Multiple Haddock comments [GHC-05641]

Flag: -Winvalid-haddock

Haddock provides multiple ways of adding documentation strings to entities such as types, constructors or functions. If more than one way is used to document an entity, all but the first documentation comment are ignored, and this warning is emitted.

Examples

The Nothing constructor is documented with two separate Haddock comments

In this example, the Nothing constructor was documented twice, once using a -- | style comment and once using a -- ^ style comment. But only one form of comment is allowed per documented entity. In order to fix this warning, only use one form of documentation comment.

messages/GHC-05641/multi-constructor-comments/before/Multi-constructor-comments.hs:6:7: warning: [GHC-05641] [-Winvalid-haddock]
    Multiple Haddock comments for a single entity are not allowed.
    The extraneous comment will be ignored.
  |
6 |       -- ^ You can use the Nothing constructor where you would use `null`in other languages.
  |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

MultiConstructorComments.hs

Before

module MultiConstructorComments where

data Maybe a =
      -- | The Nothing constructor signals the absence of a value.
      Nothing
      -- ^ You can use the Nothing constructor where you would use `null` in other languages.
    | Just a

After

module MultiConstructorComments where

data Maybe a =
      -- | The Nothing constructor signals the absence of a value.
      -- You can use the Nothing constructor where you would use `null` in other languages.
      Nothing
    | Just a