GHC Weekly News - 2014/10/31 (Halloween Edition)

Austin Seipp austin at
Fri Oct 31 23:58:00 UTC 2014

Hello *,

Welcome to the GHC Weekly news. And it's just in time before you go
out and eat lots of candy and food.

  * David Feuer and Joachim Brietner spent some time this past week
talking about more optimizations for Haskell code for fusing code, and
creating better consumers and producers. This work includes
optimizations of "One shot lambdas" (lambdas used at most once) and
Call Arity, which was implemented by Joachim at Microsoft Research.
The thread is here -
 The current situation is that Call Arity and One shot analysis tends
to have good combined results for exploiting more fusion
opportunities, but sometimes these backfire. As a result, Joachim and
David have worked on improving the situation - particularly by letting
programmers help with a new `oneShot` primitive (in Phab:D392 &
  * Herbert Valerio Riedel opened up a discussion about the origins of
code contributions. In short, we'd like to have some stronger ground
to stand on in the face of contributions from contributors - the
origin of a change and its legal status is a bit nebulous. The thread
is here:
 Overall, there's been a lot of separate points made, including CLAs
(unlikely), "Developer Certificates of Origin" a la the Linux Kernel,
and reworking how we mark up header files, and keep track of GHC's
long history of authors.
 If you work on a big project where some of these concerns are real,
we'd like to hear what you have to say!
  * Gintautas Milauskas has done some fantastic work for GHC on
Windows lately, including fixing tests, improving the build, and
making things a lot more reasonable to use. With his work, we hope GHC
7.10 will finally ship an updated MinGW compiler (a long requested
feature), and have a lot of good fixes for windows. Thank you,
  * And on that note, the call for Windows developers rages on - it
looks like Gintautaus, Austin, Simon, and others will be meeting to
discuss the best way to tackle our current woes. Are you a Windows
user? Please give us input - having input is a crucial part of the
decision making process, so let us know.
  * Jan Stolarek had a question about core2core - a lot of questions,
in fact. What's the difference between demand, strictness, and
cardinality analylsis? Does the demand analyzer change things? And
what's going on in some of the implementation? A good read if you're
interested in deep GHC optimization magic:
  * Peter Wortmann has put up the new DWARF generation patches for
review, in Phab:D396. This is one of the major components we still
plan on landing in 7.10, and with a few weeks to spare, it looks like
we can make sure it's merged for the STABLE freeze!
  * There have been a lot of good changes in the tree this past week:
 Thanks to Michael Orlitzky, we plan on adding doctest examples to
more modules in 7.10, and increase that coverage further. This is
*really* important work, but very low fruit - thanks a ton Michael!

 `Data.Bifunctor` is now inside base! (Phab:D336)

 `atomicModifyIORef'` has been optimized with excellent speedups (as
much as 1.7x to 1.4x, depending on the RTS used), thanks to some older
work by Patrick Palka (Phab:D315).
 GHC's internals have been reworked to unwire `Integer` from GHC,
leading not only to a code cleanup, but laying the foundation for
further GMP (and non-GMP!) related `Integer` improvements (Phab:D351).

 David Feuer and Joachim have been relentless in improving fusion
opportunities, including the performance of `take`, `isSuffixOf`, and
more prelude improvements, spread over nearly half a dozen patches.
And this doesn't even include the work on improving `oneShot` or Call

 In a slight change to `base` semantics, David Feuer also finally
fixed #9236. This is a change that can expose latent bugs in your
program (as it did for Haddock), so be sure to test thoroughly with
7.10 (Phab:D327).

 GHC now has support for a new `__GLASGOW_HASKELL_TH__` macro,
primarily useful for testing bootstrap compilers, or compilers which
don't support GHCi.

And there have been many closed tickets: #9549, #9593, #9720, #9031,
#8345, #9439, #9435, #8825, #9006, #9417, #9727, #2104, #9676, #2628,
#9510, #9740, #9734, #9367, #9726, #7984, #9230, #9681, #9747, and


Austin Seipp, Haskell Consultant
Well-Typed LLP,

More information about the ghc-devs mailing list