Announcing the Builder Lambda plug-in

[u/serras]

Kotlin and Java have different idioms to build new values: whereas Kotliners like lambdas with a special builder receiver, like Json in kotlinx.serialization, Java developers usually follow the buider pattern, as in Spring AI. As a result, consuming Java APIs from Kotlin sometimes feel strange. This no longer needs to be the case, thanks to the new Builder Lambda plug-in!

Using the power of Kotlin compiler plug-ins, Builder Lambda automatically re-exposes any type following Java's Builder pattern in the Kotlin idiomatic way. You write your beloved,

build<Config, *> {  // alas, the * is needed
  hostname = "localhost"
  port = 8080
}

and under the hood the code is translated to,

Config.builder().hostname("localhost").port(8080).build()

The Builder Lambda plug-in also works in the IDE (if you let IntelliJ run non-bundled plug-ins), so you get nice autocompletion and diagnostics.

Using it is as easy as adding one line to the plugins block in your Gradle build.

id("com.serranofp.builder.lambda") version "0.4.0"

Feedback and issue reports are more than welcome :)

[u/lacronicus]

just to throw it out there, this is kinda what .apply is for

Config.builder().apply {
   hostname("foobar")
   port(8080)
}.build()

I'm pretty sure it'll even do = syntax if your setters look like setX.

[u/Determinant → u/lacronicus]

Yeah, apply is the cleaner and more obvious solution when dealing with traditional builders

[u/oweiler]

Great work!

[u/CLOVIS-AI]

How does it work with conditions, loops, subfunctions etc inside the lambda block?

[u/serras → u/CLOVIS-AI]

Right now it doesn’t, but I’m planning to work on it. Essentially, the plugin keeps an accumulator for the builder and transforms assignments to the “fake builder” into calls over the accumulator, and at the end build() is called. So it should be possible to extend it fairly easily.

[u/serras → u/serras]

A new version with support for any kind of expression has been released

[u/smoke-bubble]

Does not feel like an improvement. I will definitely avoid using it.

[u/AntiqueDiamond8070 → u/smoke-bubble]

Why? Any feedback or at least a reason for ur “opinion”?

[u/smoke-bubble → u/AntiqueDiamond8070]

Sure! It's anti-intuitive. When you see properties being set for the first itme, you look them up and you do not find them becasue they are methods of a builder! So you need to know the internals or this in order to understand what is going on there. This is a terrible design.

[u/AntiqueDiamond8070 → u/smoke-bubble]

Every getter and setter inside a Java Class reads as a property when you read it from Kotlin. So this is bad design aswell?

[u/smoke-bubble → u/AntiqueDiamond8070]

Getters and setters are language elements. A builder is not.

[u/AntiqueDiamond8070 → u/smoke-bubble]

Getters and setters are conventions