spacemacs/layers/+lang/haskell/README.org

#+TITLE: Haskell layer

#+TAGS: general|layer|programming|pure

[[file:img/haskell.png]]

* Table of Contents                     :TOC_5_gh:noexport:
- [[#description][Description]]
  - [[#features][Features:]]
- [[#install][Install]]
  - [[#layer][Layer]]
  - [[#dependencies][Dependencies]]
  - [[#setup-path][Setup PATH]]
  - [[#choosing-a-backend][Choosing a backend]]
    - [[#company-ghci][=company-ghci=]]
    - [[#intero][=intero=]]
    - [[#dante][=dante=]]
    - [[#ghc-mod][=ghc-mod=]]
    - [[#lsp][=lsp=]]
  - [[#optional-extras][Optional extras]]
    - [[#structured-haskell-mode][structured-haskell-mode]]
    - [[#hindent][hindent]]
- [[#key-bindings][Key bindings]]
  - [[#documentation][Documentation]]
  - [[#debug][Debug]]
  - [[#debug-buffer][Debug Buffer]]
  - [[#repl][REPL]]
  - [[#intero-repl][Intero REPL]]
  - [[#cabal-commands][Cabal commands]]
  - [[#cabal-files][Cabal files]]
  - [[#refactor][Refactor]]
  - [[#ghc-mod-1][Ghc-mod]]
    - [[#insert-template][Insert template]]
  - [[#intero-1][Intero]]
- [[#syntax-checking][Syntax checking]]
  - [[#flycheck][Flycheck]]
  - [[#hlint][HLint]]
  - [[#ghc-mod-2][ghc-mod]]
  - [[#interactive-haskell-mode][Interactive haskell-mode]]
  - [[#flymake][Flymake]]
  - [[#troubleshooting][Troubleshooting]]
- [[#faq][FAQ]]
  - [[#the-repl-doesnt-work][The REPL doesn't work]]
  - [[#the-repl-is-stuck][The 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]]
  - [[#ghc-mod-doesnt-work][=ghc-mod= doesn't work]]
  - [[#indentation-doesnt-reset-when-pressing-return-after-an-empty-line][Indentation doesn't reset when pressing return after an empty line]]
  - [[#flycheck-displays-hlint-warnings-but-not-errors][Flycheck displays HLint warnings but not errors]]
  - [[#i-can-see-highlighted-errors-but-they-dont-appear-in-the-error-list][I can see highlighted errors but they don't appear in the error list]]
  - [[#flycheck-doesnt-work][Flycheck doesn't work]]
  - [[#flycheck-doesnt-work-with-stack][Flycheck doesn't work with =stack=]]
    - [[#the-stack-build-directory-is-wrong][The =stack= build directory is wrong]]
    - [[#the-project-root-directory-is-not-set-properly][The Project root directory is not set properly]]
  - [[#haskell-mode-commands-dont-work][haskell-mode commands don't work]]
  - [[#ghc-mod-and-haskell-mode-commands-overlap-how-do-i-know-which-command-belongs-to-what][=ghc-mod= and =haskell-mode= commands overlap. How do I know which command belongs to what?]]
  - [[#some-commands-start-with-ghc--and-some-with-haskell--what-does-that-mean][Some commands start with =ghc-= and some with =haskell-=. What does that mean?]]
  - [[#why-doesnt-the-hlint-flycheck-checker-work-when-the-intero-backend-is-enabled][Why doesn't the HLint Flycheck checker work when the Intero backend is enabled?]]

* 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 one of the selected backends (=intero=, =dante=, =ghci= or
  =ghc-mod=).

*This layer is under 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:
- =apply-refact= (required by =hlint-refactor=)
- =hlint= (required by =hlint-refactor=)
- =stylish-haskell= (optional for =haskell-mode=)
- =hasktags= (optional)
- =hoogle= (optional for =haskell-mode= and =helm-hoogle=)
- =ghc-mod= (optional for completion)
- =intero= (optional for completion)
- =hie= (optional for completion)

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

#+BEGIN_SRC sh
  $ cabal install apply-refact hlint stylish-haskell hasktags 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 only add the installation path of =stack= itself. Usually
it's =~/.local/bin=.

Then make sure that your =$PATH= contains the installation path for the =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~).

** Choosing a backend
To choose a default backend set the layer variable =haskell-completion-backend=:

#+BEGIN_SRC elisp
  (haskell :variables haskell-completion-backend 'ghci)
#+END_SRC

 Note that in order to enable completion, you'll have to enable the
=auto-completion= layer as well.

