A plugin for GADT syntax converter (#2899)

* initial hls-gadt-plugin

* Correct condition

* Render context correctly

* Fix typo

* Support from ghc-8.6 to ghc-9.0

* ghc8.6 compat

* Try to fix test for ghc-8.6

* Pretty name

* Add GADTs pragma automatically

* Enrich Readme

* Clean up

* Update hls docs

* Update CODEOWNERS

* Fix typo

* Rename withTempDir with withCanonicalTempDir

* Add @michaelpj's suggestions

* Pass PluginId through descriptor

* Explicit forall

.github/workflows/test.yml

@@ -246,6 +246,10 @@ jobs:
         name: Test hls-change-type-signature test suite
         run: cabal test hls-change-type-signature-plugin --test-options="$TEST_OPTS" || cabal test hls-change-type-signature-plugin --test-options="$TEST_OPTS" || LSP_TEST_LOG_COLOR=0 LSP_TEST_LOG_MESSAGES=true LSP_TEST_LOG_STDERR=true cabal test hls-change-type-signature-plugin --test-options="$TEST_OPTS"
 
+      - if: matrix.test
+        name: Test hls-gadt-plugin test suit
+        run: cabal test hls-gadt-plugin --test-options="$TEST_OPTS" || cabal test hls-gadt-plugin --test-options="$TEST_OPTS" || LSP_TEST_LOG_COLOR=0 LSP_TEST_LOG_MESSAGES=true LSP_TEST_LOG_STDERR=true cabal test hls-gadt-plugin --test-options="$TEST_OPTS"
+
   test_post_job:
     if: always()
     runs-on: ubuntu-latest

CODEOWNERS

@@ -15,6 +15,7 @@
 /plugins/hls-explicit-imports-plugin @pepeiborra
 /plugins/hls-floskell-plugin @Ailrun
 /plugins/hls-fourmolu-plugin @georgefst
+/plugins/hls-gadt-plugin @July541
 /plugins/hls-haddock-comments-plugin @berberman
 /plugins/hls-hlint-plugin @jneira @eddiemundo
 /plugins/hls-module-name-plugin

cabal.project

@@ -28,6 +28,7 @@ packages:
          ./plugins/hls-qualify-imported-names-plugin
          ./plugins/hls-selection-range-plugin
          ./plugins/hls-change-type-signature-plugin
+         ./plugins/hls-gadt-plugin
 
 -- Standard location for temporary packages needed for particular environments
 -- For example it is used in the project gitlab mirror to help in the MAcOS M1 build script

docs/features.md

@@ -256,7 +256,19 @@ Known Limitations:
 
 ![Change Type Signature Demo](../plugins/hls-change-type-signature-plugin/change2.gif)
 
-[Link to Docs](../plugins/hls-change-type-signature/README.md)
+![Link to Docs](../plugins/hls-change-type-signature-plugin/README.md)
+
+### Convert to GADT syntax
+
+Provided by: `hls-gadt-plugin`
+
+Code action kind: `refactor.rewrite`
+
+Convert a datatype to GADT syntax.
+
+![GADT Demo](../plugins/hls-gadt-plugin/gadt.gif)
+
+![Link to Docs](../plugins/hls-gadt-plugin/README.md)
 
 ## Code lenses
 

docs/supported-versions.md

@@ -55,6 +55,7 @@ Sometimes a plugin will be supported in the prebuilt binaries but not in a HLS b
 | `hls-stylish-haskell-plugin`        |                          |
 | `hls-tactics-plugin`                | 9.2                      |
 | `hls-selection-range-plugin`        |                          |
+| `hls-gadt-plugin`                   |                          |
 
 ### Using deprecated GHC versions
 

exe/Plugins.hs

@@ -82,6 +82,10 @@ import           Ide.Plugin.SelectionRange         as SelectionRange
 #if changeTypeSignature
 import           Ide.Plugin.ChangeTypeSignature    as ChangeTypeSignature
 #endif
+
+#if gadt
+import           Ide.Plugin.GADT                   as GADT
+#endif
 -- formatters
 
 #if floskell
@@ -190,6 +194,9 @@ idePlugins recorder includeExamples = pluginDescToIdePlugins allPlugins
 #endif
 #if changeTypeSignature
       ChangeTypeSignature.descriptor "changeTypeSignature" :
+#endif
+#if gadt
+      GADT.descriptor "gadt" :
 #endif
     -- The ghcide descriptors should come last so that the notification handlers
     -- (which restart the Shake build) run after everything else

haskell-language-server.cabal

@@ -186,6 +186,11 @@ flag changeTypeSignature
   default:     True
   manual:      True
 
+flag gadt
+  description: Enable gadt plugin
+  default:     True
+  manual:      True
+
 -- formatters
 
 flag floskell
@@ -308,6 +313,11 @@ common changeTypeSignature
     build-depends: hls-change-type-signature-plugin ^>= 1.0
     cpp-options: -DchangeTypeSignature
 
+common gadt
+  if flag(gadt)
+    build-depends: hls-gadt-plugin ^>= 1.0
+    cpp-options: -Dgadt
+
 -- formatters
 
 common floskell
@@ -359,6 +369,7 @@ executable haskell-language-server
                     , alternateNumberFormat
                     , qualifyImportedNames
                     , selectionRange
+                    , gadt
                     , floskell
                     , fourmolu
                     , ormolu

hls-plugin-api/src/Ide/PluginUtils.hs

@@ -56,7 +56,6 @@ import           Language.LSP.Types              hiding
                                                   SemanticTokensEdit (_start))
 import qualified Language.LSP.Types              as J
 import           Language.LSP.Types.Capabilities
-import           Language.LSP.Types.Lens         (uri)
 
