Forgive me, I’m coming back to android after some time away. I’ve updated to the latest stable versi...

# arrow

[Robert Menke]

Forgive me, I’m coming back to android after some time away. I’ve updated to the latest stable version of Arrow (0.13.2) and I’m trying to use monad comprehensions. I’m running into an issue where bind is not defined. I’m currently using core, optics, and meta (via kapt). What am I doing wrong?

[simon.vergauwen]

Hey @Robert Menke,

It looks like you don’t have the arrow import. The receiver type you have there OptionKt.DSL is not from Arrow.

You want to use arrow.core.computations.option, and in the case where you want to assign to a val.

val contents = option.eager { ... }

[Robert Menke]

You are a true hero @simon.vergauwen thanks for the direction! I’m not sure I understand why I need to use option.eager though.

@Suppress("ClassName")
object option {
  inline fun <A> eager(crossinline func: suspend RestrictedOptionEffect<A>.() -> A): Option<A> =
    Effect.restricted(eff = { RestrictedOptionEffect { it } }, f = func, just = { Option.fromNullable(it) })

  suspend inline operator fun <A> invoke(crossinline func: suspend OptionEffect<*>.() -> A?): Option<A> =
    Effect.suspended(eff = { OptionEffect { it } }, f = func, just = { Option.fromNullable(it) })
}

It seems that the only difference between the operator function and eager is the use of RestrictedOptionEffect, but they both return Option<A>.

[simon.vergauwen]

You’re very welcome!

The DSL uses the suspension system, but the Restricted version uses @RestrictSuspension.
This disallows any suspend function to be called inside the DSL except for the functions defined inside RestrictOptionEffect.

This means that the DSL can be evaluated immediately, or `eager`ly. Instead of requiring suspend.

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines/-restricts-suspension/

[Robert Menke]

Oh got it, I was already in a suspend context in this case so would option be preferred to option.eager here?

[simon.vergauwen]

If you’re already inside a suspend context then it makes most sense to just use the suspend version yes. That’ll also allow you to use other suspending code inside the DSL

[simon.vergauwen]

Btw there is also nullable as a DSL if you’re working with nullable already

[Robert Menke]

Awesome, any way I can tip you for your time/effort? Seriously blown away by the generosity of this community every time I’m back on an android project.

[simon.vergauwen]

‘nullable’

[Robert Menke]

Oooo nullable DSL sounds excellent

[simon.vergauwen]

Not really :) Thanks for the offer though

[simon.vergauwen]

Happy to help

[Robert Menke]

You rock ❤️