[commit: ghc] ghc-8.0: Remove superfluous code when deriving Foldable/Traversable (9d7f890)
git at git.haskell.org
git at git.haskell.org
Sat Feb 27 21:55:15 UTC 2016
Repository : ssh://git@git.haskell.org/ghc
On branch : ghc-8.0
Link : http://ghc.haskell.org/trac/ghc/changeset/9d7f890ca2408ca7671e6a6d06dff95341d869f0/ghc
>---------------------------------------------------------------
commit 9d7f890ca2408ca7671e6a6d06dff95341d869f0
Author: RyanGlScott <ryan.gl.scott at gmail.com>
Date: Wed Feb 17 12:06:17 2016 +0100
Remove superfluous code when deriving Foldable/Traversable
Currently, `-XDeriveFoldable` and `-XDeriveTraversable` generate
unnecessary `mempty` and `pure` expressions when it traverses of an
argument of a constructor whose type does not mention the last type
parameter. Not only is this inefficient, but it prevents `Traversable`
from being derivable for datatypes with unlifted arguments (see
Trac #11174).
The solution to this problem is to adopt a slight change to the
algorithms for `-XDeriveFoldable` and `-XDeriveTraversable`, which is
described in [this wiki
page](https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/DeriveFu
nctor#Proposal:alternativestrategyforderivingFoldableandTraversable).
The wiki page also describes why we don't apply the same changes to the
algorithm for `-XDeriveFunctor`.
This is techincally a breaking change for users of `-XDeriveFoldable`
and `-XDeriveTraversable`, since if someone was using a law-breaking
`Monoid` instance with a derived `Foldable` instance (i.e., one where `x
<> mempty` does not equal `x`) or a law-breaking `Applicative` instance
with a derived `Traversable` instance, then the new generated code could
result in different behavior. I suspect the number of scenarios like
this is very small, and the onus really should be on those users to fix
up their `Monoid`/`Applicative` instances.
Fixes #11174.
Test Plan: ./validate
Reviewers: hvr, simonpj, austin, bgamari
Reviewed By: simonpj, bgamari
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D1908
GHC Trac Issues: #11174
(cherry picked from commit a82956df5b34175410e0feb9e2febe7d39b60b49)
>---------------------------------------------------------------
9d7f890ca2408ca7671e6a6d06dff95341d869f0
compiler/typecheck/TcGenDeriv.hs | 478 ++++++++++++++++++----
compiler/utils/Util.hs | 19 +-
docs/users_guide/8.0.1-notes.rst | 5 +-
docs/users_guide/glasgow_exts.rst | 24 +-
testsuite/tests/deriving/should_compile/T11174.hs | 14 +
testsuite/tests/deriving/should_compile/all.T | 1 +
6 files changed, 445 insertions(+), 96 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 9d7f890ca2408ca7671e6a6d06dff95341d869f0
More information about the ghc-commits
mailing list