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
$ 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.
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,
vertical, and three modes to apply these choices,
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.
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.
-- 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,
indented, and three modes to apply these choices,
indent-by n, and
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
align-or-indent-by n will allow either choice and select the
formatting with the least penalty.
-- 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)
Some language constructs allow for tabstop alignment. Alignment is optional and subject to configurable limits, regarding the amount of added whitespace.
-- 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 the operator/enclosed item,
after, meaning after the
operator/enclosed item, or
both, meaning both before and after the
Whitespace configuration can depend on the context where an operator
or enclosing punctuation is used. The context can be one of
-- tuple with space after/before parentheses and after comma tuple = ( 1, 2 ) -- tuple without any spaces tuple = (1,2)
Preprocessor Directives (CPP)
Floskell, in general, supports Haskell source with conditional
compilation directives using the
CPP language extensions. However,
due to the way this support is implemented, some care must be taken to
not confuse the Haskell source parser.
Floskell treats conditional compilation directives as if they were
simply comments. As a consequence, the input must still be valid
Haskell when all preprocessor lines are removed. This is relevant
#endif sequences, as Floskell will see both
the if- and else-block in sequence. For example, the following cannot
be processed with Floskell, as the first declaration of
ends with an incomplete
#if MIN_VERSION_haskell_src_exts(1,21,0) prettyPrint (GadtDecl _ name _ _ mfielddecls ty) = do #else prettyPrint (GadtDecl _ name mfielddecls ty) = do #endif pretty name operator Declaration "::" mayM_ mfielddecls $ \decls -> do prettyRecordFields len Declaration decls operator Type "->" pretty ty
Instead, some of the contents of the
do block have to be duplicated,
so that the contents of the
#if are valid Haskell on their own.
#if MIN_VERSION_haskell_src_exts(1,21,0) prettyPrint (GadtDecl _ name _ _ mfielddecls ty) = do pretty name operator Declaration "::" mayM_ mfielddecls $ \decls -> do prettyRecordFields len Declaration decls operator Type "->" pretty ty #else prettyPrint (GadtDecl _ name mfielddecls ty) = do pretty name operator Declaration "::" mayM_ mfielddecls $ \decls -> do prettyRecordFields len Declaration decls operator Type "->" pretty ty #endif
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.
--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
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.jsonin the current working directory and all its parent directories,
a file called
%APPDATA%/floskell, and lastly
a file called
Only the first file found will be loaded.
floskell-mode, which provides keybindings to reindent parts
of the buffer:
M-qreformats the current declaration. When inside a comment, it fills the current paragraph instead, like the standard
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
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
((nil . ((floskell-style . "johan-tibell"))))
'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
: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
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
here is simple example of atom