Table of Contents

Haskell layer

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1. Description

This layer adds support for the Haskell language.

1.1. Features:

2. Quick start (and how to use this README)

  • Follow instructions in Install section to correctly install the layer.
  • Ensure that you have auto-completion and syntax-checking layers enabled.
  • Set backend to dante (check Configuration section for details). dante is default already, but if lsp layer is enabled, you will have to set it explicitly.
  • You are ready to go! Open any Haskell project and enjoy syntax-checking, auto-completion and more.

After that, check the rest of the README to:

  • Learn about more powerful (but more complicated to set up) backend: lsp with hie / hls: Check Configuration -> lsp.
  • Learn about all the functionalities and key bindings: Check Key bindings and Configuration -> Optional extras.
  • Learn about the details of how syntax-checking works and how it can be tweaked: Check Syntax checking.
  • Find solutions to common problems in FAQ: Check FAQ.

3. Install

3.1. 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.

3.2. 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)

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

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

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

4. Configuration

4.1. Choosing a backend

Language backend is the core component of a language layer - it has the responsibility of compiling/parsing the actual code and reporting errors, warnings, suggesting fixes, auto-completions and more.

To get the most out of the language backend, you will want to ensure that you have auto-completion and syntax-checking layers enabled.

Then, to choose a haskell backend, set the haskell layer variable haskell-completion-backend:

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

Supported values for haskell-completion-backend are dante and lsp.

If you don't specify any value for haskell-completion-backend, dante will be used as default backend, unless the layer lsp is enabled, in which case lsp is used as default 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 of .dir-locals.el 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.

There are two backends available: dante and lsp.

dante is lightweight, requires no setup and works out of the box in most cases, which is why it is also a default backend.

lsp (hie or hls) is a more ambitious, heavy-weight, cutting-edge backend that is however still somewhat rough on the edges and requires some additional setup.

4.1.1. dante

Dante is a lightweight backend which delegates most of its work directly to GHCi.

It brings features like syntax checking, auto completion, hlint suggestions, automatic error fixing, info at point, definition and use sites.

dante works for cabal, nix, sytx, and stack users and requires no additional setup.

4.1.2. lsp

Language Server Protocol is a standard for implementing language backends.

In Haskell layer, you can use a backend that implements Language Server Protocol for Haskell by specifying lsp as backend and then installing concrete backend implementation, of which there are two available at the moment: hie and hls.

Enabling the lsp backend requires the lsp layer to be enabled, and provides access to all the additional lsp-mode key bindings.

  1. hie

    Haskell Ide Engine (hie) aims to be the universal interface to a growing number of Haskell tools, providing a fully-featured Language Server Protocol server for editors and IDEs that require Haskell-specific functionality.

    This is where most of the Haskell community effort is (was - check hls) being focused regarding building Haskell IDE / language backend.

    hie 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.

    NOTE: hie is being superseded by hls, which is still in early development though.

  2. hls

    Haskell Language Server (hls) is integration point for ghcide and hie. One IDE to rule them all.

    hls is meant to supersede hie and is therefore the cutting-edge and most ambitious implementation of Haskell language backend. However, it is still in early stages of development.

    Check their docs for installation details and how to use it with emacs/spacemacs.

4.2. Optional extras

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

4.2.1. 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!

4.2.2. 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)))

5. 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)

5.1. 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

5.2. 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

5.3. 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

5.4. 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
SPC m s t change the target for the REPL
C-j switch to next history item
C-k switch to previous history item
C-l clear the REPL

5.5. 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

5.6. 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

5.7. 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

Only some of the HLint suggestions can be applied.

6. Syntax checking

There are multiple components that can indicate errors and warnings in the code. Those components are:

  • dante (via flycheck)
  • hlint (via flycheck)
  • lsp (via lsp-ui)

Since some 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 (from ghci 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.

6.1. 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, which will bring flycheck. Please read the layer's documentation on how to interact with flycheck.

Flycheck has different Haskell checkers: haskell-dante, 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.

6.2. 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.

HLint can be configured per project via .hlint.yaml (check Hlint docs for more details).

7. FAQ

7.1. Dante reports missing/hidden imports for test files

The cause might be that Dante is not loading appropriate packages for the test suite target, instead it is loading packages for the library.

Solution is to create .dir-local.el in the directory where the test suite (usually test/ or tests/) is and to put the line ((haskell-mode . ((dante-target . "--test")))) into it. This tells Dante to use test suite target when working with test files.

7.2. 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

7.3. 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)
  (define-advice haskell-interactive-switch (:after (&rest _) spacemacs/haskell-interactive-switch-advice)
    (when (eq dotspacemacs-editing-style 'vim)
      (call-interactively 'evil-insert))))

7.4. 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))
               (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)

7.5. 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.

7.6. HLint fails with parse error

If HLint is not correctly configured (e.g. does not load some extensions that you are using in your project) it might fail while parsing the file.

Check HLint docs for more details.

7.7. 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 haskell-mode. Check the Flycheck doesn't work section.

7.8. 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.

7.9. 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

7.9.1. 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.

7.9.2. 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:

cd `stack path --project-root`
stack $*

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

7.10. 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.

Author: root

Created: 2024-06-14 Fri 18:50