What is the idiomatic way of conditionally (i.e. only for some specific cases) converting a `Left` t...

# arrow

[Lukasz Kalnik]

What is the idiomatic way of conditionally (i.e. only for some specific cases) converting a Left to a Right? Like a flatMapLeft().

[Lukasz Kalnik]

api.getUser().fold(
    ifLeft = { error ->
       if (error.code == 404) EmptyState.right() else ErrorState(error).left()
    },
    ifRight = { it.right() }
)

[Alejandro Serrano.Mena]

use recover

[Youssef Shoaib [MOD]]

api.getUser().recover { error ->
  if (error.code == 404) EmptyState else raise(ErrorState(error))
}

[Lukasz Kalnik]

Can I also do it inside an either {} builder?

[Alejandro Serrano.Mena]

yep, in that case you can make recover wrap around your code

val x = recover({ api.getUser().bind() }) {
  when (it.code) {
    404 -> EmptyState
    else -> raise(EmptyState(error))
  }
}

[Lukasz Kalnik]

Ah ok, that doesn't look that nice, but I suspect it doesn't fit in the Raise DSL otherwise?

[Alejandro Serrano.Mena]

yeah, giving two functional arguments to a function still looks kind of bad in Kotlin

[Youssef Shoaib [MOD]]

You could do something like this, but idk if I'd recommend it:

either {
  withError({ error: ErrorType -> if (error.code == 404) return@either EmptyState else ErrorState(error) }) {
    api.getUser().bind()
  }
}

[Youssef Shoaib [MOD]]

This isn't in Arrow yet, but there's a PR that would allow something like:

val x = either {
  val error = attempt { return@either api.getUsers.bind() }
  if (error.code == 404) EmptyState else raise(ErrorState(error))
}

It makes the code less nested, but it's quite imperative, especially with the early-return, and hence some people will dislike it.
You can just replace attempt with merge if you want to try this today. There's a minor difference in type safety (attempt forces you to early-return, merge doesn't).

[Lukasz Kalnik]

Yes, I was actually thinking of an early return. Especially with multiple subsequent calls inside an either {} block this could be actually a clearer syntax.

[Youssef Shoaib [MOD]]

If you have more use cases for attempt, I'd love it if you replied with them here or on the PR!