Hi all! I'm a newcomer of the functional programmi...

# arrow

[HieiJ]

Hi all! I'm a newcomer of the functional programming in general; after a 3-days applied FP course (in scala) I've learned the basics of functional programming; after the course I switched to Kotlin+Arrow since I already knew the language quite well. Currently I'm facing some problems when approaching to a further steps in my understanding; I'm trying to better understand the monad transformers topic, and I've found online this little example: https://underscore.io/blog/posts/2014/07/27/readerwriterstate.html
I'm struggling to write it in arrow since I can't understand how to use the ReaderT/WriterT transformers in order to achieve the same result described in the post. Do you have any similar example but done with Kotlin+Arrow?

[Leandro Borges Ferreira]

You can take a look here: https://medium.com/@JorgeCastilloPr/kotlin-functional-programming-ii-monad-transformers-b1f020f14dd8

[Leandro Borges Ferreira]

By the way: Welcome to the FP world!

[HieiJ]

Thank you! I'll take a look at it!

[HieiJ]

I'm trying to understand how that solution works, but I can't find the complete source code and the snippets in the post are incomplete; especially, I can't understand the definition of the AsyncResult data class described there...

[raulraja]

that is old stuff, that post has not been updated.

[raulraja]

you can compose individually Reader, State and Writer and there are also MonadReader, MonadState and MonadWriter in the arrow-mtl module which avoid nesting alltogether

[raulraja]

Unfortunately those are not yet documented beyond the tests and laws we have for them

[HieiJ]

Thank you @raulraja, I've rewritten with Arrow the code of the Scala example, but I'm not sure it's the right way... You can find it here: https://gist.github.com/colomboe/6e7d9c0923f6e348edd0e5b2bc488b40

[Leandro Borges Ferreira]

If you're not using bind() inside your comprehension, you don't need it =]

[Leandro Borges Ferreira]

You could try to use polymorphic code in you example. Like:

fun <F, A> find(key: Key, monad: Monad<F>): Work<Kind<F, A>> {
    work { monad.just(Unit) })
}

[Leandro Borges Ferreira]

then, just called it like this:

val found = Database.find("foo", Option.monad())

[Leandro Borges Ferreira]

so you don't have to always count on a Option, maybe you decide to use a Either, Try... Keep it just as a monad makes it more flexible

[HieiJ]

Thank you Leandro for your suggestion! I'll try to update the code with your suggestions. I tried to keep the code as similar as possible to the one in scala. I'm wondering if the usage of the ReaderT/WriterT is correct in this sample. Also because of the fix.value.fix.value on row 34... It seems a signal of something wrong...

[Leandro Borges Ferreira]

I don't know much about the WriterT, so I can't judge if this part is correct or not.

[Leandro Borges Ferreira]

But just use monad comprehensions (the binding part) when you need to flatten a code that has many `flatMap`'s

[Leandro Borges Ferreira]

And, if you wanna get fancy, don't use concrete Wrappers, use just Kind, to use can make a more generic code. You can take a look here: https://arrow-kt.io/docs/patterns/polymorphic_programs/ (It is a bit hard to understand at first, but it is a great concept)

[HieiJ]

Yes, I'm aware of the use of Kind and higher-kinded types; as I stated, I tried to make the code more similar to the orginal example in scala in order to keep it focused on the reader/writer data types.