ArrowKT - Lens invoke giving compilation error

Question

I'm trying to follow this documentation about Lens in Arrow. However when I try to write a Lens

data class Player(val health: Int)

val playerLens: Lens<Player, Int> = Lens(
    get = { player -> player.health },
    set = { player, value -> player.copy(health = value) }
)

I get the following error:

4 type arguments expected for operator fun <S, T, A, B> invoke(): PLens<S, T, A, B>

I see that Lens<S, T> is a typealias for PLens<S, S, T, T>, so why this compilation error?

Kotlin version 1.3.50

Arrow Optics version 0.10.0

Couldn't reproduce your error, it works for me. Could you please provide kotlin and arrow version you are using? Or a full sample project — Evgeny Bovykin, Oct 05 '19 at 10:53
I've copy-pasted your code and it works fine for me: https://github.com/missingdays/ultimate-test/blob/master/src/main/kotlin/main.kt Here you can see that I'm using the same versions as you provided: https://github.com/missingdays/ultimate-test/blob/master/build.gradle#L20 — Evgeny Bovykin, Oct 06 '19 at 10:23
@EvgenyBovykin I cloned your repo and it is working fine. Is arrow-syntax needed as well? I don't have that dependency. — m0skit0, Oct 06 '19 at 13:53
Using arrow-syntax doesn't change anything, still the same error. Weird. — m0skit0, Oct 06 '19 at 13:56
Here's a sample project with the problem: https://github.com/m0skit0/test — m0skit0, Oct 06 '19 at 14:15

score 1 · Accepted Answer · answered Oct 07 '19 at 13:58

Kotlin typealias doesn't work here as expected. When calling constructor, you need to specify 4 params here:

Lens<Player, Player, String, String>(
        get = { v -> v.health },
        set = { v, value -> player.copy(health = value)  }
    )

Our don't specify them at all, compiler could infer them for you from lambda parameters types

Lens(
        get = { player: Player -> player.health },
        set = { player: Player, value: Int -> player.copy(health = value) }
    )

ArrowKT - Lens invoke giving compilation error

1 Answers1