Code messed up by FixIt to insert default implementation for Semigroup's <>

[Aster89]

Your environment

Which OS do you use?
up-to-date ArchLinux
Which version of GHC do you use and how did you install it?
GHC 9.10.1 from ghcup 0.1.50.2
How is your project built (alternative: link to the project)?
Just a 5-lines file:

data Set a = Set

instance Semigroup (Set a) where

instance Monoid (Set a) where

Which LSP client (editor/plugin) do you use?
terminal Vim + YCM
Which version of HLS do you use and how did you install it?
HLS 2.11.0.0 from ghcup 0.1.50.2
Have you configured HLS in any way (especially: a hie.yaml file)?
Not that I'm aware of

Steps to reproduce

Request the FixIt on the line defining instance Semigroup (Set a) where, and choose the first option Add placeholders for '<>'.

Expected behaviour

The buffer should become like this:

data Set a = Set

instance Semigroup (Set a) where
  (<>) = _

instance Monoid (Set a) where

Actual behaviour

The buffer content is changed to this:

data Set a = Set

instance Semigroup (Set a) where
  (<>) = _





instanceMonoid (Set a) where

Debug information

No 'hie.yaml' found. Try to discover the project type!
Run entered for haskell-language-server-wrapper(haskell-language-server-wrapper) Version 2.11.0.0 x86_64 ghc-9.10.1
Current directory: /home/enrico
Operating system: linux
Arguments: ["--debug","--lsp"]
Cradle directory: /home/enrico
Cradle type: Default

Tool versions found on the $PATH
cabal:          3.14.2.0
stack:          Not found
ghc:            9.10.1


