Equivalent of Scala cats EitherT<IO, A, B>

[TangkasOka]

Hi,

I'm porting some Scala code to .NET that uses the cats effect IO and cats data EitherT classes.

https://typelevel.org/cats-effect/docs/2.x/datatypes/io
https://typelevel.org/cats/datatypes/eithert.html

The IO is like the IO monad in Haskell in that it is for deferring actions with side effects to "the end of the world". I was thinking of replacing this with Aff.

The EitherT is an Either transformer that take a "highly kinded type" and two other types. I don't need a fully general version of this, just one specialized for Aff as the type to be transformed over.

I'm looking for suggestions on how to approach this.

• Is there already some standard equivalent of EitherT<IO, A, B> using existing LanguageExt classes/structs?
• If not, is it sensible to implement a new struct EitherAff<A, B> with the expected behavior?
• Or should the IO become a Task, and the EitherT an EitherAsync? I haven't really looked into EitherAsync.

Thanks,
Charlie

[bmazzarol]

Hi @TangkasOka,

I don't think you need a monad transformer over Aff.

Aff bakes in an error channel, and with the latest beta release it supports applicative behavior and can be extended to support custom error types.

I think it should meet all your requirements.

If you have a particular use case that you need help with (coded in a functional scala style using cats), I am more than happy to have a go (as best I can) at solving using language ext.

[louthy]

• Is there already some standard equivalent of EitherT<IO, A, B> using existing LanguageExt classes/structs?

C# doesn't have higher-kinds, so this just isn't possible. Any monad-transformer-like behaviour has to be built manually, either by creating a new type that wraps the composed monads (and then manually implementing Select, and SelectMany). Or, by writing extension methods for the nested type M<N<A>>.

If you're migrating a lot of code over from Scala, and you have a number of transformer stacks, then yes, it would make sense to wrap them in a new type and build the LINQ machinery for it.

[TangkasOka]

Thanks both. I'll start by taking a look at using Aff with an error channel.