kotlin-power-assert

Important

Starting with Kotlin 2.0.0, this compiler plugin has been replaced with an official Kotlin Power-Assert compiler plugin. As such, this compiler plugin is no longer supported, and any usage should be replaced with the official plugin. See https://kotl.in/power-assert for more information on build setup.

Kotlin Compiler Plugin which high-jacks Kotlin assert function calls and transforms them similar to Groovy's Power Assert feature. This plugin uses the IR backend for the Kotlin compiler and supports all platforms: JVM, JS, and Native!

Example

Given following code:

val hello = "Hello"
assert(hello.length == "World".substring(1, 4).length)

Normally the assertion message would look like:

java.lang.AssertionError: Assertion failed
	at <stacktrace>

A custom assertion message can be provided:

val hello = "Hello"
assert(hello.length == "World".substring(1, 4).length) { "Incorrect length" }

But this just replaces the message:

java.lang.AssertionError: Incorrect length
	at <stacktrace>

With kotlin-power-assert included, the error message for the previous example will be transformed:

java.lang.AssertionError: Incorrect length
assert(hello.length == "World".substring(1, 4).length)
       |     |      |          |               |
       |     |      |          |               3
       |     |      |          orl
       |     |      false
       |     5
       Hello
	at <stacktrace>

Complex, multi-line, boolean expression are also supported:

Assertion failed
assert(
  (text != null && text.toLowerCase() == text) ||
   |    |
   |    false
   null
      text == "Hello"
      |    |
      |    false
      null
)

Beyond Assert

The plugin by default will transform assert function calls but can also transform other functions like require, check, assertTrue, and many, many more.

Functions which can be transformed have specific requirements. A function must have a form which allows taking a String or () -> String value as the last parameter. This can either be as an overload or the original function.

For example, the assert function has 2 definitions:

• fun assert(value: Boolean)
• fun assert(value: Boolean, lazyMessage: () -> Any)

If the first function definition is called, it will be transformed into calling the second definition with the diagram message supplied as the last parameter. If the second definition is called, it will be transformed into calling the same function but with the diagram message appended to the last parameter.

This transformed function call doesn't need to throw an exception either. See Advanced Usage for some examples.

Gradle Plugin

Builds of the Gradle plugin are available through the Gradle Plugin Portal.

plugins {
  kotlin("multiplatform") version "1.8.20"
  id("com.bnorm.power.kotlin-power-assert") version "0.13.0"
}

The Gradle plugin allows configuring the functions which should be transformed with a list of fully-qualified function names.

// Kotlin DSL
configure<com.bnorm.power.PowerAssertGradleExtension> {
  functions = listOf("kotlin.assert", "kotlin.test.assertTrue")
}

// Groovy
kotlinPowerAssert {
  functions = ["kotlin.assert", "kotlin.test.assertTrue"]
}

You can also exclude Gradle source sets from being transformed by the plugin, where those source sets can be specified by name.

// Kotlin DSL
configure<com.bnorm.power.PowerAssertGradleExtension> {
  excludedSourceSets = listOf(
    "commonMain",
    "jvmMain",
    "jsMain",
    "nativeMain"
  )
}

// Groovy
kotlinPowerAssert {
  excludedSourceSets = [
    "commonMain",
    "jvmMain",
    "jsMain",
    "nativeMain"
  ]
}

Compatibility

The Kotlin compiler plugin API is unstable and each new version of Kotlin can bring breaking changes to the APIs used by this compiler plugin. Make sure you are using the correct version of this plugin for whatever version of Kotlin used. Check the table below to find when support for a particular version of Kotlin was first introduced. If a version of Kotlin or this plugin is not listed it can be assumed to maintain compatibility with the next oldest version listed.

Kotlin Version	Plugin Version
1.3.60	0.1.0
1.3.70	0.3.0
1.4.0	0.4.0
1.4.20	0.6.0
1.4.30	0.7.0
1.5.0	0.8.0
1.5.10	0.9.0
1.5.20	0.10.0
1.6.0	0.11.0
1.7.0	0.12.0
1.8.20	0.13.0
2.0.0 and beyond	Official support by Kotlin: https://kotl.in/power-assert

Kotlin IR

This plugin supports all IR based compiler backends: JVM, JS, and Native! Only Kotlin/JS still uses the legacy compiler backend by default, use the following to make sure IR is enabled.

target {
  js(IR) {
  }
}

Advanced Usage

Function Call Tracing

Similar to Rust's dbg! macro, functions which take arbitrary parameters can be transformed. For example:

fun <T> dbg(value: T): T = value

fun <T> dbg(value: T, msg: String): T {
  println(msg)
  return value
}

fun main() {
  println(dbg(1 + 2 + 3))
}

Prints the following:

dbg(1 + 2 + 3)
      |   |
      |   6
      3
6

Soft Assertion

To achieve soft assertion, the following template can be implemented:

typealias LazyMessage = () -> Any

interface AssertScope {
  fun assert(assertion: Boolean, lazyMessage: LazyMessage? = null)
}

fun <R> assertSoftly(block: AssertScope.() -> R): R = TODO("implement")

You can then use the template as follows:

val jane: Person = TODO()
assertSoftly {
  assert(jane.firstName == "Jane")
  assert(jane.lastName == "Doe")
}

A working example is available in this repository in the sample directory.