I’m having trouble continuing to use the `Reader` ...

# arrow

[jem]

I’m having trouble continuing to use the Reader type after upgrading to 0.12.0. (MTL is still on 0.11.0 though). After this upgrade:

-  val arrowFx = "io.arrow-kt:arrow-fx:0.10.5"
-  val arrowMtlData = "io.arrow-kt:arrow-mtl-data:0.10.5"
-  val arrowOptics = "io.arrow-kt:arrow-optics:0.10.5"
+  val arrowFx = "io.arrow-kt:arrow-fx:0.12.0"
+  val arrowMtlData = "io.arrow-kt:arrow-mtl-data:0.11.0"
+  val arrowOptics = "io.arrow-kt:arrow-optics:0.12.0"

I find that reader.run(thing).value() is no longer able to resolve .value(). How should it read now?
.run(thing) is returning type Kind<ForId, T>

[simon.vergauwen]

mtl is no longer actively maintained. It was an experimental module, and heavily relies on Kind and typeclasses which are unidiomatic in Kotlin and require a lot of weird machinery.

Here are a couple examples of how you can replace Reader with receiver functions and how you can scope/compose the T dependency type of your Reader.

- https://gist.github.com/raulraja/ac7a489c01193bcf5e8fdc59c89b5156
- https://gist.github.com/raulraja/97e2d5bf60e9d96680cf1fddcc90ee67
- https://gist.github.com/nomisRev/1f91710ebec1709d4ce8059812482624

[jem]

Thanks

[simon.vergauwen]

You're very welcome! 🙂

[Carlos Fernandes]

@simon.vergauwen sad to hear about ReaderT, could you please elaborate a bit on your explanation above?

I've had a look at the examples, they are quite different from one another

[raulraja]

@Carlos Fernandes ReaderT is (Dependencies) -> F<A> which is already covered in Kotlin by extension functions and suspension if your F is IO. suspend Dependencies.f(): A it’s isomorphic to ReaderT<IO<_>, Dependencies, A> , more ergonomic, faster and memory friendly since it avoids several layers of flatMap nesting. The examples above are examples of programs written in that style in which dependencies can be constrained with type bounds to enable syntax in the functions.

[simon.vergauwen]

And suspend Dependencies.f(): Either<E, A> being ReaderT<PartialEitherTOf<ForIO, E>, Dependencies, A>

[Carlos Fernandes]

Got it, thanks.

[rebyrg]

My test of functionall dependency injection. Is this approach ok when there is no MTL?

https://github.com/Rebyrg/learningarrow/blob/main/src/test/kotlin/pl/rebyrg/learningarrow/ReaderTest.kt

[raulraja]

@rebyrg that’s a great example using a custom Effect type 👏 . I still see no value for Reader compared to just making those extensions functions with your Reader argument as a receiver. Is there an advantage using Reader over extension functions?

[rebyrg]

DI implemented by extension function works fine. My test:
https://github.com/Rebyrg/learningarrow/blob/main/src/test/kotlin/pl/rebyrg/learningarrow/DependecyInjectionTest.kt