Alternatively the =lsp= backend will be automatically chosen if the layer =lsp=
is used and you did not specify any value for =haskell-completion-backend=.

Backend can be chosen on a per project basis using directory local variables
(files named =.dir-locals.el= at the root of a project), an example to use the
=lsp= backend:

#+BEGIN_SRC elisp
  ;;; Directory Local Variables
  ;;; For more information see (info "(emacs) Directory Variables")

  ((haskell-mode (haskell-completion-backend . lsp)))
#+END_SRC

*Note:* you can easily add a directory local variable with ~SPC f v d~.

Supported values for =haskell-completion-backend= arer =intero=, =dante=,
=ghci=, =ghc-mod=, and =lsp=. =ghci= (=company-ghci=) is used by default unless
the layer =lsp= is used. =ghci= is the default backend because it doesn't
require any dependencies, and it works with both =stack= and pure =cabal=
projects.

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers
                '(auto-completion
                  (haskell :variables haskell-completion-backend 'intero)))
#+END_SRC

*** =company-ghci=
[[https://github.com/juiko/company-ghci][company-ghci]] communicates directly with =ghci=, in order to provide completion.
To use it, you have to call =haskell-process-load-or-reload= (=SPC s b=).

*** =intero=
=Intero= only works for =stack= users. You can install the =intero= executable
manually, by calling =stack install intero=, but this step is optional as
=Intero= installs itself.

*** =dante=
=dante= works for =cabal=, =nix=, =sytx=, and =stack= users.
=dante= requires Emacs 25.

*** =ghc-mod=
[[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 the =stack= equivalent).

=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/DanielG/ghc-mod/wiki#user-content-known-issues-related-to-stack][document]].

Also note that =ghc-mod= only works with the =GHC= version that was used to
build =ghc-mod=. You can check which version was used by calling
=ghc-mod --version=.

*** =lsp=
=lsp= requires an appropriate installation of =hie= to provide the Haskell language server. 
=hie= is built on =ghc-mod=, so many of the same considerations apply.

Enabling the =lsp= backend requires the =lsp= layer to be enabled, and provides access to
all the additional =lsp-mode= key bindings. As such it is more of a full backend than just
a completion backend.

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

*** structured-haskell-mode
Currently there is no support for [[https://github.com/chrisdone/structured-haskell-mode][structured-haskell-mode]], since it doesn't play
very well with non-emacs editing styles ([[https://github.com/chrisdone/structured-haskell-mode/issues/81][structured-haskell-mode/#81]]). Emacs
editing style users can easily enable it by adding =structured-haskell-mode= to
the list of =dotspacemacs-additional-packages= in your =.spacemacs= file. For
more installation instructions, please refer to the official documentation at
the [[https://github.com/chrisdone/structured-haskell-mode#features][structured-haskell-mode]] page. In case you are a non-emacs editing style user
and still want to use =structured-haskell-mode= - use it at your own risk.

Any contributions that will help to solve issues with =structured-haskell-mode=
are warmly welcome!

*** hindent
[[https://github.com/commercialhaskell/hindent][hindent]] is an extensible Haskell pretty printer, which lets you reformat your
code. You need to install the executable with =cabal install hindent= or
=stack install hindent=.

To enable it you have to toggle the variable =haskell-enable-hindent=.

See examples [[https://github.com/commercialhaskell/hindent/blob/master/TESTS.md][here]].

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers
    '((haskell :variables haskell-enable-hindent t)))
#+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 a~ | abandon current process                    |
| ~SPC m d b~ | insert breakpoint at function              |
| ~SPC m d B~ | delete breakpoint                          |
| ~SPC m d c~ | continue current process                   |
| ~SPC m d d~ | start debug process, needs to be run first |
| ~SPC m d n~ | next breakpoint                            |
| ~SPC m d N~ | previous breakpoint                        |
| ~SPC m d p~ | previous breakpoint                        |
| ~SPC m d r~ | refresh process buffer                     |
| ~SPC m d s~ | step into the next function                |
| ~SPC m d t~ | trace the expression                       |

** Debug Buffer

| Key binding | Description                                 |
|-------------+---------------------------------------------|
| ~RET~       | select object at the point                  |
| ~a~         | abandon current computation                 |
| ~b~         | break on function                           |
| ~c~         | continue the current computation            |
| ~d~         | delete object at the point                  |
| ~i~         | step into the next function                 |
| ~r~         | refresh the debugger buffer                 |
| ~s~         | go to next step to inspect bindings         |
| ~S~         | go to previous step to inspect the bindings |
| ~t~         | trace the expression                        |

