Type level programming in Scala

Type Level Programming in scala
@liam.m
1

Requirements
— Scala? .
— !
— !
— !
— !
2

?
!
—
—
—
—
—
—
—
3

Once your code compiles it usually works.2
.
2
https://wiki.haskell.org/Why_Haskell_just_works
5

TDD
TDD - Type driven development3
3
https://www.manning.com/books/type-driven-development-with-idris
6

1. Type safe equality - ===
2. Builder pattern using Phantom type
3. Type class pattern - implicit
4. Literal type - 42.type
5. Dependent type - a.B
Type .
7

Equality?
val items = List(
Item(1, Some("ON_SALE")), Item(2, Some("SOLDOUT")),
Item(3, Some("ON_SALE")), Item(4, None))
items.filter(item => item.status == "ON_SALE")
?
8

Equality?
val items = List(
Item(1, Some("ON_SALE")), Item(2, Some("SOLDOUT")),
Item(3, Some("ON_SALE")), Item(4, None))
items.filter(item => item.status == "ON_SALE")
Nil .
item.status: Option[String] != "ON_SALE": String
9

.
!
test("item status filter") {
val expected = List(
Item(1, Some("ON_SALE")),
Item(3, Some("ON_SALE"))
)
assert(items == expected) // !
}
10

.
!
)
}
?
11

.
!
)
}
?
Once your code compiles it usually works.
12

Type safe equality
implicit class StrictEq[A](a: A) {
def ===(b: A) = a == b
}
13

Type safe equality
implicit class StrictEq[A](a: A) {
def ===(b: A) = a == b
}
implicit class .
A ===
14

COMPILE TIME .
val x = Some("ON_SALE")
val y = Some("SOLDOUT")
val z = "ON_SALE"
x === y // false
x === z // doesn't compile,
.
⛑
15

scalacOptions += "-Xfatal-warnings"
case class UserId(n: Int)
val john = UserId(7)
john == 7
// <console>:11: warning: comparing values of
// types UserId and Int using `=='
// will always yield false john == 7
warning -> error
17

Builder pa!ern
"The builder pattern is a good choice when designing
classes whose constructors or static factories would
have more than a handful of parameters."
Joshua Bloch, Effective Java
!
20

Builder pa!ern
"The builder pattern is a good choice when designing
classes whose constructors or static factories would
have more than a handful of parameters."
Joshua Bloch, Effective Java
!
21

// , .
public class Item {
private Integer id;
private Integer brandId;
private Integer catalogId;
private Integer supplyChannelId; // optional
private String supplyChannelItemCode;
private String supplyChannelCategoryCode;
private String name;
private String displayName;
private Integer itemType;
private Integer basicPrice;
private Integer sellingPrice;
private Integer discountRate;
private Integer feeRate;
private Byte periodType;
private Integer validPeriod;
private Byte pinIssueType;
private String pinIssueCsInfo; // optional
private Boolean isCancelable;
private String imageUrl; // optional
private Integer status;
private Boolean displayYn;
private Short distType; // optional
private String detailInfo; // optional
private String noticeInfo; // optional
private Instant couponExpiredAt; // optional
private Instant releasedAt;
private Instant expiredAt;
... // 51
}
?
22

- Joshua Bloch
public class Item {
public static class Builder {
// Builder
public Builder(Integer id, Integer brandId, Integer catalogId, String supplyChannelItemCode,
String supplyChannelCategoryCode, String name, String displayName, Integer itemType,
Integer basicPrice, Integer sellingPrice, Integer discountRate, Integer feeRate,
Byte periodType, Integer validPeriod, String pinIssueCsInfo, Byte pinIssueType,
Boolean isCancelable, Integer status, Boolean displayYn, Instant releasedAt, Instant expiredAt) {
this.id = id; this.brandId = brandId; this.catalogId = catalogId;
this.supplyChannelItemCode = supplyChannelItemCode;
this.supplyChannelCategoryCode = supplyChannelCategoryCode;
this.name = name; this.displayName = displayName; this.itemType = itemType;
this.basicPrice = basicPrice; this.sellingPrice = sellingPrice;
this.discountRate = discountRate; this.feeRate = feeRate;
this.periodType = periodType; this.validPeriod = validPeriod;
this.pinIssueType = pinIssueType; this.pinIssueCsInfo = pinIssueCsInfo;
this.isCancelable = isCancelable; this.status = status;
this.displayYn = displayYn; this.releasedAt = releasedAt; this.expiredAt = expiredAt;
}
//
public Builder withSupplyChannelId(Integer val) {
this.supplyChannelId = val;
return this;
}
public Builder withPinIssueCsInfo(String val) {
this.pinIssueCsInfo = val;
return this;
}
...
}
}
23

