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General nonsense (math.)
Abstract nonsenseFrom Wikipedia, the free encyclopedia
In mathematics, abstract nonsense, general abstract nonsense, and general nonsense are terms used facetiously by some mathematicians to describe certain kinds of arguments and methods related to category theory. (Very) roughly speaking, category theory is the study of the general form of mathematical theories, without regard to their content. As a result, a proof that relies on category theoretic ideas often seems slightly out of context to those who are not used to such abstraction, sometimes to the extent that it resembles a comicalnon sequitur. Such proofs are sometimes dubbed “abstract nonsense” as a light-hearted way of alerting people to their abstract nature.
General nonsense (math.)
Abstract nonsenseFrom Wikipedia, the free encyclopedia
In mathematics, abstract nonsense, general abstract nonsense, and general nonsense are terms used facetiously by some mathematicians to describe certain kinds of arguments and methods related to category theory. (Very) roughly speaking, category theory is the study of the general form of mathematical theories, without regard to their content. As a result, a proof that relies on category theoretic ideas often seems slightly out of context to those who are not used to such abstraction, sometimes to the extent that it resembles a comicalnon sequitur. Such proofs are sometimes dubbed “abstract nonsense” as a light-hearted way of alerting people to their abstract nature.
General nonsense (math.)
Abstract nonsenseFrom Wikipedia, the free encyclopedia
In mathematics, abstract nonsense, general abstract nonsense, and general nonsense are terms used facetiously by some mathematicians to describe certain kinds of arguments and methods related to category theory. (Very) roughly speaking, category theory is the study of the general form of mathematical theories, without regard to their content. As a result, a proof that relies on category theoretic ideas often seems slightly out of context to those who are not used to such abstraction, sometimes to the extent that it resembles a comicalnon sequitur. Such proofs are sometimes dubbed “abstract nonsense” as a light-hearted way of alerting people to their abstract nature.
General nonsense (math.)
Abstract nonsenseFrom Wikipedia, the free encyclopedia
In mathematics, abstract nonsense, general abstract nonsense, and general nonsense are terms used facetiously by some mathematicians to describe certain kinds of arguments and methods related to category theory. (Very) roughly speaking, category theory is the study of the general form of mathematical theories, without regard to their content. As a result, a proof that relies on category theoretic ideas often seems slightly out of context to those who are not used to such abstraction, sometimes to the extent that it resembles a comicalnon sequitur. Such proofs are sometimes dubbed “abstract nonsense” as a light-hearted way of alerting people to their abstract nature.
General nonsense (math.)
Abstract nonsenseFrom Wikipedia, the free encyclopedia
In mathematics, abstract nonsense, general abstract nonsense, and general nonsense are terms used facetiously by some mathematicians to describe certain kinds of arguments and methods related to category theory. (Very) roughly speaking, category theory is the study of the general form of mathematical theories, without regard to their content. As a result, a proof that relies on category theoretic ideas often seems slightly out of context to those who are not used to such abstraction, sometimes to the extent that it resembles a comicalnon sequitur. Such proofs are sometimes dubbed “abstract nonsense” as a light-hearted way of alerting people to their abstract nature.
Category theory is absolute general nonsense!
What is all the fuss with all those monoids and semigroups I keep hearing about?
“Functional Programming is academic phenomenon. Unless your product is purely
mathematical, it’s nothing but a nice idea that has nothing to do with real world”
Why not FP?
Why not FP?
● FP might be good nice academic exercise, but ..
Why not FP?
● FP might be good nice academic exercise, but ..● .. has nothing to do with real world
Why not FP?
● FP might be good nice academic exercise, but ..● .. has nothing to do with real world● The last three decades proved that OO-programming is
useful & necessary
Why not FP?
● FP might be good nice academic exercise, but ..● .. has nothing to do with real world● The last three decades proved that OO-programming is
useful & necessary ● OO has some inconveniences, but it is a de facto standard
Why not FP?
● FP might be good nice academic exercise, but ..● .. has nothing to do with real world● The last three decades proved that OO-programming is
useful & necessary ● OO has some inconveniences, but it is a de facto standard● FP is complex, I don’t understand it, I doubt it will be
applicable to real world, ever!
Edsger Dijkstra
Edsger Dijkstraˈɛtsxər ˈdɛɪkstra
Edsger Dijkstraˈɛtsxər ˈdɛɪkstra
https://en.wikipedia.org/wiki/Edsger_W._Dijkstra
This paper tries to convince us that the well-known goto statement should be eliminated from our programming languages or, at least (since I don’t think that it will ever be eliminated), that programmers should not use it. (...)
This paper tries to convince us that the well-known goto statement should be eliminated from our programming languages or, at least (since I don’t think that it will ever be eliminated), that programmers should not use it. (...) The author is a proponent of the socalled “structured programming” style, in which, if I get it right, gotos are replaced by indentation.
This paper tries to convince us that the well-known goto statement should be eliminated from our programming languages or, at least (since I don’t think that it will ever be eliminated), that programmers should not use it. (...) The author is a proponent of the socalled “structured programming” style, in which, if I get it right, gotos are replaced by indentation. Structured programming is a nice academic exercise, which works well for small examples, but I doubt that any real-world program will ever be written in such a style.
