1. Trip with monads
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 1

2. Why should I write FP code?
1. Modularity
2. Testability
3. Performance
4. Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 2

3. FP Building Blocks
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 3

4. FP Building Blocks
case class EitherT[F[_], A, B](
run: F[A / B]
)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 4

5. FP Building Blocks
case class EitherT[F[_], A, B](
run: F[A / B]
)
A / B
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 5

6. FP Building Blocks
case class EitherT[F[_], A, B](
run: F[A / B]
)
A __/ B
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 6

7. FP Building Blocks
case class EitherT[F[_], A, B](
run: F[A / B]
)
A _( )_/ B
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 7

8. FP Building Blocks
case class EitherT[F[_], A, B](
run: F[A / B]
)
A ¯_( )_/¯ B
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 8

9. FP Building Blocks
case class Reader[A, B](
run: A => B
)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 9

10. FP Building Blocks
case class Kleisli[M[_], A, B](
run: A => M[B]
)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 10

11. FP Building Blocks
case class Kleisli[M[_], A, B](
run: A => M[B]
)
type ReaderT[M[_], A, B] = Kleisli[M, A, B]
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 11

12. FP Building Blocks
case class Kleisli[M[_], A, B](
run: A => M[B]
)
type ReaderT[M[_], A, B] = Kleisli[M, A, B]
type Reader[A, B] = Kleisli[Id, A, B]
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 12

13. FP Building Blocks
case class Kleisli[M[_], A, B](
run: A => M[B]
)
type ReaderT[M[_], A, B] = Kleisli[M, A, B]
type Reader[A, B] = Kleisli[Id, A, B]
type Id[A] = A
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 13

14. FP Building Blocks
case class State[S, A](
run: S => (S, A)
)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 14

15. FP Building Blocks
case class StateT[M[_], S, A](
run: S => M[(S, A)]
)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 15

16. FP Building Blocks
case class StateT[M[_], S, A](
run: S => M[(S, A)]
)
type State[A, B] = StateT[Id, A, B]
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 16

17. FP Building Blocks
case class EitherT[F[_], A, B](run: F[A / B])
case class Kleisli[M[_], A, B](run: A => M[B])
type ReaderT[M[_], A, B] = Kleisli[M, A, B]
type Reader[A, B] = Kleisli[Id, A, B]
case class StateT[M[_], S, A](run: S => M[(S, A)])
type State[A, B] = StateT[Id, A, B]
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 17

18. And now what?
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 18

19. Learning Functional Programming
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 19

20. Let's write FP application!
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 20

21. The App
>
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 21

22. The App
> run
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 22

23. The App
> run
Using weather service at http://our-super-weather-forcast-app.com
What is the next city?
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 23

24. The App
> run
Using weather service at http://our-super-weather-forcast-app.com
What is the next city?
Wroclaw
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 24

25. The App
> run
Using weather service at http://our-super-weather-forcast-app.com
What is the next city?
Wroclaw
Forcast for City(Wroclaw) is Temperature(28, Celcius)
Hottest city found so far is (City(Wroclaw), Temperature(28, Celcius))
What is the next city?
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 25

26. The App
> run
Using weather service at http://our-super-weather-forcast-app.com
What is the next city?
Wroclaw
Forcast for City(Wroclaw) is Temperature(28, Celcius)
Hottest city found so far is (City(Wroclaw), Temperature(28, Celcius))
What is the next city?
London
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 26

27. The App
> run
Using weather service at http://our-super-weather-forcast-app.com
What is the next city?
Wroclaw
Forcast for City(Wroclaw) is Temperature(28, Celcius)
Hottest city found so far is (City(Wroclaw), Temperature(28, Celcius))
What is the next city?
London
Forcast for City(London) is Temperature(34, Celcius)
Hottest city found so far is (City(London), Temperature(34, Celcius))
What is the next city?
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 27

28. The App
> run
Using weather service at http://our-super-weather-forcast-app.com
What is the next city?
Wroclaw
Forcast for City(Wroclaw) is Temperature(28, Celcius)
Hottest city found so far is (City(Wroclaw), Temperature(28, Celcius))
What is the next city?
London
Forcast for City(London) is Temperature(34, Celcius)
Hottest city found so far is (City(London), Temperature(34, Celcius))
What is the next city?
kjsdkjssd
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 28

