FP functions

background-image: url(../img/fp-tower/website-background.svg)

class: center, middle, white

.title[Functions<br> in <br>Functional Programming]

---
# FP == Val Functions

<br>

.forty-seven-left[
```scala
val isPositive: Int => Boolean =
  number => number >= 0

val isEven: Int => Boolean =
  number => number % 2 == 0
```
]

--

<br><br><br><br><br><br><br>

.forty-seven-left[
```scala
List(-6, 2, 7, -3, 0).filter(`isPositive`)
// res0: List[Int] = List(2, 7, 0)

List(-6, 2, 7, -3, 0).filter(`isEven`)
// res1: List[Int] = List(-6, 2, 0)
```
]

---
# Function in imperative programming

<br>

.sixty-two-left[
```scala
def procedure(input1: Int, input2: String): String = {
  var var1 = 0
  var var2 = otherProcedure1(input1, 3)
  var1 = otherProcedure2(var2, input2)
  return var1
}
```
]

---
# Function in imperative programming

<br>

.sixty-two-left[
```scala
def procedure(input1: Int, input2: String): String = {
  `var var1 = 0`
  var var2 = otherProcedure1(input1, 3)
  var1 = otherProcedure2(var2, input2)
  return var1
}
```
]

---
# Function in imperative programming

<br>

.sixty-two-left[
```scala
def procedure(input1: Int, input2: String): String = {
  var var1 = 0
  var var2 = `otherProcedure1(input1, 3)`
  var1 = `otherProcedure2(var2, input2)`
  return var1
}
```
]

---
# Function in imperative programming

<br>

.sixty-two-left[
```scala
def procedure(input1: Int, input2: String): String = {
  var var1 = 0
  var var2 = otherProcedure1(input1, 3)
  var1 = otherProcedure2(var2, input2)
  `return var1`
}
```
]

---
# Function in functional programming

<br><br>

.sixty-seven-left[
.center[
<img src="../img/foundation/fp-function-transformation.svg" style="width:600px;"/>
]]

---
# Program in functional programming

<br><br><br>

<img src="../img/foundation/fp-program.svg" style="width:1000px;"/>

---
# An FP Function is an immutable Map

<br>

.forty-seven-left[
```scala
val function: Map[Input, Output] =
  Map(
    input1 -> output1,
    input2 -> output2,
    input3 -> output3,
    input4 -> output4,
    // ...
  )
```
]

---
# An FP Function is an immutable Map

.sixty-two-left[
<img src="../img/foundation/fp-function-map-1.svg" style="width:600px;"/>
]

--

.thirty-seven-right[
<br><br><br><br><br><br><br>
```scala
val getUserAge: Map[User, Int] =
  Map(
    User("Elise", ...) -> 34,
    User("Bob"  , ...) -> 52,
  )
```
]

---
# An FP Function is an immutable Map

.sixty-two-left[
<img src="../img/foundation/fp-function-map-1a.svg" style="width:600px;"/>
]

.thirty-seven-right[
<br><br><br><br><br><br><br>
```scala
val getUserAge: Map[User, Int] =
  Map(
    User("`Elise`", ...) -> `34`,
    User("Bob"  , ...) -> 52,
  )
```
]

---
# An FP Function is an immutable Map

.sixty-two-left[
<img src="../img/foundation/fp-function-map-1b.svg" style="width:600px;"/>
]

.thirty-seven-right[
<br><br><br><br><br><br><br>
```scala
val getUserAge: Map[User, Int] =
  Map(
    User("Elise", ...) -> 34,
    User("`Bob`"  , ...) -> `52`,
  )
```
]

---
# An FP Function is an immutable Map

.sixty-two-left[
<img src="../img/foundation/fp-function-map-2a.svg" style="width:600px;"/>
]

.thirty-seven-right[
<br><br><br><br><br><br><br>
```scala
val getUserAge: Map[User, Int] =
  Map(
    User("`Elise`", ...) -> `34`,
    User("Bob"  , ...) -> 52,
    User("`Zoe`"  , ...) -> `34`,
  )
```
]

---
# An FP Function is an immutable Map

.sixty-two-left[
<img src="../img/foundation/fp-function-map-3.svg" style="width:600px;"/>
]