This paper tries to convince us that the well-known goto statement should be eliminated from our programming languages or, at least (since I don’t think that it will ever be eliminated), that programmers should not use it. (...) The author is a proponent of the socalled “structured programming” style, in which, if I get it right, gotos are replaced by indentation. Structured programming is a nice academic exercise, which works well for small examples, but I doubt that any real-world program will ever be written in such a style. More than 10 years of industrial experience with Fortran have proved conclusively to everybody concerned that, in the real world, the goto is useful and necessary:
This paper tries to convince us that the well-known goto statement should be eliminated from our programming languages or, at least (since I don’t think that it will ever be eliminated), that programmers should not use it. (...) The author is a proponent of the socalled “structured programming” style, in which, if I get it right, gotos are replaced by indentation. Structured programming is a nice academic exercise, which works well for small examples, but I doubt that any real-world program will ever be written in such a style. More than 10 years of industrial experience with Fortran have proved conclusively to everybody concerned that, in the real world, the goto is useful and necessary: its presence might cause some inconveniences in debugging, but it is a de facto standard and we must live with it.
This paper tries to convince us that the well-known goto statement should be eliminated from our programming languages or, at least (since I don’t think that it will ever be eliminated), that programmers should not use it. (...) The author is a proponent of the socalled “structured programming” style, in which, if I get it right, gotos are replaced by indentation. Structured programming is a nice academic exercise, which works well for small examples, but I doubt that any real-world program will ever be written in such a style. More than 10 years of industrial experience with Fortran have proved conclusively to everybody concerned that, in the real world, the goto is useful and necessary: its presence might cause some inconveniences in debugging, but it is a de facto standard and we must live with it. It will take more than the academic elucubrations of a purist to remove it from our languages. (...)
“Semigroup, monoid, typeclass… It sounds so abstract”
“Semigroup, monoid, typeclass… It sounds so abstract”“Yes, but so do all OO-patterns!”
“The Story of a Blackout”
In 5 acts
Act.1: “Loss & Grief”
Our goal:
● DRY principle● Separation of concerns● Epic decoupling● Clean API● Minimal Boilerplate
503 Service Temporarily Unavailablenginx
503 Service Temporarily Unavailablenginx
The 5 Stages of Loss and Grief
The 5 Stages of Loss and Grief
1. Denial
The 5 Stages of Loss and Grief
1. Denial2. Anger
The 5 Stages of Loss and Grief
1. Denial2. Anger3. Bargaining
The 5 Stages of Loss and Grief
1. Denial2. Anger3. Bargaining4. Depression
The 5 Stages of Loss and Grief
1. Denial2. Anger3. Bargaining4. Depression5. Acceptance
Act.2: “The Mess”
auction
auctionify
auctionify.io
Domain & Feature RequestsDomain:
● Users place Orders in the auction system
Order
Domain & Feature RequestsDomain:
● Users place Orders in the auction system● Orders can be:
○ General - contain list of Products○ Complex - combined from two or more
other Orders○ Cancelled - used to be fully fledged
Orders, but now are cancelled
Order
General Complex Cancelled
Domain & Feature RequestsDomain:
● Users place Orders in the auction system● Orders can be:
○ General - contain list of Products○ Complex - combined from two or more
other Orders○ Cancelled - used to be fully fledged
Orders, but now are cancelled● Products can be:
○ Basic - id & price○ Discounted - wrapped Product &
discount○ OutOfStock - used to be in warehouse, but
now gone (sorry)
Order
General Complex Cancelled
Product
Basic Discounter Out of Stock
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
sealed trait Order
case class GeneralOrder(products: List[Product]) extends Order
case object CancelledOrder extends Order
case class ComplexOrder(orders: List[Order]) extends Order
sealed trait Product
case class BasicProduct(id: Int, price: BigDecimal) extends Product
case class DiscountedProduct(product: Product,
discount: Double) extends Product
case object OutOfStock extends Product
test("should evaluate order") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = GeneralOrder(DiscountedProduct(
product = BasicProduct(11, BigDecimal("1")),
discount = 0.2) :: Nil)
val o3 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: o3 :: Nil)
order.evaluate should equal (BigDecimal("11.0"))
}
test("should evaluate order") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = GeneralOrder(DiscountedProduct(
product = BasicProduct(11, BigDecimal("1")),
discount = 0.2) :: Nil)
val o3 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: o3 :: Nil)
order.evaluate should equal (BigDecimal("11.0"))
}
[error] OrderTest.scala evaluate is not a member of jw.ComplexOrder
[error] order.evaluate should equal (BigDecimal("11.0"))
[error] ^
[error] one error found
Subtype Polymorphism
test("should evaluate order") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = GeneralOrder(DiscountedProduct(
product = BasicProduct(11, BigDecimal("1")),
discount = 0.2) :: Nil)
val o3 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: o3 :: Nil)
order.evaluate should equal (BigDecimal("11.0"))
}
[error] OrderTest.scala evaluate is not a member of jw.ComplexOrder
[error] order.evaluate should equal (BigDecimal("11.0"))
[error] ^
[error] one error found
test("should evaluate order") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = GeneralOrder(DiscountedProduct(
product = BasicProduct(11, BigDecimal("1")),
discount = 0.2) :: Nil)
val o3 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: o3 :: Nil)
order.evaluate should equal (BigDecimal("11.0"))
}
sealed trait Order
case class GeneralOrder(products: List[Product]) extends Order
case object CancelledOrder extends Order
case class ComplexOrder(orders: List[Order]) extends Order
sealed trait Product
case class BasicProduct(id: Int, price: BigDecimal) extends Product
case class DiscountedProduct(product: Product,
discount: Double) extends Product
case object OutOfStock extends Product
trait Evaluatable[T] { def evaluate: T }
sealed trait Order extends Evaluatable[BigDecimal]
case object CancelledOrder extends Order {
def evaluate: BigDecimal = BigDecimal("0.0")
}
case class ComplexOrder(orders: List[Order]) extends Order {
def evaluate: BigDecimal = orders.foldLeft(BigDecimal("0.0")) {
case (acc, o) => acc + o.evaluate
}
}