Builder builder .
Item item = new Item.Builder(
10,
1000,
2000,
"I123",
"C123",
"( ) ",
" ",
101,
5000,
3000,
40,
5,
(byte)2,
365,
"CS",
(byte) 3,
true,
201,
true,
Instant.now(),
Instant.MAX
).withNoticeInfo(" ").withImageUrl("http://builder.jpg")
.build();
24

: Builder builder
— Builder withXXX
25

/* Item.scala */
case class Item(
id: Int,
brandId: Int,
catalogId: Int,
supplyChannelId: Option[Int], //
supplyChannelItemCode: String, // .
supplyChannelCategoryCode: String,
name: String,
displayName: String,
itemType: Int,
basicPrice: Int,
sellingPrice: Int,
discountRate: Int,
feeRate: Int,
periodType: Byte,
validPeriod: Int,
pinIssueType: Byte,
pinIssueCsInfo: Option[String],
isCancelable: Boolean,
imageUrl: Option[String],
status: Int,
displayYn: Boolean,
distType: Option[Short],
detailInfo: Option[String],
noticeInfo: Option[String],
couponExpiredAt: Option[Instant],
releasedAt: Instant,
expiredAt: Instant
)
26

type .
Why? Compiler type type
29

type .
Why? Compiler type type
// .
sealed trait BuildState {
type Id <: Bool // True
type Name <: Bool // True
}
30

sealed trait Bool
val True = new Bool {}
val False = new Bool {}
val hasOption: Bool = True
31

sealed trait Bool
sealed trait True extends Bool
sealed trait False extends Bool
type HasOption = True
32

— ADT Values : val → trait
val True = new Bool {} ➜ trait True extends Bool
33

— ADT Values : val → trait
val True = new Bool {} ➜ trait True extends Bool
— members : val → type X
val hasOption: Bool = True ➜ type HasOption = True
34

sealed trait Bool
//
val True = new Bool {}
val False = new Bool {}
val hasOption: Bool = True
//
sealed trait True extends Bool
sealed trait False extends Bool
type HasOption = True
35

Name, DisplayName True
class Builder[B <: BuildState] { self =>
private var name: Option[String] = None
private var displayName: Option[String] = None
def newBuilder[C <: BuildState] = this.asInstanceOf[Builder[C]]
def withName(name: String) = {
self.name = Some(name)
newBuilder[B {type Name = True}]
}
def withDisplayName(displayName: String) = {
self.displayName = Some(displayName)
newBuilder[B {type DisplayName = True}]
}
}
36

True
def build(implicit
ev1: B#Id =:= True, // True
ev2: B#BrandId =:= True,
ev3: B#CatalogId =:= True,
ev4: B#SupplyChannelItemCode =:= True,
ev5: B#SupplyChannelCategoryCode =:= True,
ev6: B#Name =:= True,
ev7: B#DisplayName =:= True,
ev8: B#ItemType =:= True,
ev9: B#BasicPrice =:= True,
ev10: B#SellingPrice =:= True,
ev11: B#DiscountRate =:= True,
ev12: B#FeeRate =:= True,
...
): Item =
Item(id.get, brandId.get, catalogId.get, supplyChannelId, supplyChannelItemCode.get,
supplyChannelCategoryCode.get, name.get, displayName.get, itemType.get, basicPrice.get,
sellingPrice.get, discountRate.get, feeRate.get, periodType.get, validPeriod.get,
pinIssueType.get, pinIssueCsInfo, isCancelable.get, imageUrl, status.get, displayYn.get,
distType, detailInfo, noticeInfo, couponExpiredAt, releasedAt.get, expiredAt.get)
}
37

BuildState .
private var name: Option[String] = None
private var detailInfo: Option[String] = None // optional
def withName(name: String) = { //
self.name = Some(name)
newBuilder[B {type Name = True}]
}
def withDetailInfo(detailInfo: String) = { //
self.detailInfo = Some(detailInfo)
newBuilder[B]
}
}
38

object Builder {
def apply() = new Builder[BuildState {}]
}
//
Builder().build // doesn't compile, .
Builder()
.withBasicPrice(5000).build // doesn't compile, .
40

