Status of the rich errors proposal

Javier Neira Sanchez atreyu.bbb at
Wed Oct 23 06:52:32 UTC 2019

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)


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

extractImportableTerm dirtyMsg = do

  (n, s) <- extractedTerm

  let n' = T.strip n

  return (n', s)


    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 =


          >>= 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:

that nicely will improve the state of things.

Otoh there are already a way to output ghc errors as json (see 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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