hindent

Haskell pretty printer

Install

$ stack install hindent

Usage

$ hindent --help
hindent --version --help --style STYLE --line-length <...> --indent-size <...> --no-force-newline [-X<...>]* [<FILENAME>]
Version 5.1.1
Default --indent-size is 2. Specify --indent-size 4 if you prefer that.
-X to pass extensions e.g. -XMagicHash etc.
The --style option is now ignored, but preserved for backwards-compatibility.
Johan Tibell is the default and only style.

hindent is used in a pipeline style

$ cat path/to/sourcefile.hs | hindent

The default indentation size is 2 spaces. Configure indentation size with --indent-size:

$ echo 'example = case x of Just p -> foo bar' | hindent --indent-size 2; echo
example =
  case x of
    Just p -> foo bar
$ echo 'example = case x of Just p -> foo bar' | hindent --indent-size 4; echo
example =
    case x of
        Just p -> foo bar

Customization

Create a .hindent.yaml file in your project directory or in your ~/ home directory. The following fields are accepted and are the default:

indent-size: 2
line-length: 80
force-trailing-newline: true

By default, HIndent preserves the newline or lack of newline in your input. With force-trailing-newline, it will make sure there is always a trailing newline.

Emacs

In elisp/hindent.el there is hindent-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/hindent/elisp")
(require 'hindent)
(add-hook 'haskell-mode-hook #'hindent-mode)

Vim

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

setlocal formatprg=hindent

Then you can format with hindent 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 hindent yourself. If that is too much trouble you can try vim-textobj-haskell which provides a text object for top level bindings.

In order to format an entire source file execute:

:%!hindent

Alternatively you could use the vim-hindent plugin which runs hindent automatically when a Haskell file is saved.

Atom

Fortunately, you can use https://atom.io/packages/ide-haskell with the path to hindent specified instead of that to stylish-haskell. Works like a charm that way!