 -- ---------------------------------------------------------------------
 

hls-test-utils/src/Test/Hls/Util.hs

@@ -40,6 +40,7 @@ module Test.Hls.Util
     , waitForDiagnosticsFromSourceWithTimeout
     , withCurrentDirectoryInTmp
     , withCurrentDirectoryInTmp'
+    , withCanonicalTempDir
   )
 where
 
@@ -54,7 +55,7 @@ import           Data.Default
 import           Data.List.Extra                 (find)
 import qualified Data.Set                        as Set
 import qualified Data.Text                       as T
-import           Development.IDE                 (GhcVersion(..), ghcVersion)
+import           Development.IDE                 (GhcVersion (..), ghcVersion)
 import qualified Language.LSP.Test               as Test
 import           Language.LSP.Types              hiding (Reason (..))
 import qualified Language.LSP.Types.Capabilities as C
@@ -62,8 +63,9 @@ import qualified Language.LSP.Types.Lens         as L
 import           System.Directory
 import           System.Environment
 import           System.FilePath
-import           System.IO.Temp
 import           System.Info.Extra               (isMac, isWindows)
+import qualified System.IO.Extra
+import           System.IO.Temp
 import           System.Time.Extra               (Seconds, sleep)
 import           Test.Tasty                      (TestTree)
 import           Test.Tasty.ExpectedFailure      (expectFailBecause,
@@ -253,7 +255,7 @@ onMatch :: [a] -> (a -> Bool) -> String -> IO a
 onMatch as predicate err = maybe (fail err) return (find predicate as)
 
 noMatch :: [a] -> (a -> Bool) -> String -> IO ()
-noMatch [] _ _ = pure ()
+noMatch [] _ _           = pure ()
 noMatch as predicate err = bool (pure ()) (fail err) (any predicate as)
 
 inspectDiagnostic :: [Diagnostic] -> [T.Text] -> IO Diagnostic
@@ -384,3 +386,10 @@ getCompletionByLabel desiredLabel compls =
         Nothing -> liftIO . assertFailure $
             "Completion with label " <> show desiredLabel
             <> " not found in " <> show (fmap (^. L.label) compls)
+
+-- ---------------------------------------------------------------------
+-- Run with a canonicalized temp dir
+withCanonicalTempDir :: (FilePath -> IO a) -> IO a
+withCanonicalTempDir f = System.IO.Extra.withTempDir $ \dir -> do
+  dir' <- canonicalizePath dir
+  f dir'

plugins/hls-call-hierarchy-plugin/test/Main.hs

@@ -19,6 +19,7 @@ import           System.Directory.Extra
 import           System.FilePath
 import qualified System.IO.Extra
 import           Test.Hls
+import           Test.Hls.Util            (withCanonicalTempDir)
 
 plugin :: PluginDescriptor IdeState
 plugin = descriptor "callHierarchy"
@@ -319,7 +320,7 @@ outgoingCallsTests =
   testGroup "Outgoing Calls"
   [ testGroup "single file"
     [
-      testCase "xdata unavailable" $ withTempDir $ \dir ->
+      testCase "xdata unavailable" $ withCanonicalTempDir $ \dir ->
         runSessionWithServer plugin dir $ do
           doc <- createDoc "A.hs" "haskell" $ T.unlines ["a=3", "b=a"]
           waitForKickDone
@@ -423,7 +424,7 @@ deriving instance Ord CallHierarchyIncomingCall
 deriving instance Ord CallHierarchyOutgoingCall
 
 incomingCallTestCase :: T.Text -> Int -> Int -> [(Int, Int)] -> [Range] -> Assertion
-incomingCallTestCase contents queryX queryY positions ranges = withTempDir $ \dir ->
+incomingCallTestCase contents queryX queryY positions ranges = withCanonicalTempDir $ \dir ->
   runSessionWithServer plugin dir $ do
     doc <- createDoc "A.hs" "haskell" contents
     waitForKickDone
@@ -465,7 +466,7 @@ incomingCallMultiFileTestCase filepath queryX queryY mp =
     closeDoc doc
 
 outgoingCallTestCase :: T.Text -> Int -> Int -> [(Int, Int)] -> [Range] -> Assertion
-outgoingCallTestCase contents queryX queryY positions ranges = withTempDir $ \dir ->
+outgoingCallTestCase contents queryX queryY positions ranges = withCanonicalTempDir $ \dir ->
   runSessionWithServer plugin dir $ do
     doc <- createDoc "A.hs" "haskell" contents
     waitForKickDone
@@ -505,7 +506,7 @@ outgoingCallMultiFileTestCase filepath queryX queryY mp =
     closeDoc doc
 
 oneCaseWithCreate :: T.Text -> Int -> Int -> (Uri -> CallHierarchyItem) -> Assertion
-oneCaseWithCreate contents queryX queryY expected = withTempDir $ \dir ->
+oneCaseWithCreate contents queryX queryY expected = withCanonicalTempDir $ \dir ->
   runSessionWithServer plugin dir $ do
     doc <- createDoc "A.hs" "haskell" contents
     waitForKickDone
@@ -544,8 +545,3 @@ mkIncomingCallsParam = CallHierarchyIncomingCallsParams Nothing Nothing
 
 mkOutgoingCallsParam :: CallHierarchyItem -> CallHierarchyOutgoingCallsParams
 mkOutgoingCallsParam = CallHierarchyOutgoingCallsParams Nothing Nothing
-
-withTempDir :: (FilePath -> IO a) -> IO a
-withTempDir f = System.IO.Extra.withTempDir $ \dir -> do
-  dir' <- canonicalizePath dir
-  f dir'

plugins/hls-gadt-plugin/LICENSE

@@ -0,0 +1,201 @@
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plugins/hls-gadt-plugin/README.md