Consulting the cradle to get project GHC version...
2025-09-22T10:22:57.671725Z | Debug | ghc --numeric-version
Project GHC version: 9.10.1
haskell-language-server exe candidates: ["haskell-language-server-9.10.1","haskell-language-server"]
Launching haskell-language-server exe at:/home/enrico/.ghcup/bin/haskell-language-server-9.10.1
2025-09-22T10:22:57.768531Z | Debug | ghc -v0 -package-env=- -ignore-dot-ghci -e Control.Monad.join (Control.Monad.fmap System.IO.putStr System.Environment.getExecutablePath)
2025-09-22T10:22:57.795532Z | Debug | ghc --print-libdir
2025-09-22T10:22:58.245224Z | Info | haskell-language-server version: 2.11.0.0 (GHC: 9.10.1) (PATH: /home/enrico/.ghcup/bin/haskell-language-server-9.10.1~2.11.0.0)
2025-09-22T10:22:58.245811Z | Info | Directory: /home/enrico
2025-09-22T10:22:58.245914Z | Info | Starting (haskell-language-server) LSP server...
  GhcideArguments {argsCommand = LSP, argsCwd = Nothing, argsShakeProfiling = Nothing, argsTesting = False, argsExamplePlugin = False, argsLogLevel = Debug, argsLogFile = Nothing, argsLogStderr = True, argsLogClient = False, argsThreads = 0, argsProjectGhcVersion = False}
  PluginIds: [ pragmas-suggest
             , pragmas-completion
             , ghcide-code-actions-bindings
             , cabalHaskellIntegration
             , ghcide-extend-import-action
             , stylish-haskell
             , explicit-fields
             , ghcide-code-actions-type-signatures
             , ghcide-code-actions-fill-holes
             , stan
             , changeTypeSignature
             , ghcide-code-actions-imports-exports
             , cabal-fmt
             , notes
             , ghcide-completions
             , eval
             , ghcide-type-lenses
             , cabal
             , overloaded-record-dot
             , gadt
             , LSPRecorderCallback
             , importLens
             , codeRange
             , class
             , ormolu
             , qualifyImportedNames
             , ghcide-hover-and-symbols
             , alternateNumberFormat
             , rename
             , moduleName
             , semanticTokens
             , fourmolu
             , cabal-gild
             , callHierarchy
             , ghcide-core
             , explicit-fixity
             , pragmas-disable ]
2025-09-22T10:22:58.247578Z | Info | Logging heap statistics every 60.00s
 2025-09-22T10:22:58.251510Z | Info | Starting LSP server...
  If you are seeing this in a terminal, you probably should have run WITHOUT the --lsp option!
  PluginIds: [ pragmas-suggest
             , pragmas-completion
             , ghcide-code-actions-bindings
             , cabalHaskellIntegration
             , ghcide-extend-import-action
             , stylish-haskell
             , explicit-fields
             , ghcide-code-actions-type-signatures
             , ghcide-code-actions-fill-holes
             , stan
             , changeTypeSignature
             , ghcide-code-actions-imports-exports
             , cabal-fmt
             , notes
             , ghcide-completions
             , eval
             , ghcide-type-lenses
             , cabal
             , overloaded-record-dot
             , gadt
             , LSPRecorderCallback
             , importLens
             , codeRange
             , class
             , ormolu
             , qualifyImportedNames
             , ghcide-hover-and-symbols
             , alternateNumberFormat
             , rename
             , moduleName
             , semanticTokens
             , fourmolu
             , cabal-gild
             , callHierarchy
             , ghcide-core
             , explicit-fixity
             , pragmas-disable ]
2025-09-22T10:22:58.251773Z | Info | Starting server
2025-09-22T10:22:58.252404Z | Debug | LSP: set new config: {  }
2025-09-22T10:22:58.252840Z | Info | Registering IDE configuration: IdeConfiguration {workspaceFolders = fromList [NormalizedUri (-7191819183657831203) "file:///home/enrico"], clientSettings = hashed (Just (Object (fromList [])))}
2025-09-22T10:22:58.254548Z | Info | Started LSP server in 0.00s
2025-09-22T10:22:58.254627Z | Debug | shouldRunSubset: True
2025-09-22T10:22:58.254739Z | Debug | Initializing exports map from hiedb
2025-09-22T10:22:58.255232Z | Debug | Done initializing exports map from hiedb. Size: 2
2025-09-22T10:22:58.255728Z | Warning | LSP: can't register dynamically for: "workspace/didChangeConfiguration"
2025-09-22T10:22:58.255792Z | Debug | LSP: not requesting configuration since the client does not support workspace/configuration
2025-09-22T10:22:58.255861Z | Debug | LSP: set new config: {  }
2025-09-22T10:22:58.255947Z | Debug | Shake session initialized
2025-09-22T10:22:58.256052Z | Debug | Finished build session
AsyncCancelled
HasCallStack backtrace:
  collectBacktraces, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:92:13 in ghc-internal:GHC.Internal.Exception
  toExceptionWithBacktrace, called at libraries/ghc-internal/src/GHC/Internal/IO.hs:260:11 in ghc-internal:GHC.Internal.IO
  throwIO, called at libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs:195:43 in ghc-internal:GHC.Internal.Control.Exception.Base


2025-09-22T10:22:58.256071Z | Debug | Configuration changed: Config {checkParents = CheckOnSave, checkProject = True, formattingProvider = "ormolu", cabalFormattingProvider = "cabal-gild", maxCompletions = 40, sessionLoading = PreferSingleComponentLoading, plugins = fromList []}
2025-09-22T10:22:58.256263Z | Debug | Restarting build session due to config change
Action Queue: []
Keys: [GetClientSettings; ]
Aborting previous build session took 0.00s 
2025-09-22T10:22:58.256388Z | Debug | LSP: not requesting configuration since the client does not support workspace/configuration
2025-09-22T10:22:58.256529Z | Debug | VFS: opening file:///home/enrico/uffa.hs
2025-09-22T10:22:58.256868Z | Debug | Finished build session
AsyncCancelled
HasCallStack backtrace:
  collectBacktraces, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:92:13 in ghc-internal:GHC.Internal.Exception
  toExceptionWithBacktrace, called at libraries/ghc-internal/src/GHC/Internal/IO.hs:260:11 in ghc-internal:GHC.Internal.IO
  throwIO, called at libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs:195:43 in ghc-internal:GHC.Internal.Control.Exception.Base


