[commit: ghc] master: Deriving for phantom and empty types (69f070d)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/69f070d8e4d6043937e3405675ac911448bfcb44/ghc

>---------------------------------------------------------------

commit 69f070d8e4d6043937e3405675ac911448bfcb44
Author: David Feuer <david.feuer at gmail.com>
Date:   Thu Mar 30 13:30:52 2017 -0400

    Deriving for phantom and empty types
    
    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


>---------------------------------------------------------------

69f070d8e4d6043937e3405675ac911448bfcb44
 compiler/typecheck/TcGenDeriv.hs                   | 114 ++++++-----
 compiler/typecheck/TcGenFunctor.hs                 | 216 +++++++++++++++++----
 compiler/typecheck/TcGenGenerics.hs                |  15 +-
 docs/users_guide/8.4.1-notes.rst                   |  78 ++++++++
 docs/users_guide/glasgow_exts.rst                  |  74 ++++++-
 docs/users_guide/index.rst                         |   1 +
 testsuite/tests/deriving/should_compile/all.T      |   5 +
 .../deriving/should_compile/drv-empty-data.hs      |  19 ++
 .../deriving/should_compile/drv-empty-data.stderr  |  68 +++++++
 .../tests/deriving/should_compile/drv-phantom.hs   |  12 ++
 .../deriving/should_compile/drv-phantom.stderr     |  18 ++
 .../tests/generics/T10604/T10604_deriving.stderr   | 128 ++++++------
 testsuite/tests/perf/compiler/T13056.hs            |   4 +
 13 files changed, 584 insertions(+), 168 deletions(-)

Diff suppressed because of size. To see it, use:

    git diff-tree --root --patch-with-stat --no-color --find-copies-harder --ignore-space-at-eol --cc 69f070d8e4d6043937e3405675ac911448bfcb44