** 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 and switch to the REPL                     |
| ~SPC m s S~ | show the REPL without switching to it           |

** Intero REPL
Intero REPL commands are prefixed by ~SPC m i~:

| Key binding | Description                                                   |
|-------------+---------------------------------------------------------------|
| ~SPC m i c~ | change directory in the backend process                       |
| ~SPC m i d~ | reload the module =DevelMain= and then run =DevelMain.update= |
| ~SPC m i k~ | stop the current worker process and kill its associated       |
| ~SPC m i l~ | list hidden process buffers created by =intero=               |
| ~SPC m i r~ | restart the process with the same configuration as before     |
| ~SPC m i t~ | set the targets to use for stack =ghci=                       |

** 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
These 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 library 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 |

** Refactor
Refactor commands are prefixed by ~SPC m r~:

| Key binding | Description                                       |
|-------------+---------------------------------------------------|
| ~SPC m r b~ | apply all HLint suggestions in the current buffer |
| ~SPC m r i~ | reformat imports from anywhere in the buffer      |
| ~SPC m r r~ | apply the HLint suggestion under the cursor       |
| ~SPC m r s~ | list all Intero suggestions                       |

Only some of the HLint suggestions can be applied.

To apply the Intero suggestions, press `C-c C-c` when the window is open.

Both the HLint and Intero suggestions appear in the same window.

** 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 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 a function without signature, the
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,
the original code is replaced with hlint's suggestion if possible.

** Intero
This command is only available when intero is enabled.

This top-level command is prefixed by ~SPC m~:

| Key binding | Description            |
|-------------+------------------------|
| ~SPC m g b~ | return from definition |

* Syntax checking
At the moment there are four components, which can check the syntax and indicate
errors and warnings in the code. Those components are:
- flycheck
- hlint (via flycheck)
- ghc-mod
- haskell-mode interactive

Since all of these components can be active at the same time, it can be tricky to
know which component is displaying which message, especially when they disagree,
or if one isn't working. Only flycheck errors (ghc and hlint) are displayed in
the error list and can be navigated between, using the standard Spacemacs key
bindings (under ~SPC e~) even though errors from other modes might highlight the
actual buffer.

** Flycheck
This is the standard Spacemacs way of syntax checking, and it's also the most
elaborate. You need to install the syntax-checking layer first. Please read the
layer's [[https://github.com/syl20bnr/spacemacs/tree/develop/layers/+checkers/syntax-checking][documentation]] on how to interact with flycheck.

Flycheck has different Haskell checkers: =haskell-ghc=, =haskell-stack-ghc= and
=haskell-hlint=. Normally it can detect the best one to use automatically, but
if it doesn't work, then you can change it with ~SPC e s~.

** HLint
HLint is a linter for Haskell. It doesn't detect errors (as long as it can parse
the file) but bad coding style and code smell. The HLint checker is called
*after* the flycheck GHC checker.

** ghc-mod
Ghc-mod, when enabled, also does syntax checking. It doesn't highlight errors,
but instead displays an exclamation point in the fringe. You can navigate
between errors using =ghc-goto-next-error= (~M-n~) and =ghc-goto-prev-error=
(~M-p~).

** Interactive haskell-mode
Finally, interactive haskell-mode (~SPC m s b~) also displays errors. These
errors can be navigated to, from the interactive buffer (by clicking on the
error) or using =haskell-goto-next-error= (~M-n~) and =haskell-goto-prev-error=
(~M-p~).

** Flymake
An alternative to syntax checking is to build your projects with
=flymake-compile=. It doesn't highlight errors in the buffer, but it's more
reliable. The error navigation is similar to interactive haskell-mode.

** Troubleshooting
Flycheck and ghc-mod can fail silently for miscellaneous reasons. See the [[#faq][FAQ]]
for troubleshooting.

* FAQ
** The 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, then 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-new-repl
- cabal-dev
- cabal-ghci
- stack-ghci

** The REPL is stuck
Make sure that there's a space between the REPL's =λ>= prompt and the cursor.
When there is no space, then the REPL will behave as if it's stuck. Usually,
when you enter normal state, the cursor moves backwards by one character, so there
is no required space when you switch to insert mode. There is a possible
workaround - just add the following snippet to your =dotspacemacs/user-config=
function:

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

It makes the cursor stay in 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 the REPL in insert mode. This is done by
placing the following snippet in your =dotspacemacs/user-config= function:

#+BEGIN_SRC emacs-lisp
  (when (configuration-layer/package-used-p '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 a =dist= directory doesn't exist in your project root. If it
exists, just remove it and try again.

