Announcing Floskell

by Enno Cramer • Programming • Haskell • Code Formatting

Table of Contents
  1. History
  2. Comparison to Other Tools
  3. Core Concepts
    1. Penalty
    2. Layout
    3. Indentation
    4. Tabstop Alignment
    5. Whitespace
  4. Benchmark
    1. src/Floskell/Pretty.hs
    2. src/Floskell/Comments.hs
  5. Missing Features and Known Issues

Floskell is a new Haskell source code formatter.

Floskell tries to be flexible, meaning that the user should be able to tune the output to match their personal style preference. To achieve this goal, Floskell defines a number of orthogonal formatting options and applies these consistently to all Haskell language constructs.

Floskell is hosted on GitHub. To get a quick impression of how Floskell output looks, check out the style demo / test harness. Or check out the configuration file used by Floskell itself.


Floskell started as a fork of version 4 of HIndent, back in mid-2016, simply to keep my personal formatting style alive. As I tried to push that style to better handle various edge cases, it became clear that the existing pretty printing infrastructure was not sufficiently flexible to support my goals.

The formatting styles of HIndent have been preserved in spirit, but generally will not produce exactly the same output. The last version to have the original formatting styles (modulo bug fixes) available is in the classic branch.

Comparison to Other Tools

Core Concepts

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


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 can be quite slow. I have made a good effort to keep runtime acceptable, but large and complex expressions may take a while to format.

The formatting algorithm is not always linear in the size of the input, though I have made an effort to avoid most cases of super-linear runtime. If you might run into cases where Floskell requires an extraordinary amount of time, please open an issue.

To give an impression of the expected runtime, I have benchmarked Floskell against both HIndent (v5.2.6) and brittany (, on the worst-performing source files (for Floskell) in this project. The benchmarks have been performed on a 2012 MacBook Air (2 GHz Intel Core i7) using hyperfine.


A very large (2087 lines), but reasonably simple file.

Benchmark #1: floskell <src/Floskell/Pretty.hs >/dev/null
  Time (mean ± σ):      1.094 s ±  0.016 s    [User: 1.035 s, System: 0.043 s]
  Range (min … max):    1.076 s …  1.120 s    10 runs

Benchmark #2: hindent <src/Floskell/Pretty.hs >/dev/null
  Time (mean ± σ):     10.293 s ±  0.055 s    [User: 10.105 s, System: 0.124 s]
  Range (min … max):   10.191 s … 10.352 s    10 runs

Benchmark #3: brittany <src/Floskell/Pretty.hs >/dev/null
  Time (mean ± σ):      2.076 s ±  0.058 s    [User: 1.964 s, System: 0.091 s]
  Range (min … max):    2.018 s …  2.214 s    10 runs

  'floskell <src/Floskell/Pretty.hs >/dev/null' ran
    1.90 ± 0.06 times faster than 'brittany <src/Floskell/Pretty.hs >/dev/null'
    9.41 ± 0.15 times faster than 'hindent <src/Floskell/Pretty.hs >/dev/null'


A very small (121 lines) file with a sufficiently complex expression to cause Floskell to explore many formatting options.

Benchmark #1: floskell <src/Floskell/Comments.hs >/dev/null
  Time (mean ± σ):     386.1 ms ±   7.7 ms    [User: 355.4 ms, System: 18.6 ms]
  Range (min … max):   375.1 ms … 398.6 ms    10 runs

Benchmark #2: hindent <src/Floskell/Comments.hs >/dev/null
  Time (mean ± σ):      85.5 ms ±   6.5 ms    [User: 65.7 ms, System: 10.8 ms]
  Range (min … max):    76.6 ms … 102.7 ms    33 runs

Benchmark #3: brittany <src/Floskell/Comments.hs >/dev/null
  Time (mean ± σ):     212.6 ms ±   6.2 ms    [User: 173.6 ms, System: 27.8 ms]
  Range (min … max):   203.6 ms … 225.2 ms    13 runs

  'hindent <src/Floskell/Comments.hs >/dev/null' ran
    2.49 ± 0.20 times faster than 'brittany <src/Floskell/Comments.hs >/dev/null'
    4.52 ± 0.35 times faster than 'floskell <src/Floskell/Comments.hs >/dev/null'

Missing Features and Known Issues

Floskell has a few known issues. I am open to suggestions and/or contributions to improve the situation.