functional-streams-for-scala/fs2

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

Bob Glamm

@glammr1

^^ one of the earliest lessons I learned by trying Scala FP is exactly that

"why am I using A => F[B] here when a simple F[B] would do"

Adam Rosien

@arosien

if you apply that literally, you get tagless-final style :)

Billzabob

@Billzabob

@ChristopherDavenport Ah ok that makes sense :) Thanks

Bob Glamm

@glammr1

@arosien yes, it was on that path of discovery

Christopher Davenport

@ChristopherDavenport

Then it all becomes kleisli.

Adam Rosien

@arosien

i'm still getting there...

Bob Glamm

@glammr1

You know Kleisli pops up a lot more than I would have expected when I write FP

Adam Rosien

@arosien

a SUPER example is scala-steward, the ENTIRE app is just tagless-final traits + mixing them all together at the top layer: https://github.com/fthomas/scala-steward/tree/master/modules/core/src/main/scala/org/scalasteward/core

oh and any of @ChristopherDavenport's libraries too

he basically takes an idea for a library, writes out the algebra, writes an impl or 2, then is done

:jazz-hands:

Christopher Davenport

@ChristopherDavenport

Thats the idea, that and minimizing allocations for the small wins in FP.

Adam Rosien

@arosien

but scala-steward is a great example of the tagless-final discipline in a big-ish app

Bob Glamm

@glammr1

That's Liskov's process for writing applications

"I think of a language that enables me to solve the problem at hand then write the interpreter for the language"

Adam Rosien

@arosien

yeah, we're all just catching up :)

Christopher Davenport

@ChristopherDavenport

^^ This is final tagless

A laguage where it all just writes like butter.

In the french style of cooking.

Bob Glamm

@glammr1

I've seen glimmerings of that in my exploration

"wow, this code reads how I would write it up in my software design docs and requirements"

Adam Rosien

@arosien

but there's also the step of integrating that language into the main program, so it pays to have a very composable return type, like F

Christopher Davenport

@ChristopherDavenport

As @arosien states, that app is a great example.

Adam Rosien

@arosien

i randomly wondered how he implemented that bot, and was totally surprised!

Bob Glamm

@glammr1

How much benefit is there to going all the way to data constructors & free algebras for implementation? I seem to recall that some of the libraries use that instead of direct final tagless impls

Christopher Davenport

@ChristopherDavenport

Don't go free in scala unless you have a real use case why you should at the moment.

Most libraries using free are exploring rewriting in tagless.

Adam Rosien

@arosien

you can go tagless-final => free with an interpreter :)

reify

Bob Glamm

@glammr1

Would it be fair to say "it's worth learning but I'm unlikely to have code that justifies the usage of free"?

Christopher Davenport

@ChristopherDavenport

Also you don't have to reinvent the entire wheel up to whatever you need.

Like Rob had to basically subsume a good chunk of cats-effect to make ConnectionIO work.

Adam Rosien

@arosien

it's an implementation of separating program from implementation, so in that sense it is important and useful. tagless final is a cheaper encoding, for some dimensions of cost

Fabio Labella

@SystemFw

@glammr1 is worth learning the general concept behind both free and final tagless because they are both more general than free or final tagless

Adam Rosien

@arosien

caller chooses the effect constraint in tagless-final (F has to be a Monad, etc.), vs. the caller acts on Free

Fabio Labella

@SystemFw

for example, implementing new complex types is better done with initial encodings (free-like)

case in point: Stream and IO

whereas application abstraction in terms of commands/interfaces is nicer in final tagless

Adam Rosien

@arosien

method of tagless final algebra =~ data type representing that method coupled with Free

Bob Glamm

@glammr1

I seem to have this vague recollection that recursion schemes & free algebras are closely related and that's the piece I'm really curious about

Fabio Labella

@SystemFw

yeah, you need Fix to start talking about cata

and Fix is half of Free (the join part of Monad, without pure)

Christopher Davenport

@ChristopherDavenport

trait StateFinal[F[_], S]{
  def modify[A](f: S => (S, A)): F[A]
}
implicit def instance[S]: StateFinal[State[S, *], S] = new StateFinal[State[S, *], S]{
    def modify[A](f: S => (S, A)): State[S, A] = State(f)
  }

Always my favorite pairing for example purposes.

Bob Glamm

@glammr1

That was my biggest stumbling block with TaPL (and still is), the notion of Fix

Fabio Labella

@SystemFw

actually initial encodings help you understand those things a lot better

for example the difference between Monad and Applicative (or Command) is literally under your eyes with free constructions

since you have a data type that you can visualise

iirc fix in TaPL is used in a different context but I don't have it at hand to check

but generally when you hear about fix in the context of language design is for languages with no general recursion, and fix enriches them with it

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