The Death of Final Tagless

The Death of Tagless Final
John A. De Goes — @jdegoes
Wiem Zine Elabidine — @wiemzin
Amsterdam.scala & FP AMS
Xebia, Amsterdam
March 2, 2019

SPECIAL THANKS
Amsterdam.scala
FP AMS

First-Class Effects Tagless Final
ZIO Environment Wrap Up

PROGRAMS AS STATEMENTS
def main: Unit = {
println("Good morning, what is your name?")
val name = readLine()
println(s"Good to meet you, $name!")
}

val example: Double = …
val value = example
val values = List.fill(10)(value)
val plusone = value => value + 1

def main: Unit = …
val program = main
val programs = List.fill(10)(main)
val retried = program =>
program.retried(2.times)

BENEFITS OF VALUES
Abstraction
Friendly
Refactor
Friendly
Test
Friendly

BENEFITS OF VALUES
def main: Unit = {
println("Good morning, what is your name?")
val name = readLine()
println(s"Good to meet you, $name!")
}
Procedural
Effects
Functional
Effects

Functional effects are immutable data structures
that merely describe sequences of operations.
At the end of the world, the data structure has to
be impurely “interpreted” to real world effects.

println("Hello World")
Procedural Effects “Do”

case class PrintLine(text: String)
Functional Effects “Describe”

PROGRAMS AS VALUES
IO[A]
A description of an effect that when unsafely
interpreted, will succeed with a value of type A

PROGRAMS AS VALUES
class IO[+A](val unsafeInterpret: () => A) { s =>
def map[B](f: A => B) = flatMap(f.andThen(IO.effect(_)))
def flatMap[B](f: A => IO[B]): IO[B] =
IO.effect(f(s.unsafeInterpret()).unsafeInterpret())
}
object IO {
def effect[A](eff: => A) = new IO(() => eff)
}

PROGRAMS AS VALUES
def putStrLn(line: String): IO[Unit] =
IO.effect(println(line))
val getStrLn: IO[String] =
IO.effect(scala.io.StdIn.readLine())

PROGRAMS AS VALUES
val main = for {
_ <- putStrLn("Good morning, " +
"what is your name?")
name <- getStrLn
_ <- putStrLn(s"Good to meet you, $name!")
} yield ()

PROGRAMS AS VALUES
val main = …
val program = main
val programs = List.fill(10)(main)
val retried = program =>
program.retried(2.times)

PROGRAMS AS VALUES
def loadTest(url: String, n: Int) =
val policy = Schedule.recurs(10).jittered
val worker = client.get(url).retry(policy)
val workers = List.fill(n)(worker)
IO.collectAllPar(workers)
}

Functional Effects
Abstraction
Friendly
Refactor
Friendly
Test
Friendly

PROGRAMS AS (OPAQUE) VALUES
def putStrLn(line: String): IO[Unit] =
IO.effect(println(line))
val getStrLn: IO[String] =
IO.effect(scala.io.StdIn.readLine())

BACK TO BASICS: JAVA 101
interface Console {
void println(String line);
String readLine();
}

public void program(Console console) {
console.println("Good morning, " +
"what is your name?");
String name = console.readLine();
console.println(s"Good to meet you, $name!");
}

class LiveConsole implements Console { … }
class TestConsole implements Console { … }
// In production:
program(new LiveConsole());
// In tests:
program(new TestConsole());

IO INTERFACES
trait Console {
def println(line: String): IO[Unit]
val readLine: IO[String]
}

PURE PROGRAM WITH INTERFACE
def program(c: Console) = for {
_ <- c.println("Good morning, " +
name <- c.readLine
_ <- c.println(s"Good to meet you, $name!")
} yield ()

POLYMORPHIC EFFECT
trait Console[IO[_]] {
def println(String line): IO[Unit]
}

MAKING IT EVEN MORE OBSCURE
trait Console[F[_]] {
def println(String line): F[Unit]
val readLine: F[String]
}

THE GLORIOUS TAGLESS-FINAL TYPE CLASS
trait Console[F[_]] {
def println(String line): F[Unit]
val readLine: F[String]
}
object Console {
def apply[F[_]](implicit F: Console[F]) = F
}

