treefmt - one CLI to format the code tree

Status: experimental -- not all the features described here are working yet.

Every project has different languages and code formatters, with possibly different configurations. When jumping between project it always takes a bit of time to get accustomed to them and update our editor configuration.

This project tries to solve that by proposing a unified CLI interface: type treefmt to format all of the files in a given project, using the project's configuration.

Design decisions

We assume that project code is checked into source control. Therefor, the default should be to write formatter changes back in place. Options like --dry-run are not needed, we rely on the source control to revert or check for code changes.

treefmt is responsible for traversing the file-system and mapping files to specific code formatters.

We want all of the project files to be formatted. treefmt should hint if any files in the project are not covered.

Only one formatter per file. treefmt should enforces that only one tool is executed per file. Guaranteeing two tools to product idempotent outputs is quite difficult.

Usage

treefmt [options] [<file>...]

• file: path to files to format. If no files are passed, format all of the files from the current folder and down.

Options

• --stdin: If this option is passed, only a single file must be passed to the command-line. Instead of reading the file, it will read the content from stdin, and write the formatted result to stdout. The file is only used to select the right formatter.

• --init: Creates a templated treefmt.toml in the current directory. In the future we want to add some heuristic for detecting the languages available in the project and make some suggestions for it.

• --show-config: Prints out the merged configuration for the current folder.

• --help: Shows this help.

Configuration format

treefmt depends on the treefmt.toml to map file extensions to actual code formatters. That file is searched for recursively from the current folder and up.

TODO: describe the actual format here

[formatters.<name>]

This section describes the integration between a single formatter and treefmt.

• files: A list of glob patterns used to select files. Usually this would be something like [ "*.sh" ] to select all the shell scripts. Sometimes, full filenames can be passed. Eg: [ "Makefile" ].

• command: A list of arguments to execute the formatter. This will be composed with the options attribute during invocation. The first argument is the name of the executable to run.

• options: A list of extra arguments to add to the command. This is typically project-specific arguments.

NOTE: Formatters SHOULD adhere to the formatter spec. If they don't, the best is to create a wrapper script that transforms the usage to match that spec.

Use cases

CLI usage

As a developer, I want to run treefmt in any folder and it would automatically format all of the code, configured for the project. I don't want to remember what tool to use, or their magic incantation.

Editor integration

Editors often want to be able to format a file, before it gets written to disk.

Ideally, the editor would pipe the code in, pass the filename, and get the formatted code out. Eg: cat ./my_file.sh | treefmt --stdin my_file.sh > formatted_file.sh

CI integration

We can assume that code lives in a source control.

For example a Git integration would look like this:

#!/usr/bin/env bash
set -euo pipefail

# Format all of the code
treefmt

# Check that there are no changes in the code
if [[ -n "$(git status --porcelain -unormal)" ]]; then
  echo "Some code needs formatting! Please run \`treefmt\`.
  git status -unormal
  exit 1
fi
echo "OK"

Interfaces

Formatter integration

Formatters can ship with a config file, that describes and simplifies the integration between the formatter and treefmt.

Whenever a new formatter is described in the project's treefmt.toml config file, treefmt will look for that formatter config in $dir/share/treefmt.d/<formatter>.toml where $dir is each folder in XDG_DATA_DIRS.

Example:

In the treefmt.toml file of the project:

[formatters.ormolu]
options = [
  "--ghc-opt", "-XBangPatterns",
  "--ghc-opt", "-XPatternSynonyms",
]

Now assuming that ormolu ships with $out/share/treefmt.d/ormolu.toml that contains:

command = ["ormolu", "--mode", "inplace"]
files = ["*.hs"]
options = []

The project will first load treefmt.toml, take note of the ormolu formatter, then load the ormolu.toml file, and merge back the treefmt.toml config for that section back on top. The end-result would be the same as if the user has this in their config:

[formatters.ormolu]
command = ["ormolu", "--mode", "inplace"]
files = ["*.hs"]
options = [
  "--ghc-opt", "-XBangPatterns",
  "--check-idempotence"
]

Note how the empty ormolu options got overwritten by the project options.

Related projects

• EditorConfig: unifies file indentations configuration on a per-project basis.

Contributing

All contributions are welcome! We try to keep the project simple and focused so not everything will be accepted. Please open an issue to discuss before working on a big item.

If you want to discuss, we have a public Matrix channel: #treefmt:numtide.com

License

MIT - (c) 2020 NumTide Ltd.