.thirty-seven-right[
<br><br><br><br><br><br><br>
```scala
val getUserAge: Map[User, Int] =
  Map(
    User("Elise", ...) -> 34,
    User("Bob"  , ...) -> 52,
    User("Zoe"  , ...) -> 34,
  )
```
]

---
# 1. All inputs must return an output

.sixty-two-left[
<img src="../img/foundation/fp-function-partial.svg" style="width:600px;"/>
]

.thirty-seven-right[
<br><br><br><br><br><br><br>
```scala
val getUserAge: Map[User, Int] =
  Map(
    User("Elise", ...) -> 34,
    User("Bob"  , ...) -> 52,
  )
```
]

---
class: medium-code

# 1. All inputs must return an output

.fifty-seven-left[
```scala
sealed trait Json

case class JsonNumber(value: Int   ) extends Json
case class JsonString(value: String) extends Json

def jsonToInt(json: Json): Int =
  json match {
    case JsonNumber(number) => number
  }
```

```scala
jsonToInt(JsonNumber(12))
// res: Int = 12
```
]

---
class: medium-code

# 1. All inputs must return an output

.fifty-seven-left[
```scala
sealed trait Json

case class JsonNumber(value: Int   ) extends Json
case class JsonString(value: String) extends Json

def `jsonToInt`(json: Json): Int =
  json match {
    case JsonNumber(number) => number
  }
```

```scala
jsonToInt(JsonNumber(12))
// res: Int = 12
```
]

---
class: medium-code

# 1. All inputs must return an output

.fifty-seven-left[
```scala
sealed trait Json

case class JsonNumber(value: Int   ) extends Json
case class JsonString(value: String) extends Json

def jsonToInt(json: Json): Int =
  json match {
    case JsonNumber(number) => number
  }
```

```scala
jsonToInt(JsonString("Hello"))
// scala.`MatchError`: JsonString(Hello)
```
]

---
class: medium-code

# 1. All inputs must return an output

.eighty-two-left[
```scala
sealed trait Json

case class JsonNumber(value: Int   ) extends Json
case class JsonString(value: String) extends Json

def jsonToInt(json: Json): Int =
  json match {
    case JsonNumber(number) => number
    case JsonString(text)   =>
      `throw new IllegalArgumentException`("cannot convert a JsonString to a number")
  }
```
]

---
# 2. Mapping is static

.forty-seven-left[
```scala
val function: Map[Input, Output] =
  scala.collection.`immutable`.Map(
    input1 -> output1,
    input2 -> output2,
    input3 -> output3,
    input4 -> output4,
    // ...
  )
```
]

---
# 2. Mapping is static

.forty-seven-left[
```scala
val function: Map[Input, Output] =
  scala.collection.immutable.Map(
    input1 -> output1,
    input2 -> output2,
    input3 -> output3,
    input4 -> output4,
    // ...
  )
```
]
<br><br><br><br><br><br><br><br><br><br><br>
.forty-seven-left[
```scala
function(input1)
// res = `output1`

function(input1) // tomorrow
// res = `output1`
```
]

---
# 2. Mapping is static

.sixty-two-left[
<img src="../img/foundation/fp-function-map-zoe.svg" style="width:600px;"/>
]

.thirty-seven-right[
<br><br><br><br><br><br>
```scala
val zoe = User("Zoe", ...)

getUserAge(zoe)
// res: Int = 34

getUserAge(zoe) // 10 years later
// res: Int = 34
```
]

---
class: medium-code

# 3. Cannot perform actions

.eighty-two-left[
```scala
def createNewBlogPost(author: User, title: String, content: String): Future[Unit] =
  httpClient.post(
    path = "/user/${author.userId}/blog",
    payload = json"""{"title" : $title, "content" : $content }"""
  )
```

```scala
def getAllUser: Future[List[User]] =
  db.query(
    sql"SELECT userId, userName, createdAt from users"
  ).as[User]
```

```scala
def auditLog(service: String, message: String): Unit =
  println(s"[$service] $message")
```
]