EFFECT-POLYMORPHIC PROGRAMS
def program[F[_]: Console: Monad] = for {
_ <- Console[F].println(
"Good morning, " +
name <- Console[F].readLine
s"Good to meet you, $name!")
} yield ()

TEST VS PRODUCTION EFFECTS
case class TestIO[A](...)
implicit val TestConsole = new Console[TestIO] { … }
implicit val LiveConsole = new Console[IO] { … }
// In production:
program[IO] : IO[Unit]
// In tests:
program[TestIO] : TestIO[Unit]

TAGLESS-FINAL IS EASY!
Functional Effects
IO[A]

TAGLESS-FINAL IS PRETTY EASY!
Functional Effects Parametric Polymorphism
F

TAGLESS-FINAL IS EASY-ISH!
F[_]
Higher-Kinded Types

TAGLESS-FINAL ISN’T THAT HARD!
trait Console[F[_]] { … }
Higher-Kinded Types Type Classes

TAGLESS-FINAL COULD BE MUCH HARDER!
implicit val TestConsole = new Console[TestIO] { … }
Type Class Instances

TAGLESS-FINAL IS SORT OF HARD!
new Console[({type F[A] = State[TestData, A]})#F] { … }
Type Class Instances Partial Type Application

OMG, TAGLESS-FINAL IS THE WORST!!!
def program[F[_]: Monad: Console: Database] = ...
Type Class Instances Partial Type Application Monad Hierarchy

TAGLESS-FINAL IS MANY THINGS
EASY IS NOT ONE OF THEM

☠ TYPE CLASS ABUSE ☠
THE DARK SIDE OF TAGLESS-FINAL
REAL TYPE CLASS
Defines common
structure across types
through lawful
operations, enabling
abstraction.
FAKE TYPE CLASS
Defines a common
interface across types
through ad hoc
polymorphism,
enabling testing.

☠ BIG BANG INTRODUCTION ☠
def program[F[_]: Console: Monad] = for {
"Good morning, " +
name <- Console[F].readLine
s"Good to meet you, $name!")
} yield ()

☠ TEDIOUS REPETITION ☠
def genFeed[F[_]: Monad:
Logging: UserDatabase:
ProfileDatabase: RedisCache:
GeoIPService: AuthService:
SessionManager: Localization:
Config: EventQueue: Concurrent:
Async: MetricsManager]: F[Feed] = ???

☠ STUBBORN REPETITION ☠
def genFeed[F[_]: Everything]: F[Feed] =
???

☠ COMPLETELY NON-INFERABLE ☠
def genFeed = ???

☠ FAKE PARAMETRIC GUARANTEES ☠
def innocent[F[_]: Monad]: F[Unit] = {
def effect[A](a: => A): F[A] =
Monad[F].point(()).map(_ => a)
println("What guarantees?")
effect(System.exit(42))
}

ZIO is a zero-dependency Scala library for
asynchronous and concurrent programming.

High-Performance
Γ
Type-Safe
ǁ
Concurrent
⇅
Asynchronous Resource-Safe
✓
Testable
Resilient
λ
Functional

BACK TO BASICS
trait Console {
}

BACK TO BASICS
def program(c: Console) = for {
"What is your name?")
name <- c.readLine
} yield ()

BACK TO BASICS
def program(c: Console, p: Persistence) = for {
name <- c.readLine
_ <- p.savePreferences(name)
} yield ()

BACK TO BASICS
def program(s1: Service1, s2: Service2,
s3: Service3, … sn: ServiceN) =
for {
a <- foo(s1, s9, s3)("localhost", 42)
b <- bar(sn, s19, s3)(a, 1024)
...
} yield z

THE MODULE PATTERN
trait HasConsole {
def console: HasConsole.Service
}
object HasConsole {
trait Service {
}
}

THE MODULE PATTERN
def program(s: HasConsole with HasPersistence) =
for {
_ <- s.console.println("What is your name?")
name <- s.console.readLine
_ <- s.persistence.savePreferences(name)
_ <- s.console.println(s"Good to meet” +
” you, $name!")
} yield ()

THE MODULE PATTERN
def program(s: HasService1 with … HasServiceN) =
for {
a <- foo(s)("localhost", 42)
b <- bar(s)(a, 1024)
...
} yield z

