Floskell is a flexible Haskell source code pretty printer.
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Floskell is a flexible Haskell source code pretty printer.



Floskell started as a fork of version 4 of Chris Done's hindent. The formatting styles present in hindent 4 have been preserved in spirit, but generally will not produce exactly the same output.


$ git clone https://github.com/ennocramer/floskell
$ cd floskell
$ stack install


Floskell can be used to reformat Haskell source files in place

$ floskell path/to/sourcefile.hs

or as a pipeline processor

$ cat path/to/sourcefile.hs | floskell > outfile.hs

One of the predefined formatting styles can be selected with the --style option

$ floskell --style cramer path/to/sourcefile.hs

Or the style can be read from a configuration file

$ floskell --config path/to/config.json path/to/sourcefile.hs

If neither style nor configuration file is given on the command line, Floskell will try to find a configuration file in the current working directory or any of its parent directories, or fall back to the users global configuration file.

Formatting Process

A style in Floskell is a set of formatting possibilities for different language constructs. Floskell formats Haskell code according to a given style by finding the combination of allowed formatting choices that result in the best overall layout.


The overall layout of the generated output is judged by a penalty function. This function takes into account the number of lines generated, whether lines are longer than a defined limit, and the indentation of each line.

In general, Floskell will try to generate

  • the smallest number of lines,

  • the least amount of indentation, and

  • the least amount of overflow.


A number of language constructs can be formatted in different ways. Floskell generally defines two layout choices for these constructs, flex and vertical, and three modes to apply these choices, flex, vertical, and try-oneline.

The layout choice flex generally tries to fit as much on each line as possible, but allows linebreaks in a number of places, while the vertical layout choice forces linebreaks in various places.

The flex and vertical layout modes simply select the respective layout choice, while try-oneline will first try flex, but replace the choice with vertical if the flex layout would more than one line or an overfull line.

An example:

-- flex layout for con-decls
data Enum = One | Two | Three

-- vertical layout for con-decls
data Enum = One
          | Two
          | Three


A number of language constructs can apply indentation to sub-elements. Floskell provides two different indentation choices, aligned and indented, and three modes to apply these choices, align, indent-by n, and align-or-indent-by n.

align will start the sub-element on the same line and raise the indentation to align following lines, while indent-by n will start the sub-element on the following line with the indentation raised by n.

align-or-indent-by n will allow either choice and select the formatting with the least penalty.

An example:

-- align for do
foo = do x <- xs
         y <- ys
         return (x, y)

-- indent-by 4 for do
foo = do
    x <- xs
    y <- ys
    return (x, y)

Tabstop Alignment

Some language constructs allow for tabstop alignment. Alignment is optional and subject to configurable limits, regarding the amount of added whitespace.

An example:

-- let without alignment
let foo = bar
    quux = quuz
in foo quux

-- let with alignment
let foo    = bar
    quuuux = quuz
in foo quuuux


Floskell allows the customization of whitespace around infix operators, as well as inside parentheses and other enclosing punctuation characters.

The presence of whitespace or linebreaks is as before, meaning before the operator/enclosed item, after, meaning after the operator/enclosed item, or both, meaning both before and after the operator/enclosed item.

Whitespace configuration can depend on the context where an operator or enclosing punctuation is used. The context can be one of declaration, type, pattern, expression, or other.

An example:

-- tuple with space after/before parentheses and after comma
tuple = ( 1, 2 )
-- tuple without any spaces
tuple = (1,2)


Floskell's behaviour and the style of its output can be modified with a configuration file.

See the documentation on the Configuration Format for a detailed description of the contents of the configuration file.

Initial Configuration

The --print-config command line option can be used to create an initial configuration file.

$ floskell --style cramer --print-config > ~/.floskell

This command will create a configuration file with all fields and the entire definition of the selected style in the formatting block.

Configuration File Location

  • If a style is given on the command line, but no explicit configuration file, the style will be used as-is and not configuration file will be loaded.

  • If both a style and an explicit configuration file are given on the command line, the explicit configuration file will be loaded and the style parameter will replace any style setting in the configuration file.

  • If neither style nor explicit configuration file are given on the command line, Floskell will try to find an applicable configuration file. Floskell will look for

    • a file called floskell.json in the current working directory and all its parent directories,

    • a file called config.json in ~/.floskell, ~/config/floskell, or %APPDATA%/floskell, and lastly

    • a file called .floskell.json in ~ or ~/.config.

    Only the first file found will be loaded.

Editor Integration


In contrib/floskell.el there is floskell-mode, which provides keybindings to reindent parts of the buffer:

  • M-q reformats the current declaration. When inside a comment, it fills the current paragraph instead, like the standard M-q.
  • C-M-\ reformats the current region.

To enable it, add the following to your init file:

(add-to-list 'load-path "/path/to/floskell/contrib")
(require 'floskell)
(add-hook 'haskell-mode-hook #'floskell-mode)

By default, Floskell uses the style called base. If you want to use another, run M-x customize-variable floskell-style or create a Floskell configuration file in your home directory. If you want to configure per-project, add a configuration file in the project root or make a file called .dir-locals.el in the project root directory like this:

((nil . ((floskell-style . "johan-tibell"))))


The 'formatprg' option lets you use an external program (like floskell) to format your text. Put the following line into ~/.vim/ftplugin/haskell.vim to set this option for Haskell files:

setlocal formatprg=floskell\ --style\ chris-done

Then you can format with floskell using gq. Read :help gq and help 'formatprg' for more details.

Note that unlike in emacs you have to take care of selecting a sensible buffer region as input to floskell yourself. If that is too much trouble you can try vim-textobj-haskell which provides a text object for top level bindings.


Basic support is provided through contrib/floskell.coffee, which adds floskell to atom menu with each available style, and Default which will use the appropriate configuration file. Mode should be installed as package into .atom\packages\${PACKAGE_NAME}, here is simple example of atom package.