@@ -0,0 +1,22 @@
+# GADT Converter Plugin
+
+The hls-gadt-plugin provides a code action that converts a datatype to GADT syntax.
+
+## Demo
+
+![GADT](gadt.gif)
+
+## Design
+The plugin works in the following steps:
+1. Get data declarations and enabled pragmas from parsed source.
+2. Response a code action with a command to convert to GADT syntax if given position is a H98 data declaration.
+3. Convert every part of H98 declaration to corresponding GADT's.
+4. Print converted declaration. (See `prettyGADTDecl` source code for details)
+5. Send edit request to LSP, the edit includes replacing origin data declaration to GADT and inserting a `GADTs` pragma if necessary.
+
+## Known limitations
+- Currently all comments missed while converting to GADT syntax.
+
+## Change log
+### 1.0.0.0
+- Release

plugins/hls-gadt-plugin/hls-gadt-plugin.cabal

@@ -0,0 +1,61 @@
+cabal-version:      2.4
+name:               hls-gadt-plugin
+version:            1.0.0.0
+synopsis:           Convert to GADT syntax plugin
+description:
+  Please see the README on GitHub at <https://github.com/haskell/haskell-language-server/tree/master/plugins/hls-gadt-plugin#readme>
+
+license:            Apache-2.0
+license-file:       LICENSE
+author:             Lei Zhu
+maintainer:         julytreee@gmail.com
+category:           Development
+build-type:         Simple
+extra-source-files:
+  LICENSE
+  test/testdata/*.hs
+
+library
+  exposed-modules:    Ide.Plugin.GADT
+  other-modules:      Ide.Plugin.GHC
+
+  hs-source-dirs:     src
+  build-depends:
+    , aeson
+    , base                  >=4.12 && <5
+    , containers
+    , extra
+    , ghc
+    , ghcide                ^>= 1.7
+    , ghc-boot-th
+    , ghc-exactprint
+    , hls-plugin-api        ^>= 1.4
+    , lens
+    , lsp                    >=1.2.0.1
+    , mtl
+    , text
+    , transformers
+    , unordered-containers
+
+  ghc-options:
+    -Wall
+    -Wno-name-shadowing
+    -Wno-unticked-promoted-constructors
+  default-language:   Haskell2010
+  default-extensions: DataKinds
+
+test-suite tests
+  type:             exitcode-stdio-1.0
+  default-language: Haskell2010
+  hs-source-dirs:   test
+  main-is:          Main.hs
+  ghc-options:      -threaded -rtsopts -with-rtsopts=-N
+  build-depends:
+    , base
+    , filepath
+    , hls-gadt-plugin
+    , hls-test-utils              ^>=1.3
+    , lens
+    , lsp
+    , lsp-test
+    , text

