Hi. I’m trying to do monad comprehension for `Opt...

# arrow

[herlevsen]

Hi.

I’m trying to do monad comprehension for OptionT. My stack is IO<Option<User>>. I cant figure out what to import to get OptionT.fx. I have tried importing arrow.mtl.extensions.fx like described here: https://arrow-kt.io/docs/0.10/arrow/mtl/optiont/, but that doesn’t work

[Jannis]

M.fx is usually an explicitly defined extension function for our core datatypes. Not sure why mtl does not define them, but M.fx is defined as an alias for monad().fx.monad {} anyway, so you can always use that.
Fyi: The full definition of the helper is:

fun <A> Option.Companion.fx(c: suspend MonadSyntax<ForOption>.() -> A): Option<A> =
  Option.monad().fx.monad(c).fix()

[Gopal S Akshintala]

import arrow.mtl.OptionT
import arrow.mtl.extensions.fx

[Jannis]

Right @Gopal S Akshintala! I just had a quick search on github and it is defined.
https://github.com/arrow-kt/arrow-incubator/blob/543b852916b2a40176c4da9f5a4cbec8e79ff2ba/arrow-mtl/src/main/kotlin/arrow/mtl/extensions/optiont.kt

[herlevsen]

Don’t know what i did wrong before, but that works. Thank you! 🙂

[herlevsen]

Aha, i imported OptionT from arrow.core.OptionT, but that doesn’t have fx defined.

Why is there an OptionT in core and in mtl?

[Jannis]

There should not be. Maybe you had Option instead of OptionT?

[pakoito]

also check that you don't have 2 versions of arrow imported through a 3rd party

[herlevsen]

I had a dependency that used arrow-extras-data:0.9.0. Thank you guys 🙂