[commit: ghc] master: Derive the definition of null (bf5e0ea)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/bf5e0eab60a11d494671793740122e381a707c1a/ghc

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

commit bf5e0eab60a11d494671793740122e381a707c1a
Author: David Feuer <david.feuer at gmail.com>
Date:   Sun Apr 2 16:20:20 2017 -0400

    Derive the definition of null
    
    We can sometimes produce much better code by deriving the
    definition of `null` rather than using the default. For example,
    given
    
        data SnocList a = Lin | Snoc (SnocList a) a
    
    the default definition of `null` will walk the whole list, but of
    course we can stop as soon as we see `Snoc`. Similarly, if a
    constructor contains some other `Foldable` type, we want to use its
    `null` rather than folding over the structure.
    
    Partially fixes Trac #13280
    
    Reviewers: austin, bgamari, RyanGlScott
    
    Reviewed By: RyanGlScott
    
    Subscribers: rwbarton, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3402


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

bf5e0eab60a11d494671793740122e381a707c1a
 compiler/prelude/PrelNames.hs                      |   5 +-
 compiler/typecheck/TcGenFunctor.hs                 | 132 ++++++++++++++++++++-
 docs/users_guide/8.4.1-notes.rst                   |  50 ++++----
 docs/users_guide/glasgow_exts.rst                  |  45 +++++--
 .../tests/perf/should_run/DeriveNullTermination.hs |  17 +++
 .../should_run/DeriveNullTermination.stdout}       |   0
 testsuite/tests/perf/should_run/all.T              |   7 +-
 7 files changed, 216 insertions(+), 40 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 bf5e0eab60a11d494671793740122e381a707c1a