plugins/hls-gadt-plugin/src/Ide/Plugin/GADT.hs

@@ -0,0 +1,131 @@
+{-# LANGUAGE DeriveAnyClass    #-}
+{-# LANGUAGE DeriveGeneric     #-}
+{-# LANGUAGE GADTs             #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RankNTypes        #-}
+{-# LANGUAGE RecordWildCards   #-}
+{-# LANGUAGE TypeOperators     #-}
+{-# LANGUAGE ViewPatterns      #-}
+module Ide.Plugin.GADT (descriptor) where
+
+import           Control.Lens                    ((^.))
+import           Control.Monad.Except
+import           Data.Aeson                      (FromJSON, ToJSON,
+                                                  Value (Null), toJSON)
+import           Data.Either.Extra               (maybeToEither)
+import qualified Data.HashMap.Lazy               as HashMap
+import qualified Data.Text                       as T
+import           Development.IDE
+import           Development.IDE.GHC.Compat
+
+import           Control.Monad.Trans.Except      (throwE)
+import           Data.Maybe                      (mapMaybe)
+import           Development.IDE.GHC.Compat.Util (toList)
+import           Development.IDE.Spans.Pragmas   (NextPragmaInfo,
+                                                  getNextPragmaInfo,
+                                                  insertNewPragma)
+import           GHC.Generics                    (Generic)
+import           GHC.LanguageExtensions.Type     (Extension (GADTSyntax, GADTs))
+import           Ide.Plugin.GHC
+import           Ide.PluginUtils
+import           Ide.Types
+import           Language.LSP.Server             (sendRequest)
+import           Language.LSP.Types
+import qualified Language.LSP.Types.Lens         as L
+
+descriptor :: PluginId -> PluginDescriptor IdeState
+descriptor plId = (defaultPluginDescriptor plId)
+    { Ide.Types.pluginHandlers =
+        mkPluginHandler STextDocumentCodeAction codeActionHandler
+    , pluginCommands =
+        [PluginCommand toGADTSyntaxCommandId "convert data decl to GADT syntax" (toGADTCommand plId)]
+    }
+
+-- | Parameter used in the command
+data ToGADTParams = ToGADTParams
+    { uri   :: Uri
+    , range :: Range
+    } deriving (Generic, ToJSON, FromJSON)
+
+toGADTSyntaxCommandId :: CommandId
+toGADTSyntaxCommandId = "GADT.toGADT"
+
+-- | A command replaces H98 data decl with GADT decl in place
+toGADTCommand :: PluginId -> CommandFunction IdeState ToGADTParams
+toGADTCommand plId state ToGADTParams{..} = response $ do
+    nfp <- getNormalizedFilePath plId uri
+    (decls, exts) <- getInRangeH98DeclsAndExts state range nfp
+    (L ann decl) <- case decls of
+        [d] -> pure d
+        _   -> throwE $ "Expected 1 declaration, but got " <> show (Prelude.length decls)
+    deps <- liftIO $ runAction "GADT.GhcSessionDeps" state $ use GhcSessionDeps nfp
+    (hsc_dflags . hscEnv -> df) <- liftEither
+        $ maybeToEither "Get GhcSessionDeps failed" deps
+    txt <- liftEither $ T.pack <$> (prettyGADTDecl df . h98ToGADTDecl) decl
+    range <- liftEither
+        $ maybeToEither "Unable to get data decl range"
+        $ srcSpanToRange $ locA ann
+    pragma <- getNextPragma state nfp
+    let insertEdit = [insertNewPragma pragma GADTs | all (`notElem` exts) [GADTSyntax, GADTs]]
+
+    _ <- lift $ sendRequest
+            SWorkspaceApplyEdit
+            (ApplyWorkspaceEditParams Nothing (workSpaceEdit nfp (TextEdit range txt : insertEdit)))
+            (\_ -> pure ())
+
+    pure Null
+    where
+        workSpaceEdit nfp edits = WorkspaceEdit
+            (pure $ HashMap.fromList
+                [(filePathToUri $ fromNormalizedFilePath nfp,
+                 List edits)])
+                 Nothing Nothing
+
+codeActionHandler :: PluginMethodHandler IdeState TextDocumentCodeAction
+codeActionHandler state plId (CodeActionParams _ _ doc range _) = response $ do
+    nfp <- getNormalizedFilePath plId (doc ^. L.uri)
+    (inRangeH98Decls, _) <- getInRangeH98DeclsAndExts state range nfp
+    let actions = map (mkAction . printOutputable . tcdLName . unLoc) inRangeH98Decls
+    pure $ List actions
+    where
+        mkAction :: T.Text -> Command |? CodeAction
+        mkAction name = InR CodeAction{..}
+            where
+                _title = "Convert \"" <> name <> "\" to GADT syntax"
+                _kind = Just CodeActionRefactorRewrite
+                _diagnostics = Nothing
+                _isPreferred = Nothing
+                _disabled = Nothing
+                _edit = Nothing
+                _command = Just
+                    $ mkLspCommand plId toGADTSyntaxCommandId _title (Just [toJSON mkParam])
+                _xdata = Nothing
+
+        mkParam = ToGADTParams (doc ^. L.uri) range
+
+-- | Get all H98 decls in the given range, and enabled extensions
+getInRangeH98DeclsAndExts :: (MonadIO m) =>
+    IdeState
+    -> Range
+    -> NormalizedFilePath
+    -> ExceptT String m ([LTyClDecl GP], [Extension])
+getInRangeH98DeclsAndExts state range nfp = do
+    pm <- handleMaybeM "Unable to get ParsedModuleWithComments"
+        $ liftIO
+        $ runAction "GADT.GetParsedModuleWithComments" state
+        $ use GetParsedModuleWithComments nfp
+    let (L _ hsDecls) = hsmodDecls <$> pm_parsed_source pm
+        decls = filter isH98DataDecl
+            $ mapMaybe getDataDecl
+            $ filter (inRange range) hsDecls
+        exts = (toList . extensionFlags . ms_hspp_opts . pm_mod_summary) pm
+    pure (decls, exts)
+
+-- Copy from hls-alternate-number-format-plugin
+getNextPragma :: MonadIO m => IdeState -> NormalizedFilePath -> ExceptT String m NextPragmaInfo
+getNextPragma state nfp = handleMaybeM "Error: Could not get NextPragmaInfo" $ do
+      ghcSession <- liftIO $ runAction "GADT.GhcSession" state $ useWithStale GhcSession nfp
+      (_, fileContents) <- liftIO $ runAction "GADT.GetFileContents" state $ getFileContents nfp
+      case ghcSession of
+        Just (hscEnv -> hsc_dflags -> sessionDynFlags, _) -> pure $ Just $ getNextPragmaInfo sessionDynFlags fileContents
+        Nothing -> pure Nothing

