Hi again, I'm going to repeat a question that may ...

# arrow

[Mark Fisher]

Hi again, I'm going to repeat a question that may have got lost in my previous one.
I'm looking at the docs at https://arrow-kt.io/docs/patterns/dependency_injection/#using-di-to-inject-any-object but can't get the following to work (towards the end):

with (Option.monadError()) {
  createId("123") shouldBe Some(123)
  // ...

In the code above it, createId is an extension of MonadThrow not MonadError so this doesn't compile.
How do I get a monadThrow from an Option to make this work?

[simon.vergauwen]

You cannot construct a MonadThrow instance for Option since Option cannot deal with Throwable as an error.

[pakoito]

It kinda did. MonadThrow is a MonadError where the error is Throwable, This is not possible with Option while respecting the test suite

[pakoito]

beaten by a few seconds!

[pakoito]

😄

[Mark Fisher]

Thanks both. It's a shame because preceding that block it says "And as a consequence to all of this, testability and refactoring possibilities are through the roof! Many of the promises of OOP, fulfilled with simple functions and interfaces.", but I don't know how to fix the code to make this statement true, as I'm trying to learn arrow through its docs.

[Mark Fisher]

Is it possible to fix that test?

[pakoito]

switching to Either: Either.monadError()

[pakoito]

Option is...nice, still doesn't have much over simple nullable types

[pakoito]

Either retains errors, and allows you to swap the implementation with Validated and Option for some use cases

[Mark Fisher]

Thanks Paco