December 2017 - Glasgow-haskell-users

[ANNOUNCE] GHC 8.4.1-alpha1 available
by Ben Gamari 22 Jan '18

22 Jan '18

The GHC development team is pleased to announce the first alpha release of the 8.4.1 release. The usual release artifacts are available from https://downloads.haskell.org/~ghc/8.4.1-alpha1 Note that this release drops compatibility with GCC 4.6 and earlier. While we generally try to place as few constraints on system toolchain as possible, this release depends upon the __atomic__ builtins provided by GCC 4.7 and later (see #14244). === Notes on release scheduling === The 8.4.1 release marks the first release where GHC will be adhering to its new, higher-cadence release schedule [1]. Under this new scheme, major releases will be made in 6-month intervals with interstitial minor releases as necessary. In order to minimize the likelihood of schedule slippage and to ensure adequate testing, each major release will be preceeded by a number of regular alpha releases. We will begin issuing these releases roughly three months before the final date of the major release and will issue roughly one every two weeks during this period. This high release cadence will allow us to quickly get fixes in to users hands and allow better feedback on the status of the release. GHC 8.4 is slated to be released in mid-February but, due to technical constraints, we are starting the alpha-release cycle a bit later than planned under the above schedule. For this reason, it would be greatly appreciated if users could put this alpha through its paces to make up for lost time. As always, do let us know if you encounter any trouble in the course of testing. Thanks for your help! Cheers, - Ben [1] https://ghc.haskell.org/trac/ghc/blog/2017-release-schedule

3 7

[ANNOUNCE] GHC 8.2.2 released
by Ben Gamari 14 Dec '17

14 Dec '17

=============================================== The Glasgow Haskell Compiler -- version 8.2.2 =============================================== The GHC Team is pleased to announce a new minor release of GHC. This release builds on the performance and stability improvements of 8.2.1, fixing a variety of correctness bugs, improving error messages, and making the compiler more portable. Notable bug-fixes include * A correctness issue resulting in segmentation faults in some FFI-users (#13707, #14346) * A correctness issue resulting in undefined behavior in some programs using STM (#14171) * A bug which may have manifested in segmentation faults in out-of-memory condition (#14329) * clearBit of Natural no longer bottoms (#13203) * A specialisation bug resulting in exponential blowup of compilation time in some specialisation-intensive programs (#14379) * ghc-pkg now works even in environments with misconfigured NFS mounts (#13945) * GHC again supports production of position-independent executables (#13702) * Better error messages around kind mismatches (#11198, #12373, #13530, #13610) A thorough list of the changes in the release can be found in the release notes, https://haskell.org/ghc/docs/8.2.2/html/users_guide/release-8-2-2.html How to get it ~~~~~~~~~~~~~ This release can be downloaded from https://www.haskell.org/ghc/download_ghc_8_2_2.html For older versions see https://www.haskell.org/ghc/ We supply binary builds in the native package format for many platforms, and the source distribution is available from the same place. Background ~~~~~~~~~~ Haskell is a standardized lazy functional programming language. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating efficient code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces. GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below). On-line GHC-related resources ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Relevant URLs on the World-Wide Web: GHC home page https://www.haskell.org/ghc/ GHC developers' home page https://ghc.haskell.org/trac/ghc/ Haskell home page https://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, is here: https://ghc.haskell.org/trac/ghc/wiki/Contributors Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: https://ghc.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC's developer's site: https://ghc.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at https://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-tickets There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see https://mail.haskell.org/cgi-bin/mailman/listinfo Many GHC developers hang out on #haskell on IRC: https://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: https://www.haskell.org/ghc/reportabug

2 2

Re: [was ghc-devs] Reasoning backwards with type families
by Anthony Clayden 14 Dec '17

14 Dec '17

> On Thu Nov 16 01:31:55 UTC 2017, David Feuer wrote: (Moving to ghc-users, there's nothing particularly dev-y.) > Sometimes it woulld be useful to be able to reason backwards > about type families. Hi David, this is a well-known issue/bit of a sore. Discussed much in the debate between type families compared to FunDeps. > For example, we have > > type family a && b where > 'False && b = 'False > 'True && b = b > a && 'False = 'False > a && 'True = a > a && a = a > ... if we know something about the *result*, > GHC doesn't give us any way to learn anything about the arguments. You can achieve the improvement you want today. You'll probably find Oleg has a solution With FunDeps and superclass constraints, it'll go like this class (OnResult r a b, OnResult r b a) => And a b r | a b -> r instance And 'False b 'False -- etc, as you'd expect following the tf equations -- the instances are overlapping but confluent class OnResult r a b | r a -> b instance OnResult 'True a 'True instance OnResult 'False 'True 'False You could equally make `OnResult` a type family. If you can trace backwards to where `&&` is used, you might be able to add suitable constraints there. So there's a couple of futures: * typechecker plugins, using an SMT solver * injective type families the next phase is supposed to allow type family a && b = r | r a -> b, r b -> a where ... That will help with some type families (such as addition of Nats), plug1 https://github.com/AntC2/ghc-proposals/blob/instance-apartness-guards/propo… but I don't see it helping here. plug2 (this example) https://github.com/AntC2/ghc-proposals/blob/instance-apartness-guards/propo… Because (for example) if you unify the first two equations, (that is, looking for coincident overlap) 'False && 'False = 'False 'True && 'False = 'False That's inconsistent on dependency ` r b -> a`. And you can't fix it by re-ordering the closed equations. > For (&&), the obvious things you'd want are ... > > There's nothing inherently impossible about this sort of reasoning. No-ish but. It relies on knowing kind `Bool` is closed. And GHC doesn't pay attention to that. So you need to bring type family `Not` into the reasoning; hence a SMT solver. > ... > I wouldn't necessarily expect GHC > to be able to work something like this out on its own. That's a relief ;-) > But it seems like there should be some way to allow the user > to guide it to a proof. Yes, an SMT solver with a model for kind `Bool`. (And a lot of hard work to teach it, by someone.) AntC

4 9

How to uninstall all cabal packages?
by Volker Wysk 03 Dec '17

03 Dec '17

Hi! I want to remove eveything which cabal has installed, and begin again with a clean installation. How is this accomplished? I've deleted ~/.cabal, but it still says that hsshellscript is already installed: desktop ~ $ cabal install hsshellscript Resolving dependencies... All the requested packages are already installed: hsshellscript-3.4.3 Use --reinstall if you want to reinstall anyway. Bye V.W.

3 4

Rewrite rules involving LHS lambda?
by Conal Elliott 03 Dec '17

03 Dec '17

Is there a written explanation and/or examples of rewrite rules involving a LHS lambda? Since rule matching is first-order, I'm wondering how terms with lambda are matched on the LHS and substituted into on the RHS. For instance, I want to restructure a lambda term as follows: > foo (\ x -> fmap u v) = bar (\ x -> u) (\ x -> v) My intent is that the terms `u` and `v` may contain `x` and that whatever variable name is actually used in a term being rewritten is preserved so as to re-capture its occurrences on the RHS. When I write this sort of rule, I get warnings about `x` being defined but not used. Thanks, -- Conal

4 4