plugins/hls-gadt-plugin/src/Ide/Plugin/GHC.hs

@@ -0,0 +1,304 @@
+{-# LANGUAGE CPP              #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE GADTs            #-}
+{-# LANGUAGE LambdaCase       #-}
+{-# LANGUAGE PatternSynonyms  #-}
+{-# LANGUAGE RankNTypes       #-}
+{-# LANGUAGE RecordWildCards  #-}
+{-# LANGUAGE TupleSections    #-}
+{-# LANGUAGE TypeApplications #-}
+{-# OPTIONS_GHC -Wno-missing-signatures #-}
+{-# OPTIONS_GHC -Wno-overlapping-patterns #-}
+module Ide.Plugin.GHC where
+
+import           Data.Functor                            ((<&>))
+import           Data.List.Extra                         (stripInfix)
+import qualified Data.Text                               as T
+import           Development.IDE
+import           Development.IDE.GHC.Compat
+import           Ide.PluginUtils                         (subRange)
+import           Language.Haskell.GHC.ExactPrint.Parsers (parseDecl)
+
+#if MIN_VERSION_ghc(9,2,1)
+import           GHC.Parser.Annotation                   (AddEpAnn (..),
+                                                          Anchor (Anchor),
+                                                          AnchorOperation (MovedAnchor),
+                                                          DeltaPos (..),
+                                                          EpAnn (..),
+                                                          EpAnnComments (EpaComments),
+                                                          EpaLocation (EpaDelta),
+                                                          SrcSpanAnn' (SrcSpanAnn),
+                                                          spanAsAnchor)
+import           Language.Haskell.GHC.ExactPrint         (showAst)
+#else
+import qualified Data.Map.Lazy                           as Map
+import           Language.Haskell.GHC.ExactPrint.Types   (AnnConName (CN),
+                                                          AnnKey (AnnKey),
+                                                          Annotation (..),
+                                                          DeltaPos (DP),
+                                                          KeywordId (G),
+                                                          deltaColumn)
+#endif
+
+type GP = GhcPass Parsed
+
+-- | Check if a given range is in the range of located item
+inRange :: HasSrcSpan a => Range -> a -> Bool
+inRange range s = maybe False (subRange range) (srcSpanToRange (getLoc s))
+
+-- | Get data decl and its location
+getDataDecl :: LHsDecl GP -> Maybe (LTyClDecl GP)
+getDataDecl (L l (TyClD _ d@DataDecl{})) = Just (L l d)
+getDataDecl _                            = Nothing
+
+isConDeclH98 :: ConDecl GP -> Bool
+isConDeclH98 ConDeclH98{} = True
+isConDeclH98 _            = False
+
+isH98DataDecl :: LTyClDecl GP -> Bool
+isH98DataDecl (L _ decl@DataDecl{}) =
+    any (isConDeclH98 . (\(L _ r) -> r)) (dd_cons $ tcdDataDefn decl)
+isH98DataDecl _ = False
+
+-- | Convert H98 data type definition to GADT's
+h98ToGADTDecl :: TyClDecl GP -> TyClDecl GP
+h98ToGADTDecl = \case
+    DataDecl{..} -> DataDecl
+        { tcdDataDefn = updateDefn tcdLName tcdTyVars tcdDataDefn
+        , ..
+        }
+    x -> x
+    where
+        updateDefn dataName tyVars = \case
+            HsDataDefn{..} -> HsDataDefn
+                { dd_cons =
+                    mapX (h98ToGADTConDecl dataName tyVars (wrapCtxt dd_ctxt)) <$> dd_cons
+                , dd_ctxt = emptyCtxt -- Context can't appear at the data name in GADT
+                , ..
+                }
+            x -> x
+
+-- | Convert H98 data constuctor to GADT data constructor
+h98ToGADTConDecl ::
+    LIdP GP -- ^Type constuctor name,
+            -- used for constucting final result type in GADT
+    -> LHsQTyVars GP
+            -- ^Type variable names
+            -- used for constucting final result type in GADT
+    -> Maybe (LHsContext GP)
+            -- ^Data type context
+    -> ConDecl GP
+    -> ConDecl GP
+h98ToGADTConDecl dataName tyVars ctxt = \case
+    ConDeclH98{..} ->
+        ConDeclGADT
+            con_ext
+            [con_name]
+#if !MIN_VERSION_ghc(9,2,1)
+            con_forall
+#endif
+            -- Ignore all existential type variable since GADT not needed
+            implicitTyVars
+            (mergeContext ctxt con_mb_cxt)
+            (renderDetails con_args)
+            renderResultTy
+            con_doc
+    x -> x
+    where
+        -- Parameters in the data constructor
+#if MIN_VERSION_ghc(9,2,1)
+        renderDetails :: HsConDeclH98Details GP -> HsConDeclGADTDetails GP
+        renderDetails (PrefixCon _ args)   = PrefixConGADT args
+        renderDetails (InfixCon arg1 arg2) = PrefixConGADT [arg1, arg2]
+        renderDetails (RecCon recs)        = RecConGADT recs
+#else
+        renderDetails (PrefixCon args)     = PrefixCon args
+        renderDetails (InfixCon arg1 arg2) = PrefixCon [arg1, arg2]
+        renderDetails (RecCon recs)        = RecCon recs
+#endif
+
+        -- | Construct GADT result type
+        renderResultTy :: LHsType GP
+        renderResultTy = case tyVars of
+            -- Without type variable
+            HsQTvs _ []   -> wrappedDataName
+            -- With type variable
+            HsQTvs _ vars -> foldl go wrappedDataName vars
+            _             -> wrappedDataName
+            where
+                wrappedDataName = wrap (HsTyVar noUsed NotPromoted dataName)
+
+                -- Bundle data name with type vars by `HsAppTy`
+                go acc (L _(UserTyVar' var)) =
+                    wrap
+                        (HsAppTy noExtField acc
+                            (wrap (HsTyVar noUsed NotPromoted var)))
+                go acc _ = acc
+
+        -- Merge data type context and constructor type context
+        mergeContext :: Maybe (LHsContext GP) -> Maybe (LHsContext GP) -> Maybe (LHsContext GP)
+        mergeContext ctxt1 ctxt2 =
+            (wrap . map wrap) . map unParTy
+                <$> (getContextType ctxt1 <> getContextType ctxt2)
+            where
+                getContextType :: Maybe (LHsContext GP) -> Maybe [HsType GP]
