1. Scalaz is a Scala library that provides functional data structures and type classes to complement the standard library. It defines foundational type classes like Functor, Monad, and provides instances for many data types. 2. The document discusses several Scalaz type classes - Equal, Semigroup, Monoid, Functor, Applicative, Monad, and Validation. It provides examples of how to use these type classes to define instances for types like Int, List, Option, etc. 3. Using Scalaz allows functional programming with types like Validation that encapsulate errors in a pure way, avoiding side effects from exceptions. It also provides syntactic sugar like |+| and <*> to make

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scalaz
En nybegynners introduksjon

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Agenda
• Bakgrunnen for at jeg har tatt i bruk
scalaz
• En vei gjennom rammeverket
• Konklusjoner
• Gjerne spørsmål underveis!

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Trond Marius Øvstetun
• Sjesfkonsulent i
Mesan
• Utvikler, arkitekt,
teamleder +++

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Mesan
... når standardsystemer ikke er nok
•Systemutvikling – skreddersøm
•Løsningsfokus
•Transaksjonssystemer
•Lang levetid – langsiktighet og
vedlikeholdbarhet

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Hvorfor scalaz?

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Hvorfor scalaz?
• If you are thinking about using Scalaz,
stop now while you still have your
sanity! ref
• Don’t listen to anyone telling you to use
Scalaz!
• Hva er greia med metodene
<:::, :::>, <+>, |@|, , , <=< ?★ ☆

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Mitt triggerpunkt

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unfiltered og json
case req @ POST(Path("/person")) => {
val x = Body.string(req)
val p: Person = read[Person](x)
val id: Int = personer.add(p)
Created ~> Json("id" -> id)
}
case class Person(name: String, age: Int)

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json
val json = """{"name": "Petter", "age": 22}"""
read[Person](json)
val json = """{"name":"Petter"}"""
read[Person](json)
=> Person(Petter, 22)
=> org.json4s.package$MappingException:
No usable value for age
Did not find value which can be converted
into int

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Validering av input
def add(p: Person): Int = {
Validate.isTrue(p.age > 18 && p.age < 60)
1
}
case req @ POST(Path("/person")) => {
val x = Body.string(req)
val p: Person = read[Person](x)
val id: Int = personer.add(p)
Created ~> Json("id" -> id)
}

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Fra en klients
perspektiv...

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Fra en utviklers
perspektiv
Dette føles ikke bra!

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validering med scalaz
case req @ POST(Path("/personz")) => {
val x = Body.string(req)
val p = fromJSON[Person](parse(x))
val id = p map personer.add
id match {
case Success(i) => Created ~> Json("id"->i)
case Failure(errors) =>
Forbidden ~>
Json("errors" ->
jsonErrors(errors).toList)
}
}

14. © Mesan AS
validering med scalaz
case req @ POST(Path("/personz")) => {
val x = Body.string(req)
val p = fromJSON[Person](parse(x))
val id = p map personer.add
id match {
case Success(i) => Created ~> Json("id"->i)
case Failure(errors) =>
Forbidden ~>
Json("errors" ->
jsonErrors(errors).toList)
}
}
implicit val personR: JSONR[Person] =
Person.applyJSON(field[String]("name"),
validate[Int]("age") >==>
min(18) >==> max(60))

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Fra en klients
perspektiv...

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Fra en utviklers
perspektiv
Mye bedre?! Men?!
>==> fromJSON[Person]
Kleisli[Result, JValue, A]
type EitherNel[+a] = NonEmptyList[Error] / a

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scalaz
import scalaz._
import Scalaz._

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Hva er scalaz?
• Scalaz is a Scala library for functional
programming.
• It provides purely functional data
structures to complement those from the
Scala standard library. It defines a set of
foundational type classes (e.g. Functor,
Monad) and corresponding instances for
a large number of data structures.

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Hva er scalaz?
1.Funksjonelle datastrukturer
2.Syntaks for utvidelse av standard
klasser
3.Typeklasser og instanser for disse

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Syntaks
utvidelser til standard klasser
og til hvordan du skriver kode

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Equal
1 == 1
=> true
1 == 1.0
=> true
val x: Int = 1
val y: Person = new Person("Petter")
x == y
warning: comparing values of types Int and Person
using `==' will always yield false
=> false
y == x
warning: Person and Int are unrelated:
they will most likely never compare equal
=> false
1 == 1
=> false

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Typesikker Equal
1 === 1
=> true
1 === 1.0
error: could not find implicit
value for parameter F0: scalaz.Equal[Any]
1 =/= 1
=> false
1 =/= 1.0
error: could not find implicit
value for parameter F0: scalaz.Equal[Any]
1 === 2
=> false
1 =/= 2
=> true

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Boolean
true && true
=> true
true / true
=> true
true && false
=> false
true / false
=> false
(if (true) "a" else "b")
=> "a"
true ? "a" | "b"
=> "a"
true option "a"
=> Some("a")
false option "a"
=> (None:Option[String])