29. The App
> run
Using weather service at http://our-super-weather-forcast-app.com
What is the next city?
Wroclaw
Forcast for City(Wroclaw) is Temperature(28, Celcius)
Hottest city found so far is (City(Wroclaw), Temperature(28, Celcius))
What is the next city?
London
Forcast for City(London) is Temperature(34, Celcius)
Hottest city found so far is (City(London), Temperature(34, Celcius))
What is the next city?
kjsdkjssd
Encountered an error: UknownCity(kjsdkjssd)
>
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 29

30. Domain
sealed trait TempUnit
case object Celcius extends TempUnit
case object Fahren extends TempUnit
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 30

31. Domain
case class Temperature(
value: Int,
unit: TempUnit = Celcius
)
case class Forcast(temperature: Temperature)
case class City(name: String)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 31

32. Third-party
class WeatherClient(host: String, port: Int) {
def forcast(city: City): Forcast = city match {
case City("Wroclaw") =>
Forcast(Temperature(28))
case City("London") =>
Forcast(Temperature(34))
}
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 32

33. Helper classes
case class Config(host: String, port: Int)
sealed trait Error
case class UnknownCity(city: String) extends Error
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 33

34. Helper classes
type Requests = Map[City, Forcast]
object Requests {
def empty: Requests = Map.empty[City, Forcast]
def hottest(requests: Requests): (City, Forcast) = ...
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 34

35. Imports
import scalaz._, Scalaz._
import monix.eval.Task
import monix.execution.Scheduler
import shims._
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 35

36. So what do we need?
1. Host & port
2. Input / output
3. City by name (errors)
4. Forcast from third-party
5. Hottest city
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 36

37. Host & port
def host: Reader[Config, String] =
Reader(_.host)
def port: Reader[Config, Int] =
Reader(_.port)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 37

38. Input / output
def printLn(line: String): Task[Unit] =
Task.delay(println(line))
def readLn: Task[String] =
Task.delay(scala.io.StdIn.readLine)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 38

39. City by name
def cityByName(cityName: String): Error / City =
cityName match {
case "Wroclaw" =>
/-(City(cityName))
case "London" =>
/-(City(cityName))
case _ =>
-/(UnknownCity(cityName))
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 39

40. Weather forcast
def weather(city: City)(
host: String, port: Int
): Task[Forcast] = Task.delay {
new WeatherClient(host, port).forcast(city)
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 40

41. Hottest city
def hottestCity: State[(City, Temperature)] =
State { reqs =>
val temper = Requests.hottest(reqs)
.map(f => f.temperature)
(reqs, temper)
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 41

42. Ask city
def askCity: Task[String] = for {
_ <- printLn("What is the next city?")
cityName <- readLn
} yield cityName
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 42

43. def fetchForcast(city: City)(host: String, port: Int): StateReq[Forcast] =
for {
mForcast <- StateT { (reqs: Requests) =>
Task.delay ((reqs, reqs.get(city)))
}
forcast <- StateT { (reqs: Requests) =>
maybeForcast
.cata(_.pure[Task], weather(city)(host, port))
.map (forcast => (reqs, forcast))
}
_ <- StateT { (reqs: Requests) =>
Task.delay ((reqs + (city -> forcast), ()))
}
} yield forcast
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 43

44. Final program
def askFetchJudge =
for {
cityName <- askCity
city <- cityByName(cityName)
h <- host
p <- port
forcast <- fetchForcast(city)(host, port)
_ <- printLn(s"Forcast for $city is ${forcast.temperature}")
hottest <- hottestCity
_ <- printLn(s"Hottest city found so far is $hottest")
} yield ()
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 44

45. Final program
def program = for {
h <- host
p <- port
_ <- printLn(s"Using weather service at http://$h:$p n")
_ <- askFetchJudge.forever
} yield ()
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 45

