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brittany

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haskell source code formatter

// repository documentation

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# brittany [![Hackage version](https://img.shields.io/hackage/v/brittany.svg?label=Hackage)](https://hackage.haskell.org/package/brittany) [![Stackage version](https://www.stackage.org/package/brittany/badge/lts?label=Stackage)](https://www.stackage.org/package/brittany) [![Build Status](https://secure.travis-ci.org/lspitzner/brittany.svg?branch=master)](http://travis-ci.org/lspitzner/brittany)

:warning:
This project is effectively unmaintained!
I ([@tfausak](https://github.com/tfausak)) would recommend switching to another formatter.
At time of writing (2022-11-11), I would suggest [Ormolu](https://github.com/tweag/ormolu).
Or if you prefer some configuration, I would suggest [Fourmolu](https://github.com/fourmolu/fourmolu).

haskell source code formatter

![Output sample](https://github.com/lspitzner/brittany/raw/master/brittany-sample.gif)

(see [more examples and comparisons](/doc/showcases))

This project's goals roughly are to:

- Always retain the semantics of the source being transformed;
- Be idempotent;
- Support the full GHC-haskell syntax including syntactic extensions
  (but excluding `-XCPP` which is too hard);
- Retain newlines and comments unmodified;
- Be clever about using the available horizontal space while not overflowing
  the column maximum unless it cannot be avoided;
- Be clever about aligning things horizontally (this can be turned off
  completely however);
- Have linear complexity in the size of the input.

In theory, the core algorithm inside brittany reaches these goals. It is rather
clever about making use of horizontal space while still being linear in the
size of the input (although the constant factor is not small). See
[these examples of clever layouting](/doc/showcases/Layout_Interactions.md).

But brittany is not finished yet, and there are some open issues that yet
require fixing:

- **only the module header (imports/exports), type-signatures and
  function/value bindings** are processed;
  other module elements (data-decls, classes, instances, etc.)
  are not transformed in any way; this extends to e.g. **bindings inside class
  instance definitions** - they **won't be touched** (yet).
- By using `ghc-exactprint` as the parser, brittany supports full GHC
  including extensions, but **some of the less common syntactic elements
  (even of 2010 haskell) are not handled**.
- **There are some known issues regarding handling of in-source comments.**
  There are cases where comments are not copied to the output (this will
  be detected and the user will get an error); there are other cases where
  comments are moved slightly; there are also cases where comments result in
  wonky newline insertion (although this should be a purely aesthetic issue.)

## Try without Installing

You can [paste haskell code over here](https://hexagoxel.de/brittany/)
to test how it gets formatted by brittany. (Rg. privacy: the server does
log the size of the input, but _not_ the full input/output of requests.)

# Other usage notes

- Supports GHC version `9.0.x`.
- included in stackage with lts>=10.0 (or nightlies dating to >=2017-11-15)
- config (file) documentation is lacking.
- some config values can not be configured via commandline yet.
- uses/creates user config file in `~/.config/brittany/config.yaml`;
  also reads (the first) `brittany.yaml` found in current or parent
  directories.

# Installation

- via `stack`

~~~~.sh
    stack install brittany # --resolver lts-16.31
    ~~~~

If you use an lts that includes brittany this should just work; otherwise
    you may want to clone the repo and try again.

- via `cabal`

Due to constant changes to the cabal UI, I have given up on making sure
    these instructions work before releases. Please do not expect these
    instructions to be up-to-date; they may produce incomprehensible error
    messages, they may be broken otherwise, they may work now but break with
    the next cabal release. Thanks for your understanding, and feel free to
    open issues for any problems you encounter. -- lennart

If you are using cabal-3.0, using
    `cabal install brittany --installdir=$HOME/.cabal/bin`
    might work. Keep in mind that cabal merely puts a symlink to the "store"
    into the installdir, so you have to re-install if you ever clean your
    store. On cabal-2.4, try `cabal v2-install brittany`. On cabal-2.2 or
    earlier you might be succesful using
    ```cabal new-build exe:brittany; cp `find dist-newstyle/ -name brittany -type f | xargs -x ls -t | head -n1` $HOME/.cabal/bin/```.
    Alternatively, you can also use the v1-approach with sandboxes as
    `cabal v1-sandbox init; cabal v1-install brittany --bindir=$HOME/.cabal/bin`.

(TODO: These instructions are more confusing than helpful. I am inclined
    to just remove them.)

- on ArchLinux:
    ~~~~.sh
    pacman -S haskell-brittany
    ~~~~

# Development tips

# Editor Integration

#### Sublime text
  [In this gist](https://gist.github.com/lspitzner/097c33177248a65e7657f0c6d0d12075)
  I have described a haskell setup that includes a shortcut to run brittany formatting.
#### VSCode
  [This extension](https://marketplace.visualstudio.com/items?itemName=MaxGabriel.brittany)
  connects commandline `brittany` to VSCode formatting API. Thanks to @MaxGabriel.
#### Via HLS
  [haskell-language-server](https://github.com/haskell/haskell-language-server)
  includes a `brittany` plugin that directly uses the brittany library.
  Relevant for any editors that properly support the language-server-protocol.
#### Neovim / Vim 8
  The [Neoformat](https://github.com/sbdchd/neoformat) plugin comes with support for
  brittany built in.
#### Atom
  [Atom Beautify](https://atom.io/packages/atom-beautify) supports brittany as a formatter for Haskell. Since the default formatter is set to hindent, you will need to change this setting to brittany, after installing the extension.
#### Emacs
  [format-all](https://github.com/lassik/emacs-format-all-the-code) support brittany as the default formatter for Haskell.

# Usage

- Default mode of operation: Transform a single module, from `stdin` to `stdout`.
  Can pass one or multiple files as input, and there is a flag to override them
  in place instead of using `stdout` (since 0.9.0.0). So:

~~~~ .sh
    brittany                           # stdin -> stdout
    brittany mysource.hs               # ./mysource.hs -> stdout
    brittany --write-mode=inplace *.hs # apply formatting to all ./*.hs inplace
    ~~~~

- For stdin/stdout usage it makes sense to enable certain syntactic extensions
  by default, i.e. to add something like this to your
  `~/.config/brittany/config.yaml` (execute `brittany` once to create default):

~~~~
    conf_forward:
      options_ghc:
      - -XLambdaCase
      - -XMultiWayIf
      - -XGADTs
      - -XPatternGuards
      - -XViewPatterns
      - -XRecursiveDo
      - -XTupleSections
      - -XExplicitForAll
      - -XImplicitParams
      - -XQuasiQuotes
      - -XTemplateHaskell
      - -XBangPatterns
    ~~~~

# Feature Requests, Contribution, Documentation

For a long time this project has had a single maintainer, and as a consequence
there have been some mildly large delays for reacting to feature requests
and even PRs.

Sorry about that.

The good news is that this project is getting sponsored by PRODA LTD, and two
previous contributors, Evan Borden and Taylor Fausak, have agreed on helping
with organisational aspects. Thanks!

Still, this project has a long queue of very sensible feature requests, so it
may take some time until new ones get our attention. But with the help of
the co-maintainers, at least the reaction-times on PRs and the frequency
of releases should improve significantly.

If you are interested in making your own contributions, there is
a good amount of high-level documentation at

[the documentation index](doc/implementation/index.md)

# License

This program is free software: you can redistribute it and/or modify
it under the terms of the
[GNU Affero General Public License, version 3](http://www.gnu.org/licenses/agpl-3.0.html),
as published by the Free Software Foundation.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

brittany

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