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Option
Some(42)
=> Some[Int] = Some(42)
None
=> None.type = None
some(42)
=> Option[Int] = Some(42)
none[Int]
=> Option[Int] = None
42.some
=> Some(42)
val o = 42.some
o getOrElse 1
o | 1
=> 42
val n = option.none[Int]
n getOrElse 1
n | 1
=> 1
~o
=> 42
~n
=> 0

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Index
val l = List(1,2,3,4)
l(0)
=> 1
l(2)
=> 3
l(-1)
=> IndexOutOfBoundsException
l(4)
=> IndexOutOfBoundsException
l index 0
=> Some(1)
l index 2
=> Some(3)
l index -1
=> None
l index 4
=> None

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Alle instanser - Id
"a" ?? "b"
=> "a"
val x: String = null
x ?? "asdf"
=> "asdf"
def f(i:Int) = i + 1
(1 |> f)
=> 2
1 + 2 + 3 |> {_ * 6}
=> 36

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Typeklasser

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Hva er en Typeklasse?
• En form for et interface som definerer
oppførsel til en type
• oppførselen er definert utenfor typen
• alle typer som er medlem i typeklassen
har implementasjoner av oppførselen
• ikke et interface som i Java

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Typeklasser i Scala
trait Truthy[A] {
def truthy(a: A): Boolean
}
implicit val intTruthy: Truthy[Int] = new Truthy[Int] {
def truthy(a: Int): Boolean = a match {
case 0 => false
case _ => true
}
}
intTruthy.truthy(1)
=> true
intTruthy.truthy(0)
=> false

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Typeklasser i Scala
object Truthy {
def apply[A](implicit A: Truthy[A]): Truthy[A] = A
}
> Truthy[Int].truthy(0)
=> false
Truthy[Int].truthy(1)
=> true
1.truthy
=> true

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Typeklasser i Scala
trait TruthyOps[A] {
def self: A
implicit def F:Truthy[A]
def truthy: Boolean = F.truthy(self)
}
object ToTruthyOps {
implicit def toTOps[A](a: A)(implicit ev: Truthy[A]) =
new TruthyOps[A] {
def self: A = a
implicit def F = ev
}
}
1.truthy
=> true

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Typeklasser i Scala
def iffy[A: Truthy, B](t: A)(ifT: =>B)(ifF: =>B): B = {
if (t.truthy) ifT else ifF
}
iffy(1)("Ja!")("Neeei!")
=> Ja!
iffy(0)("Ja!")("Neeei!")
=> Neeei!

33. © Mesan AS
Typeklasser i Scalaz

34. © Mesan AS
Typeklasser i Scalaz

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Equal
trait Equal[F] { self =>
def equal(a1: F, a2: F): Boolean
}
implicit val intInstance: Monoid[Int] with Enum[Int] with Show[Int] =
new Monoid[Int] with Enum[Int] with Show[Int] {
....
}
trait EqualOps[F] extends Ops[F] {
final def ===(other: F): Boolean = ???
final def =/=(other: F): Boolean = ???
}
1 === 1
=> true

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Equal
case class Car(val model: String)
implicit val carEqual = new Equal[Car] {
def equal(a1: Car, a2: Car): Boolean =
a1.model == a2.model
}
Car("Honda") === Car("Toyota")
=> false
Car("Honda") === Car("Honda")
=> true

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SemiGroup
trait Semigroup[F] { self =>
def append(f1: F, f2: => F): F
}
trait SemigroupOps[F] extends Ops[F] {
implicit def F: Semigroup[F]
final def |+|(other: => F): F = ???
final def mappend(other: => F): F = ???
}
1 |+| 2
=> 3
List(1,2) |+| List(3,4)
=> List(1,2,3,4)
(1,2) |+| (3,4)
=> (4,6)

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Monoid
trait Monoid[F] extends Semigroup[F] { self =>
def zero: F
}
trait MonoidOps[F] extends Ops[F] {
final def multiply(n: Int): F = ???
final def ifEmpty[A](tv: => A)(fv: => A) = ???
}
(0.ifEmpty("Empty")("NonEmpty"))
=> "Empty"
(1.ifEmpty("Empty")("NonEmpty"))
=> "NonEmpty"
"a" multiply 3
=> "aaa"
~o.none[Int]
=> 0
~3.some
=> 3

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Monoid avler Monoid!
• Option[A] er Monoid hvis A er Monoid
• Map[K, A] er Monoid hivs A er Monoid
• (A, B) er Monoid hvis A og B er Monoid!
(1,2) |+| (3,4)
=> (4,6)
Map(1->1, 2->3) |+| Map(1->4, 3->6)
=> Map(1->5, 2->3, 3->6)
(1.some, 2.some) |+| (3.some, o.none[Int])
=> (4.some, 2.some)

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Og Monider er overalt..
• Tall (Int, Double, BigDecimal....)
• Penger (Din egen klasse)
• Dato / Perioder
• Kun fantasien setter grenser...
100.NOK |+| 250.NOK
=> Money(350, NOK)
1.day |+| 1.week
=> Period(1, 1)