+                getContextType ctxt = map unWrap . unWrap <$> ctxt
+
+                -- Unparen the outmost, it only occurs at the outmost
+                -- for a valid type.
+                --
+                -- Note for context paren rule:
+                --
+                -- If only one element, it __can__ have a paren type.
+                -- If not, there can't have a parent type.
+                unParTy :: HsType GP -> HsType GP
+                unParTy (HsParTy _ ty) = unWrap ty
+                unParTy x              = x
+{- |
+We use `printOutputable` to print H98 data decl as GADT syntax,
+this print is not perfect, it will:
+
+1. Make data name and the `where` key word in different lines.
+2. Make the whole data decl prints in one line if there is only one data constructor.
+3. The ident size of every data constructor depends on its origin
+   format, and may have different ident size between constructors.
+
+Hence, we first use `printOutputable` to get an initial GADT syntax,
+then use `ghc-exactprint` to parse the initial result, and finally
+adjust the details that mentioned above.
+
+The adjustment includes:
+
+1. Make the `where` key word at the same line of data name.
+2. Remove the extra blank line caused by adjustment of `where`.
+3. Make every data constructor start with a new line and 2 spaces
+-}
+prettyGADTDecl :: DynFlags -> TyClDecl GP -> Either String String
+#if MIN_VERSION_ghc(9,2,1)
+prettyGADTDecl df decl =
+    let old = printOutputable decl
+        hsDecl = parseDecl df "unused" (T.unpack old)
+        tycld = adjustTyClD hsDecl
+    in removeExtraEmptyLine . exactPrint <$> tycld
+    where
+        adjustTyClD = \case
+                Right (L _ (TyClD _ tycld)) -> Right $ adjustDataDecl tycld
+                Right x -> Left $ "Expect TyClD but got " <> showAst x
+                Left err -> Left $ show err
+
+        adjustDataDecl DataDecl{..} = DataDecl
+            { tcdDExt = adjustWhere tcdDExt
+            , tcdDataDefn = tcdDataDefn
+                { dd_cons = map adjustCon (dd_cons tcdDataDefn)
+                }
+            , ..
+            }
+        adjustDataDecl x = x
+
+        -- Make every data constructor start with a new line and 2 spaces
+        adjustCon :: LConDecl GP -> LConDecl GP
+        adjustCon (L (SrcSpanAnn _ loc) r) =
+            L (SrcSpanAnn (EpAnn (go (spanAsAnchor loc)) (AnnListItem []) (EpaComments [])) loc) r
+            where
+                go (Anchor a _) = Anchor a (MovedAnchor (DifferentLine 1 2))
+
+        -- Adjust where annotation to the same line of the type constuctor
+        adjustWhere tcdDExt = tcdDExt <&> map
+            (\(AddEpAnn ann l) ->
+            if ann == AnnWhere
+                then AddEpAnn AnnWhere (EpaDelta (SameLine 1) [])
+                else AddEpAnn ann l
+            )
+
+        -- Remove the first extra line if exist
+        removeExtraEmptyLine s = case stripInfix "\n\n" s of
+            Just (x, xs) -> x <> "\n" <> xs
+            Nothing      -> s
+#else
+prettyGADTDecl df decl =
+    let old = printOutputable decl
+        hsDecl = parseDecl df "unused" (T.unpack old)
+        tycld = adjustTyClD hsDecl
+    in removeExtraEmptyLine . uncurry (flip exactPrint) <$> tycld
+    where
+        adjustTyClD = \case
+                Right (anns, t@(L _ (TyClD _ _))) -> Right (adjustDataDeclAnns anns, t)
+                Right _ -> Left "Expect TyClD"
+                Left err -> Left $ show err
+
+        adjustDataDeclAnns = Map.mapWithKey go
+            where
+                isDataDeclAnn (AnnKey _ (CN name)) = name == "DataDecl"
+                isConDeclGADTAnn (AnnKey _ (CN name)) = name == "ConDeclGADT"
+
+                go key ann
+                    | isDataDeclAnn key = adjustWhere ann
+                    | isConDeclGADTAnn key = adjustCon ann
+                    | otherwise = ann
+
+                -- Adjust where annotation to the same line of the type constuctor
+                adjustWhere Ann{..} = Ann
+                    { annsDP = annsDP <&>
+                        (\(keyword, dp) ->
+                            if keyword == G AnnWhere
+                                then (keyword, DP (0, 1))
+                                else (keyword, dp))
+                    , ..
+                    }
+
+                -- Make every data constructor start with a new line and 2 spaces
+                --
+                -- Here we can't force every GADT constuctor has (1, 2)
+                -- delta. For the first constructor with (1, 2), it prints
+                -- a new line with 2 spaces, but for other constructors
+                -- with (1, 2), it will print a new line with 4 spaces.
+                --
+                -- The original ann parsed with `praseDecl` shows the first
+                -- constructor has (1, 4) delta, but others have (1, 0).
+                -- Hence, the following code only deal with the first
+                -- constructor.
+                adjustCon Ann{..} = let c = deltaColumn annEntryDelta
+                    in Ann
+                    { annEntryDelta = DP $ (1,) $ if c > 0 then 2 else 0
+                    , ..
+                    }
+
+        -- Remove the first extra line if exist
+        removeExtraEmptyLine s = case stripInfix "\n\n" s of
+            Just (x, xs) -> x <> "\n" <> xs
+            Nothing      -> s
+
+#endif
+
+#if MIN_VERSION_ghc(9,2,1)
+wrap :: forall a. WrapXRec GP a => a -> XRec GP a
+wrap = wrapXRec @GP
+wrapCtxt = id
+emptyCtxt = Nothing
+unWrap = unXRec @GP
+mapX = mapXRec @GP
+noUsed = EpAnnNotUsed
+#else
+wrapCtxt = Just
+wrap = L noSrcSpan
+emptyCtxt = wrap []
+unWrap (L _ r) = r
+mapX = fmap
+noUsed = noExtField
+#endif
+
+#if MIN_VERSION_ghc(9,0,1)
+pattern UserTyVar' :: LIdP pass -> HsTyVarBndr flag pass
+pattern UserTyVar' s <- UserTyVar _ _ s
+#else
+pattern UserTyVar' :: LIdP pass -> HsTyVarBndr pass
+pattern UserTyVar' s <- UserTyVar _ s
+#endif
+
+#if MIN_VERSION_ghc(9,2,1)
+implicitTyVars = (wrapXRec @GP mkHsOuterImplicit)
+#elif MIN_VERSION_ghc(9,0,1)
+implicitTyVars = []
+#else
+implicitTyVars = HsQTvs noExtField []
+#endif