2025-09-22T10:22:58.256907Z | Debug | Set files of interst to
    [(/home/enrico/uffa.hs, Modified {firstOpen = True})]
2025-09-22T10:22:58.257123Z | Debug | Restarting build session due to /home/enrico/uffa.hs (modified)
Action Queue: []
Keys: [ GetClientSettings; 
      , GetModificationTime; /home/enrico/uffa.hs
      , IsFileOfInterest; /home/enrico/uffa.hs ]
Aborting previous build session took 0.00s 
2025-09-22T10:22:58.257199Z | Debug | Opened text document: file:///home/enrico/uffa.hs
2025-09-22T10:22:58.257556Z | Info | Cradle path: uffa.hs
2025-09-22T10:22:58.257633Z | Warning | No [cradle](https://github.com/mpickering/hie-bios#hie-bios) found for uffa.hs.
Proceeding with [implicit cradle](https://hackage.haskell.org/package/implicit-hie).
You should ignore this message, unless you see a 'Multi Cradle: No prefixes matched' error.
2025-09-22T10:22:58.259544Z | Debug | Cradle: Cradle{ cradleRootDir = "/home/enrico", cradleOptsProg = CradleAction: Default}
2025-09-22T10:22:58.259607Z | Info | invoking build tool to determine build flags (this may take some time depending on the cache)
2025-09-22T10:22:58.290384Z | Debug | ghc --print-libdir
2025-09-22T10:22:58.319649Z | Debug | ghc --numeric-version
2025-09-22T10:22:58.319843Z | Debug | Session loading result: Right (ComponentOptions {componentOptions = ["-dynamic"], componentRoot = "/home/enrico", componentDependencies = []},"/home/enrico/.ghcup/ghc/9.10.1/lib/ghc-9.10.1/lib","9.10.1")
2025-09-22T10:22:58.326563Z | Info | Interface files cache directory: /home/enrico/.cache/ghcide/main-1a596a151463f2c53ee4feb14ecd276a1ccebfda-1a596a151463f2c53ee4feb14ecd276a1ccebfda
2025-09-22T10:22:58.332026Z | Info | Making new HscEnv. In-place unit ids: [ main-1a596a151463f2c53ee4feb14ecd276a1ccebfda ]
2025-09-22T10:22:58.428986Z | Debug | New component cache HscEnvEq: (([],Just HscEnvEq 5),fromList [])
2025-09-22T10:22:58.429657Z | Debug | Finished build session
AsyncCancelled
HasCallStack backtrace:
  collectBacktraces, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:92:13 in ghc-internal:GHC.Internal.Exception
  toExceptionWithBacktrace, called at libraries/ghc-internal/src/GHC/Internal/IO.hs:260:11 in ghc-internal:GHC.Internal.IO
  throwIO, called at libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs:195:43 in ghc-internal:GHC.Internal.Control.Exception.Base


