- Glasgow-haskell-users

Re: Equality constraints (~): type-theory behind them
by Anthony Clayden 23 Dec '18

23 Dec '18

On Fri, 7 Dec 2018 at 10:57 PM, Adam Gundry wrote: > > Regarding inference, the place that comes to mind is Vytiniotis et al. > OutsideIn(X): > Thanks Adam for the references. Hmm That OutsideIn paper is formidable in so many senses ... I'm not sure I grok your response on my specific q; to adapt your example slightly, without a signature foo2 x@(_: _) = x GHC infers `foo2 :: [a] -> [a]`. That's not general enough. So I give a signature with bare `b` as the return type, using the (~) to improve it: foo2 :: [a] ~ b => [a] -> b ... and GHC infers `foo2 :: [a] -> [a]`. Oh. Is that what I should expect? I'm chiefly concerned about improving bare tyvars under a FunDep. So to take the time-worn example, I want instances morally equivalent to data TTrue = TTrue; data TFalse = TFalse class TEq a b r | a b -> r where tEq :: a -> b -> r instance TEq a a TTrue where tEq _ _ = TTrue instance TEq a b TFalse where tEq _ _ = TFalse The FunDeps via CHRs 2006 paper tells me that first instance is supposed to be as if instance (r ~ TTrue) => TEq a a r where ... but it isn't tEq 'a' 'a' :: TFalse -- returns TFalse, using the first pair of instances tEq 'a' 'a' :: TFalse -- rejected, using the (~) instance: 'Couldn't match type TFalse with TTrue ...' The 'happily' returning the 'wrong' answer in the first case has caused consternation amongst beginners, and a few Trac tickets. So what magic is happening with (~) that it can eagerly improve `foo2` but semi-lazily hold back for `tEq`? Of course I lied about the first-given two instances for TEq: instances are not consistent with Functional Dependency. So I need to resort to overlaps and a (~) and exploit GHC's bogus consistency check: instance {-# OVERLAPPABLE #-} (r ~ TFalse) => TEq a b r where ... > ... like automatically inferring where `typeCast` needs to be placed Yes the non-automatic `TypeCast` version instance (TypeCast TTrue r) => TEq a a r where tEq _ _ = TTrue -- rejected 'Couldn't match expected type 'r' with actual type 'TTrue' ...' tEq _ _ = typeCast TTrue -- accepted Even though `typeCast` is really `id` (after it's jumped through the devious indirection hoops I talked about). So what I'm trying to understand is not just "where" to place `typeCast` but also "when" in inference it unmasks the unification. AntC > About your specific question, if a wanted constraint `a ~ b` can be > solved by unifying `a` with `b`, then GHC is free to do so (in a sense, > either parameter of (~) can be improved from the other). The real > difference with `TypeCast` arises when local equality assumptions (given > constraints) are involved, because then there may be wanted constraints > that cannot be solved by unification but can be solved by exploiting the > local assumptions. > > For example, consider this function: > > foo :: forall a b . a ~ b => [a] -> [b] > foo x = x > > When checking the RHS of `foo`, a wanted constraint `[a] ~ [b]` arises > (because `x` has type `[a]` and is used in a place where `[b]` is > expected). Since `a` and `b` are universally quantified variables, this > cannot be solved by unification. However, the local assumption (given > constraint) `a ~ b` can be used to solve this wanted. > > This is rather like automatically inferring where `typeCast` needs to be > placed (indeed, at the Core level there is a construct for casting by an > equality proof, which plays much the same role). > > Hope this helps, > > Adam > > > On 06/12/2018 12:01, Anthony Clayden wrote: > > The (~) constraint syntax is enabled by either `GADTs` or `TypeFamilies` > > language extension. > > > > GADTs/Assoc Data Types/Assoc Type Synonyms/Type Families arrived in a > > series of papers 2005 to 2008, and IIRC the development wasn't finished > > in full in GHC until after that. (Superclass constraints took up to > > about 2010.) > > > > Suppose I wanted just the (~) parts of those implementations. Which > > would be the best place to look? (The Users Guide doesn't give a > > reference.) ICFP 2008 'Type Checking with Open Type Functions' shows > > uses of (~) in user-written code, but doesn't explain it or motivate it > > as a separate feature. > > > > My specific question is: long before (~), it was possible to write a > > TypeCast class, with bidirectional FunDeps to improve each type > > parameter from the other. But for the compiler to see the type > > improvement at term level, typically you also need a typeCast method and > > to explicitly wrap a term in it. If you just wrote a term of type `a` in > > a place where `b` was wanted, the compiler would either fail to see your > > `TypeCast a b`, or unify the two too eagerly, then infer an overall type > > that wasn't general enough. > > > > (For that reason, the instance for TypeCast goes through some > > devious/indirect other classes/instances or exploits separate > > compilation, to hide what's going on from the compiler's enthusiasm.) > > > > But (~) does some sort of magic: it's a kinda typeclass but without a > > method. How does it mesh with type improvement in the goldilocks zone: > > neither too eager nor too lazy? > > > > > > AntC > > >

