defer-it

A compiler plugin to enable defer statements in the Kotlin programming language.

Dissimilar to Go's defer statements, currently, function deferral only works within a block body.

The compiler plugin transforms the (Kotlin compiler backend) IR to defer the execution of deferrable functions until the surrounding block exits.

Example

Given the following Kotlin:

fun printHelloWorld() {
  defer { println("World!") }
  println("Hello")
}

Ignoring the implementation details of defer, the output of invoking printHelloWorld() is (reasonably) expected to be:

World!
Hello

With the defer-it-compiler-plugin included, the body of printHelloWorld will be transformed (transparently during compilation) to:

fun printHelloWorld() {
  try {
    println("Hello")
  } finally {
    runCatching { println("World!") }
  }
}

Thus, the output of invoking printHelloWorld() will be:

Hello
World!

Usage

The defer-it-compiler-plugin is enabled through the defer-it-gradle-plugin, which is published to Maven Central. Add the following to the build.gradle configuration to apply the plugin:

buildscript {
    dependencies {
        classpath "io.github.c-fraser:defer-it-gradle-plugin:+"
    }
}

apply plugin: 'defer-it'

The gradle plugin inserts the defer-it-api library into the project dependencies. This library defines the defer function, which is the only function that the defer-it-compiler-plugin transforms to defer the invocation of.

License

Copyright 2021 c-fraser

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

Acknowledgements

Kudos to the kotlin-power-assert project which significantly influenced the implementation of defer-it.