I'm migrating my pet project from an MTL-based imp...

# arrow

[Marcin Gryszko]

I'm migrating my pet project from an MTL-based implementation with Arrow 0.10 to new Arrow 0.11 fx. I had a function that was combining results of two computations:

fun <F> Applicative<F>.combine(
    fn1: () -> Kind<F, Any>,
    fn2: () -> Kind<F, Any>
): Kind<F, Any> =
    fn1().map2(fn2) { (a, b) ->
        // do sth with a and b
    }

With Arrow 0.11 I'm trying to pass effectful computations as suspend functions. I'm able to combine them using the either companion:

suspend fun combine(
    fn1: suspend () -> Either<Throwable, Any>,
    fn2: suspend () -> Either<Throwable, Any>
): Either<F, Any> =
    either {
        val a = !fn1()
        val b = !fn2()
        // do sth with a and b
    }

If I want to execute the effects concurrently with parMapN, I'm not able to unwrap Either values and work with them:

parMapN(fn1, fn2) { a, b ->
        // Compile error: Suspension functions can be called only within coroutine body
        either {
            // same code as in the sequential example
        }
    }

Is there a way to combine parMapN with Either comprehensions?

[raulraja]

Hi @Marcin Gryszko , the Coroutines lib provides parMapN for suspension https://arrow-kt.io/docs/fx/async/#parmapnpartupledn

[raulraja]

Is that the one you are using or the one from the Applicative instances?

[Marcin Gryszko]

This is the one I'm trying to use. However, the 3rd parameter to parMapN is not a suspend function:

parMapN(fa: suspend () -> A, fb: suspend () -> B, f: (A, B) -> C)

[Marcin Gryszko]

from the package arrow.fx.coroutines

[raulraja]

Can you surround all with either and apply the effects inside the suspended args. Sorry on my phone but something like...

[raulraja]

either { parMap({ !eitherValue }, ... }

[Marcin Gryszko]

Same error:

either { 
        parMapN(fn1, fn2) { a, b -> 
            val aUnwrapped = !a // Compile error: Suspension functions can be called only within coroutine body
        }
    }

Would it make sense to add a parMap variant accepting a suspend combining function:

parMapN(fa: suspend () -> A, fb: suspend () -> B, suspend f: (A, B) -> C)

[raulraja]

I ment applying either in the captured lambdas you pass to parMap before the final function, but yes maybe it makes sense to make the final function suspend in there

[raulraja]

I can't think of a reason why that last function is not already suspend.