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[ANNOUNCE] GHC 8.6.3 is now available
by Ben Gamari 17 Dec '18

17 Dec '18

Hello everyone, The GHC team is very happy to announce the availability of GHC 8.6.3, a bugfix release in the GHC 8.6 series. The source distribution, binary distributions, and documentation for this release are available at https://downloads.haskell.org/~ghc/8.6.3 The 8.6 release fixes several regressions present in 8.6.2 including: - A code generation bug resulting in segmentations faults in some programs (#15892) - Darwin binary distributions are now correctly built against an in-tree GMP (#15404) - Three bugs leading to linker failures on Windows (#15105, #15894, #15934) - A bug leading to programs with deep stacks crashing when run with retainer profiling enabled (#14758) - A bug resulting in potential heap corruption during stable name allocation (#15906) - Plugins are now loaded during GHCi sessions (#15633) As a few of these issues are rather serious users are strongly encouraged to upgrade. See Trac [1] for a full list of issues resolved in this release. Note that this release ships with one significant but long-standing bug (#14251): Calls to functions taking both Float# and Double# may result in incorrect code generation when compiled using the LLVM code generator. This is not a new issue, it has existed as long as the LLVM code generator has existed; however, changes in code generation in 8.6 made it more likely that user code using only lifted types will trigger it. Happy compiling! Cheers, - Ben [1] https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.6.3&col=id…

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Does anyone have much experience generating Haskell from Coq?
by Tim Watson 10 Dec '18

10 Dec '18

So far I've been reading https://www.cs.purdue.edu/homes/bendy/Fiat/FiatByteString.pdf. I'm interested in the ideas presented in https://github.com/DistributedComponents/verdi-runtime, which is OCaml based. My goal is to provide building blocks for verifying and testing Cloud Haskell programs. I've been looking at existing frameworks (such as quickcheck-state-machine/-distributed and hedgehog) for model based testing, and ways of injecting an application layer scheduler for detecting race conditions. The final bit of the puzzle is being able to apply formal methods to verify concurrent/distributed algorithms, and generate some (if not all) of the required implementation code. Any pointers to research or prior art would be greatly appreciated. Cheers, Tim Watson

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Guidelines for respectful communication
by Simon Peyton Jones 08 Dec '18

08 Dec '18

Friends As many of you will know, I have been concerned for several years about the standards of discourse in the Haskell community. I think things have improved since the period that drove me to write my Respect email<https://mail.haskell.org/pipermail/haskell/2016-September/024995.html>, but it's far from secure. We discussed this at a meeting of the GHC Steering Committee<https://github.com/ghc-proposals/ghc-proposals> at ICFP in September, and many of us have had related discussions since. Arising out of that conversation, the GHC Steering Committee has decided to adopt these Guidelines for respectful communication<https://github.com/ghc-proposals/ghc-proposals/blob/master/GRC.rst> We are not trying to impose these guidelines on members of the Haskell community generally. Rather, we are adopting them for ourselves, as a signal that we seek high standards of discourse in the Haskell community, and are willing to publicly hold ourselves to that standard, in the hope that others may choose to follow suit. We are calling them "guidelines for respectful communication" rather than a "code of conduct", because we want to encourage good communication, rather than focus on bad behaviour. Richard Stallman's recent post<https://lwn.net/Articles/769167/> about the new GNU Kind Communication Guidelines<https://gnu.org/philosophy/kind-communication.html> expresses the same idea. Meanwhile, the Stack community is taking a similar approach<https://www.snoyman.com/blog/2018/11/proposal-stack-coc>. Our guidelines are not set in stone; you can comment here<https://github.com/ghc-proposals/ghc-proposals/commit/373044b5a78519071b9a2…>. Perhaps they can evolve so that other Haskell committees (or even individuals) feel able to adopt them. The Haskell community is such a rich collection of intelligent, passionate, and committed people. Thank you -- I love you all! Simon