case class GeneralOrder(products: List[Product]) extends Order {
def evaluate: BigDecimal = products.foldLeft(BigDecimal("0.0")) {
case (acc, p) => acc + p.evaluate
}
}
trait Evaluatable[T] { def evaluate: T }
sealed trait Product extends Evaluatable[BigDecimal]
case class BasicProduct(id: Int, price: BigDecimal) extends Product {
def evaluate: BigDecimal = price
}
case class DiscountedProduct(product: Product, discount: Double) extends Product {
def evaluate: BigDecimal = product.evaluate * (1 - discount)
}
case object OutOfStock extends Product {
def evaluate: BigDecimal = BigDecimal("0.0")
}
test("should evaluate order") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = GeneralOrder(DiscountedProduct(
product = BasicProduct(11, BigDecimal("1")),
discount = 0.2) :: Nil)
val o3 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: o3 :: Nil)
order.evaluate should equal (BigDecimal("11.0"))
}
test("should evaluate order") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = GeneralOrder(DiscountedProduct(
product = BasicProduct(11, BigDecimal("1")),
discount = 0.2) :: Nil)
val o3 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: o3 :: Nil)
order.evaluate should equal (BigDecimal("11.0"))
}
[info] OrderTest:
[info] - should evaluate order
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
test("should calculate average") {
val o1 = GeneralOrder(DiscountedProduct(product =
BasicProduct(11, BigDecimal("1")), discount = 0.2) :: Nil)
val o2 = GeneralOrder(BasicProduct(10, BigDecimal("4")) :: Nil)
val o3 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
Order.average(o1 :: o2 :: o3 :: Nil) should equal(BigDecimal("5.0"))
}
test("should calculate average") {
val o1 = GeneralOrder(DiscountedProduct(product =
BasicProduct(11, BigDecimal("1")), discount = 0.2) :: Nil)
val o2 = GeneralOrder(BasicProduct(10, BigDecimal("4")) :: Nil)
val o3 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
Order.average(o1 :: o2 :: o3 :: Nil) should equal(BigDecimal("5.0"))
}
[error] OrderTest.scala average is not a member of jw.Order
[error] Order.average should equal (BigDecimal("5.0"))
[error] ^
[error] one error found
object Stat {
def mean(xs: Seq[BigDecimal]): BigDecimal =
xs.reduce(_ + _) / xs.size
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean(xs: Seq[BigDecimal]): BigDecimal =
xs.reduce(_ + _) / xs.size
}
AWESOME!I just need it to work for
Doubles and Ints as well! Can you make it
more generic?
object Stat {
def mean(xs: Seq[BigDecimal]): BigDecimal =
xs.reduce(_ + _) / xs.size
}
object Stat {
def mean(xs: Seq[Number]): Number =
xs.reduce(_ + _) / xs.size
}
trait Number[A] {
def value: A
def +(other: Number[A]): Number[A]
def /(other: Number[A]): Number[A]
def /(other: Int): Number[A]
}
trait Number[A] {
def value: A
def +(other: Number[A]): Number[A]
def /(other: Number[A]): Number[A]
def /(other: Int): Number[A]
}
case class BigDecimalNumber(value: BigDecimal) extends Number[BigDecimal] {
def +(other: Number[BigDecimal]) = BigDecimalNumber(value + other.value)
def /(other: Number[BigDecimal]) = BigDecimalNumber(value / other.value)
def /(other: Int) = BigDecimalNumber(value / other)
}
trait Number[A] {
def value: A
def +(other: Number[A]): Number[A]
def /(other: Number[A]): Number[A]
def /(other: Int): Number[A]
}
case class BigDecimalNumber(value: BigDecimal) extends Number[BigDecimal] {
def +(other: Number[BigDecimal]) = BigDecimalNumber(value + other.value)
def /(other: Number[BigDecimal]) = BigDecimalNumber(value / other.value)
def /(other: Int) = BigDecimalNumber(value / other)
}
trait Number[A] {
def value: A
def +(other: Number[A]): Number[A]
def /(other: Number[A]): Number[A]
def /(other: Int): Number[A]
}
case class BigDecimalNumber(value: BigDecimal) extends Number[BigDecimal] {
def +(other: Number[BigDecimal]) = BigDecimalNumber(value + other.value)
def /(other: Number[BigDecimal]) = BigDecimalNumber(value / other.value)
def /(other: Int) = BigDecimalNumber(value / other)
}
case class IntNumber(value: Int) extends Number[Int] {
def +(other: Number[Int]) = IntNumber(value + other.value)
def /(other: Number[Int]) = IntNumber(value / other.value)
def /(other: Int) = IntNumber(value / other)
}
trait Number[A] {
def value: A
def +(other: Number[A]): Number[A]
def /(other: Number[A]): Number[A]
def /(other: Int): Number[A]
}
case class BigDecimalNumber(value: BigDecimal) extends Number[BigDecimal] {
def +(other: Number[BigDecimal]) = BigDecimalNumber(value + other.value)
def /(other: Number[BigDecimal]) = BigDecimalNumber(value / other.value)
def /(other: Int) = BigDecimalNumber(value / other)
}
case class IntNumber(value: Int) extends Number[Int] {
def +(other: Number[Int]) = IntNumber(value + other.value)
def /(other: Number[Int]) = IntNumber(value / other.value)
def /(other: Int) = IntNumber(value / other)
}
case class DoubleNumber(value: Double) extends Number[Double] {
def +(other: Number[Double]) = DoubleNumber(value + other.value)
def /(other: Number[Double]) = DoubleNumber(value / other.value)
def /(other: Int) = DoubleNumber(value / other)
}
object Stat {
def mean(xs: Seq[BigDecimal]): BigDecimal =
xs.reduce(_ + _) / xs.size
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean[A](xs: Seq[Number[A]]): Number[A] =
xs.reduce(_ + _) / xs.size
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(o => BigDecimalNumber(o.evaluate))).value
}
object Stat {
def mean[A](xs: Seq[Number[A]]): Number[A] =
xs.reduce(_ + _) / xs.size
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(o => BigDecimalNumber(o.evaluate))).value
}
test("should calculate average") {
val o1 = GeneralOrder(DiscountedProduct(product =
BasicProduct(11, BigDecimal("1")), discount = 0.2) :: Nil)
val o2 = GeneralOrder(BasicProduct(10, BigDecimal("4")) :: Nil)
val o3 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
Order.average(o1 :: o2 :: o3 :: Nil) should equal(BigDecimal("5.0"))
}
[info] OrderTest:
[info] - should evaluate order
[info] - should calculate average
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
test("should serialize to json") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: Nil)
val expectedJson = """{
"type: "complex"",
"orders: [{
"type: "general"",
"products: [{"type: "basic"", "id: 10", "price: 10.2"}]"
},
"cancelled order"]"
}"""
JsonSerializer.write(order) should equal(expectedJson)
}
test("should serialize to json") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: Nil)
val expectedJson = """{
"type: "complex"",
"orders: [{
"type: "general"",
"products: [{"type: "basic"", "id: 10", "price: 10.2"}]"
},
"cancelled order"]"
}"""
JsonSerializer.write(order) should equal(expectedJson)
}
[error] OrderTest.scala not found: value JsonSerializer
[error] JsonSerializer.write(order) should equal(expectedJson)
[error] ^
object JsonSerializer {
def write(order: Order): String = ...
