In my project I've defined the following extensions to avoid having to declare the id monad on Reade...

# arrow

[jem]

In my project I've defined the following extensions to avoid having to declare the id monad on Reader map and flatmap.

/**
 * Extension function that fixes `Reader` so that you can `map` without needing to pass
 * `arrow.core.Id.monad()` explicitly.
 */
fun <D, A, B> Reader<D, A>.map(f: (A) -> B): Reader<D, B> {
  return this.map(arrow.core.Id.monad()) { f(it) }
}

/**
 * Extension function that fixes `Reader` so that you can `flatMap` without needing to pass
 * `arrow.core.Id.monad()` explicitly.
 */
fun <D, A, B> Reader<D, A>.flatMap(f: (A) -> KleisliOf<D, ForId, B>): Reader<D, B> {
  return this.flatMap(arrow.core.Id.monad()) { f(it) }
}

Is there any blocker for actually just making Reader work this way in the lib? Is there any other monad instance that is valid for Reader?

[simon.vergauwen]

Yes, you can use Reader with IO to combine it with effects, or any other Monad. It’s often composed together with EitherT and IO as well.

To describe a program that has a dependency, and runs effect-fully with a domain error or success value as result.

These APIs should already be available as extensions over ReaderT<ForId actually, and they should be importable as extensions.

https://github.com/arrow-kt/arrow-incubator/blob/master/arrow-mtl-data/src/main/kotlin/arrow/mtl/Reader.kt

[simon.vergauwen]

You can however also replace Reader with extensions functions.

Example/docs: https://gist.github.com/nomisRev/1f91710ebec1709d4ce8059812482624

https://github.com/arrow-kt/arrow-fx/tree/master/arrow-fx-coroutines#suspend-r---a

[jem]

👍

[jem]

@Chris Myers