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Equality constraints (~): type-theory behind them
by Anthony Clayden 07 Dec '18

07 Dec '18

The (~) constraint syntax is enabled by either `GADTs` or `TypeFamilies` language extension. GADTs/Assoc Data Types/Assoc Type Synonyms/Type Families arrived in a series of papers 2005 to 2008, and IIRC the development wasn't finished in full in GHC until after that. (Superclass constraints took up to about 2010.) Suppose I wanted just the (~) parts of those implementations. Which would be the best place to look? (The Users Guide doesn't give a reference.) ICFP 2008 'Type Checking with Open Type Functions' shows uses of (~) in user-written code, but doesn't explain it or motivate it as a separate feature. My specific question is: long before (~), it was possible to write a TypeCast class, with bidirectional FunDeps to improve each type parameter from the other. But for the compiler to see the type improvement at term level, typically you also need a typeCast method and to explicitly wrap a term in it. If you just wrote a term of type `a` in a place where `b` was wanted, the compiler would either fail to see your `TypeCast a b`, or unify the two too eagerly, then infer an overall type that wasn't general enough. (For that reason, the instance for TypeCast goes through some devious/indirect other classes/instances or exploits separate compilation, to hide what's going on from the compiler's enthusiasm.) But (~) does some sort of magic: it's a kinda typeclass but without a method. How does it mesh with type improvement in the goldilocks zone: neither too eager nor too lazy? AntC

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Summit on Advances in Programming Languages 2019
by Simon Peyton Jones 26 Nov '18

26 Nov '18

Haskellers The Summit oN Advances in Programming Languages<http://snapl.org/2017/index.html> (SNAPL) is a biennial venue for discussions about programming languages. SNAPL focuses on experience-based insight, innovation, and visionary ideas spanning from foundations to applications of programming languages. SNAPL welcome perspectives from both industry and academia. Ras Bodik and Shriram Krishnamurthi are looking for community input in where and when to hold SNAPL 2019, and how long it should be. Here is their survey<https://goo.gl/forms/pNydkzP4RwLuIa4p1>. Do consider making your views known to them. Simon

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[ANNOUNCE] GHC 8.6.2 is now available
by Ben Gamari 05 Nov '18

05 Nov '18

Hello everyone, The GHC team is very happy to announce the availability of GHC 8.6.2, a bugfix release to GHC 8.6.1. The source distribution, binary distributions, and documentation for this release are available at https://downloads.haskell.org/~ghc/8.6.2 The 8.6 release fixes several regressions present in 8.6.1 including: * A long-standing (but previously hard to manifest) bug resulting in undefined behavior for some applications of dataToTag# has been fixed (#15696) * An incorrect linker path to libgmp in the Mac OS binary distributions (#15404) * A regression rendering Windows installations to read-only directories unusable (#15667) * A regression resulting in panics while compiling some record updates of GADTs constructors (#15499) * A regression resulting in incorrect type errors when splicing types into constraint contexts has been fixed (#15815) * Around a dozen other issues. See Trac [1] for a full list of issues resolved in this release. Note that this release ships with one significant but long-standing bug (#14251): Calls to functions taking both Float# and Double# may result in incorrect code generation when compiled using the LLVM code generator. This is not a new issue, it has existed as long as the LLVM code generator has existed; however, changes in code generation in 8.6 made it more likely that user code using only lifted types will trigger it. Happy compiling! Cheers, - Ben [1] https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.6.2&col=id…

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Re: [ANNOUNCE] GHC 8.4.4 released
by Carter Schonwald 26 Oct '18

