ktfmt

Introduction: A program that reformats Kotlin source code to comply with the common community standard for Kotlin code conventions.
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ktfmt is a program that pretty-prints (formats) Kotlin code, based on google-java-format.

Note that ktfmt still has some rough edges which we're constantly working on fixing.

The minimum supported runtime version is JDK 11, released September 2018.

Demo

Before Formatting Formatted by ktfmt
Original ktfmt

For comparison, the same code formatted by ktlint and IntelliJ:

Formatted by ktlint Formatted by IntelliJ
ktlint IntelliJ

Using the formatter

from the command-line

Download the formatter and run it with:

java -jar /path/to/ktfmt-<VERSION>-jar-with-dependencies.jar [--dropbox-style] [files...]

--dropbox-style makes ktfmt use a block indent of 4 spaces instead of 2. See below for details.

Note: There is no configurability as to the formatter's algorithm for formatting (apart from --dropbox-style). This is a deliberate design decision to unify our code formatting on a single format.

using Spotless

On Gradle: https://github.com/diffplug/spotless/tree/master/plugin-gradle#applying-ktfmt-to-kotlin-files On Maven: https://github.com/diffplug/spotless/tree/master/plugin-maven#applying-ktfmt-to-kotlin-files

FAQ

ktfmt vs ktlint vs IntelliJ

ktfmt uses google-java-format's underlying engine, and as such, many items on google-java-format's FAQ apply to ktfmt as well.

In particular,

  1. ktfmt ignores most existing formatting. It respects existing newlines in some places, but in general, its output is determinstic and is independent of the input code.
  2. ktfmt exposes no configuration options that govern formatting behavior. See https://github.com/google/google-java-format/wiki/FAQ#i-just-need-to-configure-it-a-bit-differently-how for the rationale.

These two properties make ktfmt a good fit in large Kotlin code bases, where consistency is very important.

We created ktfmt because ktlint and IntelliJ sometime fail to produce nice-looking code that fits in 100 columns, as can be seen in the Demo section.

ktfmt uses a 2-space indent; why not 4? any way to change that?

Two reasons -

  1. Many of our projects use a mixture of Kotlin and Java, and we found the back-and-forth in styles to be distracting.
  2. From a pragmatic standpoint, the formatting engine behind google-java-format uses more whitespace and newlines than other formatters. Using an indentation of 4 spaces quickly reaches the maximal column width.

However, we do offer an escape-hatch for projects that absolutely cannot make the move to ktfmt because of 2-space: the --dropbox-style flag changes block indents to 4-space.

Developer's Guide

Setup

  • Open pom.xml in IntelliJ. Choose "Open as a Project"
  • The IntelliJ project will unfortunately be broken on import. To fix,

Development

  • Currently, we mainly develop by adding tests to FormatterKtTest.kt.

Building on the Command Line

  • Run mvn install
  • Run java -jar core/target/ktfmt-<VERSION>-jar-with-dependencies.jar

Releasing

See RELEASING.md.

License

Apache License 2.0

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