46. WRONG!
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 46

47. Reality
def askFetchJudge: Effect[Unit] =
for {
cityName <- askCity.liftM[ErrorEitherT].liftM[ConfigReaderT].liftM[RequestsStateT]
city <- EitherT
.fromDisjunction[Task](cityByName(cityName))
.liftM[ConfigReaderT].liftM[RequestsStateT]
h <- StateT[Effect1, Requests, String]((requests: Requests) =>
ReaderT[Effect0, Config, (Requests, String)]((config: Config) =>
EitherT(Task.delay {
((requests, host.run(config))).right[Error]
})))
p <- StateT[Effect1, Requests, Int]((requests: Requests) =>
ReaderT[Effect0, Config, (Requests, Int)]((config: Config) =>
EitherT(Task.delay {
((requests, port.run(config))).right[Error]
})))
forcast <- fetchForcast(city)(h, p)
.mapK[Effect1, Forcast, Requests](
(t: Task[(Requests, Forcast)]) =>
ReaderT[EitherT[Task, Error, ?], Config, (Requests, Forcast)](
κ(EitherT[Task, Error, (Requests, Forcast)](
t.map(_.right[Error])))))
_ <- printLn(s"Forcast for $city is ${forcast.temperature}")
.liftM[ErrorEitherT].liftM[ConfigReaderT].liftM[RequestsStateT]
hottest <- hottestCity.mapK[Effect1, (City, Temperature), Requests](
(t: Task[(Requests, (City, Temperature))]) =>
ReaderT[EitherT[Task, Error, ?],
Config,
(Requests, (City, Temperature))](
κ(EitherT[Task, Error, (Requests, (City, Temperature))](
t.map(_.right[Error])))))
_ <- printLn(s"Hottest city found so far is $hottest")
.liftM[ErrorEitherT]
.liftM[ConfigReaderT]
.liftM[RequestsStateT]
} yield ()
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 47

48. Reality
def program: Effect[Unit] = for {
h <- StateT[Effect1, Requests, String]((requests: Requests) =>
ReaderT[Effect0, Config, (Requests, String)]((config: Config) =>
EitherT(Task.delay {
((requests, host.run(config))).right[Error]
})))
p <- StateT[Effect1, Requests, Int]((requests: Requests) =>
ReaderT[Effect0, Config, (Requests, Int)]((config: Config) =>
EitherT(Task.delay {
((requests, port.run(config))).right[Error]
})))
_ <- printLn(s"Using weather service at http://$h:$p n")
.liftM[ErrorEitherT]
.liftM[ConfigReaderT]
.liftM[RequestsStateT]
_ <- askFetchJudge.forever
} yield ()
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 48

49. "The FP sounds rather like
a mediæval monk, denying
himself the pleasures of life
in the hope that it will
make him virtuous."
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 49

50. Modularity
Testability
Performance
Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 50

51. Modularity !
Testability
Performance
Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 51

52. Modularity !
Testability !
Performance
Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 52

53. Modularity !
Testability !
Performance !
Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 53

54. Modularity !
Testability !
Performance !
Maintainability !
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 54

55. def askFetchJudge: Effect[Unit] = ..
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 55

56. def askFetchJudge: Effect[Unit] = ..
type Effect0[A] = EitherT[Task, Error, A]
type Effect1[A] = ReaderT[Effect0, Config, A]
type Effect[A] = StateT[Effect1, Requests, A]
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 56

57. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
}
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 57

58. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
val app: Effect[Unit] = program
implicit val io: SchedulerService =
Scheduler.io("io-scheduler")
}
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 58

59. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
val app: Effect[Unit] = program
implicit val io: SchedulerService =
Scheduler.io("io-scheduler")
app.run(requests).run(config).run
}
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 59

60. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
val app: Effect[Unit] = program
implicit val io: SchedulerService =
Scheduler.io("io-scheduler")
app.run(requests).run(config).run >>= {
case -/(error) =>
printLn(s"Encountered an error:" +
s"${error.shows}")
case /-(_) => ().pure[Task]
}
}
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 60

61. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
val app: Effect[Unit] = program
implicit val io: SchedulerService =
Scheduler.io("io-scheduler")
(app.run(requests).run(config).run >>= {
case -/(error) =>
printLn(s"Encountered an error:" +
s"${error.shows}")
case /-(_) => ().pure[Task]
}).unsafeRunSync
}
.
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 61

