Upgrade to LanguageExt v5 (beta)

[fw2568]

Target branch: v5-beta (seperated until LanguageExt v5 is officially released)

Summary

Upgrades Dbosoft.Functional from LanguageExt v4 (4.4.2) to v5 (5.0.0-beta-77), including a comprehensive compatibility shim layer to ease migration for downstream consumers. This is a major version bump with significant breaking changes in LanguageExt itself.

Key Changes

LanguageExt v5 Upgrade

• LanguageExt.Core: 4.4.2 → 5.0.0-beta-77
• Target Framework: netstandard2.0 → net10.0 (required by LanguageExt v5)
• C# Language Version: bumped to C# 14 (for extension operators/members)

Compatibility Shim Layer (src/Dbosoft.Functional/Compat/)

A full set of backward-compatible shims that allow downstream consumers (e.g. YaNco) to migrate incrementally:

Shim	Purpose
EitherAsync<L,R>	Wraps Task<Either<L,R>> — replaces removed EitherAsync
Aff<R> / Aff<RT,R>	Wraps Eff<R> / Eff<RT,R> — replaces removed Aff
TryAsync<A>	Wraps Func<Task<Fin<A>>> — replaces removed TryAsync
NewType<NEWTYPE,A,PRED,ORD>	Record-based reimplementation of removed NewType
ClassInstances	Shims for OrdStringOrdinalIgnoreCase and related
TypeClasses	Shims for Ord<A>, Eq<A> traits
CompatPrelude	Bare TryAsync(...), RightAsync(...), LeftAsync(...) calls
CompatExtensions	IEnumerable.Map(), Task<Either>.MapAsync(), Option.Flatten(), TraverseParallel, etc.

New Packages / Projects

• Dbosoft.Functional.Json — System.Text.Json converter for LanguageExt NewType (new project)
• AwesomeAssertions.LanguageExt — Test assertion extensions for Either, Option, Fin, Validation (replaces FluentAssertions.LanguageExt which is v4-only)

Existing Code Adaptations

• EitherExtensions — updated for v5 type changes
• AffExtensions — adapted for Aff→Eff merge
• UseExtensions — adapted for removed EitherAsync overloads
• ValidatingNewType — redesigned for v5 record-based NewType
• ErrorExtensions — verified compatible (Error changed from struct to abstract record)
• Validation extensions use CombineAll() instead of | operator (semantics changed in v5)

Migration Guide

A detailed migration guide is included in MIGRATION-v5.md documenting all 33 breaking changes encountered and their resolutions.

For downstream consumers

1. Add using Dbosoft.Functional.Compat; to continue using EitherAsync, Aff, TryAsync, NewType
2. Add using static LanguageExt.CompatPrelude; for bare prelude calls (TryAsync(...), etc.)
3. Use named parameters for Match() calls — parameter order reversed in v5
4. Replace Aff with Eff in new code (shims provide bridge for existing code)
5. Replace .Traverse() / .Sequence() on IEnumerable<Either> with imperative alternatives

Why Parked

LanguageExt v5 is still in beta (5.0.0-beta-77). This branch is parked as v5-beta until:

1. LanguageExt v5 reaches a stable release
2. Any late-breaking API changes in LanguageExt are absorbed

Once LanguageExt v5 is released, this branch can be rebased onto master and finalized.