#+TITLE: Haskell contribution layer for Spacemacs

[[file:img/haskell.png]]

* Table of Contents                                                   :TOC@4:
 - [[#description][Description]]
     - [[#features][Features:]]
 - [[#install][Install]]
     - [[#layer][Layer]]
     - [[#cabal-packages][Cabal packages]]
     - [[#os-x][OS X]]
     - [[#optional-extras][Optional extras]]
         - [[#ghci-ng-support][GHCi-ng support]]
         - [[#structured-haskell-mode][structured-haskell-mode]]
         - [[#hindent][hindent]]
 - [[#key-bindings][Key bindings]]
     - [[#documentation][Documentation]]
     - [[#debug][Debug]]
     - [[#repl][REPL]]
     - [[#cabal-commands][Cabal commands]]
     - [[#cabal-files][Cabal files]]

* Description

This layer adds support for the [[https://www.haskell.org/][Haskell]] language.

** Features:
- auto-completion with [[https://github.com/iquiw/company-ghc][company-ghc]],
- syntax highlighting for [[https://github.com/bgamari/cmm-mode][C-- source]].

*This layer is in construction, it needs your contributions and bug reports.*

* Install

** Layer

To use this contribution add it to your =~/.spacemacs=

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers '(haskell))
#+END_SRC

** Cabal packages

This layer requires some [[https://www.haskell.org/cabal/][cabal]] packages:
- =ghc-mod=
- =hlint=
- =stylish-haskell=
- =hasktags=

To install them, use the following command: 

#+BEGIN_SRC sh
  cabal install stylish-haskell hlint ghc-mod hasktags
#+END_SRC

Then you have to add this path to your system =$PATH= (preferred):
Note that on *Linux* distributions the installed binaries should be in
=~/.cabal/bin= and on *OS X* the binaries are installed in
=/Users/<username>/Library/Haskell/bin=.

#+BEGIN_SRC sh
  export PATH=~/.cabal/bin/:$PATH
#+END_SRC

_or_ to the Emacs =exec-path= variable in the =dotspacemacs/init= function of
your =.spacemacs= file:

#+BEGIN_SRC emacs-lisp
  (add-to-list 'exec-path "~/.cabal/bin/")
#+END_SRC

*Note:* it is important to add the path in the =dotspacemacs/init= function,
so that the path is added before any layers is loaded.

** OS X

Note that =emacs.app= for OS X does not pick up =$PATH= from =~/.bashrc= or
=~/.zshrc= when launched from outside a terminal.

** Optional extras
The Haskell layer supports some extra features that can be enabled through
layer variables.

*** GHCi-ng support
[[https://github.com/chrisdone/ghci-ng][ghci-ng]] adds some nice features to =haskell-mode=, and is supported in
Spacemacs by a layer variable:

Follow the instructions to install [[https://github.com/chrisdone/ghci-ng][ghci-ng]] (remember to add =:set +c=
in =~/.ghci=, next set the layer variable:

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers
    '((haskell :variables haskell-enable-ghci-ng-support t)))
#+END_SRC

Once ghci-ng is enabled, two of the old keybindings are overriden with improved
versions from ghci-ng, and a new keybinding available: 

| Key Binding | Description                                                               |
|-------------+---------------------------------------------------------------------------|
| ~SPC m h t~ | gets the type of the identifier under the cursor or for the active region |
| ~SPC m g g~ | go to definition                                                          |
| ~SPC m u~   | finds uses of identifier                                                  |

*** structured-haskell-mode
[[https://github.com/chrisdone/structured-haskell-mode][structured-haskell-mode]], or shm, replaces default haskell-mode
auto-indentation and adds some nice functionalities.
To enable shm, run =cabal install structured-haskell-mode= and set the layer
variable:

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers
    '((haskell :variables haskell-enable-shm-support t)))
#+END_SRC

After shm has been enabled, some of the evil normal state bindings are overridden:

| Key Binding | Description         |
|-------------+---------------------|
| ~D~         | =shm/kill-line=     |
| ~R~         | =shm/raise=         |
| ~P~         | =shm/yank=          |
| ~(~         | =shm/forward-node=  |
| ~)~         | =shm/backward-node= |

For a nice visualization of these functions, please refer to the github page
for [structured-haskell-mode][].

