hindent

Extensible Haskell pretty printer. Both a library and an executable. Currently a work in progress (see FIXME items).

Documentation

Install

$ cabal install hindent

You might need to cabal install happy if haskell-src-exts complains.

Usage

$ hindent
hindent: arguments: --style [fundamental|chris-done|johan-tibell|gibiansky]

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-hook 'haskell-mode-hook #'hindent-mode)

By default it uses the style called fundamental, if you want to use another, johan-tibell, run M-x customize-variable hindent-style. If you want to configure per-project, make a file called .dir-locals.el in the project root directory like this:

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

Vim

Basic support is provided through vim/hindent.vim, which sets hindent as the formatter used by gq for haskell files. The formatting style defaults to fundamental but can be configured by setting g:hindent_style to the desired style.

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.

Contributing your own printer style

This package comes with a basic fundamental pretty printer, which is probably not desirable to use.

It comes with other styles implemented on top of this fundamental printer, in the modules in HIndent.Styles.*.

Make a module HIndent.Styles.YourName in which to place the printer.

To define your own, see HIndent.Styles.Fundamental for a starting point. This module defines a blank style, adds no additional extensions. Customizations are specified via the styleExtenders property. See HIndent.Styles.ChrisDone for an example of a non-trivial style.

Useful combinators can be found in HIndent.Pretty for defining printers. When you want to use a fundamental printer, use prettyNoExt instead of pretty. Comments will still be inserted by prettyNoExt.

If you want to contribute it to the package, add it to the list of styles in HIndent and export it, and open a pull request. Use the Erlang git commit guide for your commits.

Remaining issues

• Add test suite.

• Flesh out more obscure parts of the AST.

• Improve comment re-insertion.

• Possibly: Support formatting whole modules.

• Implement some operator-specific layouts: e.g.

    Foo <$> foo
        <*> bar
        <*> mu
  

Example

Input code:

foo = do print "OK, go"; foo (foo bar) -- Yep.
          (if bar then bob else pif) (case mu {- cool -} zot of
            Just x -> return (); Nothing -> do putStrLn "yay"; return 1) bill -- Etc
  where potato Cakes {} = 2 * x foo * bar / 5

Fundamental

This is an intentionally very dumb style that demands extension.

foo =
  do print
       "OK, go"
     foo
       (foo
          bar)
       (if bar
           then bob
           else pif)
       (case mu {- cool -}
               zot of
          Just x ->
            return
              ()
          Nothing ->
            do putStrLn
                 "yay"
               return
                 1)
       bill -- Etc
  where potato Cakes{} =
          2 * x
                foo * bar / 5

Johan Tibell

Documented in the style guide. This printer style uses some simple heuristics in deciding when to go to a new line or not, and custom handling of do, if, case alts, rhs, etc.

foo = do
    print "OK, go"
    foo
        (foo bar)
        (if bar
             then bob
             else pif)
        (case mu {- cool -} zot of
             Just x ->
                 return ()
             Nothing -> do
                 putStrLn "yay"
                 return 1)
        bill -- Etc
  where
    potato Cakes{} =
        2 * x foo * bar / 5

Chris Done

My style is documented in the style guide. This printer style uses some simple heuristics in deciding when to go to a new line or not.

foo =
  do print "OK, go"
     foo (foo bar)
         (if bar
             then bob
             else pif)
         (case mu {- cool -} zot of
            Just x -> return ()
            Nothing ->
              do putStrLn "yay"
                 return 1)
         bill -- Etc
  where potato Cakes{} = 2 * x foo * bar / 5

Andrew Gibiansky

foo = do
  print "OK, go"
  foo (foo bar) -- Yep.
   (if bar
       then bob
       else pif) (case mu {- cool -} zot of
                    Just x -> return ()
                    Nothing -> do
                      putStrLn "yay"
                      return 1) bill -- Etc

  where
    potato Cakes{} = 2 * x foo * bar / 5