brittany

haskell source code formatter

(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.

Other usage notes

• Requires GHC-8.0.*; support for 8.2 is on the list, but I haven't even looked at how much the GHC API changes.
• 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.

Installation

• via cabal "old-build"

  # optionally:
  # mkdir brittany
  # cd brittany
  # cabal sandbox init
  cabal install brittany --bindir=$HOME/.cabal/bin # -w $PATH_TO_GHC_8_0
  

• via cabal new-build

  cabal unpack brittany
  cd brittany-0.8.0.2
  # cabal new-configure -w $PATH_TO_GHC_8_0
  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 https://github.com/lspitzner/brittany.git
  cd brittany
  stack install
  

Usage

• 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):

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

Implementation/High-level Documentation

See the documentation index

License

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 http://www.gnu.org/licenses/.