A Kotlin compiler plugin that lets you retrieve function default argument values at compile time.

Kotlin allows defining default arguments for function parameters, but there is no standard way to access those values programmatically. For example, if you want to display default values in a CLI help message or a configuration framework, you would need to either duplicate the values or rely on reflection.

This plugin replaces defaultArgOf function calls with the actual default value expressions at compile time, providing a type-safe solution with zero runtime cost.

// build.gradle.kts
plugins {
    id("me.tbsten.defaultargcapture") version "0.1.0-alpha03"
}

Simply applying the plugin automatically adds the runtime library dependency and configures the compiler plugin.

import me.tbsten.defaultargcapture.runtime.defaultArgOf

fun greet(name: String = "World") {
    println("Hello, $name!")
}

fun main() {
    // Replaced with "World" at compile time
    val defaultName = defaultArgOf<String>(funName = "greet", argName = "name")
    println("Default value: $defaultName") // => Default value: World
}

inline fun <reified T> defaultArgOf(funName: String, argName: String): T

• All arguments must be compile-time constants
• Specifying a non-existent function/parameter name or a parameter without a default value results in a compile error

fun serve(port: Int = 8080, host: String = "localhost") { /* ... */ }

fun printHelp() {
    val defaultPort = defaultArgOf<Int>(funName = "serve", argName = "port")
    val defaultHost = defaultArgOf<String>(funName = "serve", argName = "host")
    println("  --port  Port number (default: $defaultPort)")
    println("  --host  Hostname    (default: $defaultHost)")
}

1. You call defaultArgOf<T>(funName = "f", argName = "x") in your code
2. The FIR checker validates the arguments during type analysis and reports compile errors if invalid
3. The IR Transformer finds the call and replaces it with the default value IR expression from the target function

• Kotlin 2.x (K2 compiler)

MIT