Hi fellow Arrow users! I wonder is there any examp...

# arrow

[Oğuzhan Soykan]

Hi fellow Arrow users! I wonder is there any example or intention of using Try monad from Scala in Arrow? It would be really good addition.
So far, I am using a custom implementation and conversions between arrow and Try: https://github.com/Trendyol/stove4k/blob/main/lib/stove-testing-e2e/src/main/kotlin/com/trendyol/stove/functional/Try.kt

[CLOVIS]

Hi! Try is essentially just an Either that catches exceptions. In Arrow, you can use:

val foo: Either<Throwable, Int> = Either.catch {
    5 % 0
}

to catch exceptions with regular Either .

Is there anything else from Try that you would find useful?

[Oğuzhan Soykan]

Yes you’re right. It pretty much does the same job. Do you think that it is a design decision about not implementing Try as a separate monad? Either.catch is a feature Scala does not implement as far as I know and offloads this burden to Try.

[raulraja]

We used to have Try in Arrow in the 0.x series but we removed it a some point in favor of Kotlin's Result which models the same thing https://blog.rockthejvm.com/functional-error-handling-in-kotlin-part-2/

[raulraja]

Scala         Kotlin

Try(block) -> runCatching { ... }
Success(value) -> Result.success(value)
Failure(ex) -> Result.failure(e)

[raulraja]

The main difference between Scala's Try and Result is that the Try constructor is implemented only capturing NonFatal exceptions:
https://github.com/scala/scala/blob/13a80bd6faea2d9a53f40305853debd3900e729e/src/library/scala/util/Try.scala#L220
https://github.com/scala/scala/blob/13a80bd6faea2d9a53f40305853debd3900e729e/src/library/scala/util/control/NonFatal.scala#L42
In contrast the implementation of runCatching may capture exceptions that in most cases are considered Fatal and something you should. not recover from:
https://github.com/JetBrains/kotlin/blob/e959e97992507f4ecc84eca3fab829692ec05376/libraries/stdlib/src/kotlin/util/Result.kt#L144
In Arrow for Either.catch and other variations of catch we check if the exception is fatal and if it is we rethrow it.
https://github.com/arrow-kt/arrow/blob/731a9b292ab5f2f8ba2decc251dab3f2c4a60c06/ar[…]core/arrow-core/src/commonMain/kotlin/arrow/core/raise/Raise.kt

[raulraja]

Yes you’re right. It pretty much does the same job. Do you think that it is a design decision about not implementing Try as a separate monad? Either.catch is a feature Scala does not implement as far as I know and offloads this burden to Try.

The design decision here was motivated because Try was the same as Result but Result lacks a way to bubble up fatal exceptions with nonFatalOrThrow .
We decided to remove Try , keep nonFatalOrThrow as public APIs in case you want to use them with try/catch and provide catch variations as functions and constructors for Error Handling related types.

[Oğuzhan Soykan]

Thanks a lot giving the decision history and detailed explanation!