// . withXXX . .
val item = Builder()
.withId(10)
.withBrandId(1000)
.withCatalogId(2000)
.withSupplyChannelItemCode("I123")
.withSupplyChannelCategoryCode("C123")
.withName("( ) ")
.withDisplayName(" ")
.withItemType(101)
.withBasicPrice(5000)
.withSellingPrice(3000)
.withDiscountRate(40)
.withFeeRate(5)
.withPeriodType(2)
.withValidPeriod(365)
.withPinIssueType(3)
.withIsCancelable(true)
.withStatus(201)
.withDisplayYn(true)
.withReleasedAt(now)
.withExpiredAt(Instant.MAX)
.build
41

// named argument ... ? ?
val item = Item(
id = 10, brandId = 1000, catalogId = 2000,
supplyChannelId = None, supplyChannelItemCode = "I123",
supplyChannelCategoryCode = "C123", name = "( ) ",
displayName = " ", itemType = 101,
basicPrice = 5000, sellingPrice = 3000,
discountRate = 40, feeRate = 5,
periodType = 2, validPeriod = 365,
pinIssueType = 3, pinIssueCsInfo = None,
isCancelable = true, imageUrl = None,
status = 201, displayYn = true,
distType = None, detailInfo = None,
noticeInfo = None, couponExpiredAt = None,
releasedAt = now, expiredAt = Instant.MAX
)
42

Type class Pa!ern
sum .
case class Point(x: Int, y: Int) // 2
sum(List(1, 2, 3)) => 6
sum(List(Point(1,10), Point(5, 5)) => Point(6, 15)
45

trait Adder[A] {
def zero: A
def add(x: A, y: A): A
}
46

trait Adder[A] {
def zero: A
def add(x: A, y: A): A
}
def sum[A](xs: List[A])(adder: Adder[A]): A =
xs.foldLeft(adder.zero)(adder.add)
47

val intAdder = new Adder[Int] {
def zero: Int = 0
def add(x: Int, y: Int) = x + y
}
val pointAdder = new Adder[Point] {
def zero = Point(0, 0)
def add(a: Point, b: Point) = Point(a.x + b.x, a.y + b.y)
}
48

.
sum(List(1, 2, 3))(intAdder) // 7
sum(List(Point(1, 10), Point(5, 5)))(pointAdder) // Point(6, 15)
49

. .
sum(List(1, 2, 3))(intAdder) // <= .
sum(List(Point(1, 10), Point(5, 5)))(pointAdder) // <=
50

Type class pa!ern ?
def sum[A](xs: List[A])(implicit adder: Adder[A]): A
Ad-hoc polymorphism - type parameter
implicit instance polymorphism
DI - compiler dependency injection
Spring, Guice runtime DI
51

implicit
!
// Adder implicit
def sum[A](xs: List[A])(implicit adder: Adder[A]): A =
xs.foldLeft(adder.zero)(adder.add)
// type class instance implicit
implicit val intAdder = new Adder[Int] {
def zero = 0
def add(x: Int, y: Int) = x + y
}
implicit val pointAdder = new Adder[Point] {
def zero = Point(0, 0)
def add(a: Point, b: Point) = Point(a.x + b.x, a.y + b.y)
}
52

sum(List(1, 2, 3))(intAdder)
sum(List(Point(1, 10), Point(5, 5)))(pointAdder)
53

sum(List(1, 2, 3))
sum(List(Point(1, 10), Point(5, 5)))
54

sum(List(1, 2, 3))
sum(List(Point(1, 10), Point(5, 5)))
// String compile .
sum(List("Hello", "World")) // doesn't compile
//
!
55

String ?
// string instance .
implicit val stringAdder = new Adder[String] {
def zero = ""
def add(x: String, y: String) = x + y
}
sum(List("Hello", "World")) // => "HelloWorld"
56

Literal type - 42.type
? .
!
val t: 42 = 42
val x: "Jedi" = "Jedi"
60

? .
!
val t: 42 = 42
?
61

? .
!
val t: 42 = 42
? .
!
def f(t: Double): t.type = t
val a: 1.2 = f(1.2)
62

Literal type in scala
Scala http://docs.scala-lang.org/sips/pending/42.type.html
Typelevel Scala https://typelevel.org/scala/
Dotty http://dotty.epﬂ.ch/docs/reference/singleton-types.html
.
Literal singleton type library
.
63

true.type if condition
trait Cond[T] { type V ; val value: V }
implicit val condTrue = new Cond[true] { type V = String ; val value = "foo" }
implicit val condFalse = new Cond[false] { type V = Int ; val value = 23 }
def cond[T](implicit cond: Cond[T]): cond.V = cond.value
// true is type!
!
cond[true] // "foo"
// flase is type!
"
cond[false] // 23
64