THE READER MONAD
case class Reader[-R, +A](provide: R => A) { self =>
def map[B](f: A => B) = flatMap(a => Reader.point(f(a)))
def flatMap[R1 <: R, B](f: A => Reader[R1, B]) =
Reader[R, B](r => f(self.provide(r)).provide(r))
}
object Reader {
def point[A](a: => A): Reader[Any, A] = Reader(_ => a)
def environment[R]: Reader[R, R] = Reader(identity)
}

THE READER MONAD
def program: Reader[HasService1 with ..., Unit] =
for {
a <- foo("localhost", 42)
b <- bar(a, 1024)
...
} yield z

THE READER MONAD TRANSFORMER
ReaderT[IO, R, A]

THE READER MONAD TRANSFORMER
ReaderT[IO, R, A]
IO
ReaderT

ZIO ENVIRONMENT
ZIO[R, E, A]*
type UIO [ +A] = ZIO[Any, Nothing, A]
type Task[ +A] = ZIO[Any, Throwable, A]
type IO [+E, +A] = ZIO[Any, E, A]
*

ZIO ENVIRONMENT
ZIO[R, E, A]
Environment Type

ZIO ENVIRONMENT
ZIO[R, E, A]
Failure Type

ZIO ENVIRONMENT
ZIO[R, E, A]
Success Type

ZIO ENVIRONMENT
ZIO[Console, IOException, Unit]
Requires Console

ZIO ENVIRONMENT
Might Fail with
IOException

ZIO ENVIRONMENT
Might Succeed
with Unit

ZIO ENVIRONMENT: EXAMPLE
val program: ZIO[Console with Persistence, IOException, Unit] =
for {
_ <- putStrLn("Good morning, what is your name?")
name <- getStrLn
_ <- savePreferences(name)
} yield ()

ZIO ENVIRONMENT: CORE
sealed trait ZIO[-R, +E, +A] {
...
def provide(environment: R): ZIO[Any, E, A] = ...
}
object ZIO {
def accessM[R, E, A](f: R => ZIO[R, E, A]): ZIO[R, E, A] = ...
def access[R, E, A](f: R => A): ZIO[R, Nothing, A] =
accessM(ZIO.succeed(_))
def environment[R]: ZIO[R, Nothing, R] = access(identity)
}

ZIO ENVIRONMENT: TUTORIAL
// Module (contains service)
trait Console {
def console: Console.Service
}

object Console {
// Service definition:
trait Service {
def println(line: String): ZIO[Any, Nothing, Unit]
val readLine: ZIO[Any, IOException, String]
}
}

object ConsoleLive extends Console {
val console = new Console.Service {
def println(line: String): UIO[Unit] =
ZIO.effectTotal(scala.io.StdIn.println(line))
val readLine: IO[IOException, String] =
ZIO.effect(scala.io.StdIn.readLine()).refineOrDie {
case e : IOException => e
}
}
}

// Optional (but handy!) helpers:
package object console {
def println(line: String): ZIO[Console, Nothing, Unit] =
ZIO.accessM(_.console println line)
val readLine: ZIO[Console, IOException, String] =
ZIO.accessM(_.console.readLine)
}

val program: ZIO[Console, IOException, Unit] =
for {
name <- getStrLn
} yield ()
DefaultRuntime.unsafeRun(program.provide(ConsoleLive))

object ConsoleTest extends Console {
...
}
DefaultRuntime.unsafeRun(program.provide(ConsoleTest))

val MyEnvironment: R = ...
val MyRuntime: Runtime[R] =
Runtime(MyEnvironment, PlatformLive)
Your Own R Platform (thread pool, etc.)

ZIO ENVIRONMENT: TEACHABLE
trait Console { def console: Console.Service }
object Console {
trait Service {
}
}
object ConsoleLive extends Console.Service {
def println(line: String) = ZIO.effectTotal(scala.io.StdIn.println(line))
val readLine =
ZIO.effect(scala.io.StdIn.readLine()).refineOrDie(JustIOException)
}
Module

object Console {
trait Service {
}
}
val readLine =
}
Service

object Console {
trait Service {
}
}
val readLine =
}
Implementation

ZIO ENVIRONMENT: COMPOSABLE
trait Logging { def logging: Logging.Service }
trait Persistence { def persistence: Persistence.Service }
...
val program: ZIO[Console with Logging with Persistence,
AppError, Unit] = ...

