phase_scalafmt.md

Phase Scalafmt

Contents

• Overview
• How to set up
• IntelliJ plugin support

Overview

A phase extension phase_scalafmt can format Scala source code via Scalafmt.

How to set up

Add this snippet to MODULE.bazel:

# MODULE.bazel
scala_deps = use_extension(
    "@rules_scala//scala/extensions:deps.bzl",
    "scala_deps",
)
scala_deps.scalafmt()

or to WORKSPACE:

# WORKSPACE
load(
    "@rules_scala//scala:toolchains.bzl",
    "scala_register_toolchains",
    "scala_toolchains",
)

scala_toolchains(
    # Other toolchains settings...
    scalafmt = True,
)

scala_register_toolchains()

To add this phase to a rule, you have to pass the extension to a rule macro. Take scala_binary for example,

load("//scala:advanced_usage/scala.bzl", "make_scala_binary")
load("//scala/scalafmt:phase_scalafmt_ext.bzl", "ext_scalafmt")

scalafmt_scala_binary = make_scala_binary(ext_scalafmt)

Then use scalafmt_scala_binary as normal.

The extension adds 2 additional attributes to the rule

• format: enable formatting
• config: the Scalafmt configuration file

When format is set to true, you can do

bazel run <TARGET>.format

to format the source code, and do

bazel run <TARGET>.format-test

to check the format (without modifying source code).

The extension provides a default configuration. To use a custom configuration, pass its path or target Label string to the toolchain configuration. The file must exist within a package; the following examples presume a BUILD file exists in the repository root:

# MODULE.bazel
scala_deps.scalafmt(
    default_config = "path/to/my/custom/.scalafmt.conf",
)

# WORKSPACE
scala_toolchains(
    # Other toolchains settings...
    # This _must_ be a target Label under WORKSPACE.
    scalafmt = {"default_config": "//:path/to/my/custom/.scalafmt.conf"},
)

When using Scala 3, you must append runner.dialect = scala3 to .scalafmt.conf.

IntelliJ plugin support

If you use IntelliJ Bazel plugin, then you should check the Customizable Phase page.

TL;DR: you should try naming your scalafmt rules the same way as the default rules_scala rules are named (in your own scope), otherwise external dependency loading won't work in IntelliJ for your Scala targets. E.g.:

# Using this rule won't let you see external dependencies:
scalafmt_scala_binary = make_scala_binary(ext_scalafmt)

# But this will work:
scala_binary = make_scala_binary(ext_scalafmt)