plugins/hls-gadt-plugin/test/Main.hs

@@ -0,0 +1,81 @@
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RecordWildCards   #-}
+{-# LANGUAGE TypeOperators     #-}
+{-# OPTIONS_GHC -Wall #-}
+
+module Main where
+
+import           Control.Monad   (void)
+import           Data.Either     (rights)
+import qualified Data.Text       as T
+import qualified Ide.Plugin.GADT as GADT
+import           System.FilePath ((</>))
+import           Test.Hls
+
+main :: IO ()
+main = defaultTestRunner tests
+
+gadtPlugin :: PluginDescriptor IdeState
+gadtPlugin = GADT.descriptor "GADT"
+
+tests :: TestTree
+tests = testGroup "GADT"
+    [ runTest "range" "SimpleData" 2 0 2 1
+    , runTest "SimpleData" "SimpleData" 2 0 2 10
+    , runTest "SimpleNewtype" "SimpleNewtype" 2 0 2 17
+    , runTest "Data" "Data" 2 0 2 36
+    , runTest "Newtype" "Newtype" 2 0 2 21
+    , runTest "Deriving" "Deriving" 2 0 2 56
+    , runTest "Infix" "Infix" 2 0 2 35
+    , runTest "Record" "Record" 2 0 5 1
+    , runTest "TypeVariable" "TypeVariable"  2 0 2 32
+    , runTest "DataContext" "DataContext" 2 0 2 31
+    , runTest "DataContextParen" "DataContextParen" 2 0 3 6
+    , runTest "Forall" "Forall" 2 0 2 44
+    , runTest "ConstuctorContext" "ConstructorContext" 2 0 2 38
+    , runTest "Context" "Context" 2 0 4 41
+    , runTest "Pragma" "Pragma" 2 0 3 29
+    , onlyWorkForGhcVersions [GHC92] "Single deriving has different output on ghc9.2" $
+        runTest "SingleDerivingGHC92" "SingleDerivingGHC92" 2 0 3 14
+    , knownBrokenForGhcVersions [GHC92] "Single deriving has different output on ghc9.2" $
+        runTest "SingleDeriving" "SingleDeriving" 2 0 3 14
+    , onlyWorkForGhcVersions [GHC92] "only ghc-9.2 enabled GADTs pragma implicitly" $
+        gadtPragmaTest "ghc-9.2 don't need to insert GADTs pragma" False
+    , knownBrokenForGhcVersions [GHC92] "ghc-9.2 has enabled GADTs pragma implicitly" $
+        gadtPragmaTest "insert pragma" True
+    ]
+
+gadtPragmaTest :: TestName -> Bool -> TestTree
+gadtPragmaTest title hasGADT = testCase title
+    $ withCanonicalTempDir
+    $ \dir -> runSessionWithServer gadtPlugin dir $ do
+        doc <- createDoc "A.hs" "haskell" (T.unlines ["module A where", "data Foo = Bar"])
+        _ <- waitForProgressDone
+        (act:_) <- findGADTAction <$> getCodeActions doc (Range (Position 1 0) (Position 1 1))
+        executeCodeAction act
+        let expected = T.unlines $
+                ["{-# LANGUAGE GADTs #-}" | hasGADT] ++
+                ["module A where", "data Foo where", "  Bar :: Foo"]
+        contents <- skipManyTill anyMessage (getDocumentEdit doc)
+        liftIO $ contents @?= expected
+
+runTest :: TestName -> FilePath -> UInt -> UInt -> UInt -> UInt -> TestTree
+runTest title fp x1 y1 x2 y2 =
+    goldenWithHaskellDoc gadtPlugin title testDataDir fp "expected" "hs" $ \doc -> do
+        _ <- waitForProgressDone
+        (act:_) <- findGADTAction <$> getCodeActions doc (Range (Position x1 y1) (Position x2 y2))
+        executeCodeAction act
+        void $ skipManyTill anyMessage (getDocumentEdit doc)
+
+findGADTAction :: [a |? CodeAction] -> [CodeAction]
+findGADTAction = filter isGADTCodeAction . rights . map toEither
+
+isGADTCodeAction :: CodeAction -> Bool
+isGADTCodeAction CodeAction{..} = case _kind of
+    Nothing -> False
+    Just kind -> case kind of
+        CodeActionRefactorRewrite -> True
+        _                         -> False
+
+testDataDir :: FilePath
+testDataDir = "test" </> "testdata"

plugins/hls-gadt-plugin/test/testdata/ConstructorContext.expected.hs

@@ -0,0 +1,4 @@
+module ConstructorContext where
+
+data Foo where
+  Bar :: Show a => a -> Foo

plugins/hls-gadt-plugin/test/testdata/ConstructorContext.hs

@@ -0,0 +1,3 @@
+module ConstructorContext where
+
+data Foo = forall a. (Show a) => Bar a

plugins/hls-gadt-plugin/test/testdata/Context.expected.hs

@@ -0,0 +1,5 @@
+module Context where
+
+data Foo a where
+  Bar :: (Eq a, Show b, Show a) => a -> b -> Foo a
+  Baz :: (Eq a, Show c) => c -> c -> Foo a

plugins/hls-gadt-plugin/test/testdata/Context.hs

@@ -0,0 +1,5 @@
+module Context where
+
+data (Eq a) => Foo a =
+    forall b c. (Show b, Show a) =>
+  Bar a b | forall c. (Show c) => Baz c c

plugins/hls-gadt-plugin/test/testdata/Data.expected.hs

@@ -0,0 +1,5 @@
+module Data where
+
+data Foo where
+  Bar :: Int -> Foo
+  Baz :: Char -> String -> Foo

plugins/hls-gadt-plugin/test/testdata/Data.hs

@@ -0,0 +1,3 @@
+module Data where
+
+data Foo = Bar Int | Baz Char String

plugins/hls-gadt-plugin/test/testdata/DataContext.expected.hs

@@ -0,0 +1,5 @@
+module DataContext where
+
+data T a b where
+  F :: Ord a => a -> T a b
+  G :: Ord a => b -> T a b

plugins/hls-gadt-plugin/test/testdata/DataContext.hs

@@ -0,0 +1,3 @@
+module DataContext where
+
+data Ord a => T a b = F a | G b

plugins/hls-gadt-plugin/test/testdata/DataContextParen.expected.hs

@@ -0,0 +1,4 @@
+module DataContextParen where
+
+data F a where
+  G :: Eq a => a -> F a

plugins/hls-gadt-plugin/test/testdata/DataContextParen.hs

@@ -0,0 +1,4 @@
+module DataContextParen where
+
+data (Eq a) => F a
+ = G a

plugins/hls-gadt-plugin/test/testdata/Deriving.expected.hs

@@ -0,0 +1,6 @@
+module Deriving where
+
+data Foo where
+  Bar :: Int -> Foo
+  Baz :: Char -> String -> Foo
+  deriving (Show, Eq)

