#+TITLE: Haskell contribution layer for Spacemacs

[[file:img/haskell.png]]

* Table of Contents                                                   :TOC@4:
 - [[#description][Description]]
   - [[#features][Features:]]
 - [[#install][Install]]
   - [[#layer][Layer]]
   - [[#dependencies][Dependencies]]
   - [[#setup-path][Setup PATH]]
   - [[#os-x][OS X]]
   - [[#optional-extras][Optional extras]]
     - [[#ghc-mod-support][ghc-mod support]]
     - [[#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]]
 - [[#faq][FAQ]]
   - [[#repl-doesnt-work][REPL doesn't work.]]
   - [[#i-am-using-stack-and-ghc-mod-but-ghc-mod-doesnt-work][I am using =stack= and =ghc-mod=, but =ghc-mod= doesn't work.]]
   - [[#repl-is-stuck][REPL is stuck.]]

* 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

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

You can install them using =cabal= or =stack=. =Cabal= users should use
following command:

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

=Stack= users should use following command:

#+BEGIN_SRC sh
  $ stack install stylish-haskell hlint hasktags
#+END_SRC

** Setup PATH

Make sure that your =$PATH= contains installation path for =cabal= packages. If
you are using =cabal= it should be =~/.cabal/bin= or
=/Users/<username>/Library/Haskell/bin= (for Haskell for Mac users). If you are
using =stack= then it should be =~/.local/bin=. So if this path is not in
=$PATH=, just add it and Spacemacs will automatically pick up shell =$PATH=.

_Alternatively_, you can add it to the Emacs =exec-path= variable in the
=dotspacemacs/user-config= function of your =.spacemacs= file:

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

Or for =stack= users:

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

** 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.

*** ghc-mod support
[[http://www.mew.org/~kazu/proj/ghc-mod/][ghc-mod]] enhances =haskell-mode= with for example templates, case-splitting and
much more. In order to use it you need to install the executable with

#+BEGIN_SRC sh
  $ cabal install ghc-mod
#+END_SRC

=Stack= users should use following command:

#+BEGIN_SRC sh
  $ stack install ghc-mod --resolver nightly-2015-10-07
#+END_SRC

and set the layer variable:

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

=Stack= users also should make sure that =dist/setup-config= doesn't exist in
the project root. As it will confuse =ghc-mod=. For more troubleshooting,
checkout this [[https://github.com/kazu-yamamoto/ghc-mod/wiki#known-issues-related-to-stack][document]].

*** 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 Install =shm= with cabal run following command:

#+BEGIN_SRC sh
  $ cabal install structured-haskell-mode
#+END_SRC

=Stack= users should use following command:

#+BEGIN_SRC sh
  $ stack install structured-haskell-mode
#+END_SRC

To enable =shm= 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 [[https://github.com/chrisdone/structured-haskell-mode#features][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= or =stack 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
- 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 H~ | do a local 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 |

* FAQ

** REPL doesn't work.

Usually =haskell-mode= is great at figuring out which interactive process to
bring up. But if you are experiencing problems with it you can help
=haskell-mode= by setting =haskell-process-type= as in following code:

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers
    '((haskell :variables haskell-process-type 'stack-ghci)))
#+END_SRC

Available options are:

- ghci
- cabal-repl
- cabal-dev
- cabal-ghci
- stack-ghci

** I am using =stack= and =ghc-mod=, but =ghc-mod= doesn't work.

Make sure that =dist= directory doesn't exist in your project root. So if it
exists, just remove it and try again.

** REPL is stuck.

Make sure that when you are typing anything in REPL there is a space between
what you type and =λ=. This problem usually happens when you switch from normal
mode to edit mode.