Hello Javier,

Ben and I have indeed been thinking about how to improve the situation, and have been talking to a few fellow Haskellers in the tooling/IDE space to get a first round of feedback on the ideas that we came up with. I will soon be writing down a proposal for one of those ideas, that is likely to work well for everyone (GHC devs, tooling authors and more generally GHC API consumers).

Once that document is ready and up on github, I wil make sure to send the link here on ghc-devs@.

Hi, lately i am being collaborated in the haskell-ide-engine (hie) repo and one of the issues i get to fix was related with the parsing of ghc errors.
It turns out that delimiters of terms in errors are different in windows (`term') and *nix (‘term’) systems and that drive to parse errors.
Quite code actions and diagnostics are based in parsing them and they were broken in windows.
I am afraid that the code is very brittle and close to human readable error messages. the actual code look like this:

-- | Extract a term from a compiler message.

-- It looks for terms delimited between '‘' and '’' falling back to '`' and '\''

-- (the used ones in Windows systems).

extractTerm :: T.Text -> T.Text

extractTerm txt =

  case extract '‘' '’' txt of

    ""  -> extract '`' '\'' txt -- Needed for windows

    term -> term

  where extract b e = T.dropWhile (== b)

                    . T.dropWhileEnd (== e)

                    . T.dropAround (\c -> c /= b && c /= e)

or

extractImportableTerm :: T.Text -> Maybe (T.Text, SymbolImport SymbolType)

extractImportableTerm dirtyMsg = do

  (n, s) <- extractedTerm

  let n' = T.strip n

  return (n', s)

  where

    importMsg = S.headMay

      -- Get rid of the rename suggestion parts

      $ T.splitOn "Perhaps you meant "

      $ T.replace "\n" " "

      -- Get rid of trailing/leading whitespace on each individual line

      $ T.unlines

      $ map T.strip

      $ T.lines

      $ T.replace "* " "" -- Needed for Windows

      $ T.replace "• " "" dirtyMsg

    extractTerm prefix symTy =

      importMsg

          >>= T.stripPrefix prefix

          >>= \name -> Just (name, Import symTy)

    extractType b =

      extractTerm ("Not in scope: type constructor or class " <> b) Type

    extractedTerm = asum

      [ extractTerm "Variable not in scope: " Symbol

      , extractType "‘"

      , extractType "`" -- Needed for windows

      , extractTerm "Data constructor not in scope: " Constructor]

It is clearly unsatisfactory but hard to improve without changing the messages to make it more structured.
Moreover any legitimate change on errors to make it better will likely break it.

After exposing my worries in the hie irc channel, @mpickering pointed out that it is already a proposal to improve error messages:

https://github.com/bgamari/ghc-proposals/blob/rich-errors-proposal/proposals/0000-rich-errors-proposal.rst

that nicely will improve the state of things.

Otoh there are already a way to output ghc errors as json (see https://gitlab.haskell.org/ghc/ghc/issues/13190). It contains valuable info about the error in specific fields but the message itself is in plain text.
So merging both features will let tools to handle compiler errors without use the ghc api directly if needed.

what is the status of the proposal? hie an other tooling developers will welcome it very heartly.

Thanks in advance!

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