Black vs Fourmolu

Black is a free, MIT-licensed formatter for Python code that follows PEP 8. It reformats whole files in place and uses a default line length of 88 characters. Its limited style choices are intended to keep code consistent across projects; indentation style cannot be configured. After formatting, Black checks whether the abstract syntax tree remains effectively equivalent and raises an error if the check fails. Install it with pip; it requires Python 3.10 or newer. The documentation lists editor integrations including Emacs, PyCharm, IntelliJ IDEA, Vim, Neovim, Visual Studio Code and Sublime Text, as well as GitHub Actions and doctest workflows. Notebook formatting is available with the jupyter extra, though Black skips cells containing certain magics or invalid syntax. It also includes blackd, an HTTP server that binds to localhost by default. Black is primarily a command-line tool and does not provide a Python interface. Stable releases preserve formatting within the same calendar year when the options match.

Fourmolu is a configurable formatter for Haskell source code and a fork of Ormolu. It uses GHC’s parser and aims to keep diffs small, produce idempotent output, and handle large projects. Set formatting options in `fourmolu.yaml` or pass them on the command line. Controls include indentation, line length, import grouping, record layout, Haddock comments, Unicode output, and operator fixities. Fourmolu can find the relevant Cabal file to read default extensions and dependencies that help determine fixities. Magic comments can disable formatting for selected code, and command-line options can target a line region. There is an interactive browser demo with configurable formatting and program options. Installation instructions cover GHCup, Cabal, Stack, and dotslash; editor integration is documented through Haskell Language Server. The project recommends a GitHub Action to keep projects formatted. Its listed current release is v0.21.0.0. CPP handling is experimental and limited to simple cases where conditionals surround top-level declarations. Output always uses LF line endings, which can cause check mode to fail on CRLF files.