7. Simple data structure
case class Player(name: String, age: Int)
val player = Player("aoino", 25)
scala> player.name
res0: String = aoino
scala> player.copy(age = 26)
res1: Player = Player(aoino,26)
8. Nested data structure
case class Game(stageId: Int, player: Player)
val game = Game(999, player)
// get
scala> game.player.name
res6: String = aoino
// set
scala> game.copy(
| player = game.player.copy(
| name = "Aoyama"
| )
| )
res7: Game = Game(999,Player(Aoyama,25))
10. Motivation
• We need the simple syntax like updating
mutable objects
-> want to avoid the copy method hell
• We always want a "composability" and
"reusability"
-> compose of a getter/settter defined in
the class is difficult
12. What s the Lens?
• something like a getter/setter such as Java
• functional reference
13. What s the Lens?
getter: S => A
e.g. player.name
// => "aoino"
setter: A => S => S
e.g. player.copy(age = 26)
// => Player("aoino",26)
14. What s the Lens?
case class Lens[S, A](get: S => A, set: A => S => S)
15. Let s define some Lenses
case class Lens[S, A](get: S => A, set: A => S => S)
case class Player(name: String, age: Int)
val _name: Lens[Player, String] =
Lens(
_.name,
str => player => player.copy(name = str)
)
val _age: Lens[Player, Int] =
Lens(
_.age,
num => player => player.copy(age = num)
)
16. Give it a try
scala> _name.get(player)
res2: String = aoino
scala> _name.set("Aoyama")(player)
res3: Player = Player(Aoyama,25)
scala> _age.get(player)
res4: Int = 25
scala> _age.set(26)(player)
res5: Player = Player(aoino,26)
17. We need `composability`
// in Function
f: A => B
g: B => C
g compose f : A => C
// in Lens
sa: Lens[S, A]
ab: Lens[A, B]
sa compose ab : Lens[S, B]
Note: Typically, Lenses `compose` is reverse compared to function.
18. Let s define `compose` method
case class Lens[S, A](get: S => A, set: A => S => S){
def compose[B](other: Lens[A, B]): Lens[S, B] =
Lens(
s => other.get(this.get(s)),
b => s => this.set(
other.set(b)(this.get(s))
)(s)
)
}
19. Give it a try
val _player: Lens[Game, Player] = ...
scala> (_player compose _name).get(game)
res8: String = aoino
scala> (_player compose _name).set("Aoyama")(game)
res9: Game = Game(999,Player(Aoyama,25))