I want to init a class `val` property in the `init...

# arrow

[Lukasz Kalnik]

I want to init a class val property in the init block of a class based on some Either value. However, whenever I use any of the Either related functions like fold or either.eager block (which take a lambda as parameter) and I try to set the class property inside the lambda I get the error "Captured member variables initialization is forbidden due to possible reassignment".
Is my only option really something like:

myProperty = if (either.isRight()) either.value else defaultValue

?

[simon.vergauwen]

myProperty = either.fold({ defaultValue }, { it })

This doesn't work?

[Lukasz Kalnik]

Hmm, yes, that would work. Thanks!

[simon.vergauwen]

What about .orNull() ?: defaultValue or getOrElse ?

[Lukasz Kalnik]

For some reason I didn't think about pulling myProperty out of the lambda...

[simon.vergauwen]

Ah I see. Yes, that is not allowed by the Kotlin compiler... a bit strange though since with inline fun it should be perfectly valid.

[Lukasz Kalnik]

I was doing either.fold({ myProperty = defaultValue }, { myProperty = it })

[Lukasz Kalnik]

But your syntax is more concise anyway.

[stojan]

any reason you are not doing it the other way around?

e.g. etiher.fold({}, {}) and creating your class inside the lambda... that way your class doesn't have to know about Either

[Lukasz Kalnik]

No, actually the init block is inside the class

[Lukasz Kalnik]

The background is, the class being initialized is a viewmodel. It depends on a repository which returns an Either. I want to initialize the UI state of the viewmodel based on the value returned by the repository.

[Youssef Shoaib [MOD]]

@simon.vergauwen the reason it's not allowed by the compiler is the aforementioned functions are missing a callsInPlace contract with AT_MOST_ONCE or EXACTLY_ONCE

[simon.vergauwen]

Aha, then we should definitely add those! Thanks for the tip @Youssef Shoaib [MOD]. I'm going to create a ticket for this.

[simon.vergauwen]

@Youssef Shoaib [MOD] this is not valid for fold since we need to specify AT_MOST_ONCE for both lambdas but the compiler now still won't now which one of the two lambdas will be executed, and that both will not be executed.

inline fun <E, A, B> Either<E, A>.fold(
  error: (E) -> B,
  transform: (A) -> B
): B {
  contract {
    callsInPlace(error, InvocationKind.AT_MOST_ONCE)
    callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
  }

So it will complain that either.fold2({ myProperty = "defaultValue" }, { myProperty = it }) the second assignment is invalid since val can only be assigned once.

[simon.vergauwen]

Created this ticket for further investigations. https://github.com/arrow-kt/arrow/issues/2806

[simon.vergauwen]

@Lukasz Kalnik regardless if whether this can be fixed with Kotlin Contracts I think the nicest pattern is fold/transform the value and then assign in a single statement like the fix that we discussed above. myProperty = either.fold({ defaultValue }, { it }).

[Lukasz Kalnik]

Thanks Simon, I actually also think so and already implemented it in my project. 🙏