26 Oct '18

Hey David, i'm looking at the git history andit doesn't seem to have any commits between 8.4.3 and 8.4.4 related to the dataToTag issue does any haskell code in the while trigger the bug on 8.4 series? On Sun, Oct 21, 2018 at 11:06 AM David Feuer <david(a)well-typed.com> wrote: > Did this release fix the dataToTag# issue? I think that has a number of > people concerned. > On Oct 18, 2018, at 11:46 AM, Ben Gamari <ben(a)well-typed.com> wrote: >> >> Jens Petersen <juhpetersen(a)gmail.com> writes: >> >> On Mon, 15 Oct 2018 at 07:17, Ben Gamari <ben(a)well-typed.com> wrote: >>> >>>> The GHC team is pleased to announce the availability of GHC 8.4.4 >>>> >>> >>> Thank you >>> >>> As always, the full release notes can be found in the users guide, >>>> >>> >>> https://downloads.haskell.org/~ghc/8.4.4/docs/html/users_guide/8.4.4-notes.… >>> >>> I think this base text is out of date, and could be dropped, right? >> >> >> Indeed it is. >> >> I see that stm was also bumped: though it is not listed in >>> https://downloads.haskell.org/~ghc/8.4.4/docs/html/users_guide/8.4.4-notes.… >> >> >> Also true. However, in my mind this isn't nearly as significant as the >> `text` bump, which affects many users and fixes extremely bad >> misbehavior. >> >> Thanks for noticing these! >> >> Cheers >> >> - Ben >> >> ------------------------------ >> >> ghc-devs mailing list >> ghc-devs(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >> >> _______________________________________________ > ghc-devs mailing list > ghc-devs(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >

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Happy / Alex description for Haskell2010
by Sam Halliday 25 Oct '18

25 Oct '18

Hello all, Is there a Happy / Alex file describing Haskell2010? I can see the versions in the ghc source tree, but of course they support all the ghc extensions. The link to the hsparser page [1] in the Happy documentation doesn't seem to exist anymore. I would also love to hear if there is a Bison definition. Best regards, Sam [1] http://www.pms.informatik.uni-muenchen.de/mitarbeiter/panne/haskell_libs/hs…

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[ANNOUNCE] GHC 8.6.1 released
by Ben Gamari 21 Oct '18

21 Oct '18

Hello everyone, The GHC team is pleased to announce the availability of GHC 8.6.1, the fourth major release in the GHC 8 series. The source distribution, binary distributions, and documentation for this release are available at https://downloads.haskell.org/~ghc/8.6.1 The 8.6 release fixes over 400 bugs from the 8.4 series and introduces a number of exciting features. These most notably include: * A new deriving mechanism, `deriving via`, providing a convenient way for users to extend Haskell's typeclass deriving mechanism * Quantified constraints, allowing forall quantification in constraint contexts * An early version of the GHCi `:doc` command * The `ghc-heap-view` package, allowing introspection into the structure of GHC's heap * Valid hole fit hints, helping the user to find terms to fill typed holes in their programs * The BlockArguments extension, allowing the `$` operator to be omitted in some unambiguous contexts * An exciting new plugin mechanism, source plugins, allowing plugins to inspect and modify a wide variety of compiler representations. * Improved recompilation checking when plugins are used * Significantly better handling of macOS linker command size limits, avoiding linker errors while linking large projects * The next phase of the MonadFail proposal, enabling -XMonadFailDesugaring by default A full list of the changes in this release can be found in the release notes: https://downloads.haskell.org/~ghc/8.6.1/docs/html/users_guide/8.6.1-notes.… Perhaps of equal importance, GHC 8.6 is the second major release made under GHC's accelerated six-month release schedule and the first set of binary distributions built primarily using our new continuous integration scheme. While the final 8.6 release is around three weeks later than initially scheduled due to late-breaking bug reports, we expect that the 8.8 release schedule shouldn't be affected. Thanks to everyone who has contributed to developing, documenting, and testing this release! As always, let us know if you encounter trouble. How to get it ~~~~~~~~~~~~~ The easy way is to go to the web page, which should be self-explanatory: 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. Packages will appear as they are built - if the package for your system isn't available yet, please try again later. Background ~~~~~~~~~~ Haskell is a standard 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 run by Trac: 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 Some GHC developers hang out on #haskell on IRC, too: 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

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