plugins/hls-gadt-plugin/test/testdata/Deriving.hs

@@ -0,0 +1,3 @@
+module Deriving where
+
+data Foo = Bar Int | Baz Char String deriving (Show, Eq)

plugins/hls-gadt-plugin/test/testdata/Forall.expected.hs

@@ -0,0 +1,4 @@
+module Forall where
+
+data Foo where
+  Bar :: Show a => a -> b -> a -> Foo

plugins/hls-gadt-plugin/test/testdata/Forall.hs

@@ -0,0 +1,3 @@
+module Forall where
+
+data Foo = forall a b. (Show a) => Bar a b a

plugins/hls-gadt-plugin/test/testdata/Infix.expected.hs

@@ -0,0 +1,5 @@
+module Infix where
+
+data Foo where
+  (:->) :: Int -> Char -> Foo
+  deriving ()

plugins/hls-gadt-plugin/test/testdata/Infix.hs

@@ -0,0 +1,3 @@
+module Infix where
+
+data Foo = Int :-> Char deriving ()

plugins/hls-gadt-plugin/test/testdata/Newtype.expected.hs

@@ -0,0 +1,4 @@
+module Newtype where
+
+newtype Foo where
+  Bar :: Int -> Foo

plugins/hls-gadt-plugin/test/testdata/Newtype.hs

@@ -0,0 +1,3 @@
+module Newtype where
+
+newtype Foo = Bar Int

plugins/hls-gadt-plugin/test/testdata/Pragma.expected.hs

@@ -0,0 +1,5 @@
+module Pragma where
+
+data F where
+  G :: {-# UNPACK #-}Int -> F
+  H :: {-# NOUNPACK #-}Char -> F

plugins/hls-gadt-plugin/test/testdata/Pragma.hs

@@ -0,0 +1,4 @@
+module Pragma where
+
+data F = G{-# UNPACK #-}Int
+    | H {-# NOUNPACK #-} Char

plugins/hls-gadt-plugin/test/testdata/Record.expected.hs

@@ -0,0 +1,4 @@
+module Record where
+
+data Foo where
+  Foo :: {bar :: Char, baz :: Int} -> Foo

plugins/hls-gadt-plugin/test/testdata/Record.hs

@@ -0,0 +1,6 @@
+module Record where
+
+data Foo = Foo {
+    bar :: Char,
+    baz :: Int
+}

plugins/hls-gadt-plugin/test/testdata/SimpleData.expected.hs

@@ -0,0 +1,4 @@
+module SimpleData where
+
+data A where
+  B :: A

plugins/hls-gadt-plugin/test/testdata/SimpleData.hs

@@ -0,0 +1,3 @@
+module SimpleData where
+
+data A = B

plugins/hls-gadt-plugin/test/testdata/SimpleNewtype.expected.hs

@@ -0,0 +1,4 @@
+module SimpleNewtype where
+
+newtype A where
+  B :: Int -> A

plugins/hls-gadt-plugin/test/testdata/SimpleNewtype.hs

@@ -0,0 +1,3 @@
+module SimpleNewtype where
+
+newtype A = B Int

plugins/hls-gadt-plugin/test/testdata/SingleDeriving.expected.hs

@@ -0,0 +1,5 @@
+module SingleDeriving where
+
+data Foo a b where
+  Bar :: b -> a -> Foo a b
+  deriving Eq

plugins/hls-gadt-plugin/test/testdata/SingleDeriving.hs

@@ -0,0 +1,4 @@
+module SingleDeriving where
+
+data Foo a b = Bar b a
+ deriving (Eq)

plugins/hls-gadt-plugin/test/testdata/SingleDerivingGHC92.expected.hs

@@ -0,0 +1,5 @@
+module SingleDerivingGHC92 where
+
+data Foo a b where
+  Bar :: b -> a -> Foo a b
+  deriving (Eq)

plugins/hls-gadt-plugin/test/testdata/SingleDerivingGHC92.hs

@@ -0,0 +1,4 @@
+module SingleDerivingGHC92 where
+
+data Foo a b = Bar b a
+ deriving (Eq)

plugins/hls-gadt-plugin/test/testdata/TypeVariable.expected.hs

@@ -0,0 +1,5 @@
+module TypeVariable where
+
+data Foo a f where
+  Foo :: a -> Foo a f
+  Bar :: (f a) -> Foo a f

plugins/hls-gadt-plugin/test/testdata/TypeVariable.hs

@@ -0,0 +1,3 @@
+module TypeVariable where
+
+data Foo a f = Foo a | Bar (f a)

plugins/hls-gadt-plugin/test/testdata/hie.yaml

@@ -0,0 +1,3 @@
+cradle:
+  direct:
+    arguments: ["-XHaskell2010", "-XExistentialQuantification", "-XGADTs"]

stack-lts16.yaml

@@ -30,6 +30,7 @@ packages:
   - ./plugins/hls-alternate-number-format-plugin
   - ./plugins/hls-selection-range-plugin
   - ./plugins/hls-change-type-signature-plugin
+  - ./plugins/hls-gadt-plugin
 
 ghc-options:
   "$everything": -haddock

stack-lts19.yaml

@@ -30,6 +30,7 @@ packages:
   - ./plugins/hls-alternate-number-format-plugin
   - ./plugins/hls-selection-range-plugin
   - ./plugins/hls-change-type-signature-plugin
+  - ./plugins/hls-gadt-plugin
 
 ghc-options:
   "$everything": -haddock

stack.yaml

@@ -31,6 +31,7 @@ packages:
 - ./plugins/hls-alternate-number-format-plugin
 - ./plugins/hls-selection-range-plugin
 - ./plugins/hls-change-type-signature-plugin
+- ./plugins/hls-gadt-plugin
 
 extra-deps:
 - Chart-1.9.3@sha256:640a38463318b070d80a049577e4f0b3322df98290abb7afcf0cb74a4ad5b512,2948