kotlinlang #arrow

data class SaveUserScope(
  val userRepository: UserRepository
)

fun saveUserReader(model: UserModel) = Reader.ask<IdHK, SaveUserScope>().map({
  IO { it.userRepository.save(model) }
}, Id.functor())

isto

02/19/2018, 7:12 AM

am I doing something wrong as the compiler requires me to explicitly provide the functor instance

Id.functor()

isto

02/19/2018, 7:13 AM

in the examples that I've seen (caster io course, and this https://medium.com/@JorgeCastilloPr/kotlin-functional-programming-i-monad-stack-518d1bd8fbee) it has not been necessary

themishkun

02/19/2018, 7:16 AM

That’s exactly why I’m interested in Applicative builder pattern. I noticed that in our project we already have very similar constructs, but they are ad-hoc and cumbersome, so I turned my head to the theory to make my life as developer easier

👍 1

simon.vergauwen

02/19/2018, 7:17 AM

@isto some changes have been made a while back.

simon.vergauwen

02/19/2018, 7:18 AM

If you need a normal

Reader

and not

ReaderT

you can use it like the following snippet

simon.vergauwen

02/19/2018, 7:18 AM

Copy code

fun saveUserReader(model: UserModel) = Reader().ask<SaveUserScope>().map {
    IO { it.userRepository.save(model) }
}

simon.vergauwen

02/19/2018, 7:19 AM

Problem is that we can not create extension functions on

typealias Reader<D, A> = ReaderT<IdHK, D, A>

simon.vergauwen

02/19/2018, 7:20 AM

You’re probably missing the instances which are in the dependency

compile 'io.arrow-kt:arrow-instances:0.6.1'

isto

02/19/2018, 7:21 AM

changing to

Reader().ask<...

(was

Reader.ask<...

) seems to do the trick at least partly, I'll add the dependency also

isto

02/19/2018, 7:24 AM

by using

Reader().ask

the

IdHK

is not required any more, it still seems to require the functor instance

cedric

02/19/2018, 7:27 AM

Personal nitpick:

Reader

is "dependency passing", not "dependency injection" 🙂

➕ 1

✅ 1

simon.vergauwen

02/19/2018, 7:30 AM

@cedric So what is DI? Using dagger is also dependency passing unless you use it for field injection? ;p

cedric

02/19/2018, 7:30 AM

I'd argue Dagger is dependency injection: the framework injects these fields or values for you. With

Reader

, you're still passing these dependencies yourself

cedric

02/19/2018, 7:31 AM

Implicitly in Scala, with evidences in Arrow

simon.vergauwen

02/19/2018, 7:31 AM

But if you use it for constructor injection you're also just doing dependency passing only the code gen takes some boilerplate of your hands.

👍 1

simon.vergauwen

02/19/2018, 7:32 AM

And field injection is frowned upon by a lot of people.

simon.vergauwen

02/19/2018, 7:33 AM

I am not disagreeing with you. Just wondering what you'd consider DI.

cedric

02/19/2018, 7:33 AM

I like field injection (in Java). Cuts down so much boiler plate with a tiny downside

👍 1

chadmorrow

02/21/2018, 5:27 AM

Hey is the course up on caster.io available for free through tomorrow or does it go behind the pay wall early in the morning PST?

pablisco

02/21/2018, 10:40 AM

New tag available on Stackoverflow:

arrow-kt

https://stackoverflow.com/tags/arrow-kt/info IT would be good to have public Q+A about arrow 🙂

arrow 4

👍 2

pablisco

02/21/2018, 10:41 AM

@pablisco pinned a message to this channel.

pakoito

02/21/2018, 10:46 AM

@Jorge Castillo for maxpower