Upgrade to Pro
— share decks privately, control downloads, hide ads and more …
Speaker Deck
Features
Speaker Deck
PRO
Sign in
Sign up for free
Search
Search
Monads in Scala
Search
Alexey Novakov
March 06, 2020
Programming
0
82
Monads in Scala
Monad as an abstract interface. Usage of Monads in Scala
Alexey Novakov
March 06, 2020
Tweet
Share
More Decks by Alexey Novakov
See All by Alexey Novakov
Особенности разработки Flink джобов на Scala
alexeyn
0
3
Streamhouse Architecture with Flink and Paimon
alexeyn
0
200
Flink Forward 2023: State of Scala API in Apache Flink
alexeyn
0
130
State of Scala API in Apache Flink
alexeyn
0
380
Deployment of Streaming Application with Ververica Platform on Kubernetes
alexeyn
0
190
Rapid Deployment with Apache Flink and Ververica Platform
alexeyn
0
120
Scalacon 2021 - Deep Learning in Scala
alexeyn
0
89
AWS Airflow and EMR
alexeyn
0
92
EMR Data Ingestion with Apache Hudi
alexeyn
0
36
Other Decks in Programming
See All in Programming
ファインディ株式会社におけるMCP活用とサービス開発
starfish719
0
1.9k
今だからこそ入門する Server-Sent Events (SSE)
nearme_tech
PRO
3
240
CJK and Unicode From a PHP Committer
youkidearitai
PRO
0
110
より安全で効率的な Go コードへ: Protocol Buffers Opaque API の導入
shwatanap
2
360
Reading Rails 1.0 Source Code
okuramasafumi
0
250
概念モデル→論理モデルで気をつけていること
sunnyone
3
290
The Past, Present, and Future of Enterprise Java
ivargrimstad
0
400
旅行プランAIエージェント開発の裏側
ippo012
2
920
さようなら Date。 ようこそTemporal! 3年間先行利用して得られた知見の共有
8beeeaaat
3
1.5k
The Past, Present, and Future of Enterprise Java with ASF in the Middle
ivargrimstad
0
160
How Android Uses Data Structures Behind The Scenes
l2hyunwoo
0
480
Oracle Database Technology Night 92 Database Connection control FAN-AC
oracle4engineer
PRO
1
460
Featured
See All Featured
Easily Structure & Communicate Ideas using Wireframe
afnizarnur
194
16k
Connecting the Dots Between Site Speed, User Experience & Your Business [WebExpo 2025]
tammyeverts
8
530
Unsuck your backbone
ammeep
671
58k
実際に使うSQLの書き方 徹底解説 / pgcon21j-tutorial
soudai
PRO
188
55k
For a Future-Friendly Web
brad_frost
180
9.9k
Done Done
chrislema
185
16k
Visualizing Your Data: Incorporating Mongo into Loggly Infrastructure
mongodb
48
9.7k
The MySQL Ecosystem @ GitHub 2015
samlambert
251
13k
[Rails World 2023 - Day 1 Closing Keynote] - The Magic of Rails
eileencodes
36
2.5k
StorybookのUI Testing Handbookを読んだ
zakiyama
31
6.1k
Writing Fast Ruby
sferik
628
62k
JavaScript: Past, Present, and Future - NDC Porto 2020
reverentgeek
52
5.6k
Transcript
Monads in Scala Alexey Novakov, Ultra Tendency, 2020
"A monad is just a monoid in the category of
endofunctors”
“Monad is an abstract interface” Book: Functional Programming in Scala
(Red Book)
Option Either List Future Map Set Stream Vector Future Try
And others … Monads in standard Scala
flatMap unit a.k.a bind a.k.a pure What makes thing a
Monad? “apply” in Scala
class Box[A](v: A) { def flatMap[B](f: A => Box[B]): Box[B]
= f(v) } def map[B](f: A => B): Box[B] = flatMap(a => new Box(f(a))) A Monad modeled as a class minimum set
scala> new Box(1) res2: Box[Int] = 1 scala> res2.map(_ +
1) res3: Box[Int] = 2 scala> res3.flatMap(i => new Box(1 + i)) res5: Box[Int] = 3
scala> val l = List(1,2,3) l: List[Int] = List(1, 2,
3) scala> l.map(_ + 1) res0: List[Int] = List(2, 3, 4) scala> l.flatMap(i => List(i + 1)) res1: List[Int] = List(2, 3, 4) List
val isOn = Some(1) val isBlack = None def makeCoffee:
Option[String] = Some(1) scala> isOn .flatMap(_ => isBlack .flatMap(_ => makeCoffee)) res0: Option[String] = None Option stopped here
trait Functor[F[_]] { def map[A,B](fa: F[A])(f: A => B): F[B]
} trait Monad[F[_]] extends Functor[F] { def unit[A](a: => A): F[A] def flatMap[A,B](ma: F[A])(f: A => F[B]): F[B] def map[A,B](ma: F[A])(f: A => B): F[B] = flatMap(ma)(a => unit(f(a))) } Generic Monad another abstract interface
Monads are like burritos
“f” function application in flatMap & map depends on the
concrete Monad instance “f” applied when: - Option[A]: is Some(A) - Either[A, B]: is Right(B) - List[A]: is non-empty - Future[A]: is ready
Moreover, Monad laws are there
Monad law 1. Identity Example def f(x: Int): Option[Int] =
Some(x) scala> Some(1).flatMap(f) == f(1) res0: Boolean = true scala> f(1) == Some(1).flatMap(f) res1: Boolean = true
Monad law 1. Left Identity def f[A](x: A): Monad[A] =
??? flatMap(unit(x))(f) == f(x) Right Identity f(x) == flatMap(unit(x))(f)
Monad law 2. Associative Example def f1(a: Int): Option[Int] =
Some(a + 1) def f2(a: Int): Option[Int] = Some(a * 2) scala> Some(1).flatMap(f1).flatMap(f2) res0: Option[Int] = Some(4) scala> Some(1).flatMap(a => f1(a).flatMap(f2)) res1: Option[Int] = Some(4)
