March 2019 - Glasgow-haskell-users

GHC HEAD documentation once again available
by Ben Gamari 31 Mar '19

31 Mar '19

TL;DR. A snapshot of GHC's documentation from the master branch can always be found at [2]. Hi everyone, Quite a while ago I made it a habit of periodically pushing documentation snapshots from GHC's master branch to downloads.haskell.org [1]. Unfortunately, despite some attempts at automating this process, this frequently grew out-of-date. I am happy to report that documentation snapshots are now generated as a product of GHC's CI process and made available here [2]. The old downloads.haskell.org URL redirects to [2] and consequently should now always be up-to-date. Let me know if you notice anything amiss. Cheers, - Ben [1] https://downloads.haskell.org/ghc/master/ [2] https://ghc.gitlab.haskell.org/ghc/doc/

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Re: Equality constraints (~): type-theory behind them
by Anthony Clayden 26 Mar '19

26 Mar '19

> On Mon, Dec 10, 2018 at 8:36:42 AM Tom Schrijvers wrote: > Maybe our Haskell'17 paper about Elaboration on Functional Dependencies sheds some more light on your problem: Thanks Tom, I've also been working through the 2011 expanded version of 'System F with Type Equality Coercions' that Adam suggested. I'm finding your 2017 paper's proposals not up to the job, because it doesn't consider FunDeps for Overlapping Instances; and unnecessary because the examples it's addressing can be fixed purely using FunDeps, with their semantics per the 2006 'via CHRs' paper. The chief problems with using FunDeps in GHC code is GHC doesn't follow that semantics; neither does it follow any other well-principled/documented semantics.You can get it to accept a bunch of instances then find that taken together they're not consistent and confluent, in the sense of the 'via CHRs' paper. Specifically, re the 2017's paper Challenge 1: Enforcing (essentially Trac #10675). That set of instances in the example is not valid by the 'via CHRs' rules, and should be rejected. Hugs indeed rejects that code. GHC accepts it and #10675 shows what sort of incoherence can result. I'm not seeing we need to translate to Type Families/System FC to explain it. Re Challenge 2: Elaborating (which is Trac #9627 that gives the main example for the paper). We can fix that code with an extra `~` constraint: class C a b | a -> b instance C Int Bool f :: (C Int b, b ~ Bool) => b -> Bool f x = x (Or in Hugs with a `TypeCast` instead of the `~`.) Is that extra constraint onerous? The signature already has `Bool` hard-coded as the return type, so I don't see it's any more onerous. Or put that signature as f :: (C Int b) => b -> b Which also has the effect `b` must be `Bool`. I'd far rather see GHC's implementation of FunDeps made more coherent (and learning from Hugs) than squeezing it into the straitjacket of System FC and thereby lose expressivity. AntC

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props for Hugs over System FC
by Anthony Clayden 25 Mar '19

25 Mar '19

heh heh. I've got to record this for posterity. I've never known an example before. Some code that compiles in Hugs and works fine; but GHC (8.6.4) can't typecheck so rejects. It's an example in the 2011 'System F with Type Equality Coercions', section 2.3 discussing FunDeps; and used to justify the extra power of type inference in Type Families as opposed to FunDeps. Full details discussed here: https://gitlab.haskell.org/ghc/ghc/issues/16430#note_189393 class F a b | a -> b instance F Int Bool class D a where { op :: F a b => a -> b } instance D Int where { op _ = True } True that doesn't compile as given. Hugs says: 'Inferred type is not general enough'. GHC says 'Couldn't match expected type `b' with actual type `Bool''/ '`b' is a rigid type variable'. (So essentially the same failure of typechecking.) With a little help for the type inference, this compiles in Hugs. class C a b | a -> b instance C Int Bool f :: (C Int b, TypeCast b Bool) => b -> Bool f x = typeCast x With `TypeCast` defined as for HList. But GHC still rejects it; and rejects a version with a `(~)` constraint instead of the `TypeCast`. AntC

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GitLab updates
by Ben Gamari 17 Mar '19

