is there a more functional version of this typical...

# announcements

[iex]

is there a more functional version of this typical "while loop with early exit"?

fun foo(start: Foo): Result<List<MyRes>> {
    val list: MutableList<MyRes> = mutableListOf()
    var bar = start
    while (bar < condition) {
        when (val result = retrieveSomething(bar)) {
            is Success -> list.addAll(result.success)
            is Failure -> return Failure(Throwable("Error: ${result.error}"))
        }
        bar = bar.next()
    }
    return Success(list)
}

[Dominaezzz]

I would've suggested using generateSequence but that failure case is quite troublesome.

[Dominaezzz]

generateSequence(start) { it.next() }
    .takeWhile { it < condition }
    .map { retrieveSomething(it) }
    .flatMap {
        when (it) {
            is Success -> it.success
            is Failure -> throw Throwable("Error: ${it.error}")
        }
    }
    .toList() // Catch exception here and wrap in Failure.

[iex]

interesting. It's more functional (the throwing of the exception may be discussable)

[iex]

but I'd probably not replace my version. It seems easier to understand.

[Saku Ytti]

I greatly prefer Dominic’s version, but I’m looking through ruby classes but I think idiomatic ruby is close to idiomatic kotlin

[Saku Ytti]

Glasses

[iex]

I don't know, throwing an exception seems like cheating. But that's probably because I dislike throwing exceptions in general

[iex]

and "catch exception here" means that I have to wrap the whole thing in a try block, right? Or is there a better way?

[Dominaezzz]

This might work in theory.

generateSequence(start) { it.next() }
    .takeWhile { it < condition }
    .map { retrieveSomething(it) }
    .asIterable()
    .flatMap {
        when (it) {
            is Success -> it.success
            is Failure -> return Failure(Throwable("Error: ${it.error}"))
        }
    }

[iex]

ooh

[iex]

well, I like this one haha

[iex]

copying it 🙂 thanks!

[Dominaezzz]

Haha, don't thank me yet. It might not compile 😆 .

[iex]

right, I noticed the "in theory" later

[iex]

it seems not to work indeed. It says "return not allowed here"

[Dominaezzz]

Try return@YourFunctionsName .

[iex]

though wait a sec

[iex]

okay, it's compiling without the @YourFunctionsName . I just had to add a `

.let { Success(it) }

at the end

[iex]

so... awesome, thanks (this time for real) 🙂

[iex]

(I'm not sure if this return without @YourFunctionsName is returning from the function or only flatmap though. Will have to confirm that. It also compiles with it)

[Dominaezzz]

It definitely can't be only flatmap because we'd need to return a List<T> to flatmap and we're not.

[Derek Peirce]

You may be able to use `tailrec`:

tailrec fun foo(start: Foo, prior: List<MyRes> = mutableListOf()): Result<List<MyRes>> {
        return if (start < condition) {
            when (val result = retrieveSomething(start)) {
                is Result.Success -> foo(start.next(), prior.apply { addAll(result.success) })
                is Result.Failure -> Failure(Throwable("Error: ${result.error}"))
            }
        } else {
            Result.Success(prior)
        }
    }

[Jakub Pi]

A slight improvement to Dominic's functional approach would be to have a sequence function with the signature

fun <A> sequence(xs: List<Result<A>>): Result<List<A>>

This should capture the short circuiting and also replace the when expression. There's no reason to map the failure, you are only losing type information if you do it that way.

I'm also curious if you are using kotlin.Result since the implementation doesn't currently like for it to be used as a return value. BTW, as an alternative to Result, arrow's Either has the short circuiting semantics that you want (including a sequence operation already defined), but learning how to actually use arrow properly has been challenging at least for me.

I'll also mention the condition is a bit suspect. I realize the code is intentionally obfuscated, but remember to avoid referencing anything outside the function to keep it pure.