Haskell-ghc-mod

Haskell ghc-mod opens pipe to ghc-mod and queries types, info, typechecks and lints current file.

When using with stack, see Using with stack

Configuration

Only configuration option you will likely need to set is additionalPathDirectories. It needs to be set to full path to the directory containing ghc-mod executable, if it is not in your PATH.

For example, if you have ghc-mod in /home/user/.cabal/bin/, you need to write /home/user/.cabal/bin in additionalPathDirectories. Note that shell expansions are not suported, i.e. you can’t use ~ or $HOME.

You really shouldn’t touch ghcModPath setting, unless you know precisely what you’re doing. If you specify full path there, Atom won’t be able to grab ghc-mod from stack or cabal sandbox!

You may also consider adding path to directory containing ghc/ghci executable to additionalPathDirectories. The list is comma-separated. For example, if you have ghc executable installed to /usr/local/bin, then you could extend additionalPathDirectories to be /home/user/.cabal/bin,/usr/local/bin.

Keybindings

Haskell-ghc-mod comes with little pre-specified keybindings, so you will need to specify your own, if you want those.

You can edit Atom keybindings by opening ‘Edit → Open Your Keymap’. Here is a template for all commands, provided by haskell-ghc-mod:

'atom-text-editor[data-grammar~="haskell"]':
  '': 'haskell-ghc-mod:check-file'
  '': 'haskell-ghc-mod:lint-file'
  'ctrl-alt-t': 'haskell-ghc-mod:show-type' #this is an example binding
  'ctrl-alt-i': 'haskell-ghc-mod:show-info' #this is an example binding
  'ctrl-alt-T': 'haskell-ghc-mod:insert-type' #this is an example binding
  '': 'haskell-ghc-mod:case-split'
  '': 'haskell-ghc-mod:sig-fill'
  '': 'haskell-ghc-mod:show-info-fallback-to-type'
  '': 'haskell-ghc-mod:show-type-fallback-to-info'
  '': 'haskell-ghc-mod:show-type-and-info'
  '': 'haskell-ghc-mod:insert-import'
  '': 'haskell-ghc-mod:go-to-declaration'

'atom-workspace':
  '': 'haskell-ghc-mod:shutdown-backend'

Advanced configuration

In some cases, it could be useful to disable ghc-mod completely for a given project (e.g. GHCJS), or suppress error pop-ups (e.g. in case of known ghc-mod bugs where some features don’t work, or don’t always work).

You can create .haskell-ghc-mod.json file in project root (i.e. directory containing a *.cabal file, or – in case of plain projects – Atom’s project root directory).

.haskell-ghc-mod.json from Atom project root will also be honored, if it exists, with lower priority.

You can also create a global config file in ${ATOM_CONFIG_DIR}/haskell-ghc-mod.json. ${ATOM_CONFIG_DIR} is usually ${HOME}/.atom, but you can check it’s path by running atom.getConfigDirPath() in Atom’s developer console (View → Developer → Toggle Developer Tools → Console).

Config file is a JSON file with the following fields:

  • "disable"true/false. Will disable all ghc-mod functions entirely. If omitted, defaults to false.
  • "suppressErrors"true/false. Will suppress error pop-ups. Those still will be displayed in Atom’s console (View → Developer → Toggle Developer Tools), so if someting seems wierd, one could check there.
  • "ghcOptions" – Array of Strings. Options to pass to GHC. Can be useful to explicitly suppress warnings, e.g. -fno-warn-unused-do-bind or anything else.
  • "ghcModOptions" – Array of Strings. Arbitrary options to pass to ghc-mod. Bear in mind that you shouldn’t really change most ghc-mod options, since the package makes some assumptions on that part. Also only global ghc-mod options will work (i.e. no command-specific ones)

Example:

{
  "disable": false,
  "suppressErrors": true,
  "ghcOptions": ["-fno-warn-unused-do-bind", "-fno-warn-name-shadowing"],
  "ghcModOptions": ["--with-ghc", "/path/to/custom/ghc"]
}

Using with stack

Stack support is limited. This section contains some tricks that can help to coerce ghc-mod into cooperation.

DO NOT specify full path to ghc-mod/ghc-modi in haskell-ghc-mod settings. Do not add anything to ‘Additional Path Directories’ as well, unless you need to (i.e. know what you’re doing). Make sure ‘Stack Sandbox’ option is enabled (this is the default)

Remove dist folder in project root

Ghc-mod assumes you want to use cabal if it finds dist/setup-config in project root. In any case, if you want to use stack, you don’t need dist directory anyway (unless you’re using it to store sources, in which case, it’s strongly advised you don’t)

Maintain a separate ghc-mod installation for each stack resolver

Ghc-mod requires that it must be built with the same version of GHC that you use to build your project. In most cases this means that you need a separate ghc-mod installation for every distinct stack resolver you’re using.

Simplest way to achieve this would be to install ghc-mod locally for every stack project you want to use it with, i.e. run stack build ghc-mod in project directory (if you have multiple stack.yaml configs, do this for each one). If this fails for some reason, try to use stack-installed ghc (with stack --no-system-ghc --install-ghc build ghc-mod). Note that some resolvers seemingly can’t build ghc-mod at all. You might also want to install newer ghc-mod version than a given resolver offers. Refer to stack documentation on how exactly you could do that.

If you enable ‘Stack Sandbox’ in haskell-ghc-mod settings (enabled by default), and leave ‘Ghc Mod Path’ as default ghc-mod (i.e. no actual path), Atom should automatically pick up local stack installation.

With lts-4.1 resolver (and probably later), you can also install ghc-mod into ‘global project’ (essentially run stack build ghc-mod outside any project directory). This will automatically provide ghc-mod executables to all projects using this resolver (so you don’t need to install it per-project). Note this does not work with lts-3.22.

Later stack versions (and resolvers) install packages per-resolver globally, regardless of if you run stack build ghc-mod in stack project directory or not. Not sure which version changed that, but stack 1.0.2 with lts-5.6 resolver does that.

Avoid mixing stack- and cabal-installed packages

This should go without saying. It’s always a bad idea to mix packages installed with stack and cabal-install. At least avoid mixing those in the same project, i.e. if you’re using stack, use stack-installed ghc-mod. If you’re using cabal-install, use cabal-installed ghc-mod. Yes, it is not exactly simple.

Run Atom with stack exec atom (deprecated)

This is a last-ditch workaround, is deprecated and can lead to unexpected problems.

Should not be required with haskell-ghc-mod 1.6.0 and up. Please create issue if it doesn’t work

This can lead to ghc-mod complaining about GHC_PACKAGE_PATH, so avoid this workaround if at all possible

Stack manages multiple ghc installations using environment hacking. So, in order to bring this environment into scope when using Atom, the most straightforward way is to run Atom with stack exec atom from project directory. Under Windows it will be something like: stack exec "%USERPROFILE%\AppData\Local\atom\app-1.7.3\atom.exe"