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

Haskell layer

• Description
  • Features:
• Install
  • Layer
  • Dependencies
  • Setup PATH
  • Choosing a backend
    • company-ghci
    • intero
    • dante
    • ghc-mod
    • lsp
  • Optional extras
    • structured-haskell-mode
    • hindent
• Key bindings
  • Documentation
  • Debug
  • Debug Buffer
  • REPL
  • Intero REPL
  • Cabal commands
  • Cabal files
  • Refactor
  • Ghc-mod
    • Insert template
  • Intero
• Syntax checking
  • Flycheck
  • HLint
  • ghc-mod
  • Interactive haskell-mode
  • Flymake
  • Troubleshooting
• FAQ
  • The REPL doesn't work
  • The REPL is stuck
  • I am using stack and ghc-mod, but ghc-mod doesn't work
  • ghc-mod doesn't work
  • Indentation doesn't reset when pressing return after an empty line
  • Flycheck displays HLint warnings but not errors
  • I can see highlighted errors but they don't appear in the error list
  • Flycheck doesn't work
  • Flycheck doesn't work with stack
    • The stack build directory is wrong
    • The Project root directory is not set properly
  • haskell-mode commands don't work
  • 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?
  • Why doesn't the HLint Flycheck checker work when the Intero backend is enabled?

Description

This layer adds support for the Haskell language.

Features:

• syntax highlighting for haskell source, cabal files, 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 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)

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

  $ cabal install apply-refact hlint stylish-haskell hasktags hoogle

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:

  (haskell :variables haskell-completion-backend 'ghci)

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:

  ;;; Directory Local Variables
  ;;; For more information see (info "(emacs) Directory Variables")

  ((haskell-mode (haskell-completion-backend . lsp)))

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.

  (setq-default dotspacemacs-configuration-layers
                '(auto-completion
                  (haskell :variables haskell-completion-backend 'intero)))

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

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 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 the official Haskell language server to work properly. hie is like an improved version of ghc-mod it is best installed by building it locally as it requires that the same GHC version has been used to compile your code as has been used for hie.

To install it please refer to the official installation instructions here.

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 structured-haskell-mode, since it doesn't play very well with non-emacs editing styles (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 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

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 here.

  (setq-default dotspacemacs-configuration-layers
    '((haskell :variables haskell-enable-hindent t)))

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
C-j	switch to next history item
C-k	switch to previous history item
C-l	clear the REPL

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 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 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:

  (setq-default dotspacemacs-configuration-layers
    '((haskell :variables haskell-process-type 'stack-ghci)))

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:

  (when (configuration-layer/layer-used-p 'haskell)
    (add-hook 'haskell-interactive-mode-hook
              (lambda ()
                (setq-local evil-move-cursor-back nil))))

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:

  (when (configuration-layer/layer-used-p 'haskell)
      (defadvice haskell-interactive-switch (after spacemacs/haskell-interactive-switch-advice activate)
        (when (eq dotspacemacs-editing-style 'vim)
          (call-interactively 'evil-insert))))

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 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.

  (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)

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 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 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 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:

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

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 Choosing a backend.

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

  (defun dotspacemacs/user-init ()
    (with-eval-after-load 'intero
      (flycheck-add-next-checker 'intero '(warning . haskell-hlint))))