62. Can we do better?
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 62

63. So what do we need?
1. Host & port
2. Input / output
3. City by name (errors)
4. Forcast from third-party
5. Hottest city
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 63

64. Host & port
def host: Reader[Config, String] =
Reader(_.host)
def port: Reader[Config, Int] =
Reader(_.port)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 64

65. Host & port
def host[F[_]]: F[String] = ?
def port[F[_]]: F[Int] = ?
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 65

66. trait ApplicativeAsk[F[_], E] {
val applicative: Applicative[F]
def ask: F[E]
def reader[A](f: E => A): F[A]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 66

67. trait ApplicativeAsk[F[_], E] {
val applicative: Applicative[F]
def ask: F[E]
def reader[A](f: E => A): F[A]
}
type ConfigAsk[F[_]] = ApplicativeAsk[F, Config]
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 67

68. def host[F[_]]: F[String] = ?
def port[F[_]]: F[Int] = ?
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 68

69. def host[F[_]: ConfigAsk]: F[String] =
ConfigAsk[F].reader(_.host)
def port[F[_]: ConfigAsk]: F[Int] =
ConfigAsk[F].reader(_.port)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 69

70. def host[F[_]: ConfigAsk]: F[String] =
ConfigAsk[F].reader(_.host)
def port[F[_]: ConfigAsk]: F[Int] =
ConfigAsk[F].reader(_.port)
trait ApplicativeAsk[F[_], E] {
val applicative: Applicative[F]
def ask: F[E]
def reader[A](f: E => A): F[A]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 70

71. City by name
def cityByName(cityName: String): Error / City =
cityName match {
case "Wroclaw" => /-(City(cityName))
case "London" => /-(City(cityName))
case _ => -/(UnknownCity(cityName))
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 71

72. City by name
def cityByName[F[_]](
cityName: String
): F[City] =
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 72

73. City by name
def cityByName[F[_]:Applicative](
cityName: String
): F[City] = cityName match {
case "Wroclaw" => City(cityName).pure[F]
case "London" => City(cityName).pure[F]
case _ => ???
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 73

74. trait ApplicativeError[F[_], E] {
val applicative: Applicative[F]
def raiseError[A](e: E): F[A]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 74

75. trait ApplicativeError[F[_], E] {
val applicative: Applicative[F]
def raiseError[A](e: E): F[A]
}
type ErrorHandler[F[_]] = ApplicativeError[F, Error]
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 75

76. City by name
def cityByName[F[_]:Applicative](
cityName: String
): F[City] = cityName match {
case "Wroclaw" => City(cityName).pure[F]
case "London" => City(cityName).pure[F]
case _ => ???
} .
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 76

77. City by name
def cityByName[F[_]: Applicative: ErrorHandler](
cityName: String
): F[City] = cityName match {
case "Wroclaw" => City(cityName).pure[F]
case "London" => City(cityName).pure[F]
case _ => ErrorHandler[F].raiseError(UnknownCity(cityName))
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 77

78. Hottest city
def hottestCity: State[(City, Temperature)] =
State { reqs =>
val temper = Requests.hottest(reqs)
.map(f => f.temperature)
(reqs, temper)
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 78

79. Hottest city
def hottestCity[F[_]]: F[(City, Temperature)]= ?
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 79

80. trait MonadState[F[_], S] {
val monad: Monad[F]
def get: F[S]
def set(s: S): F[Unit]
def inspect[A](f: S => A): F[A]
def modify(f: S => S): F[Unit]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 80

81. trait MonadState[F[_], S] {
val monad: Monad[F]
def get: F[S]
def set(s: S): F[Unit]
def inspect[A](f: S => A): F[A]
def modify(f: S => S): F[Unit]
}
type RequestsState[F[_]] = MonadState[F, Requests]
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 81

82. Hottest city
def hottestCity[F[_]]: F[(City, Temperature)]= ?
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 82

83. Hottest city
def hottestCity[
F[_]: RequestsState
]: F[(City, Temperature)] =
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 83

84. Hottest city
def hottestCity[
F[_]: RequestsState
]: F[(City, Temperature)] =
RequestsState[F].inspect(reqs =>
Requests.hottest(reqs).map(_.temperature)
)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 84