Path Dependent type -
class A {
class B
var b: Option[B] = None
}
65

class A {
class B
var b: Option[B] = None
}
val a1: A = new A
val a2: A = new A
val b1: a1.B = new a1.B // a1.B .
val b2: a2.B = new a2.B // a1.B a2.B .
b1 === b2 //
66

val b1: a1.B = new a1.B // a1.B .
val b2: a2.B = new a2.B // a1.B a2.B .
a1.b = Some(b1)
a2.b = Some(b1) // does not compile
Dependent Type Programming
67

Map - .
val strIntMap: Map[String, Int] = Map("width" -> 120)
68

Map - .
Sting Map Value ?
69

Map - .
Sting Map Value ? Any
val strAnyMap: Map[String, Any] = strIntMap + ("sort" -> "time")
70

Map - .
Sting Map Value ? Any
val strAnyMap: Map[String, Any] = strIntMap + ("sort" -> "time")
// ?
val width: Option[Any] = map2.get("width")
val sort: Option[Any] = map2.get("sort")
71

HMap - Heterogenous Map1
Dependent type !
1
Dotty : https://www.slideshare.net/Odersky/from-dot-to-dotty
72

HMap - Heterogenous Map1
Dependent type !
trait Key { type Value }
trait HMap {
def get(key: Key): Option[key.Value] // key Value, dependent type!
def add(key: Key)(value: key.Value): HMap
}
1
Dotty : https://www.slideshare.net/Odersky/from-dot-to-dotty
73

val sort = new Key { type Value = String }
val width = new Key { type Value = Int }
74

Key Value
val hmap: HMap = HMap.empty
.add(width)(120)
.add(sort)("time")
.add(width)(true) // doesn't compile, width Int Value .
75

Key Value
val hmap: HMap = HMap.empty
.add(width)(120)
.add(sort)("time")
.add(width)(true) // doesn't compile, width Int Value .
Value .
val optionInt: Option[Int] = hmap.get(width)
val optionString: Option[String] = hmap.get(sort)
76

HMap ! 9
trait HMap { self =>
val underlying: Map[Any, Any]
def get(key: Key): Option[key.Value] =
underlying.get(key).map(_.asInstanceOf[key.Value])
def add(key: Key)(value: key.Value): HMap =
new HMap {
val underlying = self.underlying + (key -> value)
}
}
78

Believing that: Life is Study!4
4
https://twitter.com/ktosopl
79

NilOnce your code compiles it usually works.
80

Scala in Real World.
Parallel Programming in Micro Service Architecture.5
5
http://tech.kakao.com/2017/09/02/parallel-programming-and-applicative-in-scala/
85

.
SELECT *
FROM
items, catalogs, wishes, categories, details, certificiations
WHERE
items.id = ?
AND items.id = catalogs.itemId
AND items.id = wishes.itemId
...
87

.
val product: Future[ProductDto] = for {
item <- itemReposotory.findByid(itemId) //
catalog <- catalogRepository.findById(item.catalogId) //
brand <- brandRepository.findById(item.brandId) //
wish <- itemWishCountRepository.findByItemId(item.id) //
category <- categoryRepository.findOneByBrandId(item.brandId) //
detail <- itemDetailRepository.findByItemId(item.id) //
cert <- itemCertificationRepository.findByItemId(item.id) //
} yield ProductFactory.of(item, brand, catalog, wish, category, detail, cert)
88

Applicative -
trait Applicative[F[_]] {
def pure[A](a: A): F[A]
def ap[A, B](fa: F[A])(ff: F[A => B]): F[B]
}
95

Applicative - ap
import scala.concurrent.Future
val futureAp = new Applicative[Future] {
def pure[A](a: A) = Future.successful(a)
def ap[A, B](fa: Future[A])(ff: Future[A => B]): Future[B] =
ff.zip(fa).map { case (f, a) => f(a)}
}
96

