["Request"] Specify AT_MOST_ONCE and EXACTLY_ONCE in applicable lambdas

[nomisRev]

In most HOF in Arrow we can specify AT_MOST_ONCE and EXACTLY_ONCE.
Currently, we're not doing so, and this prevents certain code to be written that would otherwise be valid.

[poseidon2060]

Hey, can I work on this issue?
I am relatively new to Android and I hope this can be a good beginner issue for me

[nomisRev]

Hey @poseidon2060,

Yes, this is a great issue to get started with since you can also start small and add AT_MOST_ONCE and EXACTLY_ONCE only in a couple or a single place at the same time. I would suggest getting familiar with Kotlin contracts and callsInPlace, and looking for any file in libs/arrow-core/commonMain and find any lamda were you can add these annotations.

This doesn't have to be done in the entire project in 1 PR or contribution. Feel free to add it only to a single lambda where you think it's applicable and opening a PR 👍

[poseidon2060]

Okay thankyou
Can I ask here for my help and doubts, or do I have to join Slack?

[nomisRev]

Feel free to ask here @poseidon2060, or in a DRAFT PR if you prefer 👍

[poseidon2060]

Hey @nomisRev

I have familiarized myself with Kotlin contracts and callsInPlace but I am unable to figure out where to apply them in the project
I browsed the path that you mentioned but am unable to understand exactly where I can add these annotations
Can you guide me slightly on what to look for, help appreciated

[nomisRev]

Hey @poseidon2060,

If we look for example at Either, there are many places where AT_MOST_ONCE can be added. I.e. for fold you know that either the function for ifLeft or ifRight will be called AT_MOST_ONCE

To add this annotations for Either.kt, you need to inspect the method body and see if the lambda is called AT_LEAST_ONCE or AT_MOST_ONCE and add that into the method body. This should only be done for methods that are not @Deprecated.

If you any more questions be sure to ask them ☺️ I hope this helps.

[poseidon2060]

Hey, @nomisRev

Thank you for being so helpful!
I tried out what you said for fold and this is what I changed

@OptIn(ExperimentalContracts::class)
  public inline fun <C> fold(ifLeft: (left: A) -> C, ifRight: (right: B) -> C): C =
    when (this) {
      is Right -> {
        contract { callsInPlace(ifRight,InvocationKind.AT_MOST_ONCE) }
        ifRight(value)
      }
      is Left -> {
        contract { callsInPlace(ifLeft,InvocationKind.AT_MOST_ONCE) }
        ifLeft(value)
      }
    }

Am I doing it correctly?
I hope I didn't do something terribly wrong😅

[hoc081098]

I think should move all contract blocks to the top body of function.

contract {
  callInPlace(ifRight, 
...)
  callInPlace(ifLeft, ...)
}
when(this) { ... }

[poseidon2060]

Hey @hoc081098

Do you mean like this?

@OptIn(ExperimentalContracts::class)
  public inline fun <C> fold(ifLeft: (left: A) -> C, ifRight: (right: B) -> C): C {
    contract {
      callsInPlace(ifLeft, InvocationKind.AT_MOST_ONCE)
      callsInPlace(ifRight, InvocationKind.AT_MOST_ONCE)
    }
    return when (this) {
      is Right -> ifRight(value)
      is Left -> ifLeft(value)
    }
  }

[nomisRev]

Yes, the second snippet as suggested by @hoc081098 is the correct encoding. contract needs to be the first statement in the function body.

[poseidon2060]

Yes, the second snippet as suggested by @hoc081098 is the correct encoding. contract needs to be the first statement in the function body.

So can I open a PR?

[nomisRev]

Absolutely ☺️

[poseidon2060]

Done!
Thank you @nomisRev and @hoc081098 for being so helpful😊

[poseidon2060]

Hey @nomisRev, I have created a Draft PR adding Invocation at 4 more places, can you please let me know if it's correct so that I can mark it Ready for Review🤞

[poseidon2060]

Hey @nomisRev, should I continue working on this issue or get started on a new one?
I don't know if I am supposed to complete this for all the files or not, I'd do it if you want🤔

[nomisRev]

Hey @poseidon2060,
This needs to happen for all files (you can ignore @Deprecated code but also feel free to work on an other issue if you like ☺️

[poseidon2060]

Hey @poseidon2060, This needs to happen for all files (you can ignore @Deprecated code but also feel free to work on an other issue if you like ☺️

Okay, I'll start working on a new issue while also continuing working on this issue in the background. I think this way I'll learn the most.😄
Do you have an issue to recommend for my skill level?

[nomisRev]

@poseidon2060 awesome 🙌 Thank you for taking so much interest to contributing!
These might be some interesting tickets to look into:

• Investigate 25 slowest test in Arrow #2908
• Increase test coverage Arrow Core #2894

Feel free to ask any questions on the respective issues directly, or on any other issues you find interesting and tag me 😉
I'll do my best to add some more beginner friendly tickets in the coming weeks.

[poseidon2060]

@nomisRev Okay thank you, I will look into these issues and try to solve them
Contributing to open source is really fun, I am glad I started with this repository😊

[serras]

Closed by and #2901 and #2915