2025-09-22T10:22:58.429722Z | Debug | Known files updated:
  fromList [(TargetFile NormalizedFilePath "/home/enrico/uffa.hs",fromList ["/home/enrico/uffa.hs"])]
2025-09-22T10:22:58.429900Z | Debug | Restarting build session due to new component
Action Queue: []
Keys: [ IsFileOfInterest; /home/enrico/uffa.hs
      , GhcSessionIO; 
      , GetKnownTargets;  ]
Aborting previous build session took 0.00s 
2025-09-22T10:22:58.453591Z | Debug | Finished: InitialLoad Took: 0.02s
2025-09-22T10:23:17.675151Z | Debug | Finished: GhcideCodeActions.GetFileContents Took: 0.00s
2025-09-22T10:23:17.675735Z | Debug | Finished: cabal.cabal-add Took: 0.00s
2025-09-22T10:23:17.676997Z | Debug | eval: fp /home/enrico/uffa.hs
2025-09-22T10:23:17.678147Z | Debug | Finished: Pragmas.GhcSession Took: 0.00s
2025-09-22T10:23:17.678432Z | Debug | Finished: classplugin.codeAction.getVersionedTextDoc Took: 0.00s
2025-09-22T10:23:17.678516Z | Debug | Finished: changeTypeSignature.GetParsedModule Took: 0.00s
2025-09-22T10:23:17.678738Z | Debug | Finished: GhcideCodeActions.GetFileContents Took: 0.00s
2025-09-22T10:23:17.679058Z | Debug | Finished: QualifyImportedNames.TypeCheck Took: 0.00s
2025-09-22T10:23:17.679114Z | Debug | Finished: Pragmas.GetFileContents Took: 0.00s
2025-09-22T10:23:17.679408Z | Debug | Finished: classplugin.findClassIdentifier.GetHieAst Took: 0.00s
2025-09-22T10:23:17.679419Z | Debug | Finished: Pragmas.GetParsedModule Took: 0.00s
2025-09-22T10:23:17.679440Z | Debug | Finished: Pragmas.GhcSession Took: 0.00s
2025-09-22T10:23:17.679985Z | Debug | Finished: Pragmas.GetFileContents Took: 0.00s
2025-09-22T10:23:17.680244Z | Debug | Finished: classplugin.findClassFromIdentifier.GhcSessionDeps Took: 0.00s
2025-09-22T10:23:17.680295Z | Debug | Finished: Pragmas.GetParsedModule Took: 0.00s
2025-09-22T10:23:17.680637Z | Debug | overloaded-record-dot: Collected record selectors: []
2025-09-22T10:23:17.681264Z | Debug | Finished: classplugin.findClassFromIdentifier.TypeCheck Took: 0.00s
2025-09-22T10:23:17.681376Z | Debug | Finished: GADT.GetParsedModuleWithComments Took: 0.00s
2025-09-22T10:23:17.681382Z | Debug | Finished: GhcideCodeActions.GetParsedModuleWithComments Took: 0.00s
2025-09-22T10:23:17.682060Z | Debug | Finished: GhcideCodeActions.GetGlobalBindingTypeSigs Took: 0.01s
2025-09-22T10:23:17.683317Z | Debug | Finished: GhcideCodeActions.GetFileContents Took: 0.00s
2025-09-22T10:23:17.684632Z | Debug | explicit-fields: Collected records with wildcards: []
2025-09-22T10:23:17.684684Z | Debug | Finished: GhcideCodeActions.GetAnnotatedParsedSource Took: 0.01s
2025-09-22T10:23:17.685603Z | Debug | Finished: GhcideCodeActions.getIdeOptions Took: 0.00s
2025-09-22T10:23:17.685645Z | Debug | Finished: GhcideCodeActions.GhcSession Took: 0.00s
2025-09-22T10:23:17.686879Z | Debug | Finished: GhcideCodeActions.GetParsedModuleWithComments Took: 0.00s
2025-09-22T10:23:17.689328Z | Debug | Finished: eval.GetParsedModuleWithComments Took: 0.01s
