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Haskell library for the Microsoft Language Server Protocol

Version on this page:
LTS Haskell 18.28:
Stackage Nightly 2021-06-14:
Latest on Hackage:

See all snapshots haskell-lsp appears in

MIT licensed by Alan Zimmerman
Maintained by [email protected]
This version can be pinned in stack with:haskell-lsp-,5315

CircleCI Hackage


Haskell library for the Microsoft Language Server Protocol

Warning: this library and its associated ecosystem is under development at the moment. So do not have high expectations, it is not ready for casual use.


To see this library in use you need to install the haskell-ide-engine

git clone --recursive
cd haskell-ide-engine
stack install

This will put the hie executable in your path.

Then, run the plugin in vscode:

git clone
cd vscode-hie-server
code .

In vscode, press F5 to run the extension in development mode.

You can see a log from hie by doing

tail -F /tmp/hie-vscode.log

There are also facilities on the code to send back language-server-protocol log and show events.

It can also be used with emacs, see

Using the example server

stack install :lsp-hello --flag haskell-lsp:demo

will generate a lsp-hello executable.

Changing the server to be called in the vscode-hie-server plugin from hie to lsp-hello will run the example server instead of hie.

Likewise, changing the executable in lsp-haskell for emacs.

Useful links

Other resource

See #haskell-ide-engine on IRC freenode


Revision history for haskell-lsp

  • Add and ‘isPreferred’ and ‘disabled’ fields to CodeAction

  • Add runWith for transporots other than stdio (@paulyoung)
  • Fix race condition in event captures (@bgamari)
  • Tweak the sectionSeparator (@alanz)
  • Add hashWithSaltInstances (@ndmitchell)
  • Fix CompletionItem.tags not being optional (@bubba)
  • Avoid unnecessary normalisation in Binary instance for NormalizedFilePath (@cocreature)
  • Fix ordering of TH splices (@fendor)

  • ResponseMessage results are now an Either type (@greenhat)
  • Support for GHC 8.10.1

  • Stop getCompletionPrefix from crashing if beforePos is empty
  • Add DidChangeWatchedFilesRegistrationOptions
  • Add NormalizedFilePath from ghcide
  • Add diagnostic and completion tags
  • Fix language server example
  • Correctly fix the problem with ‘$/’ notifications
  • Add azure ci

  • Fix Haddock generation syntax error

  • Force utf8 encoding when writing vfs temp files
  • Don’t log errors for ‘$/’ notifications (@jinwoo)
  • Force utf8 encoding when writing vfs temp files (@jneira)
  • Store a hash in a NormalizedUri (@mpickering)
  • Move “Semigroup WorkspaceEdit” instance (@sheaf)
  • Fix vfs line endings (@jneira) – 2019-12-14

  • Fix vfs line endings (@jneira)
  • Fix typo in .cabal (@turion)
  • Normalize file paths before converting to Uri’s (@jneira)
  • Fixes to persistVirtualFile (@mpickering) – 2019-11-17

  • Explain the use of NonEmpty in documentOnTypeFormattingTriggerCharacters (@bubba)
  • Fix response type for CodeLensResolve, add the ContentModified error code (@SquidDev)
  • Virtual file fixes, removing race conditions and other cleanups (@mpickering)
  • Add missing fmClientPrepareRenameRequest to MessageFuncs export (@alanz)
  • Make explicit GHC 8.6.5 stack file (@alanz)
  • Rework Core.Options and infer all server capabilities from handlers (@bubba)
  • Generate lenses for WorkDoneProgress data types (@alanz) – 2019-10-18

  • Update progress reporting to match the LSP 3.15 specification (@cocreature)
  • Fix progress cancellation action being retained (@mpickering)
  • Respect both codeActionProvider and codeActionHandler in server capabilities (@fendor)
  • Ensure ResponseMessage has either a result or an error (@cocreature) – 2019-09-07

  • Add support for CodeActionOptions (@thomasjm)
  • Add support for textDocument/prepareRename request (@thomasjm)
  • Fix diagnostic code parsing (@thomasjm)
  • Fix shutdown response type (@bubba)
  • Relax base constraints for GHC 8.8 (@bubba) – 2019-07-01

  • Fix decoding of ResponseMessage to account for null messages (@cocreature)
  • Normalize URIs to avoid issues with percent encoding (@cocreature)
  • Changed the initial callbacks type to also capture initial config (@lorenzo)
  • Improved documentation (@bubba) – 2019-06-13

  • Add support for custom request and notification methods (@cocreature)
  • Use attoparsec to parse message headers incrementally (@cocreature)
  • Only build lsp-hello when -fdemo flag is set (@bubba) – 2019-05-18

  • Fix relative posix URIs (@DavidM-D)
  • Make sure that markedUpContent always starts on a newline (@alanz) – 2019-05-08

  • Bring over functions from @mpickering’s hie-bios. So LspFuncs now includes
    , persistVirtualFileFunc       :: !(J.Uri -> IO FilePath)
    , reverseFileMapFunc           :: !(IO (FilePath -> FilePath))
  • Fix exception on empty filepaths

  • Migrate some utility functions from haskell-ide-engine, for the benefit of other language servers.

