["Request"] Algebraic Effects

[tapegram]

What version are you currently using?
1.0.0

What would you like to see?
An Effect system similar like Frank, Unison Abilities, or Freer monads.

@raulraja and @nomisRev gave additional context in the slack thread about this.

I would like to be able to create custom "Capabilities" or Effects if possible, but in general, the kind of standards like Writer, State, etc. would Immediately be useful. Most of my real-life use cases are Logging, Save/Fetching from a data store, or making a query to an external api.

Thank you!

[nomisRev]

Hey @tapegram,

I think this is quite easily possible with the new encodings, and context receivers. Although context receivers are only partially released. Perhaps we can write an additional document around this in the new website, I don't think this requires any new code or changes inside Arrow. So moving this to the arrow-website repo.

We could consider introducing a new module, or separate library (repo) to expose some bases interfaces / implementations later on if needed.

I.e. I think for State context(Atomic<State>) is probably sufficient, and that'd be safe in concurrent environments and everything. Writer is probably just a context(Atomic<List<String>>), or a super tiny wrapper around it that combines W with (W, W) -> W.

@JvmInline
value class MyWriter(private val atom: Atomic<MyValue>) {
  fun write(value: MyValue): Unit = atom.update { current -> current + value }
  fun get(): MyValue = atom.value
}

typealias RandomState = Atomic<SEED>(INITIAL_SEED)

context(RandomState, MyWriter, Raise<Error>)
fun myProgram(): Int {
  ...
}

cc\ @serras

[serras]

In theory this information should be present in the Effects and contexts section of the website.

@tapegram don't hesitate to let us know if you think that page could be improved, or any concept made more clear.