Let’s write a Haskell Language Server plugin

Haskell Language Server is an LSP server for the Haskell programming language. It builds on several previous efforts to create a Haskell IDE, you can find many more details on the history and architecture in the IDE 2020 community page.

In this article we are going to cover the creation of an HLS plugin from scratch: a code lens to display explicit import lists. Along the way we will learn about HLS, its plugin model, and the relationship with ghcide and LSP.


Writing plugins for HLS is a joy. Personally, I enjoy the ability to tap into the gigantic bag of goodies that is GHC, as well as the IDE integration thanks to LSP.

In the last couple of months I have written various HLS (and ghcide) plugins for things like:

  1. Suggest imports for variables not in scope,

  2. Remove redundant imports,

  3. Evaluate code in comments (a la doctest),

  4. Integrate the retrie refactoring library.

These plugins are small but meaningful steps towards a more polished IDE experience, and in writing them I didn’t have to worry about performance, UI, distribution, or even think for the most part, since it’s always another tool (usually GHC) doing all the heavy lifting. The plugins also make these tools much more accessible to all the users of HLS.

The task

Here is a visual statement of what we want to accomplish:

Imports code lens

And here is the gist of the algorithm:

  1. Request the type checking artefacts from the ghcide subsystem

  2. Extract the actual import lists from the type checked AST,

  3. Ask GHC to produce the minimal import lists for this AST,

  4. For every import statement without a explicit import list, find out the minimal import list, and produce a code lens to display it together with a command to graft it on.


To get started, let’s fetch the HLS repo and build it. You need at least GHC 9.0 for this:

git clone --recursive http://github.com/haskell/haskell-language-server hls
cd hls
cabal update
cabal build

If you run into any issues trying to build the binaries, the #haskell-language-server IRC chat room in Libera Chat is always a good place to ask for help.

Once cabal is done take a note of the location of the haskell-language-server binary and point your LSP client to it. In VSCode this is done by editing the “Haskell Server Executable Path” setting. This way you can simply test your changes by reloading your editor after rebuilding the binary.


Anatomy of a plugin

HLS plugins are values of the Plugin datatype, which is defined in Ide.Plugin as:

data PluginDescriptor =
  PluginDescriptor { pluginId                 :: !PluginId
                   , pluginRules              :: !(Rules ())
                   , pluginCommands           :: ![PluginCommand]
                   , pluginCodeActionProvider :: !(Maybe CodeActionProvider)
                   , pluginCodeLensProvider   :: !(Maybe CodeLensProvider)
                   , pluginHoverProvider      :: !(Maybe HoverProvider)
                   , pluginSymbolsProvider    :: !(Maybe SymbolsProvider)
                   , pluginFormattingProvider :: !(Maybe (FormattingProvider IO))
                   , pluginCompletionProvider :: !(Maybe CompletionProvider)
                   , pluginRenameProvider     :: !(Maybe RenameProvider)

A plugin has a unique id, a set of rules, a set of command handlers, and a set of “providers”:

  • Rules add new targets to the Shake build graph defined in ghcide. 99% of plugins need not define any new rules.

  • Commands are an LSP abstraction for actions initiated by the user which are handled in the server. These actions can be long running and involve multiple modules. Many plugins define command handlers.

  • Providers are a query-like abstraction where the LSP client asks the server for information. These queries must be fulfilled as quickly as possible.

The HLS codebase includes several plugins under the namespace Ide.Plugin.*, the most relevant are:

  • The ghcide plugin, which embeds ghcide as a plugin (ghcide is also the engine under HLS).

  • The example and example2 plugins, offering a dubious welcome to new contributors

  • The ormolu, fourmolu, floskell and stylish-haskell plugins, a testament to the code formatting wars of our community.

  • The eval plugin, a code lens provider to evaluate code in comments

  • The retrie plugin, a code actions provider to execute retrie commands

I would recommend looking at the existing plugins for inspiration and reference.