Applicative can be parallel
itemRepository.findById(itemId).flatMap { item =>
(
catalogRepository.findById(item.catalogId) |@|
brandRepository.findById(item.brandId) |@|
itemWishCountRepository.findByItemId(item.id) |@|
categoryRepository.findOneByBrandId(item.brandId) |@|
itemDetailRepository.findByItemId(item.id) |@|
itemCertificationRepository.findByItemId(item.id)
).map { case (catalog, brand, wish, category, detail, cert) =>
List(brand, catalog, wish, category, detail, cert)
}
}
97

Backgroud - Phantom type
!
Phantom( ) type Compile time
Runtime
trait Phantom //
type MyInt = Int with Phantom // Phantom type mixin
100

Int has no meaning
A Int can contains anything
Int 42 9496 7296 .
101

def findItemsBy(brandId: Int, itemId: Int,
status: Int, page: Int, size: Int = 20) =
items.filter(_.brandId == brandId).drop(page * size).take(size)
// ItemService.scala
// .
findItemsByBrandId(brandId, itemId, status, page, size)
102

?
// catalogId , Id
def findItemsBy(brandId: Int, itemId: Int,catalogId: Int,
// BrandService.scala
// API .
findItemsByBrandId(brandId, itemId, catalogId, status, page, size)
103

?
// catalogId , Id
def findItemsBy(brandId: Int, itemId: Int,catalogId: Int,
// BrandService.scala
// API .
findItemsByBrandId(brandId, itemId, catalogId, status, page, size)
// ItemService.scala
// ?
findItemsByBrandId(brandId, itemId, status, page, size)
104

User deﬁned type
Compiler .
case class ItemId(value: Int) extends AnyVal
case class CatalogId(value: Int) extends AnyVal
case class BrandId(value: Int) extends AnyVal
case class Status(value: Int) extends AnyVal
case class Page(value: Int) extends AnyVal
case class Size(value: Int) extends AnyVal
106

API
def findItemsByBrandId(
brandId: BrandId, itemId: ItemId, catalogId: CatalogId,
status: Status, page: Page, size: Size = Size(20)
): List[Item] =
items.filter(_.brandId == brandId.value)
.drop(page.value * size.value)
.take(size.value)
findItemsByBrandId(brandId, itemId, status, page, size) // doesn't compile,
107

? .
!
// before
def findItemsBy(
brandId: Int, itemId: Int,catalogId: Int,
status: Int, page: Int, size: Int = 20): List[Item] =
items
.filter(_.brandId == brandId)
.drop(page * size)
.take(size)
// after
def findItemsByBrandId(
brandId: BrandId, itemId: ItemId, catalogId: CatalogId,
status: Status, page: Page, size: Size = Size(20)): List[Item] =
items.filter(_.brandId == brandId.value)
.drop(page.value * size.value) //
.take(size.value) //
?
!
?
"
108

Page type
Int Int .
case class Page(value: Int) extends AnyVal
Page Int ? Page extends Int
Page Int .
val page: Int = Page(1) // .
109

shapeless .
import shapeless._
val shapely =
"Dependent type programming"
:: "Generic type programming" :: HNil
https://github.com/milessabin/shapeless
.
110

Tagged type
!
Phantom( ) type Compile time ( )
TypeTag @@ Phantom type
import shapeless.tag.@@
type Page = Int @@ PageTag // Page Int PageTag
val page: Page = tag[PageTag][Int](1) // ,
111

Tagged type
!
Phantom( ) type Compile time
TypeTag @@ Phantom type
type Page = Int @@ PageTag // Page Int PageTag
val page: Page = tag[PageTag][Int](1) // ,
// PageTag Int
page.getClass // => class java.lang.Integer
// Int .
val int : Int = page
112

Phantom type
— .
— (ﬁnal) .
— API .
— .
113

trait BrandIdTag; trait ItemIdTag
trait BrandTag; trait ItemTag
trait PageTag; trait SizeTag
object Pagable {
type Page = Int @@ PageTag // Page Int
type Size = Int @@ SizeTag // Size Int
}
def findItemsByBrandId(brandId: Int, page: Page, size: Size): List[Item] =
items.filter(_.brandId == brandId)
.drop(page * size) // Page => Int !
.take(size) //
val page: Page = tag[PageTag][Int](1)
val size: Size = tag[SizeTag][Int](20)
findItemsByBrandId(1, page, size) // works
// page size .
findItemsByBrandId(1, size, page) // doesn't compile
114

Type level programming in Scala

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