GHC Weekly News - 2015/02/10

Austin Seipp austin at
Tue Feb 10 23:17:36 UTC 2015

Hi *,

Welcome! This is the first GHC Weekly news of February 2015. You might
be wondering what happened to the last one. Well, your author was just
in New York for the past week attending
[ Compose Conference], making friends
and talking a lot about Haskell (luckily we missed a snow storm that
may have messed it up quite badly!)

The conference was great. I got to have some interesting discussions
about GHC and Haskell with many friendly faces from all around at an
incredibly well run conference with a stellar set of people. Many
thanks to NY Haskell (organizing), Spotify (hosting space), and to all
the speakers for a wonderful time. (And of course, your editor would
like to thank his employer Well-Typed for sending him!)

But now, since your author has returned, GHC HQ met back up this week
for some discussion, with some regularly scheduled topics. For the
most part it was a short meeting this week - our goals are pretty well

  - GHC HQ and the Core Libraries Committee have posted a survey on
the future of the 7.10 prelude and the FTP/BBP discussion. The
deadline is February 20th, so please vote if the discussion is of
interest to you. Simon Peyton-Jones and Simon Marlow will be making
the final decision.

  - It's likely GHC HQ will do a third 7.10.1 Release Candidate at the
very end of February after the votes are included. We missed some
patches in RC2 (such as Phab:D347) and incorporated even more
bugfixes, so this is worth a test drive by users.

  - For the most part, things for 7.10 have been going very smoothly
other than the debates and a few bugs trickling in - there has not
been much ticket activity the past two weeks, so things feel pretty
good right now. Austin will mostly be focused on shipping 7.10 and
keeping the normal review/patch/triaging cycles going until it's done.
We're on track to fix all the major bugs we've assigned (see

Since my last post, we've also had other random assorted chatter on
the mailing lists by the dev team:

  - In light of a recent large bug in GHC which can be used to derive
    `unsafeCoerce`, GHC HQ '''has decided to push back the 7.10
    release a bit longer to about March''', in order to fix this bug
    and ferret out the little fallout afterwords. It turns out this
    isn't a simple bug to fix, but luckily a fix is being worked on

  - Luckily, Iavor has started work on fixing this nasty bug, and had
    a few questions for the list:

  - Iavor Diatchki has raised a new topic about a simpler
    OverloadedRecordsField proposal. Adam swooped in to address some
    points about the

  - Herbert Valerio Riedel posted about a huge (76x) regression
    between GHC 7.11 and GHC 7.10, but strangely nobody has picked up
    as to why this is the case yet!

  - David Feuer has a question: why is `undefined` so special? In
    particular, it seems as if `undefined` can be specially used as a
    value with a type of kind `#` as well as `*`. It turns out GHC has
    a special notion of subkinding, and `undefined` has a type more
    special than meets the eye which allows this, as Adam Gundry

  - Merijn Verstraaten has started up a discussion about a new
    proposal of his, ValidateMonoLiterals. The proposal revolves
    around the idea of using GHC to enforce compile-time constraints
    on monomorphic literals, whose type may have invariants enforced
    on them. While this is doable with Template Haskell, Merijn would
    like to see something inside GHC instead.

  - David Feuer asked: can we merge `FlexibleContexts` with
    `FlexibleInstances`? The proposal seems to be relatively
    undiscussed at the moment with a neutral future, but perhaps
    someone would like to chime in on this minor issue.

  - Greg Weber opened up a discussion about 'Restricted Template
    Haskell', which would hopefully make it easier for users to see
    what a TH computation is actually doing. It turns out - as noted
    by Simon - that Typed Template Haskell is perhaps closer to what
    Greg wants. The proposal and discussion then resulted in us
    realising that the typed TH documentation is rather poor!
    Hopefully Greg or someone can swing in to improve things.

Closed tickets the past two weeks include: #10028, #10040, #10031,
#9935, #9928, #2615, #10048, #10057, #10054, #10060, #10017, #10038,
#9937, #8796, #10030, #9988, #10066, #7425, #7424, #7434, #10041,
#2917, #4834, #10004, #10050, #10020, #10036, #9213, and #10047.


Austin Seipp, Haskell Consultant
Well-Typed LLP,

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