Plugins are “linked” in the HlsPlugins module, so we will need to add our plugin there once we have defined it:

idePlugins = pluginDescToIdePlugins allPlugins
    allPlugins =
      [ GhcIde.descriptor "ghcide"
      , Pragmas.descriptor "pragmas"
      , Floskell.descriptor "floskell"
      , Fourmolu.descriptor "fourmolu"
      , Ormolu.descriptor "ormolu"
      , StylishHaskell.descriptor "stylish-haskell"
      , Retrie.descriptor "retrie"
      , Eval.descriptor "eval"

To add a new plugin, simply extend the list of allPlugins and rebuild.


99% of plugins will want to define at least one type of provider. But what is a provider? Let’s take a look at some types:

type CodeActionProvider =  LSP.LspFuncs Config
                        -> IdeState
                        -> PluginId
                        -> TextDocumentIdentifier
                        -> Range
                        -> CodeActionContext
                        -> IO (Either ResponseError (List CAResult))

type CompletionProvider = LSP.LspFuncs Config
                        -> IdeState
                        -> CompletionParams
                        -> IO (Either ResponseError CompletionResponseResult)

type CodeLensProvider = LSP.LspFuncs Config
                      -> IdeState
                      -> PluginId
                      -> CodeLensParams
                      -> IO (Either ResponseError (List CodeLens))

type RenameProvider = LSP.LspFuncs Config
                    -> IdeState
                    -> RenameParams
                    -> IO (Either ResponseError WorkspaceEdit)

Providers are functions that receive some inputs and produce an IO computation that returns either an error or some result.

All providers receive an LSP.LspFuncs value, which is a record of functions to perform LSP actions. Most providers can safely ignore this argument, since the LSP interaction is automatically managed by HLS. Some of its capabilities are:

  • Querying the LSP client capabilities

  • Manual progress reporting and cancellation, for plugins that provide long running commands (like the Retrie plugin),

  • Custom user interactions via message dialogs. For instance, the Retrie plugin uses this to report skipped modules.

The second argument plugins receive is IdeState, which encapsulates all the ghcide state including the build graph. This allows to request ghcide rule results, which leverages Shake to parallelize and reuse previous results as appropriate. Rule types are instances of the RuleResult type family, and most of them are defined in Development.IDE.Core.RuleTypes. Some relevant rule types are:

-- | The parse tree for the file using GetFileContents
type instance RuleResult GetParsedModule = ParsedModule

-- | The type checked version of this file
type instance RuleResult TypeCheck = TcModuleResult

-- | A GHC session that we reuse.
type instance RuleResult GhcSession = HscEnvEq

-- | A GHC session preloaded with all the dependencies
type instance RuleResult GhcSessionDeps = HscEnvEq

-- | A ModSummary that has enough information to be used to get .hi and .hie files.
type instance RuleResult GetModSummary = ModSummary

The use family of combinators allow to request rule results. For example, the following code is used in the Eval plugin to request a GHC session and a module summary (for the imports) in order to set up an interactive evaluation environment

  let nfp = toNormalizedFilePath' fp
  session <- runAction "runEvalCmd.ghcSession" state $ use_ GhcSessionDeps nfp
  ms <- runAction "runEvalCmd.getModSummary" state $ use_ GetModSummary nfp

There are three flavours of use combinators:

  1. use* combinators block and propagate errors,

  2. useWithStale* combinators block and switch to stale data in case of error,

  3. useWithStaleFast* combinators return immediately with stale data if any, or block otherwise.

LSP abstractions

If you have used VSCode or any other LSP editor you are probably already familiar with the capabilities afforded by LSP. If not, check the specification for the full details. Another good source of information is the haskell-lsp-types package, which contains a Haskell encoding of the protocol.

The haskell-lsp-types package encodes code lenses in Haskell as:

data CodeLens =
    { _range   :: Range
    , _command :: Maybe Command
    , _xdata   :: Maybe A.Value
    } deriving (Read,Show,Eq)

That is, a code lens is a triple of a source range, maybe a command, and optionally some extra data. The specification clarifies the optionality:

 * A code lens represents a command that should be shown along with
 * source text, like the number of references, a way to run tests, etc.
 * A code lens is _unresolved_ when no command is associated to it. For performance
 * reasons the creation of a code lens and resolving should be done in two stages.

To keep things simple our plugin won’t make use of the unresolved facility, embedding the command directly in the code lens.

