How can I `mapLeft` in the new `EffectScope`? ```context(EffectScope<Throwable>) fun count(): ...

# arrow

[thanh]

How can I mapLeft in the new EffectScope?

context(EffectScope<Throwable>)
fun count(): Int = TODO()

sealed interface DomainError

context(EffectScope<DomainError>)
fun logic(): Int {
  // I want to map Throwable to DomainError here
  return count()
}

[simon.vergauwen]

Hey @thanh,

Currently your best bet is to capture it into an effect and then call fold. Or turn it into an Either and call mapLeft and rebind.

effect<Throwable, Int> {
  count()
}.fold({ shift(transform(it)) }, { it })

[simon.vergauwen]

We've been working on new APIs to cover MonadError/ApplicativeError, etc.

Not sure if you've seen these APIs, https://github.com/arrow-kt/arrow/pull/2797.

I'm planning to port them back to 1.x.x soon. Any feedback would be great.

This would replace everything you find under these error APIs, since they cover all use-cases.

[simon.vergauwen]

Additionally, I've also been working on a new API for debugging and tracing purposes.

Any feedback here would also be great. Currently it's still in the process of API design since the functionality works already.

https://github.com/arrow-kt/arrow/pull/2816
This tracing support will offer tracing stacktraces for shift and thus also bind, ensure, ensureNotNull, ... everything in the Effect DSL and everything that is build on top 🥳

[thanh]

oh, that looks great! I'll check that. Do we have a snapshot version of Arrow 2.0?

[simon.vergauwen]

There is a SNAPSHOT for Arrow 2.0, at least we've enabled it. I haven't tried it myself yet.

[simon.vergauwen]

You were wrapping some side-effects in count() ?
The API designed for this is:

context(EffectScope<String>)
fun count(): Int =
  catch({ TODO() }) { ex: IllegalStateException ->
    shift(ex.message)
  }

And here I am using the reified variant to select IllegalStateException as a partial function 😉

[thanh]

So in count , there is some Error , but depend on each logic function, I want to map it to some different DomainError . That's why mapLeft here is so convenient.

[simon.vergauwen]

Okay, if you're mapping between typed errors you want to use recover.

context(Shift<E>)
suspend fun count() = 1

context(Shift<E2>)
suspend fun logic(): Int =
  recover({ count() }) { e: E ->
    shift(e.toE2())
  }

fun E.toE2(): E2 = TODO()

[simon.vergauwen]

recover allows you to do what mapLeft does but in a DSL style

[simon.vergauwen]

It will also exist as an extension on Effect, derived from the DSL method 😉

[simon.vergauwen]

So catch for Throwable and recover for typed errors, and recover allows you to transform the error type.

[simon.vergauwen]

You can see them as the uber functions from MonadError, all use-cases can be derived from them.

[thanh]

thanks Simon, I understand those functions. But still seems a bit more work than before 😂 .

[simon.vergauwen]

It requires you to call shift yes 😅

[simon.vergauwen]

How would you use mapLeft otherwise?

[thanh]

I meant before ScopeEffect , with EitherScope, we can just mapLeft directly, right?

[simon.vergauwen]

You can do:

val x: Either<String, Unit> = ...
val y: Either<Int, Unit> = 
  x.mapLeft { it.length }

val x2: Effect<String, Unit> = ...
val y2: Effect<Int, Unit> =
  x2.recover { shift(it.length) }

[simon.vergauwen]

I would say the difference is more in the passing of values vs DSL style of coding.

When working in DSL style you need to "wrap" in DSL blocks, just like you do with KotlinX.

withTimeout
coroutineScope
withContext
...

All these Kotlin APIs are DSL blocks, rather than extension methods like we used to do before or often see in Scala.

[thanh]

Thanks Simon, I'll experiment with your suggestions.

[simon.vergauwen]

If you have more feedback it would be really great to hear 🙏

[simon.vergauwen]

This is going to be important to get right for Arrow 2.0

[thanh]

yeah, definitely, I'll may try Arrow 2.0 soon 😄

[Stylianos Gakis]

Hey stumbled upon this by chance, and I wonder, what is the final design for this for 2.0?

[simon.vergauwen]

The final design is what is in 1.2.0, so basically everything that I proposed in this thread. recover for types errors, and catch for exceptions.