- The document describes a lightning talk on Lens, a data access abstraction that allows getting and setting nested values in immutable data structures in a simple way. - Lens is defined as a class with get and set methods, where get retrieves a value and set updates a value while returning a new instance. - Composing lenses allows accessing deeply nested values by chaining gets and sets across different parts of a nested data structure. - This avoids the "copy method hell" of repeatedly calling copy on nested case classes to update values.

1. Lightning Talk by
presented by Naoki Aoyama (@aoiroaoino)
in ScalaMatsuri 2016

2. > whoami
• Naoki Aoyama
• Twitter: @AoiroAoino
• GitHub: @aoiroaoino
• Scala Exp: about 4 years
• committer

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6. try {
Tour of Lens
in 3 minutes

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))

9. Copy method hell ...
aaa.copy (
bbb = aaa.bbb.copy (
ccc = aaa.bbb.ccc.copy (
ddd = aaa.bbb.ccc.ddd.copy (
eee = aaa.bbb.ccc.ddd.eee.copy (
fff = aaa.bbb.ccc.ddd.eee.fff.copy (
ggg =
)
)
)
)
)
)

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

11. ＿人人人人＿
＞ Lens ＜
￣Y^Y^Y￣

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))

20. (_bbb compose _ccc
compose _ddd
compose _eee
compose _fff
compose _ggg).set( )(aaa)
Lens for the win!!

21. Example in Maverick
• We use Monocle in test codes.

22. } finally {
–Naoki Aoyama
Lens is Awesome!
}