Funcky.Async 2.0.0 | Funcky.Xunit 3.0.0 | Funcky.Xunit.v3 2.0.0 | Funcky.Analyzers 3.0.0.

1. Mechanical breaking changes (no design dependency, start now)

• Build with the .NET 11 SDK and add the net11.0 asset #1031 Build with the .NET 11 SDK, add the net11.0 asset
• Drop the target frameworks announced in SupportPolicy.md (net7.0, net6.0, net5.0, netcoreapp3.1) #1032 Drop net7.0, net6.0, net5.0 and netcoreapp3.1
• Update Roslyn packages to their latest version #814 SDK baseline .NET 10, Roslyn 4.14 for generators and analyzers
• Remove Option.FromBoolean #901 Remove Option.FromBoolean
• Funcky.Async 2.0: require System.Linq.Async 7 and drop the pre-7 operator shims #925 + Remove the obsolete Await/AwaitWithCancellation async operators #981 Funcky.Async 2.0: System.Linq.Async 7 floor, remove the obsolete async operators (one PR)
• Parameter order of Match parameters is inconsistent #683 Result.Match(error, ok)
• 4.0.0: decide the fate of the classic forms behind λ1201–λ1203 and the rules' severity #1003 Remove FunctionalAssert and the Sequence constructors, keep the string parse extensions; retire λ1202/λ1203
• Assertion failure label must become Assert.X() when FunctionalAssert is removed #1019 Failure label Assert.X() (goes with 4.0.0: decide the fate of the classic forms behind λ1201–λ1203 and the rules' severity #1003)
• Deprecate Funcky.DiscriminatedUnion with .NET11 #900 Retire Funcky.DiscriminatedUnion

2. Design spikes (time-boxed, same branch)

• Spike: model Option, Either and Result on C# unions and closed hierarchies #1034 Spike: Option, Either, Result on C# unions and closed hierarchies
• Research the Type-Traits idea from lang-ext (HKT via static abstract) #862 Spike: higher-kinded traits via static abstract members (expected to be rejected, but worth seeing)
• Source generator for the monad cross products (Traverse/Sequence, Partition, Task binds, applicative lifting) #1036 Source generator for the monad cross products: the "else" branch if Research the Type-Traits idea from lang-ext (HKT via static abstract) #862 is rejected (Traverse/Sequence, Partition, Task binds, applicative lifting)

3. Design-dependent, after the spikes decide

• Simpler Monad Constructors #809 Simpler monad constructors
• collection expressions for Option<T> #760 Collection expressions for Option<T>
• Add extension versions of UpCast and DownCast #868 Extension versions of UpCast/DownCast

4. New types and additive work

• Implement a Validation Applicative #802 Validation applicative (stays in 4.0.0: a big change to the type system, and the only applicative if traits land)
• Make Task a Monadic type #864 Task as a monadic type (stays in 4.0.0 with the lowest priority; both may move to 4.1 if the date gets tight)
• Support xUnit.v3 version 2, 3, and 4. #1015 xunit.v3 matrix leg for 2.x and 3.x
• Documentation: upgrade guide for 4.0.0 (and the older upgrades the book never got) #1033 Documentation: upgrade guide for 4.0.0 and the older upgrades

5. Release

• global.json to the .NET 11 GA SDK, release prep PR (changelog fold, PublicAPI fold, versions), tag on the merge commit.

Decisions taken on 29 Oct 2026

• Classic forms (4.0.0: decide the fate of the classic forms behind λ1201–λ1203 and the rules' severity #1003): FunctionalAssert and Sequence.* go, string.Parse…OrNone() stays. No 3.9 release: the migration recipe is "fix-all on 3.8.0, then upgrade", documented in Documentation: upgrade guide for 4.0.0 (and the older upgrades the book never got) #1033.
• The trait experiment (Research the Type-Traits idea from lang-ext (HKT via static abstract) #862) runs despite the recorded assessment, because 4.0.0 is the only moment it can be tried.
• Implement a Validation Applicative #802 and Make Task a Monadic type #864 stay in the milestone with the lowest priority.