brittany Hackage version Stackage version Build Status

haskell source code formatter

Output sample

(see more examples and comparisons)

This project's goals roughly are to:

  • Always retain the semantics of the source being transformed;
  • Be idempotent (this also directly ensures that only valid haskell is produced);
  • 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 it if 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.

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

  • only type-signatures and function/value bindings are processed; other module elements (data-decls, classes, instances, imports/exports 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.)
  • ~~There is an open performance issue on large inputs (due to an accidentally quadratic sub-algorithm); noticable for inputs with >1k loc.~~ (fixed in

Other usage notes

  • Supports GHC versions 8.0.* and 8.2.*.
  • as of November'17, brittany is available on stackage nightly.
  • config (file) documentation is lacking.
  • some config values can not be configured via commandline yet.
  • uses/creates user config file in ~/.brittany/config.yaml; also reads brittany.yaml in current dir if present.


  • via cabal "old-build" # optionally: # mkdir brittany # cd brittany # cabal sandbox init cabal install brittany --bindir=$HOME/.cabal/bin # -w $PATHTOGHC80 ~~~~

  • via cabal new-build cabal unpack brittany cd brittany- # cabal new-configure -w $PATHTOGHC80 cabal new-build exe:brittany # and it should be safe to just copy the executable, e.g. cp ./find dist-newstyle/build/ -type f -name brittany $HOME/.cabal/bin/ ~~~~

  • via stack git clone cd brittany stack install ~~~~


  • Currently one mode of operation: Transform a single module. By default read from stdin and written to stdout, but commandline arguments allow to read/write from/to files.
  • For stdin/stdout usage it makes sense to enable certain syntactic extensions by default, i.e. to add something like this to your ~/.brittany/config.yaml (execute brittany once to create default):

    ~~~~ confforward: optionsghc: - -XLambdaCase - -XMultiWayIf - -XGADTs - -XPatternGuards - -XViewPatterns - -XRecursiveDo - -XTupleSections - -XExplicitForAll - -XImplicitParams - -XQuasiQuotes - -XTemplateHaskell - -XBangPatterns ~~~~

Feature Requests, Contribution, Documentation

This currently is a one-person project in the sense that 90% of the code is written by one person. And (unfortunately) it is not my job to keep improving this project. Please forgive that as a consequence my time to invest on new features is rather limited.

Nonetheless I consider it "in active development" :)

One way of speeding things up is to make your own contributions. There is a good amount of high-level documentation at

the documentation index

Note that most development happens on the dev branch of this repository!


Copyright (C) 2016-2017 Lennart Spitzner

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, 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


Revision history for brittany -- December 2017

  • Change default global config path (use XDG spec) Existing config should still be respected, so this should not break compatibility
  • ! Slight rework of the commandline interface:
    • Support multiple inputs and outputs
    • Support inplace-transformation for multiple files via --write-mode=inplace
  • Implement IndentPolicyLeft - the indentation mode that never adds more than the base indentation for nested parts (no hanging indentation)

    (thanks to Evan Borden)

  • Fix bug that manifested in bad output for (top-level) template haskell splices
  • Extension support:
    • RecordWildCards
    • RecursiveDo (was only partially supported previously)
  • Layouting Bugfixes:
    • Properly reproduce parentheses around kind signatures
    • Fix issue around promoted lists (example good: '[ 'True] bad: '['True])
    • Trim text from exactprint used as workaround for unknown nodes (unsupported extension workaround)
  • Layouting changes
    • Insert spaces around operator in sections -- September 2017

  • Support for ghc-8.2.1
  • Bugfixes:
    • Fix quadratic performance issue
    • Fix special "where" indentation with indentAmount /= 2
    • Fix negative literals in patterns
    • Support type applications
  • Accept -h for --help and improve help layouting (via butcher-
  • Add continuous integration via travis (cabal, cabal-new, stack) (brittle due compilation time limit)
  • Reduce compilation memory usage a bit -- August 2017

  • Add library interface, to be used by haskell-ide-engine.
  • Publish to hackage. -- May 2017

  • Document the high-level design of the program
  • Improve layouting for many different cases, too many to list here. Brittany still does only reformat top-level type signatures and bindings.
  • Publish all dependencies on hackage; ghc-exactprint adaptions got merged upstream as well.
  • Reduce the aggressiveness of horizontal alignment; this is configurable via the lconfig_alignmentLimit and lconfig_alignmentBreakOnMultiline values (config file only for now).
  • (!) Breaking change to the config file format: The keys previously contained underscore (e.g. _econf_Werror) but do not anymore (econf_Werror). Add config version; also
  • Move config value conf_errorHandling.econf_CPPMode to conf_preprocessor.ppconf_CPPMode.
  • Cope with unhandled syntactical constructs more gracefully by falling back on the ghc-exactprint output; Brittany simply won't touch certain subexpressions instead of aborting. This is further configurable via the econf_omit_output_valid_check config value.
  • Due to improvements to the butcher library: Accept --key=value on commandline (only --key value was supported previously).
  • Improve testsuite setup: The tests.blt file contains many different testcases that allow quick addition - feel free to report bugs directly by making PRs with additions to that file.
  • Release under the terms of the AGPLv3 -- 2016-09-06

  • Support stack
  • Fix --ghc-options handling
  • Add commandline param to allow shortcut brittany Foo.hs meaning brittany -i Foo.hs -- 2016-09-04

  • First official alpha release
  • Fix commandline flag parsing
  • Implement/Improve horizontal aligning
  • Various minor fixes and layouting improvements -- 2016-08-12

  • Add check that output is syntactically valid
  • (!) Change config format, breaking previous configs
  • Various layouting additions/imporements
  • Various minor fixes -- 2016-08-08

  • Support --ghc-options
  • Support user and local config files: Local config file is not mandatory anymore. -- 2016-08-06

  • Make comment handling a bit more clever
  • Various layouting additions/imporements -- 2016-08-04

  • Various layouting additions/imporements -- 2016-07-30

  • Basic and partial implementation for expression/decl/statement/pattern layouting
  • Prevent exponential behaviour using manual stablenames -- 2016-06-05

  • First working code for type signature layouting
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