Haskell source code formatter https://github.com/evolutics/haskell-formatter

Latest on Hackage:1.0.0

This package is not currently in any snapshots. If you're interested in using it, we recommend adding it to Stackage Nightly. Doing so will make builds more reliable, and allow stackage.org to host generated Haddocks.

GPL-3 licensed by Benjamin Fischer
Maintained by Benjamin Fischer
Haskell Formatter

The Haskell Formatter formats Haskell source code. It is strict in that it fundamentally rearranges code.




$ cabal install haskell-formatter

to create an executable, which is referred to as ``haskell-formatter`` in the following. On Linux, for example, you should now be able to call it as


$ ~/.cabal/bin/haskell-formatter --help

In case of issues with dependencies, try to use a Cabal sandbox. For convenience, this can be done by


$ make sandbox



Read source code from ``Input.hs``, format it, and write it to ``Output.hs`` by


$ haskell-formatter --input Input.hs --output Output.hs

If the input or output file is not given, it defaults to the corresponding standard stream. This allows commands like


$ haskell-formatter < Input.hs

For more help about the usage, call


$ haskell-formatter --help

Style Configuration

The formatting style can be configured with a file referred by the ``--style`` option. For instance, the call


$ haskell-formatter --style my_style.yaml --input Input.hs --output Output.hs

uses ``my_style.yaml`` as a style file. Such files are in the `YAML format <http://en.wikipedia.org/wiki/YAML>`_. The following is an `example style file <testsuite/resources/examples/default_style.yaml>`_, which at the same time shows the available keys with their default values.

.. GitHub does currently not allow to include files with the reStructuredText directive ``include`` (https://github.com/github/markup/issues/172).

Thus, the file content is replicated here. There is a test which checks that the strings of both sources are equal.

.. code:: yaml

# Lines should be no longer than this length in characters.
line_length_limit: 80

# How much to spread code over multiple lines instead of trying to fill a single
# line. More precisely, this guides the ratio of "line_length_limit" to the
# ribbon length (the number of characters on a line without leading and trailing
# whitespace). Only the lowest value of 1 forces "line_length_limit" to be
# applied strictly.
# Reference: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=
ribbons_per_line: 1

# More than this number of empty lines in succession are merged.
successive_empty_lines_limit: 1

# Indentation lengths in characters.
class: 8 # "class" and "instance" declarations.
do: 3 # "do" notation.
case: 4 # Body of "case" expressions.
let: 4 # Declarations in "let" expressions.
where: 6 # Declarations in "where" clauses.
onside: 2 # Continuation lines which would otherwise be offside.

# Decides which parts of the code to sort.
# Sequence of import declarations.
import_declarations: true

# Entities of import lists.
import_entities: true

Related Projects

You may like to have a look at the following projects, which aim at formatting Haskell code, too.

* `hindent <https://github.com/chrisdone/hindent>`_
* `stylish-haskell <https://github.com/jaspervdj/stylish-haskell>`_


See the `LICENSE file <LICENSE>`_.
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