Haddock comments on un-named fields are not handled correctly

[neongreen]

Parsing of formatted code failed

Input:

data Foo
  = -- | Bar
    Bar
      Field1 -- ^ Field 1
      Field2 -- ^ Field 2

Error:

Parsing of formatted code failed:
  <stdin><rendered>:5:7-12
  parse error on input ‘Field1’

Output with --unsafe:

data Foo
  = -- | Bar
    Bar
      -- | Field 1
      Field1
      -- | Field 2
      Field2

The behavior of the Haddock parser is horrifying.

Lost comments

Input:

data Foo
  = -- | Bar
    Bar
      Field1 -- ^ Field 1

Output:

data Foo
  = -- | Bar
    Bar
      Field1

[neongreen]

Note: this is a minimized real-world example taken from GHC's sources.

[mrkkrp]

Is this a bug in Ormolu or in Haddock? If latter, there is not much we can do except for reporting it.

[neongreen]

Well, it's a bug in GHC, essentially: haskell/haddock#1065.

The only way to work around it is to use -- ^ instead of -- |.

data Foo
  = Bar
    -- ^ Constructor
      Int
      -- ^ Field 1

I.e. for any constructors that have unnamed fields with haddocks, we will have to use this alternative syntax.

[neongreen]

As for the second issue (lost comments), yeah, it's possible that there's nothing we can do about it. We might be able to parse with and without Opt_Haddock and compare the trees, but I guess it's too much work.

[neongreen]

https://gitlab.haskell.org/ghc/ghc/issues/17419

[mrkkrp]

Related (almost the same issue): #445.