Data constructors made of symbols have their brackets stripped, leading to syntax errors

[kozross]

Problem

If you define a data constructor which is made up of : followed by symbols (that is, designed for infix use), hindent will strip away the parentheses around it, which becomes a syntax error.

• I checked the issue tracker for existing issues about this problem.
• I'm using the latest version of hindent.

Input haskell

data HList (ts :: [Type]) where
  HNil :: HList '[]
  (:#) :: t -> HList ts -> HList (t ': ts)

Expected output haskell

data HList (ts :: [Type]) where
  HNil :: HList '[]
  (:#) :: t -> HList ts -> HList (t  ':  ts)

Actual output haskell

data HList (ts :: [Type]) where
  HNil :: HList '[]
  :# :: t -> HList ts -> HList (t ': ts)

[kozross]

I just observed that this also happens to functions named to be infix.