1. 30 Mar, 2017 1 commit
    • David Feuer's avatar
      Deriving for phantom and empty types · 69f070d8
      David Feuer authored
      Make `Functor`, `Foldable`, and `Traversable` take advantage
      of the case where the type parameter is phantom. In this case,
      
      * `fmap _ = coerce`
      * `foldMap _ _ = mempty`
      * `traverse _ x = pure (coerce x)`
      
      For the sake of consistency and especially simplicity, make other types
      with no data constructors behave the same:
      
      * `fmap _ x = case x of`
      * `foldMap _ _ = mempty`
      * `traverse _ x = pure (case x of)`
      
      Similarly, for `Generic`,
      
      * `to x = case x of`
      * `from x = case x of`
      
      Give all derived methods for types without constructors appropriate
      arities. For example,
      
      ```
          compare _ _ = error ...
      ```
      
      rather than
      
      ```
          compare = error ...
      ```
      
      Fixes #13117 and #13328
      
      Reviewers: austin, bgamari, RyanGlScott
      
      Reviewed By: RyanGlScott
      
      Subscribers: ekmett, RyanGlScott, rwbarton, thomie
      
      Differential Revision: https://phabricator.haskell.org/D3374
      69f070d8