    • rangeLinesFromVfs
    • PosPrefixInfo(..)
    • getCompletionPrefix
  • Remove HoverContentsEmpty. It is unnecessary, and generated illegal JSON on the wire. – 2019-05-05

  • Added A NFData instance for Diagnostics (@DavidM-D/@ndmitchell)
  • Switch to using the rope-utf16-splay library for ropes (@ollef) – 2019-04-28

  • Add support for cancellable requests within withProgress and withIndefiniteProgress
  • Align withProgress and withIndefiniteProgress types to be in IO like the rest of the library. (Look at using monad-control and unliftio if you need to use them with a Monad transformer stack) – 2019-04-22

  • Add withProgress and withIndefiniteProgress functions for sending window/progress notifications.

  • Add MarkupContent to HoverResponse, and (some) json roundtrip tests. – 2019-04-11

  • Add applyTextEdit and editTextEdit helpers
  • Set the typedefinitionProvider capability if it has a handler
  • Add stack files for GHC 8.4.4 and 8.6.4 – 2019-02-28

  • Update Handler to delegate to typeDefinitionHandler instead of definitionHandler. by @fendor – 2018-10-27

  • Support GHC 8.6.1 by loosening constraints. Via @domenkozar – 2018-09-08

  • Update Hover to be a nullable according to spec
  • Move Lenses into a separate module, Language.Haskell.LSP.Types.Lens – 2018-08-14

  • Update CompletionItem
  • Add commitCharacters field
  • Add MarkupContent option for documentation
  • Add preselect field
  • Add CompletionContext
  • Add new server capabilities
  • Add workspace folder support
  • Add document color and color presentation
  • Add folding range support
  • Add goto type support
  • s/TH/Types/g
  • Move all types into haskell-lsp-types
  • Hide Language.Haskell.LSP.TH.DataTypesJSON - Use Language.Haskell.LSP.Types instead
  • Add lenses for Language.Haskell.LSP.Types.Capabilities – 2018-08-06

  • Add new DocumentSymbol type and heirarchal support
  • Rename CommandOrCodeAction to CAResult
  • Add handler for ‘textDocument/implementation’ request from client
  • Bump stack resolvers for lts 11 and lts 12 – 2018-08-03

  • Update Command.arguments to match specification
  • Update ClientCapabilities to v3.10
  • Add MarkupContent
  • Add new CompletionKinds
  • Add new SymbolKinds
  • Add preset version capabilities – 2018-07-10

  • CodeAction support as per v3.8 of the specification, by @Bubba
  • Update VersionedTextDocumentIdentifier to match specification, by @Bubba

  • Handle TextDocumentSync fallbacks with new TDS type. – 2018-99-99

  • GHC 8.4.3 support
  • Apply changes to the VFS in the order received in a message. This fixes vscode undo behaviour. By @Bubba
  • Introduce additional error codes as per the LSP spec. By @Bubba
  • Add preliminary support for recording LSP traffic for later playback in test scenarios. By @Bubba – 2018-05-04

  • Make Diagnostic relatedInformation optional, as per the LSP Spec. By @Bubba. – 2018-05-02

  • Support GHC 8.4.2
  • Split into two packages
  • Language.Haskell.LSP.TH.DataTypesJSON becomes Language.Haskell.LSP.Types
  • Diagnostic now has _relatedInformation. Can default it to mempty. via @AlexeyRaga
  • Correct the name of the DidChangeWatchedFilesParams field, by @robrix
  • Make sure to escape URIs properly for Windows file paths Fixes #75. Also added a couple of pretty dumb tests!, by @johnsonw – 2017-12-27

  • Built with LTS 10.1 (stack)
  • Don’t escape semicolons after drive letters by @nponeccop
  • Add Foldable and Traversable instance to List by @noughtmare – 2017-11-23

  • Major changes as implementation continued. Now seems stable, used in haskell-ide-engine – 2017-07-19

  • First version. Implements version 3 of the Microsoft Language Server Protocol