Hello! I'm trying to understand how to mark side e...

# arrow

[Sergei Zubov]

Hello! I'm trying to understand how to mark side effects correctly. Documentation to Arrow Fx describes using suspend () -> A instead of IO<A>. Does this literally means that signature of the function that produces a side-effect should be suspend () -> A, or it can be suspend (parameter1, parameter2...) -> A? Which one is correct and why:

data class Message(val content: String)
suspend fun sideEffectProducer(message: Message) = println(message)

or

data class Message(val content: String)
suspend fun Message.sideEffectProducer() = println(this)

And a follow-up question - how functions with side effects differ from pure functions that use either block and .bind() results? Both of them are suspend

[simon.vergauwen]

Hey @Sergei Zubov,

In the Arrow Fx Documentation when we refer to suspend () -> A we talk about suspend fun f(): A, or val f: suspend () -> A since both are the same.

They're just 2 different ways to describe the same thing, and you can move back and forth between the two different notations easily. (For example using method referencing).

So both example you shared is correct and depends on personal preference and code styles used. I personally almost always use suspend fun example(): Unit and when I need to pass it somewhere as a lambda I reference it as a method reference ?let(::example).

either { } has two different versions:
• either.eager which works doesn't allow suspension. So you cannot call any other suspend fun inside.
• either { } which does allow suspension, so you can call other suspending functions like withContext inside.
So using either.eager you can only write pure functions since you cannot call suspend inside.

either { } on the other hand, does allow suspension, so we can call side-effecting code from inside.

It's similar to List#map.
Since it's inline it allows for suspension to go through, but depending on if you call a suspending function inside then the result is considered not-pure.

[Sergei Zubov]

Thank you, Simon. So, in general, it goes down to a "Mark functions with side effects as `suspend`" rule. No matter of style - function with parameters, extension function or any other thing 😀

But shouldn't I also make all of my functions suspend to make my program run in a declarative deferred way? Or as far as higher-level function is suspend, calling non-suspend function from it is fine?

[simon.vergauwen]

But shouldn't I also make all of my functions suspend to make my program run in a declarative deferred way?

Yes, that's very typical. You could say that marking all your functions with suspend allows you to run your program in a declarative deferred way. You could also that's the result of being referential-transparent and purity. These are all properties we care about in FP, and so you could also use different arguments for this. They all achieve the same goal I think, which is being able to reason about code in small blocks.

Or as far as higher-level function is suspend, calling non-suspend function from it is fine?

Calling pure code from side-effecting code is always fine. So calling non-suspend functions from suspend fun is fine 👍
Actually, I always try to make as many non-suspend functions as possible. I am thinking now specifically about domain mappers, computations, combining data, ... etc.

[simon.vergauwen]

For example:

suspend fun fetchUser(id: Int): User = ...
suspend fun fetchProfile(id: Int): Profile = ...
suspend fun fetchAvatar(id: Int): Avatar = ...

fun createUserProfile(user: User, profile: Profile, avatar: Avatar): UserProfile = ...

suspend fun fetchUserProfile(id: Int): UserProfile =
  createUserProfile(fetchUser(id), fetchProfile(id), fetchAvatar(id))

If later I want to refactor my network calls to return Either<Throwable, A> my code changes.

suspend fun fetchUserProfile(id: Int): UserProfile =
  fetchUser(id).zip(fetchProfile(id), fetchAvatar(id), ::createUserProfile)

[Sergei Zubov]

Yes, domain mappers are exactly what I am dealing with right now😄 Thank you

[simon.vergauwen]

You're very welcome @Sergei Zubov! 🙌