---
class: medium-code

# 3. Cannot perform actions

.eighty-two-left[
```scala
def `createNewBlogPost`(author: User, title: String, content: String): Future[Unit] =
  httpClient.post(
    path = "/user/${author.userId}/blog",
    payload = json"""{"title" : $title, "content" : $content }"""
  )
```

```scala
def getAllUser: Future[List[User]] =
  db.query(
    sql"SELECT userId, userName, createdAt from users"
  ).as[User]
```

```scala
def auditLog(service: String, message: String): Unit =
  println(s"[$service] $message")
```
]

---
class: medium-code

# 3. Cannot perform actions

.eighty-two-left[
```scala
def createNewBlogPost(author: User, title: String, content: String): Future[Unit] =
  httpClient.post(
    path = "/user/${author.userId}/blog",
    payload = json"""{"title" : $title, "content" : $content }"""
  )
```

```scala
def `getAllUser`: Future[List[User]] =
  db.query(
    sql"SELECT userId, userName, createdAt from users"
  ).as[User]
```

```scala
def auditLog(service: String, message: String): Unit =
  println(s"[$service] $message")
```
]

---
class: medium-code

# 3. Cannot perform actions

.eighty-two-left[
```scala
def createNewBlogPost(author: User, title: String, content: String): Future[Unit] =
  httpClient.post(
    path = "/user/${author.userId}/blog",
    payload = json"""{"title" : $title, "content" : $content }"""
  )
```

```scala
def getAllUser: Future[List[User]] =
  db.query(
    sql"SELECT userId, userName, createdAt from users"
  ).as[User]
```

```scala
def `auditLog`(service: String, message: String): Unit =
  println(s"[$service] $message")
```
]

---
# 3. Cannot perform actions

.forty-seven-left[
```scala
val function: Map[Input, Output] =
  Map(
    input1 -> output1,
    input2 -> output2,
    input3 -> output3,
    input4 -> output4,
    // ...
  )
```
]

---
# 3. Cannot perform actions

.forty-seven-left[
```scala
val function: Map[Input, `Unit`] =
  Map(
    input1 -> (),
    input2 -> (),
    input3 -> (),
    input4 -> (),
    // ...
  )
```
]

---
# 3. Cannot perform actions

.forty-seven-left[
```scala
val function: Map[Input, Unit] =
  Map(
    input1 -> `()`,
    input2 -> `()`,
    input3 -> `()`,
    input4 -> `()`,
    // ...
  )
```
]

---
# 3. Cannot perform actions

.forty-seven-left[
```scala
val function: Map[Input, Unit] =
  Map(
    input1 -> (),
    input2 -> (),
    input3 -> (),
    input4 -> (),
    // ...
  )
```

```scala
function(input1)
// res = `()`

function(input3)
// res = `()`
```
]

---
# 3. Cannot perform actions

.forty-seven-left[
```scala
val function: Map[Input, Unit] =
  Map(
    input1 -> (),
    input2 -> (),
    input3 -> (),
    input4 -> (),
    // ...
  )
```

```scala
val function: Input => Unit =
  `_ => ()`
```
]

---
class: medium-code

# 3. Cannot perform actions

.eighty-two-left[
```scala
def createNewBlogPost(author: User, title: String, content: String): `Future[Unit]` =
  httpClient.post(
    path = "/user/${author.userId}/blog",
    payload = json"""{"title" : $title, "content" : $content }"""
  )
```

```scala
def getAllUser: Future[List[User]] =
  db.query(
    sql"SELECT userId, userName, createdAt from users"
  ).as[User]
```

```scala
def auditLog(service: String, message: String): `Unit` =
  println(s"[$service] $message")
```
]

---
class: white

background-image: url(../img/fp-tower/website-background.svg)

<br><br>

# .white[FP functions]

<br>

.large[
1. Return a value for all inputs

1. Be a static mapping

1. Cannot perform actions
]

---
class: white

background-image: url(../img/fp-tower/website-background.svg)

<br><br>

# .white[FP functions]

<br>

.large[
1. Total

1. Deterministic

1. Pure or without side effect
]

---
# Is the function isWeekend an FP function?

.forty-seven-left[
```scala
sealed trait Day

case object Monday    extends Day
case object Tuesday   extends Day
case object Wednesday extends Day
case object Thursday  extends Day
case object Friday    extends Day
case object Saturday  extends Day
case object Sunday    extends Day
```
]

.forty-seven-right[
<br>
```scala
def isWeekend(day: Day): Boolean =
  day match {
    case Saturday | Sunday => true
    case _                 => false
  }
```
]

---
# Is the function isWeekend an FP function?