}
I have few objects I also need to serialize to JSON. Can you make
your code a bit more generic? Thanks!!!
object JsonSerializer {
def write(order: Order): String = ...
}
object JsonSerializer {
def write(sth: ???): String = ...
}
object JsonSerializer {
def write(serializable: JsonSerializable) = ...
}
object JsonSerializer {
def write(serializable: JsonSerializable) = ...
}
trait JsonSerializable {
def toJson: JsonValue
}
object JsonSerializer {
def write(serializable: JsonSerializable) =
JsonWriter.write(serializable.toJson)
}
trait JsonSerializable {
def toJson: JsonValue
}
object JsonSerializer {
def write(serializable: JsonSerializable) =
JsonWriter.write(serializable.toJson)
}
trait JsonSerializable {
def toJson: JsonValue
}
sealed trait JsonValue
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
object JsonWriter {
def write(json: JsonValue): String =
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
object JsonWriter {
def write(json: JsonValue): String = json match {
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
object JsonWriter {
def write(json: JsonValue): String = json match {
case JsonNumber(value) => value.toString
}
}
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
object JsonWriter {
def write(json: JsonValue): String = json match {
case JsonString(value) => s""""$value""""
case JsonNumber(value) => value.toString
}
}
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
object JsonWriter {
def write(json: JsonValue): String = json match {
case JsonArray(elems) => "[" + elems.map(write).mkString(", ") + "]"
case JsonString(value) => s""""$value""""
case JsonNumber(value) => value.toString
}
}
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
object JsonWriter {
def write(json: JsonValue): String = json match {
case JsonObject(elems) =>
val entries = for {
(key, value) <- elems
} yield s""""$key: ${write(value)}""""
"{" + entries.mkString(", ") + "}"
case JsonArray(elems) => "[" + elems.map(write).mkString(", ") + "]"
case JsonString(value) => s""""$value""""
case JsonNumber(value) => value.toString
}
}
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
object JsonWriter {
def write(json: JsonValue): String = ...
}
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
object JsonWriter {
def write(json: JsonValue): String = ...
}
trait JsonSerializable {
def toJson: JsonValue
}
object JsonSerializer {
def write(serializable: JsonSerializable) =
JsonWriter.write(serializable.toJson)
}
sealed trait Order extends Evaluatable[BigDecimal] with JsonSerializable {
def evaluate: BigDecimal
}
sealed trait Order extends Evaluatable[BigDecimal] with JsonSerializable {
def evaluate: BigDecimal
}
case class GeneralOrder(products: List[Product]) extends Order {
def evaluate: BigDecimal = products.foldLeft(BigDecimal("0.0")) {
case (acc, p) => acc + p.evaluate
}
def toJson: JsonValue = JsonObject(Map(
"type" -> JsonString("general"),
"products" -> JsonArray(products.map(_.toJson))
))
}
sealed trait Order extends Evaluatable[BigDecimal] with JsonSerializable {
def evaluate: BigDecimal
}
case object CancelledOrder extends Order {
def evaluate: BigDecimal = BigDecimal("0.0")
def toJson: JsonValue = JsonString("cancelled order")
}
sealed trait Order extends Evaluatable[BigDecimal] with JsonSerializable {
def evaluate: BigDecimal
}
case class ComplexOrder(orders: List[Order]) extends Order {
def evaluate: BigDecimal = orders.foldLeft(BigDecimal("0.0")) {
case (acc, o) => acc + o.evaluate
}
override def toJson: JsonValue = JsonObject(Map(
"type" -> JsonString("complex"),
"orders" -> JsonArray(orders.map(_.toJson)))
)
}
sealed trait Product extends Evaluatable[BigDecimal] with JsonSerializable {
def evaluate: BigDecimal
}
case class BasicProduct(id: Int, price: BigDecimal) extends Product {
def evaluate: BigDecimal = price
def toJson: JsonValue = JsonObject(Map(
"type" -> JsonString("basic"),
"id" -> JsonNumber(BigDecimal(id)),
"price" -> JsonNumber(price)
))
}
sealed trait Product extends Evaluatable[BigDecimal] with JsonSerializable {
def evaluate: BigDecimal
}
case class DiscountedProduct(product: Product, discount: Double) extends Product {
def evaluate: BigDecimal = product.evaluate * (1 - discount)
def toJson: JsonValue = JsonObject(Map(
"type" -> JsonString("discounted"),
"product" -> product.toJson,
"discount" -> JsonNumber(discount)
))
}
sealed trait Product extends Evaluatable[BigDecimal] with JsonSerializable {
def evaluate: BigDecimal
}
case object OutOfStock extends Product {
def evaluate: BigDecimal = BigDecimal("0.0")
def toJson: JsonValue = JsonString("out of stock")
}
test("should serialize to json") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: Nil)
val expectedJson = """{
"type: "complex"",
"orders: [{
"type: "general"",
"products: [{"type: "basic"", "id: 10", "price: 10.2"}]"
},
"cancelled order"]"
}"""
JsonSerializer.write(order) should equal(expectedJson)
}
[info] OrderTest:
[info] - should evaluate order
[info] - should calculate average
[info] - should serialize to json
