How to use ghc-exactprint for code transformations?

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The context is as follows: I am working on a code action in haskell-language-server to automatically fix “Data constructor is not in scope” errors. To give an example, in the following code

module Parser where

data ParseError

data NestedBrackets
  = Empty
  | Nested NestedBrackets
  | Concat NestedBrackets NestedBrackets

parseBrackets :: Int -> String -> Either ParseError NestedBrackets
parseBrackets depth chars = case chars of
  [] ->
    if depth == 0
      then pure Empty
      else Left $ UnexpectedEof depth

token UnexpectedEof is going to be highlighted with red squiggly lines in VSCode, and the error message suffices to make it possible to add a new data constructor to ParseError automatically. The core part of the code that does the transformation looks as follows:

addConstructor :: MissingConstructor -> GenLocated SrcSpanAnnA (HsDecl GhcPs) -> GenLocated SrcSpanAnnA (HsDecl GhcPs)
addConstructor mc decl = case unLoc decl of
  TyClD a (DataDecl {tcdDataDefn=tcdDataDefn, tcdLName=tcdLName, ..}) ->
    case tcdDataDefn of
      HsDataDefn {dd_cons=(DataTypeCons b cs), ..} -> flip mapLoc decl $ \_ ->
        let newConBuilder :: DataConBuilder = PrefixDataConBuilder (toOL $ _conArgTypes mc) (_conName mc)  in
        let newCon :: ConDecl GhcPs = ConDeclH98 EpAnnNotUsed (_conName mc) False [] Nothing (dataConBuilderDetails newConBuilder) Nothing in
        let dataDefn = HsDataDefn { dd_cons=DataTypeCons b (L noSrcSpanA newCon : cs), .. } in
        TyClD a (DataDecl { tcdDataDefn = dataDefn, .. })
      _ -> decl
  _ -> decl

createTransform :: HasDecls a => MissingConstructor -> a -> TransformT Maybe a
createTransform mc a = do
  decls <- hsDecls a
  (prefix, right : suffix) <- pure $ break (isRightDecl mc) decls
  replaceDecls a (prefix ++ addConstructor mc right : suffix)

The problem is that when I use the modified tree to get the concrete text out of it, it does not render correctly. Specifically, the “source” I get in the end looks like that:

data ParseErrorUnexpectedEofInt

I was expecting the contract of ghc-exactprint to be that if I have a modified tree that it’s going to be printed to a correct syntax, is that not the case? Or maybe I’m breaking some invariants by constructing the parse tree the way that I do and I should use some smart constructors instead?

I have looked around for documentation on how to perform the transformations, but haven’t found anything so far. Haddock is not particularly informative.

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