Before any of this Emacs jazz, we have to get Haskell, of course. The easiest way to do is to download the Haskell Platform, a "Batteries Included" package of the Glasgow Haskell Compiler.

The Haskell Mode for Emacs page at the Haskell wiki is a nice place to start your explorations, and describes how to install haskell-mode. I found it easier to use ELPA, the Emacs Lisp Package Archive (install instructions here). If you're using the Emacs starter kit, you've already got ELPA.

Once elpa's all set, install haskell-mode with the following:

1. run M-x package-list-packages in Emacs
2. tap i by haskell-mode
3. hit x to the start the install.

Before we get into linting or any other customizations, Add the following to your emacs config (~/.emacs, or ~/.emacs.d/init.el, if you're using the Starter kit):

(add-hook 'haskell-mode-hook 'turn-on-haskell-doc-mode)

;; hslint on the command line only likes this indentation mode;
;; alternatives commented out below.
(add-hook 'haskell-mode-hook 'turn-on-haskell-indentation)
;;(add-hook 'haskell-mode-hook 'turn-on-haskell-indent)
;;(add-hook 'haskell-mode-hook 'turn-on-haskell-simple-indent)

;; Ignore compiled Haskell files in filename completions
(add-to-list 'completion-ignored-extensions ".hi")

At this point, you should be able to start using Haskell in emacs. Let's write our first function.

• Create a file called test.hs.
• type C-c C-z to get to the Haskell REPL, supplied by inf-haskell mode
• Now, add the following to test.hs:

module Examples where

square :: Integral a => a -> a
square x = x * x

Then type C-c C-l in that buffer to load the file's contents into the REPL. C-c C-z over to the repl again and try it out:

*Main> square 10
100
*Main> square 34
1156

Nice.

Next, we're going to add support for Flymake, the emacs syntax-checker. This is a piece of Emacs functionality that'll run our Haskell files through the compiler every few seconds and associate any warnings and errors with some specific line in our Haskell file.

While we're at it, we'll configure hs-lint, which we can run periodically on our Haskell files for a list of hints as to how to structure code better. For example, running hs-lint on a Haskell file containing times2 x = (*) 2 x yields a buffer with:

hlint /Users/sritchie/test.hs
/Users/sritchie/test.hs:1:1: Error: Eta reduce
Found:
  times2 x = (*) 2 x
Why not:
  times2 = (*) 2

 1 suggestion

Definitely helpful in the learning process.

Flymake depends on an external perl script, hslint, to pass the contents of the current buffer into the Haskell compiler.

• Download hslint from this gist and place it somewhere on your path.
• run chmod a+x hslint to give the file executable privileges
• Add the following to .emacs:

(defun flymake-haskell-init ()
  "When flymake triggers, generates a tempfile containing the
  contents of the current buffer, runs `hslint` on it, and
  deletes file. Put this file path (and run `chmod a+x hslint`)
  to enable hslint: https://gist.github.com/1241073"
  (let* ((temp-file   (flymake-init-create-temp-buffer-copy
                       'flymake-create-temp-inplace))
         (local-file  (file-relative-name
                       temp-file
                       (file-name-directory buffer-file-name))))
    (list "hslint" (list local-file))))

(defun flymake-haskell-enable ()
  "Enables flymake-mode for haskell, and sets <C-c d> as command
  to show current error."
  (when (and buffer-file-name
             (file-writable-p
              (file-name-directory buffer-file-name))
             (file-writable-p buffer-file-name))
    (local-set-key (kbd "C-c d") 'flymake-display-err-menu-for-current-line)
    (flymake-mode t)))

;; Forces flymake to underline bad lines, instead of fully
;; highlighting them; remove this if you prefer full highlighting.
(custom-set-faces
 '(flymake-errline ((((class color)) (:underline "red"))))
 '(flymake-warnline ((((class color)) (:underline "yellow")))))

I hope this helped those of you looking to get started exploring Haskell! Please let me know in the comments if anything could be clearer; I'll be posting more down the road, and all requests are welcome.