I’ve been playing around with arrow to compose fun...

# arrow

[jean]

I’ve been playing around with arrow to compose functions with a simple use case where I want to fetch different values in order to build a final object. Something like this :

data class GetUserImageContext(val apiClient: ApiClient)

typealias GetUserImage = suspend GetUserImageContext.(path: String) -> Either<ApiError, UserImage>

fun getUserImage(): GetUserImage = { path -> 
    try {
        // some executiuon
        Either.Right(image)
    } catch(e: Exception) {
        Either.Left(ApiError.SomethingWentWrong)
    }
}

This suits me well for unit testing but the problem comes where a function is composed of several like that one. The context object ends up containing too many things :

data class GetUserContext(
    val getUserImageContext: GetUserImageContext,
    val getUserImage: GetUserImage,
   // keeps on going based on the functions needed for building the final object
)
...
// I need both the context object and the alias object to be able to get the final value I want
val image = getUserImage(getUserImageContext, "/some/path")

I did watch some of the video on youtube talking about this recommended the “Context” approach but they are ~3 years old.
Is there a better way to approach this now?

[jean]

@simon.vergauwen maybe you have an opinion on this?

[Peter]

our conversation here is somewhat related: https://kotlinlang.slack.com/archives/C5UPMM0A0/p1634238366252000

[Peter]

also have you seen Either.catch { }?

[jean]

Reading this thread, I think I took the “pure function” thing too far. While using a variable not passed as parameter makes the function impure, calling other functions is okay as long as those functions are also pure. That simplifies a lot of thing and redundant dependencies in what I had.
And no I didn’t know about Either.catch that’s a nice complement to other features I use 🙂

[simon.vergauwen]

Sorry for the late reply.
I think this is the important bit here: “While using a variable not passed as parameter makes the function impure, calling other functions is okay as long as those functions are also pure.”

Composing pure functions, is how we build/compose larger programs from smaller ones.

[jean]

right, I just made my life way harder than it needed 😅