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Functor
trait Functor[F[_]] { self =>
def map[A, B](fa: F[A])(f: A => B): F[B]
}
List(1, 2, 3) map {_ + 1}
=> List(2,3,4)
trait FunctorOps[F[_],A] extends Ops[F[A]] {
final def map[B](f: A => B): F[B] = ???
final def fpair: F[(A, A)] = ???
final def fproduct[B](f: A => B): F[(A, B)] = ???
final def >|[B](b: => B): F[B] = ???
}
List(1,2,3) >| "a"
=> List("a","a","a")
(List(1,2,3) fpair)
=> List((1,1),(2,2),(3,3))
List(1,2,3) fproduct {_*2}
=> List((1,2), (2,4), (3,6))

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Applicative
trait Applicative[F[_]] extends Apply[F] { self =>
def point[A](a: => A): F[A]
}
trait Apply[F[_]] extends Functor[F] { self =>
def ap[A,B](fa: => F[A])(f: => F[A => B]): F[B]
}
trait ApplyOps[F[_],A] extends Ops[F[A]] {
final def <*>[B](f: F[A => B]): F[B] = ???
final def |@|[B](fb: F[B]):ApplicativeBuilder= ???
}

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Applicative
val f = (_:Int) * 3
2.some <*> f.some
=> 6.some
9.some <*> { (_: Int) + 3 }.some
=> 12.some
^(1.some, 2.some) {_ + _}
=> 3.some
(1.some |@| 2.some) {_ + _}
=> 3.some

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Applicative
val f: (Int, Int) => Int = (x:Int, y:Int) => x + y
f(1,2)
=> 3
f(1.some, 2.some)
=> 3.some
f(1.some, o.none[Int])
=> o.none[Int]

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Monad
trait BindOps[F[_],A] extends Ops[F[A]] {
def flatMap[B](f: A => F[B]) = ???
def >>=[B](f: A => F[B]) = ???
def >>[B](b: => F[B]): F[B] = ???
}
val f: Int => Option[Int] = (x:Int) => (x * 10).some
(9.some >>= f)
(9.some flatMap f)
(for {
x <- 9.some
y <- f(x)
} yield y)
=> 90.some
(9.some >> 4.some)
=> 4.some

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Validation
endelig

47. © Mesan AS
Validation
sealed trait Validation[+E, +A]
case class Success[E, A](a: A)
extends Validation[E, A]
case class Failure[E, A](e: E)
extends Validation[E, A]
type ValidationNel[+E, +X] =
Validation[NonEmptyList[E], X]

48. © Mesan AS
Validation
1.success[String]
=> scalaz.Validation[String,Int] = Success(1)
1.successNel[String]
=> scalaz.ValidationNel[String,Int] = Success(1)
1.successNel
=> scalaz.ValidationNel[Nothing,Int] = Success(1)

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Validation
"Too young!".failNel[Int]
=> Failure(NonEmptyList("Too young!"))
"Too young!".fail[Int]
=> Failure("Too young!")
"Too young!".fail
=> Failure("Too young!")

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Validation
case class User(username:String)
def addUser(username: String): ValidationNel[String, User] = {
findUser(username) match {
case Some(_) => s"Username '${username}' taken!".failNel
case _ => User(username).success
}
}
def findUser(username: String): Option[User] = {
(username === "ovstetun") ? User("ovstetun").some | none
}
addUser("per")
=> Success(User("per"))
addUser("ovstetun")
=> Failure(NonEmptyList("Username 'ovstetun' taken!"))

51. © Mesan AS
Validation
val e: Equal[Validation[String, Int]] =
Equal[Validation[String, Int]]
val sg: Semigroup[Validation[String, Int]] =
Semigroup[Validation[String, Int]]
val m: Monoid[Validation[String, Int]] =
Monoid[Validation[String, Int]]
val a: Applicative[({type l[a] = Validation[String, a]})#l] =
Validation.ValidationApplicative[String]
val t: Traverse[({type l[a] = Validation[String, a]})#l] =
Traverse[({type l[a] = Validation[String, a]})#l]

52. © Mesan AS
Validation
case req @ POST(Path("/personz")) => {
val x = Body.string(req)
val p = fromJSON[Person](parse(x))
val id = p map personer.add
id match {
case Success(i) => Created ~> Json("id"->i)
case Failure(errors) =>
Forbidden ~>
Json("errors" ->
jsonErrors(errors).toList)
}
}
def add(p: Person): Int = ???

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Konklusjoner

54. © Mesan AS
Konklusjoner
• Scalaz er ikke farlig
• Gir en felles måte å løse like problemer
• Man trenger ikke skjønne hvordan et
konsept er implementert for å bruke det
• Validation er killer feature for meg
• Gjenbrukbar, komponerbar og
vedlikeholdbar kode

55. © Mesan AS
Referanser
• Scalaz på github
• http://eed3si9n.com/learning-scalaz/

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Q & A

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Takk for meg!
tmo@mesan.no
@ovstetun
github.com/ovstetun/beginner-scalaz