#+TITLE: Haskell layer

[[file:img/haskell.png]]

* Table of Contents                                         :TOC_4_gh:noexport:
 - [[#description][Description]]
   - [[#features][Features:]]
 - [[#install][Install]]
   - [[#layer][Layer]]
   - [[#dependencies][Dependencies]]
   - [[#setup-path][Setup PATH]]
   - [[#ghc-mod-support][ghc-mod support]]
     - [[#stack-users][Stack users]]
   - [[#optional-extras][Optional extras]]
     - [[#ghci-ng-support][GHCi-ng support]]
       - [[#stack-users][Stack users]]
       - [[#ghc-mod-users][ghc-mod users]]
     - [[#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]]
   - [[#ghc-mod][Ghc-mod]]
     - [[#insert-template][insert template]]
 - [[#faq][FAQ]]
   - [[#repl-doesnt-work][REPL doesn't work]]
   - [[#repl-is-stuck][REPL is stuck]]
   - [[#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]]
   - [[#indentation-doesnt-reset-when-pressing-return-after-empty-line][Indentation doesn't reset when pressing return after empty line]]

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

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

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

* Install
** Layer
To use this configuration layer, add it to your =~/.spacemacs=. You will need to
add =haskell= to the existing =dotspacemacs-configuration-layers= list in this
file.

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

To install them, use following command (or the =stack= equivalent):

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

** Setup PATH
First of all make sure that your =$PATH= contains the installation path for
Haskell tools like =ghc=, =ghci= etc. It depends on how you have installed
=ghc=, but you can always check it by running =which ghc= in your terminal.
=Stack= users should add only the installation path of =stack= itself. Usually
it's =~/.local/bin=.

Then make sure that your =$PATH= contains the installation path for =cabal=
packages. If you are using =cabal= it should be =~/.cabal/bin= or
=~/Library/Haskell/bin= (for 'Haskell for Mac' users). If you
are using =stack= then it should be =~/.local/bin=.

For information about setting up =$PATH=, check out the corresponding section in
the FAQ (~SPC h SPC $PATH RET~).

** ghc-mod support
[[http://www.mew.org/~kazu/proj/ghc-mod/][ghc-mod]] enhances =haskell-mode= with for example code completion, templates,
case-splitting and much more. In order to use it you need to install the
executable with =cabal install ghc-mod= (or =stack= equivalent). Note that
=ghc-mod= support is enabled by default, so if you want to disable it, you must
do it explicitly in your =.spacemacs= file.

#+BEGIN_SRC emacs-lisp
(setq-default dotspacemacs-configuration-layers
  '((haskell :variables haskell-enable-ghc-mod-support nil)))
#+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]].

*** Stack users

Time to time =ghc-mod= is not available via latest Stackage LTS version. So if
you have problems with calling =stack install ghc-mod=, try to use =stack
install ghc-mod --resolver lts-3.1= (the last known LTS version that had
=ghc-mod=). But even if it doesn't work, don't panic, it's easy to install it
from sources.

#+BEGIN_SRC sh
# clone cabal-helper
$ git clone https://github.com/DanielG/cabal-helper.git
# cd into cloned repository
$ cd cabal-helper
# init stack, so cabal-helper can be installed using stack
$ stack init
# now install it
$ stack install
# go back where you was
$ cd ..
# and remove cabal-helper repository since you don't need it
$ rm -rf cabal-helper
# now clone ghc-mod
$ git clone https://github.com/kazu-yamamoto/ghc-mod.git
# and cd into it
$ cd ghc-mod
# again, let's init stack
$ stack init
# now install it
$ stack install
# go back
$ cd ..
# and remove ghc-mod since you don't need it
$ rm -rf ghc-mod
#+END_SRC

** 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                                                  |

**** Stack users

=Stack= and =ghci-ng= doesn't play well with each other, so the general advice
is to avoid mixing them. But, if you want mix them anyway, you'll need to set
=haskell-process-type= explicitly:

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

This is needed, because by default =haskell-process-type= is set to =auto= and
if you have =stack.yaml= file in the root of your project, =stack-ghci= will be
used as process type.