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
Our goal:
● DRY principle● Separation of concerns● Epic decoupling● Clean API● Minimal Boilerplate
Act.3: “Re-inventing the Wheel”
trait Number[A] {
def value: A
def +(other: Number[A]): Number[A]
def /(other: Number[A]): Number[A]
def /(other: Int): Number[A]
}
case class BigDecimalNumber(value: BigDecimal) extends Number[BigDecimal] {
def +(other: Number[BigDecimal]) = BigDecimalNumber(value + other.value)
def /(other: Number[BigDecimal]) = BigDecimalNumber(value / other.value)
def /(other: Int) = BigDecimalNumber(value / other)
}
case class IntNumber(value: Int) extends Number[Int] {
def +(other: Number[Int]) = IntNumber(value + other.value)
def /(other: Number[Int]) = IntNumber(value / other.value)
def /(other: Int) = IntNumber(value / other)
}
case class DoubleNumber(value: Double) extends Number[Double] {
def +(other: Number[Double]) = DoubleNumber(value + other.value)
def /(other: Number[Double]) = DoubleNumber(value / other.value)
def /(other: Int) = DoubleNumber(value / other)
}
object Stat {
def mean[A](xs: Seq[Number[A]]): Number[A] =
xs.reduce(_ + _) / xs.size
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(o => BigDecimalNumber(o.evaluate))).value
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(o => BigDecimalNumber(o.evaluate))).value
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean[A](xs: Seq[Number[A]]): Number[A] =
xs.reduce(_ + _) / xs.size
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean[A](xs: Seq[A]): A =
xs.reduce(_ + _) / xs.size
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
number.divide(xs.reduce(number.plus),xs.size)
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
trait Number[A] {
def plus(n1: A, n2: A): A
def divide(n1: A, n2: A): A
def divide(n1: A, n2: Int): A
}
trait Number[A] {
def plus(n1: A, n2: A): A
def divide(n1: A, n2: A): A
def divide(n1: A, n2: Int): A
}
object Number {
implicit object BigDecimalNumber extends Number[BigDecimal] {
def plus(n1: BigDecimal, n2: BigDecimal): BigDecimal = n1 + n2
def divide(n1: BigDecimal, n2: BigDecimal): BigDecimal = n1 / n2
def divide(n1: BigDecimal, n2: Int): BigDecimal = n1 / n2
}
trait Number[A] {
def plus(n1: A, n2: A): A
def divide(n1: A, n2: A): A
def divide(n1: A, n2: Int): A
}
object Number {
implicit object BigDecimalNumber extends Number[BigDecimal] {
def plus(n1: BigDecimal, n2: BigDecimal): BigDecimal = n1 + n2
def divide(n1: BigDecimal, n2: BigDecimal): BigDecimal = n1 / n2
def divide(n1: BigDecimal, n2: Int): BigDecimal = n1 / n2
}
implicit object IntNumber extends Number[Int] {
def plus(n1: Int, n2: Int): Int = n1 + n2
def divide(n1: Int, n2: Int): Int = n1 / n2
}
}
object Stat {
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
number.divide(xs.reduce(number.plus),xs.size)
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
number.divide(xs.reduce(number.plus),xs.size)
}
object Order {
def average(orders: Seq[Order])(BigDecimalNumber): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
trait Number[A] {
def plus(n1: A, n2: A): A
def divide(n1: A, n2: A): A
def divide(n1: A, n2: Int): A
}
object Number {
implicit object BigDecimalNumber extends Number[BigDecimal] {
def plus(n1: BigDecimal, n2: BigDecimal): BigDecimal = n1 + n2
def divide(n1: BigDecimal, n2: BigDecimal): BigDecimal = n1 / n2
def divide(n1: BigDecimal, n2: Int): BigDecimal = n1 / n2
}
implicit object IntNumber extends Number[Int] {
def plus(n1: Int, n2: Int): Int = n1 + n2
def divide(n1: Int, n2: Int): Int = n1 / n2
}
}
object Stat {
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
number.divide(xs.reduce(number.plus),xs.size)
}
object Order {
def average(orders: Seq[Order])(BigDecimalNumber): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
number.divide(xs.reduce(number.plus),xs.size)
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
number.divide(xs.reduce(number.plus),xs.size)
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
implicit class NumberOps[A](a: A)(implicit number: Number[A]) {
def +(other: A) = number.plus(a, other)
def /(other: A) = number.divide(a, other)
def /(other: Int) = number.divide(a, other)
}
}
object Stat {
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
number.divide(xs.reduce(number.plus),xs.size)
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
xs.reduce(number.plus) / xs.size
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
object Stat {
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
xs.reduce(_ + _) / xs.size
}
object Order {
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
}
[info] OrderTest:
[info] - should evaluate order
[info] - should calculate average
[info] - should serialize to json
sealed trait JsonValue
case class JsonObject(elem: Map[String, JsonValue]) extends JsonValue
case class JsonArray(elems: List[JsonValue]) extends JsonValue
case class JsonString(value: String) extends JsonValue
case class JsonNumber(value: BigDecimal) extends JsonValue
object JsonWriter {
def write(json: JsonValue): String = ...
