typelevel/cats-effect

Most discussion is on Typelevel Discord: https://discord.gg/bSQBZA3Ced

Fabio Labella

@SystemFw

I love types, and in the right amount, they tremendously help with modelling

however, when using them to encode correctness, it's easy to hit diminishing returns

for example

findUser returns User

authenticate also returns User

so types aren't helping distinguish an authenticated from a non authenticated User

so if you want to "type to the max", you might introduce a AuthedUser

but then you realise a bunch of functions need to work with both types of users

so you introduce a phantom type User[A <: AuthStatus]

so that you can check at compile time

oh, but now lots of ops become typeclasses

and then you realise you aren't tracking Banned at the type level!

and so on

codebases that do this are among the most brittle and fiddly to work with among the things that I've seen

and then eventually you hit a case where you need dependent types (real ones), and the whole castle crumbles

so, type safety definitely doesn't have the same tradeoff profile of, say, referential transparency

being 100% referentially transparent with IO is the correct choice for a lot of code

being 100% type safe is often counterproductive instead

daenyth

@daenyth:matrix.org

[m]

👍️

mpilquist

@mpilquist:matrix.org

[m]

someone really needs to publish a book of your gitter storms Fabio

Fabio Labella

@SystemFw

:grimacing:

I thought about a book a few times tbh

but it fits into those "philosophy of CS" stuff which is hard to justify

Maatary

@Maatary

👍️ really good insight indeed. A lot to think about and play with :)

Brian P. Holt

@bpholt

I have a function A => Resource[F, B] but I don’t have a value for A yet. Can I transform that into Resource[F, A => B]?

daenyth

@daenyth:matrix.org

[m]

Gut feeling: no

I think if you squint, that's actually a traverse

between Resource and Reader

and Resource isn't traversable

though.. maybe I'm wrong?

Actually, I think it's this: You can do it when F is traversable

because it's almost the same thing as having A => F[B] and wanting A => B

(IO etc are not traversable)

Brian P. Holt

@bpholt

haha well I’m glad I’m not the only one who thought it looked familiar but not immediately obvious

ritschwumm

@ritschwumm

with less squinting: in the first example, the allocation of the resource can make use of the A - in the second, it can't.

Fabio Labella

@SystemFw

your question is equivalent to saying: can I get a monad out of an applicative? and the question is you can't

monad, applicative and functor can all be seen as producing an F[A] => F[B] starting from different arrows

map: (A => B) => (F[A] => F[B])
ap: F[A => B] => (F[A] => F[B])
flatMap: (A => F[B]) => (F[A] => F[B])

A => F[B] is the essence of being monadic, i.e. context sensitivity, I can use the result of a computation to decide the shape of the next

this is what A => F[B] says: => means "depends on", so F depends on A (the next computation depends on the result)

in F[A => B], A cannot decide F, since it doesn't even see it (it's on the outside, in F[A => B])

daenyth

@daenyth:matrix.org

[m]

in F[A => B], A cannot decide F

ahhhhhh

That's a very clear way to lay that out

teo danciu

@teozaurus_twitter

your question is equivalent to saying: can I get a monad out of an applicative

Sorry, just curious, going from A => Resource[F, B] to Resource[F, A => B] - how come it's not "can i get an applicative out of a monad" , based on the definitions above?

Fabio Labella

@SystemFw

Hey Teo :)

Applicative lets you work with F[A => B], so if you could transform A => F[B] into F[A => B], that would mean that monadic operations could be expressed with applicative operations, i.e. that given an applicative, you could get all that monad can do

Jozef Koval

@jozanek

Hi guys, I am struggling a bit with following signature: f: List[Resource[F, A]] => Resource[F, List[A]] I wanted to use traverse but could not make it compile. Do you think it is even possible?

Fabio Labella

@SystemFw

.sequence should do just fine there (or traverse(x => x))

let me know if you still can't make it compile

Jozef Koval

@jozanek

do I need some special imports? currently I only have import cats.implicits._ but I get error:

polymorphic expression cannot be instantiated to expected type;
 found   : [G[_], B]G[List[B]]
 required: cats.effect.Resource[F,List[Foo]]

Fabio Labella

@SystemFw

what scala version are you on?

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