.forty-seven-left[
```scala
def isWeekend(day: Day): Boolean =
  day match {
    case Saturday | Sunday => true
    case _                 => false
  }
```

```scala
val isWeekend: Map[Day, Boolean] =
  Map(
    Monday    -> false,
    Tuesday   -> false,
    Wednesday -> false,
    Thursday  -> false,
    Friday    -> false,
    Saturday  -> true,
    Sunday    -> true,
  )
```
]

---
class: medium-code

# Is the function createBlogEntry an FP function?

.fifty-two-left[
```scala
import java.time.LocalDate
import java.util.UUID

case class BlogEntry(
  id         : UUID,
  title      : String,
  publishedAt: LocalDate,
)

def createBlogEntry(title: String): BlogEntry =
  BlogEntry(
    id          = UUID.randomUUID(),
    title       = title,
    publishedAt = LocalDate.now()
  )
```
]

---
class: medium-code

# Is the function createBlogEntry an FP function?

.fifty-two-left[
```scala
import java.time.LocalDate
import java.util.UUID

case class BlogEntry(
  id         : UUID,
  title      : String,
  publishedAt: LocalDate,
)

def createBlogEntry(title: String): BlogEntry =
  BlogEntry(
    id          = `UUID.randomUUID()`,
    title       = title,
    publishedAt = LocalDate.now()
  )
```
]

.forty-seven-right[
```scala
createBlogEntry("Hello")
// res: BlogEntry = BlogEntry(
//   `e200c6c0-c034-4b30-8f13-be44cc927739`,
//   "Hello",
//   2021-01-06
// )

createBlogEntry("Hello")
// res: BlogEntry = BlogEntry(
//   `bc899606-da5e-4c1f-a143-0deae306651b`,
//   "Hello",
//   2021-01-06
// )
```
]

---
class: medium-code

# Is the function createBlogEntry an FP function?

.fifty-two-left[
```scala
import java.time.LocalDate
import java.util.UUID

case class BlogEntry(
  id         : UUID,
  title      : String,
  publishedAt: LocalDate,
)

def createBlogEntry(title: String): BlogEntry =
  BlogEntry(
    id          = UUID.randomUUID(),
    title       = title,
    publishedAt = `LocalDate.now()`
  )
```
]

.forty-seven-right[
```scala
createBlogEntry("Hello")
// res: BlogEntry = BlogEntry(
//   e200c6c0-c034-4b30-8f13-be44cc927739,
//   "Hello",
//   `2021-01-06`
// )

createBlogEntry("Hello") // tomorrow
// res: BlogEntry = BlogEntry(
//   bc899606-da5e-4c1f-a143-0deae306651b,
//   "Hello",
//   `2021-01-07`
// )
```
]

---
class: medium-code

# Is the function addUser an FP function?

<br>

```scala
import scala.concurrent.Future

case class User(id: Long, name: String)

class UserRepository(db: Db) { // assume some database library

def addUser(user: User): Future[Unit] =
    db.run(sql"""INSERT INTO users (id, name) VALUES (${user.id}, ${user.name})""")

}
```

---
class: medium-code

# Is the function addUser an FP function?

<br>

```scala
import scala.concurrent.Future

case class User(id: Long, name: String)

class UserRepository(db: Db) { // assume some database library

def addUser(user: User): Future[`Unit`] =
    db.`run`(sql"""INSERT INTO users (id, name) VALUES (${user.id}, ${user.name})""")

}
```

---
class: medium-code

# Is the function max an FP function?

<br>

```scala
def max(list: List[Int]): Option[Int] = {
  var state = Option.empty[Int]

for (element <- list)
    state match {
      case None             => state = Some(element)
      case Some(currentMax) => if (currentMax < element) state = Some(element)
    }

state
}
```

---
class: medium-code

# Is the function max an FP function?

<br>

```scala
def max(list: List[Int]): Option[Int] = {
  `var state` = Option.empty[Int]

for (element <- list)
    state match {
      case None             => `state =` Some(element)
      case Some(currentMax) => if (currentMax < element) `state =` Some(element)
    }

state
}
```