LanguageExt - "Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement" in Either.Match

This is a follow-up to a question I asked earlier, but is (I think) much simpler, so I'm asking it separately. To clarify the question, I've stripped this one down even further.

The code here uses the Language-Ext Nuget package.

The only data structure needed here is the following...

record Trans(int Id, decimal Amount)

I have a (very much simplified) method that calls an external service to approve a transaction...

static async Task<Either<int, string>> CallService(Trans transaction) =>
  transaction.Amount < 1.5M
    ? "Success"
    : -1;

I want to call this as part of code like this (again, very much simplified)...

static async Task<Either<int, string>> ApproveRefund(Trans transaction) =>
  await (
    from result in CallService(transaction).ToAsync()
    select result
  )
  .Match(result => result, s => s);

However, I get a compiler error "Error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement" on the two lambdas I pass to the Match method.

I can't work out what I've done wrong here. Anyone able to help? Thanks

Solution

I think you'd need to be more explicit with the return types...

.Match(Right<int, string>, Left<int, string>);

I'm not actually sure why you need to do this, but I have hit this issue occasionally, and this fixed it.

Maybe one of the experts can explain why, but that should get you going.