2025-09-22T10:23:17.690561Z | Debug | eval: comments Comments {lineComments = fromList [], blockComments = fromList []}
2025-09-22T10:23:17.690612Z | Debug | Finished: alternateNumberFormat.CollectLiterals Took: 0.01s
2025-09-22T10:23:17.690702Z | Debug | Finished: GhcideCodeActions.GetParsedModuleWithComments Took: 0.00s
2025-09-22T10:23:17.690723Z | Debug | Finished: ExplicitFields.CollectRecords Took: 0.02s
2025-09-22T10:23:17.691028Z | Debug | Finished: GhcideCodeActions.GetAnnotatedParsedSource Took: 0.00s
2025-09-22T10:23:17.691105Z | Debug | eval: Tests 0 tests in 0 sections 0 setups 0 lenses.
2025-09-22T10:23:17.691319Z | Debug | Finished: overloadedRecordDot.collectRecordSelectors Took: 0.01s
2025-09-22T10:23:17.691565Z | Debug | Finished: alternateNumberFormat.GhcSession Took: 0.00s
2025-09-22T10:23:17.691680Z | Debug | Finished: classplugin.codeAction.GetInstanceBindTypeSigs Took: 0.01s
2025-09-22T10:23:17.691730Z | Debug | Finished: alternateNumberFormat.GetFileContents Took: 0.00s
2025-09-22T10:23:17.691797Z | Debug | Finished: ImportActions Took: 0.01s
2025-09-22T10:23:17.691844Z | Debug | Finished: classplugin.codeAction.TypeCheck Took: 0.00s
2025-09-22T10:23:17.691873Z | Debug | Finished: classplugin.codeAction.GhcSession Took: 0.00s
2025-09-22T10:23:17.691880Z | Info | class: Detected implemented methods for class "Semigroup": []
2025-09-22T10:23:17.692243Z | Debug | eval: tests: 0.00s
2025-09-22T10:23:17.693035Z | Debug | eval: evalMkRangeCommands: 0.02s
2025-09-22T10:23:17.737121Z | Debug | Finished: GhcideCodeActions.GetAnnotatedParsedSource Took: 0.00s
2025-09-22T10:23:17.738288Z | Debug | Finished: GhcideCodeActions.TypeCheck Took: 0.00s
2025-09-22T10:23:17.738352Z | Debug | Finished: GhcideCodeActions.GetHieAst Took: 0.00s
2025-09-22T10:23:17.742407Z | Debug | Finished: hls-refactor-plugin.codeAction.getUriContents Took: 0.00s
2025-09-22T10:23:17.742507Z | Debug | Finished: GhcideCodeActions.getParsedModule Took: 0.00s
2025-09-22T10:23:18.522359Z | Debug | Finished: classplugin.addMethodPlaceholders.GetParsedModule Took: 0.00s
2025-09-22T10:23:18.522499Z | Debug | Finished: classplugin.addMethodPlaceholders.GhcSessionDeps Took: 0.00s
2025-09-22T10:23:18.523227Z | Debug | Finished: classplugin.insertPragmaIfNotPresent.GhcSession Took: 0.00s
2025-09-22T10:23:18.523363Z | Debug | Finished: classplugin.insertPragmaIfNotPresent.GetFileContents Took: 0.00s
2025-09-22T10:23:18.523647Z | Debug | Finished: classplugin.insertPragmaIfNotPresent.GetParsedModuleWithComments Took: 0.00s
2025-09-22T10:23:18.561087Z | Debug | Finished build session
AsyncCancelled
HasCallStack backtrace:
  collectBacktraces, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:92:13 in ghc-internal:GHC.Internal.Exception
  toExceptionWithBacktrace, called at libraries/ghc-internal/src/GHC/Internal/IO.hs:260:11 in ghc-internal:GHC.Internal.IO
  throwIO, called at libraries/ghc-internal/src/GHC/Internal/Control/Exception/Base.hs:195:43 in ghc-internal:GHC.Internal.Control.Exception.Base


