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stylish-haskell

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Introduction

A simple Haskell code prettifier. The goal is not to format all of the code in a file, since I find those kind of tools often "get in the way". However, manually cleaning up import statements etc. gets tedious very quickly.

This tool tries to help where necessary without getting in the way.

You can install it using cabal install stylish-haskell.

Features

  • Aligns and sorts import statements
  • Groups and wraps {-# LANGUAGE #-} pragmas, can remove (some) redundant pragmas
  • Removes trailing whitespace
  • Replaces tabs by four spaces (turned off by default)
  • Replaces some ASCII sequences by their Unicode equivalents (turned off by default)

Feature requests are welcome! Use the issue tracker for that.

Example

Turns:

{-# LANGUAGE ViewPatterns, TemplateHaskell #-}
{-# LANGUAGE GeneralizedNewtypeDeriving,
            ViewPatterns,
    ScopedTypeVariables #-}

module Bad where

import Control.Applicative ((<$>))
import System.Directory (doesFileExist)

import qualified Data.Map as M
import      Data.Map    ((!), keys, Map)   

data Point = Point
    { pointX, pointY :: Double
    , pointName :: String
    } deriving (Show)

into:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE ScopedTypeVariables        #-}
{-# LANGUAGE TemplateHaskell            #-}

module Bad where

import           Control.Applicative ((<$>))
import           System.Directory    (doesFileExist)

import           Data.Map            (Map, keys, (!))
import qualified Data.Map            as M

data Point = Point
    { pointX, pointY :: Double
    , pointName      :: String
    } deriving (Show)

Configuration

The tool is customizable to some extent. It tries to find a config file in the following order:

  1. A file passed to the tool using the -c/--config argument
  2. .stylish-haskell.yaml in the current directory (useful for per-directory settings)
  3. .stylish-haskell.yaml in the nearest ancestor directory (useful for per-project settings)
  4. .stylish-haskell.yaml in your home directory (useful for user-wide settings)
  5. The default settings.

Use stylish-haskell --defaults > .stylish-haskell.yaml to dump a well-documented default configuration to a file, this way you can get started quickly.

VIM integration

Since it works as a filter it is pretty easy to integrate this with VIM. Just call

:%!stylish-haskell

or add a keybinding for it.

There is also the vim-stylish-haskell plugin, which runs stylish-haskell automatically when you save a Haskell file.

Emacs integration

haskell-mode for Emacs supports stylish-haskell. For configuration, see Emacs/Formatting on the HaskellWiki.

Credits

Written and maintained by Jasper Van der Jeugt.

Contributors:

  • Chris Done
  • Hiromi Ishii
  • Leonid Onokhov
  • Michael Snoyman
  • Mikhail Glushenkov

Changelog

  • 0.5.9.0
    • Add compact_line setting for Language Pragma styling
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