85. Hottest city
def hottestCity[
F[_]: RequestsState
]: F[(City, Temperature)] =
RequestsState[F].inspect(reqs =>
Requests.hottest(reqs).map(_.temperature)
)
trait MonadState[F[_], S] {
// (...)
def inspect[A](f: S => A): F[A]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 85

86. MTL© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 86

87. Input / output
def printLn(line: String): Task[Unit] =
Task.delay(println(line))
def readLn: Task[String] =
Task.delay(scala.io.StdIn.readLine)
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 87

88. Input / output
trait Console[F[_]] {
def readLine: F[String]
def printLn(line: String): F[Unit]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 88

89. Ask City
def askCity[F[_]: Console: Monad]: F[String] =
for {
_ <- Console[F].printLn("What is the next city?")
cityName <- Console[F].readLine
} yield cityName
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 89

90. Weather forcast
def weather(city: City)(
host: String, port: Int
): Task[Forcast] = Task.delay {
new WeatherClient(host, port).forcast(city)
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 90

91. Weather forcast
trait Weather[F[_]] {
def forcast(city: City): F[Forcast]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 91

92. Fetch forcast
def fetchForcast[F[_]: Weather: RequestsState: Monad](city: City): F[Forcast] =
for {
maybeForcast <- RequestsState[F].inspect(_.get(city))
forcast <- maybeForcast.cata(
_.pure[F],
Weather[F].forcast(city)
)
_ <- RequestsState[F].modify(_ + (city -> forcast))
} yield forcast
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 92

93. Tagless final
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 93

94. Final program
def askFetchJudge[F[_]: Console: Weather :
RequestsState: ErrorHandler:Monad]: F[Unit] =
for {
cityName <- askCity[F]
city <- cityByName[F](cityName)
forcast <- fetchForcast[F](city)
_ <- Console[F].printLn(s"Forcast for $city is ${forcast.temperature}")
hottest <- hottestCity[F]
_ <- Console[F].printLn(s"Hottest city found so far is $hottest")
} yield ()
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 94

95. Final program
def program[F[_]: ConfigAsk: Console: Weather:
RequestsState:ErrorHandler:Monad]: F[Unit] =
for {
h <- host[F]
p <- port[F]
_ <- Console[F].printLn(s"Using weather service at http://$h:$p")
_ <- askFetchJudge[F].forever
} yield ()
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 95

96. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = ???
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 96

97. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = Task[A]
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 97

98. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = Task[A]
implicit val weather = ???
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 98

99. object Weather {
def monixWeather(config: Config): Weather[Task] =
new Weather[Task] {
val client: WeatherClient = new
WeatherClient(config.host, config.port)
def forcast(city: City): Task[Forcast] =
Task.delay {
client.forcast(city)
}
}
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 99

100. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = Task[A]
implicit val weather = ???
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 100

101. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = Task[A]
implicit val weather = Weather.monixWeather(config)
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 101

102. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = Task[A]
implicit val weather = Weather.monixWeather(config)
implicit val console = ???
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 102

103. object Console {
val monixConsole: Console[Task] =
new Console[Task] {
def readLine: Task[String] =
Task.delay(scala.io.StdIn.readLine)
def printLn(line: String): Task[Unit] =
Task.delay(println(line))
}
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 103

104. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = Task[A]
implicit val weather = Weather.monixWeather(config)
implicit val console = ???
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 104

105. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = Task[A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 105

106. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = StateT[Task, Requests, A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 106

107. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect1[A] = ReaderT[Task, Config, A]
type Effect[A] = StateT[Effect1, Requests, A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 107

108. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect0[A] = EitherT[Task, Error, A]
type Effect1[A] = ReaderT[Effect0, Config, A]
type Effect[A] = StateT[Effect1, Requests, A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
val app: Effect[Unit] = program[Effect]
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 108

109. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect0[A] = EitherT[Task, Error, A]
type Effect1[A] = ReaderT[Effect0, Config, A]
type Effect[A] = StateT[Effect1, Requests, A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
val app: Effect[Unit] = program[Effect]
(app.run(requests).run(config).run >>= {
case -/(error) => console.printLn(s"Encountered an error: ${error.shows}")
case /-(_) => ().pure[Task]
}).unsafeRunSync
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 109