}
trait JsonSerializable {
def toJson: JsonValue
}
object JsonSerializer {
def write(serializable: JsonSerializable) =
JsonWriter.write(serializable.toJson)
}
sealed trait Order extends Evaluatable[BigDecimal] with JsonSerializable
case class GeneralOrder(products: List[Product]) extends Order {
def evaluate: BigDecimal = products.foldLeft(BigDecimal("0.0")) {
case (acc, p) => acc + p.evaluate
}
def toJson: JsonValue = JsonObject(Map(
"type" -> JsonString("general"),
"products" -> JsonArray(products.map(_.toJson))
))
}
test("should serialize to json") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: Nil)
val expectedJson = """{
"type: "complex"",
"orders: [{
"type: "general"",
"products: [{"type: "basic"", "id: 10", "price: 10.2"}]"
},
"cancelled order"]"
}"""
JsonSerializer.write(order) should equal(expectedJson)
}
trait JsonSerializable {
def toJson: JsonValue
}
object JsonSerializer {
def write(serializable: JsonSerializable) =
JsonWriter.write(serializable.toJson)
}
trait Json[-A] {
def toJson(a: A): JsonValue
}
object JsonSerializer {
def write(serializable: JsonSerializable) =
JsonWriter.write(serializable.toJson)
}
trait Json[-A] {
def toJson(a: A): JsonValue
}
object JsonSerializer {
def write[A](a: A)(implicit json: Json[A]) =
JsonWriter.write(json.toJson(a))
}
object OrderJson {
implicit object ProductToJson extends Json[Product] {
def toJson(product: Product): JsonValue = ...
}
implicit object OrderToJson extends Json[Order] {
def toJson(order: Order): JsonValue = ...
}
}
implicit object ProductToJson extends Json[Product] {
def toJson(product: Product): JsonValue = product match {
case BasicProduct(id, price) =>
JsonObject(Map(
"type" -> JsonString("basic"),
"id" -> JsonNumber(BigDecimal(id)),
"price" -> JsonNumber(price)
))
case DiscountedProduct(product, discount) =>
JsonObject(Map(
"type" -> JsonString("discounted"),
"product" -> toJson(product),
"discount" -> JsonNumber(discount)
))
case OutOfStock() =>
JsonString("out of stock")
}
}
implicit object OrderToJson extends Json[Order] {
def toJson(order: Order): JsonValue = order match {
case GeneralOrder(products) =>
JsonObject(Map(
"type" -> JsonString("general"),
"products" -> JsonArray(products.map(ProductToJson.toJson))
))
case ComplexOrder(orders) =>
JsonObject(Map(
"type" -> JsonString("complex"),
"orders" -> JsonArray(orders.map(toJson))
))
case CancelledOrder() =>
JsonString("cancelled order")
}
}
implicit object OrderToJson extends Json[Order] {
def toJson(order: Order): JsonValue = order match {
case GeneralOrder(products) =>
JsonObject(Map(
"type" -> JsonString("general"),
"products" -> JsonArray(products.map(ProductToJson.toJson))
))
case ComplexOrder(orders) =>
JsonObject(Map(
"type" -> JsonString("complex"),
"orders" -> JsonArray(orders.map(toJson))
))
case CancelledOrder() =>
JsonString("cancelled order")
}
}
object OrderJson {
implicit object ProductToJson extends Json[Product] {
def toJson(product: Product): JsonValue = ...
}
implicit object OrderToJson extends Json[Order] {
def toJson(order: Order): JsonValue = ...
}
}
test("should serialize to json") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: Nil)
val expectedJson = """{
"type: "complex"",
"orders: [{
"type: "general"",
"products: [{"type: "basic"", "id: 10", "price: 10.2"}]"
},
"cancelled order"]"
}"""
JsonSerializer.write(order) should equal(expectedJson)
}
test("should serialize to json") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: Nil)
val expectedJson = """{
"type: "complex"",
"orders: [{
"type: "general"",
"products: [{"type: "basic"", "id: 10", "price: 10.2"}]"
},
"cancelled order"]"
}"""
JsonSerializer.write(order) should equal(expectedJson)
}
[error] OrderTest.scala could not find implicit value for parameter json: json.Json[jw.