Monad law 2. Associative def f1[A](a: A): Monad[A] def f2[A](a:
A): Monad[A] if x is a Monad instance, flatMap(flatMap(x)(f1))(f2) == flatMap(x)(a => flatMap(f1(a))(f2))
Functor law 1. Identity map(x)(a => a) == x Example:
map(Some(1))(a => a) == Some(1) the same value returned
Functor law 2. Associative Example: val f1 = (n: Int)
=> n + 1 val f2 = (n: Int) => n * 2 map(map(Some(1))(f1))(f2) // Some(4) == map(Some(1))(f2 compose f1) // Some(4)
Functor law 2. Associative map(map(x)(f1))(f2) == map(x)(f2 compose f1) one
by one apply f1 then f2 in one step
None
final case class Coffee(name: String) val isOn = Some(1) val
coffeeName = Some("black") val makeCoffee = (name: String) => Some(Coffee(name)) for { _ <- isOn name <- coffeeName coffee <- makeCoffee(name) } yield coffee scala> Op)on[Coffee] = Some(Coffee(black)) Compose Option
case class Cluster(pods: Int) def validateNamespace(ns: String): Either[String, Unit] =
Right(()) def clusterExists(ns: String): Either[Cluster, Unit] = Right(()) def createCluster(ns: String, cluster: Cluster): Either[String, Cluster] = Right(Cluster(cluster.pods)) val ns = "my-cluster" for { _ <- validateNamespace(ns) _ <- clusterExists(ns).left.map(c => s"Cluster with ${c.pods} pods already exists") newCluster <- createCluster(ns, Cluster(4)) } yield newCluster Compose Either scala> Either[String,Cluster] = Right(Cluster(4))
scala> Either[String,Cluster] = LeC( Cluster namespace is not valid name,
choose another name ) def validNamespace(ns: String): Either[String, Unit] = if (ns == "my-cluster") Left( “Cluster namespace is not valid name, choose another name” ) else Right(())
for {…} yield is a syntactic sugar for a sequence
of calls: flatMap1(… + flatMapN(.. + map(…))) https://en.wikipedia.org/wiki/Powdered_sugar#/media/File:Powdered_Sugar_-_Macro.jpg
validNamespace("my-cluster") .flatMap(_ => clusterExists(ns) .left .map(c => s"Cluster with ${c.pods}
pods already exists” ).flatMap(_ => createCluster(ns, Cluster(4)) .map(newCluster => newCluster) ) ) Desugared
for { _ <- validNamespace("my-cluster") _ <- clusterExists(ns) .left.map(c =>
s"Cluster with ${c.pods} pods already exists") newCluster <- createCluster(ns, Cluster(4)) } yield newCluster Sugared
Caveat: different Monads do not compose.
def validateNamespace(ns: String): Either[String, Unit] def clusterExists(ns: String): Option[Either[String, Cluster]]
def createCluster(ns: String, cluster: Cluster): Either[String, Cluster] Problem for { _ <- validateNamespace(ns) cluster <- clusterExists(ns) updated <- createCluster(ns, cluster) } yield updated two monad stacks
updated <- createCluster(ns, cluster) ^ <pas)e>:4: error: type mismatch; found
: Either[String,Cluster] required: Cluster cluster <- clusterExists(ns) ^ <pas)e>:3: error: type mismatch; found : Op)on[Nothing] required: scala.u)l.Either[?,?] Op)on[Nothing] <: scala.u)l.Either[?,?]? false
Monad Transformers
- custom-written monad - specifically constructed for composition OptionT: Option
+ Any other Monad EitherT: Either + Any other Monad ReaderT: Reader + Any other Monad WriterT: Writer + Any other Monad … others
EitherT *Example: unwrap second stack * from cats.data.EitherT.scala final case
class EitherT[F[_], A, B](value: F[Either[A, B]]) { def flatMap[AA >: A, D]( f: B => EitherT[F, AA, D])( implicit F: Monad[F]): EitherT[F, AA, D] = EitherT(F.flatMap(value) { case l @ Left(_) => F.pure(l.rightCast) case Right(b) => f(b).value }) }
case class Cluster(pods: Int, updated: Long) def validateNamespace(ns: String): Either[String,
Unit] = Right(()) def clusterExists(ns: String): Option[Either[String, Cluster]] = Some(Right(Cluster(3, System.currentTimeMillis()))) def updateCluster(ns: String, cluster: Cluster): Either[String, Cluster] = Right(Cluster(cluster.pods, System.currentTimeMillis())) Usage
import cats.implicits._ import cats.data.EitherT val cluster = for { _
<- validateNamespace(ns).toEitherT[Option] cluster<- EitherT(clusterExists(ns)) updated <- updateCluster(ns, cluster).toEitherT[Option] } yield updated scala> cluster.value Some(Right(Cluster(3,1583095558496))) EitherT: Either + Option common ground
Error case def clusterExists(ns: String): Option[Either[String, Cluster]] = Left("Cluster is
invalid").some scala> cluster.value res4: Op)on[Either[String,Cluster]] = Some(LeC(Cluster is invalid))
Alexey Novakov email: - alexey.novakov at ultratendency.com - novakov.alex at
gmail.com Blog: https://medium.com/se-notes-by-alexey-novakov https://novakov-alexey.github.io/ Code: https://github.com/novakov-alexey Twitter: @alexey_novakov Thank you! Questions?