The explicit imports plugin

To provide code lenses, our plugin must define a code lens provider as well as a Command handler. The code at Ide.Plugin.Example shows how the convenience defaultPluginDescriptor function is used to bootstrap the plugin and how to add the desired providers:

descriptor :: PluginId -> PluginDescriptor
descriptor plId = (defaultPluginDescriptor plId) {
    -- This plugin provides code lenses
    pluginCodeLensProvider = Just provider,
    -- This plugin provides a command handler
    pluginCommands = [ importLensCommand ]

The command handler

Our plugin provider has two components that need to be fleshed out. Let’s start with the command provider, since it’s the simplest of the two.

importLensCommand :: PluginCommand

PluginCommand is a type synonym defined in LSP.Types as:

data PluginCommand = forall a. (FromJSON a) =>
  PluginCommand { commandId   :: CommandId
                , commandDesc :: T.Text
                , commandFunc :: CommandFunction a

The meat is in the commandFunc field, which is of type CommandFunction, another type synonym from LSP.Types:

type CommandFunction a =
  LSP.LspFuncs Config
  -> IdeState
  -> a
  -> IO (Either ResponseError Value, Maybe (ServerMethod, ApplyWorkspaceEditParams))

CommandFunction takes in the familiar LspFuncs and IdeState arguments, together with a JSON encoded argument. I recommend checking the LSP spec in order to understand how commands work, but briefly the LSP server (us) initially sends a command descriptor to the client, in this case as part of a code lens. When the client decides to execute the command on behalf of a user action (in this case a click on the code lens), the client sends this descriptor back to the LSP server which then proceeds to handle and execute the command. The latter part is implemented by the commandFunc field of our PluginCommand value.

For our command, we are going to have a very simple handler that receives a diff (WorkspaceEdit) and returns it to the client. The diff will be generated by our code lens provider and sent as part of the code lens to the LSP client, who will send it back to our command handler when the user activates the code lens:

importCommandId :: CommandId
importCommandId = "ImportLensCommand"

importLensCommand :: PluginCommand
importLensCommand =
    PluginCommand importCommandId "Explicit import command" runImportCommand

-- | The type of the parameters accepted by our command
data ImportCommandParams = ImportCommandParams WorkspaceEdit
  deriving Generic
  deriving anyclass (FromJSON, ToJSON)

-- | The actual command handler
runImportCommand :: CommandFunction ImportCommandParams
runImportCommand _lspFuncs _state (ImportCommandParams edit) = do
    return (Right Null, Just (WorkspaceApplyEdit, ApplyWorkspaceEditParams edit))

The code lens provider

The code lens provider implements all the steps of the algorithm described earlier:

  1. Request the type checking artefacts from the ghcide subsystem

  2. Extract the actual import lists from the type checked AST,

  3. Ask GHC to produce the minimal import lists for this AST,

  4. For every import statement without a explicit import list, find out what’s the minimal import list, and produce a code lens to display it together with a diff to graft the import list in.

The provider takes the usual LspFuncs and IdeState argument, as well as a CodeLensParams value containing the URI for a file, and returns an IO action producing either an error or a list of code lenses for that file.

provider :: CodeLensProvider
provider _lspFuncs          -- LSP functions, not used
         state              -- ghcide state, used to retrieve typechecking artifacts
         pId                -- plugin Id
         CodeLensParams{_textDocument = TextDocumentIdentifier{_uri}}
  -- VSCode uses URIs instead of file paths
  -- haskell-lsp provides conversion functions
  | Just nfp <- uriToNormalizedFilePath $ toNormalizedUri _uri
  = do
    -- Get the typechecking artifacts from the module
    tmr <- runAction "importLens" state $ use TypeCheck nfp
    -- We also need a GHC session with all the dependencies
    hsc <- runAction "importLens" state $ use GhcSessionDeps nfp
    -- Use the GHC api to extract the "minimal" imports
    (imports, mbMinImports) <- extractMinimalImports hsc tmr

    case mbMinImports of
        Just minImports -> do
            let minImportsMap =
                    Map.fromList [ (srcSpanStart l, i) | L l i <- minImports ]
            lenses <- forM imports $
              -- for every import, maybe generate a code lens
              generateLens pId _uri minImportsMap
            return $ Right (List $ catMaybes lenses)
        _ ->
            return $ Right (List [])
  | otherwise
  = return $ Right (List [])

Note how simple it is to retrieve the type checking artifacts for the module as well as a fully setup Ghc session via the Ghcide rules.

