v4.x to v5.x

This page describes a historical upgrade. Its replacement code was correct for 5.x, and later majors have removed some of the API it names. Land on 5.x first if you are following these steps, then read the pages after this one in order. Deprecations lists what has gone since.

Upgrade with an agent — copy this prompt

Pointed at your solution, in Claude Code or a similar tool.

Upgrade this solution from Waystone.Monads v4 to v5. Report what you changed.

1. Build and capture every CS0029 and CS1503 involving Task and ValueTask on a
   Waystone async extension. In v5 several of those extensions return ValueTask<T>
   where they returned Task<T>.

2. For each one, prefer changing the declared type of the local or field to
   ValueTask<T>, or simply awaiting the value. Add .AsTask() only where the value is
   passed to something that demands a Task — Task.WhenAll is the usual case.
   .AsTask() allocates; do not reach for it first.

3. Find every async lambda passed to a Waystone async extension that returns a
   ValueTask<T>. v5 no longer accepts those. Change the lambda to return T or Task<T>.

4. Build again and report anything left. Do not suppress a diagnostic or add a
   null-forgiving ! to make one go away.

Simplified the API for chaining async extension methods on Option<T> and Result<TOk, TErr> , removing support for async lambdas that returned a ValueTask<T>. Additionally, optimized the return type of specific extensions that would return either a synchronous value or a Task to have a type of ValueTask.

-Task<string> output = result.MatchAsync(async x => await doWork(x), e => e.ToString());
+ValueTask<string> output = result.MatchAsync(async x => await doWork(x), e => e.ToString());