What would be the equivalent of `onLeft {}` when w...

# arrow

[sindrenm]

What would be the equivalent of onLeft {} when working with the Raise DSL?

Given the following using when dealing with an API that exposes `Either`s:

fun getIntFromDb(): Either<DbError, Int> = 0.right()

One could “forward” the Either and also intercept `Left`s for logging or whatever:

fun getInt(): Either<DbError, Int> =
  getIntFromDb()
    .onLeft { log(it.message) }

However, if the API uses the Raise DSL, it feels a little more weird:

fun Raise<DbError>.getIntFromDb(): Int = 0

fun Raise<DbError>.getInt(): Int =
  withError({
    log(it.message)
    it
  }) { getIntFromDb() }

While writing this, I realized also could clear some things up, but I'm still curious if this is the Correct™ approach:

fun Raise<DbError>.getInt(): Int =
  withError({ it.also { log(it.message) } }) { getIntFromDb() }

[sindrenm]

I guess it's also quite convenient to move between the Either and Raise DSLs, so something like this is also definitely a possibility:

fun Raise<DbError>.getInt(): Int =
  either { getIntFromDb() }
    .onLeft { log(it.message) }
    .bind()

Feel like I should've thought about that before asking. rubber duck

[Youssef Shoaib [MOD]]

You could roll your own onError I guess. The only API I know that does something similar is traced, which is useful if you want to log the stack trace as well.

[Alejandro Serrano.Mena]

I think this is the best you can do with the Raise DSL, indeed. We decided not to include functions to "just inspect" the error in the API, so you need to roll your own it.also { ... }.

Feel free to open a discussion in the repo; if more people have this use case, we're happy to explore ways to include it in Arrow 2.0 🙂

[sindrenm]

I think I'm fine with moving between Raise and Either using either {} and bind(), tbh., but if you think it's worth it, I'd be happy to start a discussion. 👍

[Wesley Hartford]

What about something like this?

fun Raise<DbError>.getInt(): Int =
  either { getIntFromDb() }
    .onLeft { log(it.message }
    .bind()

[Wesley Hartford]

LOL, sorry, I missed the message with that exact code 😞.

[sindrenm]

Yeah, that's what we've ended up with doing in most cases. 👍