Hi, is there a way to call suspend function inside `eval's` map and flatmap. I have this code but i...

# arrow

[ds3fdfd3]

Hi, is there a way to call suspend function inside eval's map and flatmap. I have this code but it doesn't compile as doSomething() can only be called withing coroutine body.

import arrow.core.Eval
import kotlinx.coroutines.runBlocking

fun main(): Unit = runBlocking {
    Eval.now(10)
        .map { doSomething() }
        .value()
}

private suspend fun doSomething(): Int {
    println("do something")
    return 20
}

[simon.vergauwen]

Hey @ds3fdfd3,

It's not possible to call `suspend`'ing code inside Eval since it's a deferred execution monad.

What is your use case of combining Eval with suspend?

[ds3fdfd3]

Hi @simon.vergauwen,
my use case is something like this. Underlying database library that I
am using requires that for transactions, we pass on all the items that
we want to put/delete together. To solve this I was trying to write a
domain specific method which would take multiple 'Evals' generating put/delete items.
However, in certain cases, we have to make another suspending database call from within the
domain specific method. E.g. caller provides an Eval which gives a 'deviceId' to be deleted, however, to properly delete this deviceId we need its user, hence we make an internal call to database to get the user.
Basically:
eval.map { deviceId ->
val user = getUserForDevice(deviceId) //suspend call
doSomethingWithUserAndDevice(user, deviceId)
}

[ds3fdfd3]

I hope this makes some sense. 😁

[simon.vergauwen]

I think instead of passing Eval around, in this case you'd just want to pass around suspend () -> A instead.

That way everything is still lazy, and you have full suspend support without using a data type like Eval. Which doesn't support this usecase.

[simon.vergauwen]

Eval is meant to build pure stacksafe functions that cannot be written with tailrec.

[ds3fdfd3]

Thanks, I am updating the function with suspend functions rather than Evals now. It seems better now.