A great example by <@U0RM4EPC7> using the new Effe...

# arrow

[raulraja]

A great example by @simon.vergauwen using the new Effect interface and arrow-continuations to create a DB effect block that can rollback transactions and handle DB results in the context of Either 👏 https://twitter.com/vergauwen_simon/status/1402897198296276992

[thanh]

@simon.vergauwen I just noticed a small error: https://gist.github.com/nomisRev/b6aced8ce552ae718791e187ebd6cdd4#file-transactioneither-kt-L33

should it be listOf(Either.Right("test-1"), Either.Right("test-2"), Either.Right("test-3"))

instead of listOf(Either.Right("test-1"), Either.Left("test-2"), Either.Right("test-3"))

[Pedro Sena]

that looks quite interesting, I was looking for something like this.
Any idea on how complicated would be to incorporate that into some javax.transaction implementation so we could mark methods with @Transactional but having Either return types ? (not sure if I make myself clear)

[simon.vergauwen]

@Pedro Sena does this have any programmatic API?

I think with Spring we can easily do this, and IIRC the programmatic API from Spring automatically works with any surrounding @Transactional.

[simon.vergauwen]

Thank you @thanh for pointing that out!!

[leonhardt]

@simon.vergauwen I'm trying to adapt your transactionEither example to recent versions of Arrow and SQLDelight but thought I'd ask, do you already have an updated example? Or has this idea ever made its way into a library?

[simon.vergauwen]

This has never made it into a library. Are you looking at this gist? https://gist.github.com/nomisRev/b6aced8ce552ae718791e187ebd6cdd4

[simon.vergauwen]

You don't really need that anymore, as you could simply do this. Important to note is that when the either { } is on the outside it will rollback on Left, and exceptions, when it's on the inside it will never rollback on Left but only on exceptions.

either {
  transactionWithResult { 
   
  }
}

[leonhardt]

Hey @simon.vergauwen! Yes we were looking at that gist. I know it's 2 years old but it's the only reference we could find for using Arrow Either in tandem with SQLDelight transactions. Thanks for explaining it really is that simple now, without the historical perspective on how the libraries have changed we ended up over complicating it.

Moving forward, we're using either { } on the outside of transactionWithResult { } as you suggested! Thank you. However we missed the simplicity of the implied right/left typing in the either DSL – working within the transactionWithResult DSL we need to provide "rolled back" results wrapped in a Left and successful results wrapped in a Right. So we made another extension function for our project that makes the types as simple as the either DSL but automatically rolls back the transaction if any error is raised.

import app.cash.sqldelight.Transacter
import arrow.core.Either
import arrow.core.left
import arrow.core.raise.Raise
import arrow.core.raise.either

fun <Error, A> Transacter.transactionEither(block: Raise<Error>.() -> A): Either<Error, A> {
  return transactionWithResult { either { block() }.onLeft { error -> rollback(error.left()) } }
}

It's making it easier to establish a convention in our codebase. I thought I'd share as an example for the search results and in case anyone has an even better way to share.