ZIO ENVIRONMENT: PERFORMANT
scala.concurrent.Future

ZIO ENVIRONMENT: FULLY INFERABLE
val program =
for {
name <- getStrLn
_ <- savePreferences(name)
_ <- log.debug("Saved $name to configuration")
} yield ()

ZIO ENVIRONMENT: FULLY INFERABLE
val program: ZIO[Console, Error, Unit] = ...
^^^^^^^
Found: Console
Expected: Console with Persistence with Logging with
Config with Auth

ZIO ENVIRONMENT: CONCISE
...
type ProgramEnv = Console with Logging with Persistence
val program: ZIO[ProgramEnv, AppError, Unit] = ...

ZIO ENVIRONMENT: CONCISE
...
type Program[A] = ZIO[Console with Logging with Persistence,
AppError, A]
val program: Program[Unit] = ...

ZIO ENVIRONMENT: MODULAR
def fn1: ZIO[R1, E, A] = {
def fn2: ZIO[R2, E, B] = ...
val localEnvironment: R2 = ...
val v1 = fn2.provide(localEnvironment)
...
}
val globalEnvironment: R1 = ...
val v2 = fn1.provide(globalEnvironment)
...

ZIO ENVIRONMENT: INCREMENTAL
// Deeply nested code:
val myCode: Task[Unit] = …
for {
...
result <- database.query(q)
...
} yield ()

ZIO ENVIRONMENT: INCREMENTAL
type TaskDB[A] = ZIO[Database, Throwable, A]
// Now fully testable!
def myCodeV2: TaskDB[Unit] = …
for {
...
result <- database.query(q)
...
} yield ()

THE REANIMATION OF TAGLESS-FINAL

trait HasState[S] {
def state: HasState.Service[S]
}
object HasState {
trait Service[S] {
def state: Ref[S]
}
}

implicit def MSZIO[S, R <: HasState[S], E]:
MonadState[ZIO[R, E, ?], S] =
new MonadState[ZIO[R, E, ?], S] {
...
}
...
def program[F[_]: MonadState[S, ?]]: F[Unit] = …
program[ZIO[HasState[S], E]]

implicit def MSZIO[S, R <: HasState[S], E]:
MonadState[ZIO[R, E, ?], S] =
new MonadState[ZIO[R, E, ?], S] {
...
}
...
def program[F[_]: MonadState[MySt, ?]]: F[Unit] = …
program[ZIO[HasState[MySt], Err]]

THE (RE)RISING OF CAKE
// Traditional MONOLITHIC cake
trait MyModule extends
CacheModule with LoggingModule with
DatabaseModule with AuthModule {
def refreshCache: Unit = ...
def genFeed : Feed = ...
def trace : Unit = ...
}

THE (RE)RISING OF CAKE
// Next-generation MODULAR cake
object MyFunctions {
val refreshCache: ZIO[Cache, Error, Unit] = ...
val genFeed: ZIO[Database with Auth, Error, Feed] = ...
val trace: ZIO[Debug, Nothing, Unit] = ...
}

COMPARISON MATRIX
Transformers Free Monads Eff Tagless Final
Environmental
Effects
Composable ✔ ✔ ✔ ✔ ✔
Performance 𐄂 𐄂 𐄂 ✔ ✔
Reasoning Up to Discipline Up to Discipline Up to Discipline Up to Discipline Up to Discipline
Easily Teachable 𐄂 ? ✔ 𐄂 ✔
Concision 𐄂 𐄂 𐄂 𐄂 ✔
Full Type Inference 𐄂 𐄂 𐄂 𐄂 ✔
Modularity ✔ ✔ ✔ 𐄂 ✔
Pinpoint Testability 𐄂 𐄂 𐄂 𐄂 ✔

GETTING STARTED WITH ZIO
✨ github.com/scalaz/scalaz-zio
✨ gitter.im/scalaz/scalaz-zio
✨ scalaz.github.io/scalaz-zio

Thank You!
Any questions?
Stalk me online at @jdegoes
Follow Wiem online @wiemzin

The Death of Final Tagless

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