ComplexOrder]
[error] JsonSerializer.write(order) should equal(expectedJson)
test("should serialize to json") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: Nil)
val expectedJson = """{
"type: "complex"",
"orders: [{
"type: "general"",
"products: [{"type: "basic"", "id: 10", "price: 10.2"}]"
},
"cancelled order"]"
}"""
JsonSerializer.write(order) should equal(expectedJson)
}
test("should serialize to json") {
val o1 = GeneralOrder(BasicProduct(10, BigDecimal("10.2")) :: Nil)
val o2 = CancelledOrder
val order = ComplexOrder(o1 :: o2 :: Nil)
val expectedJson = """{
"type: "complex"",
"orders: [{
"type: "general"",
"products: [{"type: "basic"", "id: 10", "price: 10.2"}]"
},
"cancelled order"]"
}"""
import OrderJson._
JsonSerializer.write(order) should equal(expectedJson)
}
[info] OrderTest:
[info] - should evaluate order
[info] - should calculate average
[info] - should serialize to json
sealed trait Product extends Evaluatable[BigDecimal]
case class BasicProduct(id: Int, price: BigDecimal) extends Product {
def evaluate: BigDecimal = price
}
case class DiscountedProduct(product: Product, discount: Double) extends Product {
def evaluate: BigDecimal = product.evaluate * (1 - discount)
}
case class OutOfStock() extends Product {
def evaluate: BigDecimal = BigDecimal("0.0")
}
sealed trait Order extends Evaluatable[BigDecimal]
case class GeneralOrder(products: List[Product]) extends Order {
def evaluate: BigDecimal = products.foldLeft(BigDecimal("0.0")) {
case (acc, p) => acc + p.evaluate
}
}
case class CancelledOrder() extends Order {
def evaluate: BigDecimal = BigDecimal("0.0")
}
case class ComplexOrder(orders: List[Order]) extends Order {
def evaluate: BigDecimal = orders.foldLeft(BigDecimal("0.0")) {
case (acc, o) => acc + o.evaluate
}
trait Evaluate[-A, T] { def evaluate(a: A): T }
object Order {
implicit object ProductEvaluate extends Evaluate[Product, BigDecimal] {
def evaluate(product: Product): BigDecimal = product match {
case BasicProduct(id, price) => price
case DiscountedProduct(discounted, discount) => evaluate(discounted) * (1 - discount)
case OutOfStock() => BigDecimal("0.0")
}}
implicit object OrderEvaluate extends Evaluate[Order, BigDecimal] {
def evaluate(order: Order): BigDecimal = order match {
case GeneralOrder(products) => products.foldLeft(BigDecimal("0.0")) {
case (acc, p) => acc + ProductEvaluate.evaluate(p)
}
case ComplexOrder(orders) => orders.foldLeft(BigDecimal("0.0")) {
case (acc, o) => acc + evaluate(o)
}
case CancelledOrder() => BigDecimal("0.0")
}}}
trait Evaluate[-A, T] { def evaluate(a: A): T }
object Evaluate {
implicit class EvaluateOps[-A, T](a: A)(implicit ev: Evaluate[A, T]) {
def evaluate = ev.evaluate(a)
}
}
import Evaluate._
def average(orders: Seq[Order]): BigDecimal =
Stat.mean(orders.map(_.evaluate))
sealed trait Order
case class GeneralOrder(products: List[Product]) extends Order
case object CancelledOrder extends Order
case class ComplexOrder(orders: List[Order]) extends Order
sealed trait Product
case class BasicProduct(id: Int, price: BigDecimal) extends Product
case class DiscountedProduct(product: Product,
discount: Double) extends Product
case object OutOfStock extends Product
Act.4: “Renaissance”
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
Domain & Feature RequestsFeature Requests:
● Present current state of orders (JSON)● Calculate final checkout● Calculate average Order price● “Simplify” Orders
def simplify(orders: Seq[Order]): Order =
orders.foldLeft(CancelledOrder()) { ?? }
def simplify(orders: Seq[Order]): Order =
orders.foldLeft(CancelledOrder()) {
case (acc, o) => acc + o
}
def simplify(orders: Seq[Order]): Order =
orders.foldLeft(CancelledOrder()) {
case (acc, o) => acc + o
}
trait Addable[A] {
def add(a1: A, a2: A): A
}
def simplify(orders: Seq[Order]): Order =
orders.foldLeft(CancelledOrder()) {
case (acc, o) => acc + o
}
trait Addable[A] {
def add(a1: A, a2: A): A
}
implicit object OrderAddable extends Addable[Order] {
def add(o1: Order, o2: Order): Order = (o1, o2) match {
case (CancelledOrder(), o2) => o2
case (o1, CancelledOrder()) => o1
case (GeneralOrder(ps1), GeneralOrder(ps2)) => GeneralOrder(ps1 ++ ps2)
case (ComplexOrder(os1), ComplexOrder(os2)) => ComplexOrder(os1 ++ os2)
case (o1, ComplexOrder(orders)) => ComplexOrder(o1 :: orders)
case (ComplexOrder(orders), o2) => ComplexOrder(o2 :: orders)
}
}
import Addable._
def simplify(orders: Seq[Order]): Order =
orders.foldLeft(CancelledOrder()) {
case (acc, o) => acc + o
}
trait Addable[A] {
def add(a1: A, a2: A): A
}
import Addable._
def simplify(orders: Seq[Order]): Order =
orders.foldLeft(CancelledOrder()) {
case (acc, o) => acc + o
}
trait Addable[A] {
def add(a1: A, a2: A): A
}
object Addable {
implicit class AddableOps[A](a: A) {
def |+|(other: A)(implicit addable: Addable[A]) = addable.add(a, other)
}
}
import Addable._
def simplify(orders: Seq[Order]): Order =
orders.foldLeft(CancelledOrder()) {
case (acc, o) => acc |+| o
}
trait Addable[A] {
def add(a1: A, a2: A): A
}
object Addable {
implicit class AddableOps[A](a: A) {