The function extractMinimalImports extracts the import statements from the AST and generates the minimal import lists, implementing steps 2 and 3 of the algorithm. The details of the GHC api are not relevant to this tutorial, but the code is terse and easy to read:

  :: Maybe HscEnvEq
  -> Maybe TcModuleResult
  -> IO ([LImportDecl GhcRn], Maybe [LImportDecl GhcRn])
extractMinimalImports (Just hsc)) (Just (tmrModule -> TypecheckedModule{..})) = do
    -- extract the original imports and the typechecking environment
    let (tcEnv,_) = tm_internals_
        Just (_, imports, _, _) = tm_renamed_source
        ParsedModule{ pm_parsed_source = L loc _} = tm_parsed_module
        span = fromMaybe (error "expected real") $ realSpan loc

    -- GHC is secretly full of mutable state
    gblElts <- readIORef (tcg_used_gres tcEnv)

    let usage = findImportUsage imports gblElts
    (_, minimalImports) <-
    -- getMinimalImports computes the minimal explicit import lists
      initTcWithGbl (hscEnv hsc) tcEnv span $ getMinimalImports usage
    return (imports, minimalImports)
extractMinimalImports _ _ = return ([], Nothing)

The function generateLens implements the last piece of the algorithm, step 4, producing a code lens for an import statement that lacks an import list. Note how the code lens includes an ImportCommandParams value that contains a workspace edit that rewrites the import statement, as expected by our command provider.

-- | Given an import declaration, generate a code lens unless it has an explicit import list
generateLens :: PluginId
             -> Uri
             -> Map SrcLoc (ImportDecl GhcRn)
             -> LImportDecl GhcRn
             -> IO (Maybe CodeLens)
generateLens pId uri minImports (L src imp)
  -- Explicit import list case
  | ImportDecl{ideclHiding = Just (False,_)} <- imp
  = return Nothing
  -- No explicit import list
  | RealSrcSpan l <- src
  , Just explicit <- Map.lookup (srcSpanStart src) minImports
  , L _ mn <- ideclName imp
  -- (almost) no one wants to see an explicit import list for Prelude
  , mn /= moduleName pRELUDE
  = do
        -- The title of the command is just the minimal explicit import decl
    let title = T.pack $ prettyPrint explicit
        -- the range of the code lens is the span of the original import decl
        _range :: Range = realSrcSpanToRange l
        -- the code lens has no extra data
        _xdata = Nothing
        -- an edit that replaces the whole declaration with the explicit one
        edit = WorkspaceEdit (Just editsMap) Nothing
        editsMap = HashMap.fromList [(uri, List [importEdit])]
        importEdit = TextEdit _range title
        -- the command argument is simply the edit
        _arguments = Just [toJSON $ ImportCommandParams edit]
    -- create the command
    _command <- Just <$> mkLspCommand pId importCommandId title _arguments
    -- create and return the code lens
    return $ Just CodeLens{..}
  | otherwise
  = return Nothing

Wrapping up

There’s only one haskell code change left to do at this point: “link” the plugin in the HlsPlugins HLS module. However integrating the plugin in haskell-language-server itself will need some changes in config files. The best way is looking for the id (f.e. hls-class-plugin) of an existing plugin:

  • ./cabal*.project and ./stack*.yaml: add the plugin package in the packages field

  • ./haskell-language-server.cabal: add a conditional block with the plugin package dependency

  • ./.github/workflows/test.yml: add a block to run the test suite of the plugin

  • ./.github/workflows/hackage.yml: add the plugin to the component list to release the plugin package to hackage

  • ./*.nix: add the plugin to nix builds

The full code as used in this tutorial, including imports, can be found in this Gist as well as in this branch

I hope this has given you a taste of how easy and joyful it is to write plugins for HLS. If you are looking for ideas for contributing, here are some cool ones found in the HLS issue tracker.