** =ghc-mod= doesn't work
First of all - make sure that the version of =ghc= matches the version of =ghc=
that was used to build =ghc-mod=. To get the latter, call =ghc-mod --version= in
the terminal. If they don't match then you'll have to rebuild =ghc-mod=.

=Stack= provides the ability to use different =ghc= versions across different
projects. If you're using this feature, then you'll have to rebuild =ghc-mod=
quite often. If you only use =ghc-mod= for completion, and don't want to rebuild
=ghc-mod= every time you switch projects, then you'd better disable =ghc-mod=
support, so that =company-ghci= will be used for completion.

The second thing to do if it's still not working - is to call =ghc-mod debug= in
the root of the project that you're currently working on. Make sure that it
doesn't show any errors. If there are errors that you can't solve - then it's
better to report them [[https://github.com/DanielG/ghc-mod][upstream]].

** Indentation doesn't reset when pressing return after an empty line
This is the intended behavior in =haskell-indentation-mode=. If you want to
reset the indentation when pressing return after an empty line, add the
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

** Flycheck displays HLint warnings but not errors
The HLint checker is called *after* the normal flycheck checker, even if the
checker fails. Check the [[#flycheck-doesnt-work][Flycheck doesn't work]] section.

** I can see highlighted errors but they don't appear in the error list
The error list is only set by flycheck. You are probably seeing errors
highlighted by either ghc-mode or haskell-mode. Check the [[#flycheck-doesnt-work][Flycheck doesn't work]]
section.

** Flycheck doesn't work
You can use the =flycheck-compile= command to check what's wrong with flycheck.
This will show you the exact command line that's used, and its output.

If you are using =stack=, check the [[#flycheck-doesnt-work-with-stack][Flycheck doesn't work with =stack=]] section.

** Flycheck doesn't work with =stack=
First check that flycheck uses the correct checker, and all the paths are
properly configured using =flycheck-verify-setup= (~SPC e v~). You can force the
checker with =flycheck-select-checker= (~SPC e s~) to ensure that it uses
=haskell-stack-ghc=. If it still doesn't work, then it could be one of the
following problems:
- The =stack= build directory is wrong
- The project root is not set properly

*** The =stack= build directory is wrong
The path to the build directory, which contains some generated files, is
normally under =.stack-work/install/<os>/Cabal-<version>/build=.

However the version of the cabal library that's used by =stack= to generate the
directory name is not the version of the cabal library that's installed by
=stack= but rather the version of cabal that's associated to the GHC version.
This error can happen after upgrading cabal or cabal-install. To check if this
is the problem, compare the path name of the build path that's used by flycheck
using =flycheck-compile= and compare it to the actual path in the =.stack-work=
directory. If they are different, then you'll need to reinstall ghc using the
command =stack setup --upgrade-cabal=.

*** The Project root directory is not set properly
Flycheck launches the GHC command, not from the project root directory, but from
the directory of the file that's being checked. This is normally not a problem,
as all the paths are set properly, however it could be a problem if some
template Haskell functions use relative paths (e.g. in Yesod scaffolded
projects).

Until it's fixed in flycheck, the workaround is to wrap the =stack= command in
order to run all subcommands from the project's root directory. You can do so
with the following script:

#+BEGIN_SRC bash
  #!/bin/bash
  cd `stack --project-root`
  stack $*
#+END_SRC

Make sure you set =flycheck-haskell-stack-ghc-executable= to this script.

** haskell-mode commands don't work
Some (most) of the haskell-mode commands only work when haskell-mode is in
interactive mode, i.e. has an interactive session associated with it. Load it
using ~SPC m s b~.

** =ghc-mod= and =haskell-mode= commands overlap. How do I know which command belongs to what?
ghc-mod commands are prefixed with =ghc-=, haskell-mode ones are prefixed with
=haskell-=.

** Some commands start with =ghc-= and some with =haskell-=. What does that mean?
Commands starting with =ghc-= are ghc-mod commands. Commands starting with
=haskell-= are haskell-mode commands.

** Why doesn't the HLint Flycheck checker work when the Intero backend is enabled?
By default, only the Intrero checker will be enabled if you manually set the
Intero backend as described in [[#completion-support][completion support]].

To also enable the HLint checker, use =flycheck-add-next-checker= to set the
HLint checker after the Intero checker.

#+BEGIN_SRC emacs-lisp
  (defun dotspacemacs/user-init ()
    (with-eval-after-load 'intero
      (flycheck-add-next-checker 'intero '(warning . haskell-hlint))))
#+END_SRC