2025-09-22T10:23:18.561218Z | Debug | Set files of interst to
    [(/home/enrico/uffa.hs, Modified {firstOpen = False})]
2025-09-22T10:23:18.561730Z | Debug | Restarting build session due to /home/enrico/uffa.hs (modified)
Action Queue: []
Keys: [ GetModificationTime; /home/enrico/uffa.hs
      , IsFileOfInterest; /home/enrico/uffa.hs ]
Aborting previous build session took 0.00s 
2025-09-22T10:23:18.561902Z | Debug | Modified text document: file:///home/enrico/uffa.hs

[Aster89]

I think I attached the wrong log above.

Looking at the other logs, I found that the request that my client sends to HLS is the following:

{
  "id":5,
  "jsonrpc":"2.0",
  "method":"workspace/executeCommand",
  "params":{
    "arguments":[
      {
        "methodGroup":[
          [
            "<>",
            "(<>) :: forall k (a :: k). Set a -> Set a -> Set a"
          ]
        ],
        "range":{
          "end":{
            "character":26,
            "line":2
          },
          "start":{
            "character":9,
            "line":2
          }
        },
        "verTxtDocId":{
          "uri":"file:///home/enrico/uffa.hs",
          "version":5
        },
        "withSig":false
      }
    ],
    "command":"722683:class:classplugin.codeaction"
  }
}

which looks correct to me, and the response from HLS is this:

{
  "id":2,
  "jsonrpc":"2.0",
  "method":"workspace/applyEdit",
  "params":{
    "edit":{
      "documentChanges":[
        {
          "edits":[
            {
              "newText":"  (<>) = _\\n",
              "range":{
                "end":{
                  "character":0,
                  "line":3
                },
                "start":{
                  "character":0,
                  "line":3
                }
              }
            },
            {
              "newText":"\\n\\n\\n\\ninstanceMonoid (Set a) where",
              "range":{
                "end":{
                  "character":29,
                  "line":4
                },
                "start":{
                  "character":0,
                  "line":4
                }
              }
            }
          ],
          "textDocument":{
            "uri":"file:///home/enrico/uffa.hs",
            "version":5
          }
        }
      ]
    }
  }
}

which has clearly 2 entries in "edits", where the second one is the one that screws up the code.

[Aster89]

Made some more tests:

GHC	HLS	Reproducible
9.8.4	2.10.0.0	no
9.8.4	2.11.0.0	no
9.10.1	2.10.0.0	yes
9.10.1	2.11.0.0	yes

This suggests the bug is in GHC 9.10.1, rather in HLS. As suggested on https://web.libera.chat/#haskell-language-server, the bug might well be in HLS, which has lots of #if branching on GHC versions in files like https://github.com/haskell/haskell-language-server/blob/master/plugins/hls-class-plugin/src/Ide/Plugin/Class/ExactPrint.hs

[Aster89]

I investigated a bit more, and I think the bug is somewhere in here:

haskell-language-server/plugins/hls-class-plugin/src/Ide/Plugin/Class/ExactPrint.hs

Lines 72 to 82 in 91d2b48

	instRow = srcSpanEndLine instSpan
	methodEpAnn = noAnnSrcSpanDP $ deltaPos 1 (instCol + defaultIndent)
	-- Put each TyCl method/type signature on separate line, indented by 2 spaces relative to instance decl
	newLine (L _ e) = L methodEpAnn e
	-- Set DeltaPos for following declarations so they don't move undesirably
	resetFollowing =
	over _head (\followingDecl ->
	let followingDeclRow = srcSpanStartLine $ realSrcSpan $ getLoc followingDecl
	delta = DifferentLine (followingDeclRow - instRow) instCol
	in setEntryDP followingDecl delta)

At least, I've verified that playing around with followingDeclRow - instRow, e.g. by subtracting 1 or 2, there's a change in the spuriously added empty lines. I've not really understood where instance Monoid is screwed into instanceMonoid, though.

I've looked up the history, and it seems like the bug was introduced with 3009a45, which was precisely aimed at supporting GHC 9.10.

@fendor , since you're among the authors of 3009a45, do you have any clue?

I'm a bit out of ideas, as I don't know how to experiment with the code. What I mean is that I know the bug is in addMethodDecls, but I don't know how to experiment with it in isolation. Do you have any guidance?