**** ghc-mod users

If you want to use ~SPC m h t~ from =ghc-mod= instead of =ghci-ng= - then you
need to add following line in your =dotspacemacs/user-config=:

#+BEGIN_SRC emacs-lisp
(spacemacs/set-leader-keys-for-major-mode 'haskell-mode
        "mht"  'ghc-show-type)
#+END_SRC

This might be useful, because =ghc-mod= doesn't require active REPL in order to
get type of symbol.

*** 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= run =cabal install structured-haskell-mode= (or =stack=
equivalent).

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]].

*Warning* structured-haskell-mode doesn't play very well with =evil=
([[https://github.com/chrisdone/structured-haskell-mode/issues/81][structured-haskell-mode/#81]]). So it's better to be used with =emacs= edit
style.

*** 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 g i~ | cycle the Haskell import lines or return to point (with prefix arg) |
| ~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 f~ | do a helm-hoogle lookup                                                    |
| ~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 without switching to it           |
| ~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 s c~ | clear the REPL                              |
| ~SPC m s s~ | show the REPL without switching to it       |
| ~SPC m s S~ | show and switch to the REPL                 |
| ~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 |

** Ghc-mod
These commands are only available when ghc-mod is enabled.

For more info, see
http://www.mew.org/~kazu/proj/ghc-mod/en/emacs.html

ghc-mod commands are prefixed by ~SPC m m~:

| Key Binding | Description                               |
|-------------+-------------------------------------------|
| ~SPC m m t~ | insert template                           |
| ~SPC m m u~ | insert template with holes                |
| ~SPC m m a~ | select one of possible cases (~ghc-auto~) |
| ~SPC m m f~ | replace a hole (~ghc-refine~)             |
| ~SPC m m e~ | expand template haskell                   |
| ~SPC m m n~ | go to next type hole                      |
| ~SPC m m p~ | go to previous type hole                  |
| ~SPC m m >~ | make indent deeper                        |
| ~SPC m m <~ | make indent shallower                     |

*** insert template
~SPC m m t~ inserts a template. What this means is that
In the beginning of a buffer, "module Foo where" is
inserted. On the function without signature, inferred
type is inserted. On a symbol "foo" without definition,
"foo = undefined" is inserted or a proper module is imported.
~SPC m m u~ inserts a hole in this case. On a variable,
the case is split. When checking with hlint, original code
is replaced with hlint's suggestion if possible.

* 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

** REPL is stuck
Make sure that when you are typing anything in REPL there is a space between
what you type and =λ>=. When there is no space - REPL will behave as it's stuck.
Usually, when you enter normal state, cursor is moved back, so there is no
required space when you switch to insert mode. There is possible workaround -
just add following snippet to your =dotspacemacs/user-config= function:

#+BEGIN_SRC emacs-lisp
(when (configuration-layer/layer-usedp 'haskell)
  (add-hook 'haskell-interactive-mode-hook
            (lambda ()
              (setq-local evil-move-cursor-back nil))))
#+END_SRC

It will make cursor stay at the right place in the REPL buffer when you enter
normal state. Which in most cases helps you to avoid the problem with 'stuck'
REPL.

Also, some users might want to start REPL in insert mode. For this to happen you
could place following snippet in your =dotspacemacs/user-config= function:

#+BEGIN_SRC emacs-lisp
(when (configuration-layer/layer-usedp 'haskell)
    (defadvice haskell-interactive-switch (after spacemacs/haskell-interactive-switch-advice activate)
      (when (eq dotspacemacs-editing-style 'vim)
        (call-interactively 'evil-insert))))
#+END_SRC

** 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.

** Indentation doesn't reset when pressing return after empty line

This is intended behavior in =haskell-indentation-mode=. If you want to reset indentation when pressing return after empty line, add following snippet into your =dotspacemacs/user-config= function.

#+BEGIN_SRC emacs-lisp
(defun haskell-indentation-advice ()
  (when (and (< 1 (line-number-at-pos))
             (save-excursion
               (forward-line -1)
               (string= "" (s-trim (buffer-substring (line-beginning-position) (line-end-position))))))
    (delete-region (line-beginning-position) (point))))

(advice-add 'haskell-indentation-newline-and-indent
            :after 'haskell-indentation-advice)
#+END_SRC