*** hindent
[[https://github.com/chrisdone/hindent][hindent]] is an extensible Haskell pretty printer, which let's you
reformat your code. You need to install the executable with =cabal
install hindent=.

To enable it you have to set the variable =haskell-enable-hindent-style=
to a supported style. The available styles are:
- fundamental
- johan-tibell
- chris-done
- andrew-gibiansky

See examples [[https://github.com/chrisdone/hindent#example][here]]

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers
    '((haskell :variables haskell-enable-hindent-style "johan-tibell")))
#+END_SRC

* Key bindings

All Haskell specific bindings are prefixed with the major-mode leader
~SPC m~.

Top-level commands are prefixed by ~SPC m~:

| Key Binding | Description                                   |
|-------------+-----------------------------------------------|
| ~SPC m g g~ | go to definition or tag                       |
| ~SPC m f~   | format buffer using haskell-stylish           |
| ~SPC m F~   | format declaration using hindent (if enabled) |

** Documentation

Documentation commands are prefixed by ~SPC m h~

| Key Binding | Description                                                                |
|-------------+----------------------------------------------------------------------------|
| ~SPC m h d~ | find or generate Haddock documentation for the identifier under the cursor |
| ~SPC m h h~ | do a Hoogle lookup                                                         |
| ~SPC m h i~ | gets information for the identifier under the cursor                       |
| ~SPC m h t~ | gets the type of the identifier under the cursor                           |
| ~SPC m h y~ | do a Hayoo lookup                                                          |

** Debug

Debug commands are prefixed by ~SPC m d~:

| Key Binding  | Description                                |
|--------------+--------------------------------------------|
| ~SPC m d d~ | start debug process, needs to be run first |
| ~SPC m d b~ | insert breakpoint at function              |
| ~SPC m d n~ | next breakpoint                            |
| ~SPC m d N~ | previous breakpoint                        |
| ~SPC m d B~ | delete breakpoint                          |
| ~SPC m d c~ | continue current process                   |
| ~SPC m d a~ | abandon current process                    |
| ~SPC m d r~ | refresh process buffer                     |

** REPL

REPL commands are prefixed by ~SPC m s~:

| Key Binding | Description                                     |
|-------------+-------------------------------------------------|
| ~SPC m s b~ | load or reload the current buffer into the REPL |
| ~SPC m s c~ | clear the REPL                                  |
| ~SPC m s s~ | show the REPL                                   |
| ~SPC m s S~ | show and switch to the REPL                     |

** Cabal commands

Cabal commands are prefixed by ~SPC m c~:

| Key Binding | Description                                                |
|-------------+------------------------------------------------------------|
| ~SPC m c a~ | cabal actions                                              |
| ~SPC m c b~ | build the current cabal project, i.e. invoke =cabal build= |
| ~SPC m c c~ | compile the current project, i.e. invoke =ghc=             |
| ~SPC m c v~ | visit the cabal file                                       |

** Cabal files

This commands are available in a cabal file.

| Key Binding | Description                                 |
|-------------+---------------------------------------------|
| ~SPC m d~   | add a dependency to the project             |
| ~SPC m b~   | go to benchmark section                     |
| ~SPC m e~   | go to executable section                    |
| ~SPC m t~   | go to test-suite section                    |
| ~SPC m m~   | go to exposed modules                       |
| ~SPC m l~   | go to libary section                        |
| ~SPC m n~   | go to next subsection                       |
| ~SPC m p~   | go to previous subsection                   |
| ~SPC m N~   | go to next section                          |
| ~SPC m P~   | go to previous section                      |
| ~SPC m f~   | find or create source-file under the cursor |