def |+|(other: A)(implicit addable: Addable[A]) = addable.add(a, other)
}
}
def simplify(orders: Seq[Order]): Order =
orders.foldLeft(CancelledOrder()) {
case (acc, o) => acc |+| o
}
def simplify(orders: Seq[Order]): Order = fold(orders)
def simplify(orders: Seq[Order]): Order = fold(orders)
trait AddableWithZero[A] {
def zero: A
def add(a1: A, a2: A): A
}
def simplify(orders: Seq[Order]): Order = fold(orders)
trait AddableWithZero[A] {
def zero: A
def add(a1: A, a2: A): A
}
object AddableWithZero {
def fold[A](values: Seq[A])(implicit addableWithZ: AddableWithZero[A]): A = {
values.fold(addableWithZ.zero){
case (acc, v) => addableWithZ.add(acc, v)
}
}
}
def simplify(orders: Seq[Order]): Order = fold(orders)
trait AddableWithZero[A] {
def zero: A
def add(a1: A, a2: A): A
}
def simplify(orders: Seq[Order]): Order = fold(orders)
trait AddableWithZero[A] {
def zero: A
def add(a1: A, a2: A): A
}
implicit object OrderAddableWithZero extends AddableWithZero[Order] {
def zero = CancelledOrder()
def add(o1: Order, o2: Order): Order = OrderAddable.add(o1,o2)
}
def simplify(orders: Seq[Order]): Order = fold(orders)
trait AddableWithZero[A] {
def zero: A
def add(a1: A, a2: A): A
}
object AddableWithZero {
def fold[A](values: Seq[A])(implicit addableWithZ: AddableWithZero[A]): A = {
values.fold(addableWithZ.zero){
case (acc, v) => addableWithZ.add(acc, v)
}
}
}
trait Evaluate[-A, T] { def evaluate(a: A): T }
object Order {
implicit object ProductEvaluate extends Evaluate[Product, BigDecimal] {
def evaluate(product: Product): BigDecimal = product match {
case BasicProduct(id, price) => price
case DiscountedProduct(discounted, discount) => evaluate(discounted) * (1 - discount)
case OutOfStock() => BigDecimal("0.0")
}}
implicit object OrderEvaluate extends Evaluate[Order, BigDecimal] {
def evaluate(order: Order): BigDecimal = order match {
case GeneralOrder(products) => products.foldLeft(BigDecimal("0.0")) {
case (acc, p) => acc + ProductEvaluate.evaluate(p)
}
case ComplexOrder(orders) => orders.foldLeft(BigDecimal("0.0")) {
case (acc, o) => acc + evaluate(o)
}
case CancelledOrder() => BigDecimal("0.0")
}}}
trait Evaluate[-A, T] { def evaluate(a: A): T }
object Order {
implicit object ProductEvaluate extends Evaluate[Product, BigDecimal] {
def evaluate(product: Product): BigDecimal = product match {
case BasicProduct(id, price) => price
case DiscountedProduct(discounted, discount) => evaluate(discounted) * (1 - discount)
case OutOfStock() => BigDecimal("0.0")
}}
implicit object OrderEvaluate extends Evaluate[Order, BigDecimal] {
def evaluate(order: Order): BigDecimal = order match {
case GeneralOrder(products) => products.foldLeft(BigDecimal("0.0")) {
case (acc, p) => acc + ProductEvaluate.evaluate(p)
}
case ComplexOrder(orders) => orders.foldLeft(BigDecimal("0.0")) {
case (acc, o) => acc + evaluate(o)
}
case CancelledOrder() => BigDecimal("0.0")
}}}
Act.5: “Enlightenment”
Research
Definitions
Definitions● Type classes
Definitions● Type classes
○ Ad-hoc polymorphism
Definitions● Type classes● Abstract Data Type (ADT)
Definitions● Type classes● Abstract Data Type (ADT)● Addable → Semigroup
Definitions● Type classes● Abstract Data Type (ADT)● Addable → Semigroup● AddableWithZero → Monoid
Definitions● Type classes● Abstract Data Type (ADT)● Addable → Semigroup● AddableWithZero → Monoid● Type classes come with laws!
Where to go next1. Functor2. Applicative3. Monad!
All are just type classes with some laws. That’s it!
Before we go, a bit of history...
Links & Resources● https://github.com/rabbitonweb/scala_typeclasses ● https://inoio.de/blog/2014/07/19/type-class-101-semigroup/ ● http://danielwestheide.com/blog/2013/02/06/the-neophytes-guide-to-scala-
part-12-type-classes.html ● https://www.youtube.com/watch?v=sVMES4RZF-8 ● http://homepages.cwi.nl/~storm/teaching/reader/Dijkstra68.pdf● https://www.destroyallsoftware.com/misc/reject.pdf● http://southpark.cc.com/avatar ● http://www.tandemic.com/wp-content/uploads/Definition.png
And that’s all folks!
And that’s all folks!
Paweł Szulc
And that’s all folks!
Paweł Szulc twitter: @rabbitonweb
And that’s all folks!
Paweł Szulc twitter: @rabbitonweb
blog: http://rabbitonweb.com
And that’s all folks!
Paweł Szulc twitter: @rabbitonweb
blog: http://rabbitonweb.com
github: https://github.com/rabbitonweb
And that’s all folks!
Paweł Szulc twitter: @rabbitonweb
blog: http://rabbitonweb.com
github: https://github.com/rabbitonweb
Questions?
Thank you!
Thank you!Bonus?
object Stat {
import Number._
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
xs.reduce(_ + _) / xs.size
}
object Stat {
import Number._
def mean[A](xs: Seq[A])(implicit number: Number[A]): A =
xs.reduce(_ + _) / xs.size
}
object Stat {
import Number._
def mean[A : Number](xs: Seq[A]): A =
xs.reduce(_ + _) / xs.size
}
object JsonSerializer {
def write[A](a: A)(implicit json: Json[A]) =
JsonWriter.write(json.toJson(a))
}
object JsonSerializer {
def write[A](a: A)(implicit json: Json[A]) =
JsonWriter.write(json.toJson(a))
}
object JsonSerializer {
def write[A : Json](a: A) =
JsonWriter.write(implicitly[Json[A]].toJson(a))
}