110. Modularity
Testability
Performance
Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 110

111. Modularity ✅
Testability
Performance
Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 111

112. def askFetchJudge[F[_]: Console: Weather :
RequestsState: ErrorHandler:Monad]: F[Unit] =
for {
cityName <- askCity[F]
city <- cityByName[F](cityName)
forcast <- fetchForcast[F](city)
_ <- Console[F].printLn(s"Forcast for $city is ${forcast.temperature}")
hottest <- hottestCity[F]
_ <- Console[F].printLn(s"Hottest city found so far is $hottest")
} yield ()
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 112

113. Modularity ✅
Testability
Performance
Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 113

114. Modularity ✅
Testability ✅
Performance
Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 114

115. Modularity ✅
Testability ✅
Performance "
Maintainability
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 115

116. Modularity ✅
Testability ✅
Performance "
Maintainability ✅
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 116

117. Can we do better?
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 117

118. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect0[A] = EitherT[Task, Error, A]
type Effect1[A] = ReaderT[Effect0, Config, A]
type Effect[A] = StateT[Effect1, Requests, A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 118

119. private[twm] class AtomicMonadState[S](atomic: Atomic[S])
extends MonadState[Task, S] {
val monad: Monad[Task] = Monad[Task]
def get: Task[S] = Task.delay(atomic.get)
def set(s: S): Task[Unit] = Task.delay(atomic.set(s))
def inspect[A](f: S => A): Task[A] = Task.delay(f(atomic.get))
def modify(f: S => S): Task[Unit] = Task.delay(atomic.transform(f))
}
object AtomicMonadState {
def create[S <: AnyRef](s: S): AtomicMonadState[S] =
new AtomicMonadState(AtomicAny[S](s))
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 119

120. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect0[A] = EitherT[Task, Error, A]
type Effect1[A] = ReaderT[Effect0, Config, A]
type Effect[A] = StateT[Effect1, Requests, A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 120

121. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect0[A] = EitherT[Task, Error, A]
type Effect[A] = ReaderT[Effect0, Config, A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
implicit val requestsState =
AtomicMonadState.create(Requests.empty)
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 121

122. object ApplicativeAsk {
def constant[
F[_]:Applicative, E
](e: E): ApplicativeAsk[F, E] =
new DefaultApplicativeAsk[F, E] {
val applicative: Applicative[F] = Applicative[F]
def ask: F[E] = applicative.point(e)
}
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 122

123. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect0[A] = EitherT[Task, Error, A]
type Effect[A] = ReaderT[Effect0, Config, A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
implicit val requestsState =
AtomicMonadState.create(Requests.empty)
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 123

124. def main(args: Array[String]): Unit = {
val config = Config("localhost", 8080)
val requests = Requests.empty
type Effect[A] = EitherT[Task, Error, A]
implicit val weather = Weather.monixWeather(config)
implicit val console = Console.monixConsole
implicit val requestsState =
AtomicMonadState.create(Requests.empty)
implicit val configAsk =
ApplicativeAsk.constant[Task, Config](config)
}
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 124

125. Modularity ✅
Testability ✅
Performance "
Maintainability ✅
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 125

126. Modularity ✅
Testability ✅
Performance ✅ *
Maintainability ✅
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 126

127. Can we do better?
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 127

128. type Effect[A] = EitherT[Task, Error, A]
Last frontier
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 128

129. Why?
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 129

130. Why?
1. Performance
2. Two channels error handling
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 130

131. Introducing IO[E, A]
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 131

132. Introducing IO[E, A]
https://github.com/scalaz/scalaz-zio
https://github.com/LukaJCB/cats-bio
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 132

133. Modularity ✅
Testability ✅
Performance ✅ *
Maintainability ✅
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 133

134. Modularity ✅
Testability ✅
Performance ✅
Maintainability ✅
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 134

135. Announcements
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 135

136. Announcements
1. https://github.com/scalaz/scalaz-mtl
2. http://rabbitonweb.com/
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 136

137. Thank you very much
Pawel Szulc
@rabbitonweb
© Pawel Szulc, @rabbitonweb, paul.szulc@gmail.com 137