Generic functions: Part 1

---
# List is a generic data structure

List[String]

List[User]
```
]

.fifty-seven-right[
<img src="../img/foundation/parametric-list.svg" alt="Generic list" style="width:600px;"/>
]

---
# How to avoid duplication?

def size(list: List[String]): Int

def size(list: List[User]  ): Int
```
]

.fifty-seven-right[
<img src="../img/foundation/parametric-list.svg" alt="Parametric list" style="width:600px;"/>
]

---
# Generic function

```scala
def size[A](list: List[A]): Int
```

```scala
size(List(1, 4, 8, -3))
// res1: Int = 4

size(List("Hello", "World", "!"))
// res2: Int = 3
```
]

.fifty-seven-right[
<img src="../img/foundation/parametric-list.svg" alt="Generic list" style="width:600px;"/>
]

---
# Generic function

```scala
def size[Elem](list: List[Elem]): Int
```

```scala
size(List(1, 4, 8, -3))
// res3: Int = 4

size(List("Hello", "World", "!"))
// res4: Int = 3
```
]

.fifty-seven-right[
<img src="../img/foundation/parametric-list.svg" alt="Generic list" style="width:600px;"/>
]

---
# Generic function

```scala
def map[A](list: List[A], update: A => A): List[A]
```

```scala
map(List(1,2,3,4), (x: Int) => x + 1)
// res5: List[Int] = List(2, 3, 4, 5)

map(List("Hello", "World"), (x: String) => x.reverse)
// res6: List[String] = List("olleH", "dlroW")
```

---
# Generic function

```scala
def map[A](list: List[A], update: A => A): List[A]
```

```scala
val users = List(User("John", 23), User("Eda", 37), User("Bob", 18))
```

```scala
map(users, (user: User) => user.name)
// error: type mismatch;
//  found   : MdocApp0.this.User => String
//  required: java.io.Serializable => java.io.Serializable
// map(users, (user: User) => user.name)
//            ^^^^^^^^^^^^^^^^^^^^^^^^^
```

---
# Generic function

```scala
def map[A](list: List[A], update: A => A): List[A]
```

```scala
val users = List(User("John", 23), User("Eda", 37), User("Bob", 18))
```

```scala
map[User](users, (user: User) => user.name)
// error: type mismatch;
//  found   : String
//  required: MdocApp0.this.User
// map[User](users, (user: User) => user.name)
//                                  ^^^^^^^^^
```

---
# Generic function

```scala
def map[From, To](list: List[From], update: From => To): List[To]
```

```scala
val users = List(User("John", 23), User("Eda", 37), User("Bob", 18))
```

```scala
map(users, (user: User) => user.name)
// res10: List[String] = List("John", "Eda", "Bob")

map(List(1,2,3,4), (x: Int) => x + 1)
// res11: List[Int] = List(2, 3, 4, 5)
```

---
# Generic function

```scala
def map[From, To](list: List[From], update: From => To): List[To]
```

```scala
val users = List(User("John", 23), User("Eda", 37), User("Bob", 18))
```

```scala
map(users, user => user.name)
// error: missing parameter type
// map(users, user => user.name)
//            ^^^^

map(users, _.name)
// error: missing parameter type for expanded
// function ((<x$1: error>) => x$1.name)
// map(users, _.name)
// ^
```

---
# Generic function

```scala
def map[From, To](list: List[From])(update: From => To): List[To]
```

```scala
val users = List(User("John", 23), User("Eda", 37), User("Bob", 18))
```

```scala
map(users)(user => user.name)
// res14: List[String] = List("John", "Eda", "Bob")

map(users)(_.name)
// res15: List[String] = List("John", "Eda", "Bob")
```

---
# Generic function in Dotty/Scala 3

```scala
def map[From, To](list: List[From], update: From => To): List[To]
```

```scala
val users = List(User("John", 23), User("Eda", 37), User("Bob", 18))
```

```scala
map(users, user => user.name)
// res16: List[String] = List("John", "Eda", "Bob")

map(users, _.name)
// res17: List[String] = List("John", "Eda", "Bob")
```

---
# All generic types are not data structure

```scala
trait JsonDecoder[A]{
  def decode(value: Json): A
}

case class Predicate[A](value: A => Boolean)
```

---
class: white

# .white[Summary]

* We can have several type parameters

* We split parameters into different set of parentheses to help type inference
]

---
# Least Upper Bound (LUB)

```scala
map(users, (user: User) => user.name)
// error: type mismatch;
//  found   : MdocApp5.this.User => String
//  required: java.io.Serializable => java.io.Serializable
// map(users, (user: User) => user.name)
//            ^^^^^^^^^^^^^^^^^^^^^^^^^
```
]

.thirty-seven-right[
<img src="../img/foundation/list-inference.svg" alt="Parametric list" style="width:400px;"/>
]

---
# Generic functions in Dotty

```scala
def map[A](list: List[A], update: A => A): List[A]
```

```scala
val users = List(User("John", 23), User("Eda", 37), User("Bob", 18))
```

```scala
map(users, (user: User) => user.name)
// Type Mismatch Error:
// map(users, (user: User) => user.name)
//                            ^^^^^^^^^
//                            Found:    (user.name : String)
//                            Required: User
```