17 Mar '19

Hi everyone, I want to share a few developments on the GitLab migration front: * URLs pointing to tickets or the Wiki which were previously served by Trac (https://ghc.haskell.org/) now redirect to their corresponding GitLab URL. Wiki page names are redirected via this mapping [1]. If you find any cases were this doesn't work as you would expect then please do let me know or open a ticket [4]. If you do want to view Trac you may do so (for the time being) at https://trac.haskell.org/. In the coming days I will be taking a final static archival snapshot of our Trac instance and in the next few weeks we will likely shut it down. * This afternoon I imported [2] a few dozen keywords from Trac that were omitted from the initial import. At this point the ghc/ghc project has nearly 100 labels. It's unclear how usable this will be without either paring down the label space or imposing some sort of order (e.g. adding categorical prefixes to label names) but time will tell. * Our GitLab installation has been upgraded to GitLab 11.8. The changes are relatively minor but I am sure you will spot them. * The source for GHC's homepage (https://www.haskell.org/ghc) is now hosted in the ghc/homepage project [3]. If you find anything wrong with the website feel free to submit a merge request. As always, if you find anything amiss don't hesitate to ping me or open a ticket [4]. Note that the Wiki is still in need of quite a bit of cleanup [5]. If you have some time and want to help out either pick up a ticket or just start browsing and clean-up the first page you find. Many hands makes for light work. However, if you end up renaming any Wiki pages please do open a merge request updating `wiki-mapping.json` [1]. Thank you for your patience while we push through the remaining parts of the migration. Cheers, - Ben [1] https://gitlab.haskell.org/bgamari/gitlab-migration/blob/master/wiki-mappin… [2] https://gitlab.haskell.org/bgamari/trac-to-remarkup/commit/4104086bca53528b… [3] https://gitlab.haskell.org/ghc/homepage [4] https://gitlab.haskell.org/bgamari/gitlab-migration/issues [5] https://gitlab.haskell.org/bgamari/gitlab-migration/issues?label_name%5B%5D…

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Re: [Haskell-cafe] Final steps in GHC's Trac-to-GitLab migration
by Anthony Clayden 13 Mar '19

13 Mar '19

>On March 6, 2019 11:21:34 UTC, Ben Gamari wrote: >>On March 6, 2019 6:11:49 AM EST, Ara Adkins wrote:>>*One question: what is happening with the trac tickets mailing list? I *>>*imagine it’ll be going away, but for those of us that use it to keep *>>*track of things is there a recommended alternative? *>> Yes I look at that as a way to keep a finger lightly on the pulse - chiefly for what users think ghc should be doing. >The ghc-commits list will continue to work. But that's only fixes, not explanations. >The ghc-tickets list is a good question. I suspect that under gitlab there will be less need for this list but we may still want to continue maintaining it regardless for continuity's sake. Thoughts? The gitlab issues list can be sorted by latest update, which should be roughly equiv. But it's showing weird stuff at the moment. (Like decades-old closed tickets with activity 1 day ago.) I guess that's a 'feature' of the migration(?) AntC

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Trac to GitLab migration underway
by Ben Gamari 10 Mar '19

10 Mar '19

Hello everyone, I have started the process of migrating GHC's Trac content to GitLab. GitLab (gitlab.haskell.org) and Trac (ghc.haskell.org) will be down until this process has finished. I will post updates as necessary. However, if you do need to refer to a ticket, you are welcome to use the staging server [1], which has a reasonably up-to-date (read-only) snapshot of our Trac tickets. However, do note that unfortunately authentication via GitHub will not work on this site. Cheers, - Ben [1] https://gitlab.staging.haskell.org/

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Re: [Haskell-cafe] Final steps in GHC's Trac-to-GitLab migration
by Ben Gamari 06 Mar '19

06 Mar '19

On March 6, 2019 6:11:49 AM EST, Ara Adkins <me(a)ara.io> wrote: >Super excited for this! Thank you to everyone whose put in so much hard >work to get it done! > >One question: what is happening with the trac tickets mailing list? I >imagine it’ll be going away, but for those of us that use it to keep >track of things is there a recommended alternative? > The ghc-commits list will continue to work. The ghc-tickets list is a good question. I suspect that under gitlab there will be less need for this list but we may still want to continue maintaining it regardless for continuity's sake. Thoughts? Cheers, - Ben >Best, >_ara > >> On 6 Mar 2019, at 01:21, Ben Gamari <ben(a)well-typed.com> wrote: >> >> Hi everyone, >> >> Over the past few weeks we have been hard at work sorting out the >> last batch of issues in GHC's Trac-to-GitLab import [1]. At this >point I >> believe we have sorted out the issues which are necessary to perform >the >> final migration: >> >> * We are missing only two tickets (#1436 and #2074 which will require >a >> bit of manual intervention to import due to extremely large >> description lengths) >> >> * A variety of markup issues have been resolved >> >> * More metadata is now preserved via labels. We may choose to >> reorganize or eliminate some of these labels in time but it's >easier >> to remove metadata after import than it is to reintroduce it. The >> logic which maps Trac metadata to GitLab labels can be found here >[2] >> >> * We now generate a Wiki table of contents [3] which is significantly >> more readable than GitLab's default page list. This will be updated >> by a cron job until underlying GitLab pages list becomes more >> readable. >> >> * We now generate redirects for Trac ticket and Wiki links (although >> this isn't visible in the staging instance) >> >> * Milestones are now properly closed when closed in Trac >> >> * Mapping between Trac and GitLab usernames is now a bit more robust >> >> As in previous test imports, we would appreciate it if you could have >a >> look over the import and let us know of any problems your encounter. >> >> If no serious issues are identified with the staging site we plan to >> proceed with the migration this coming weekend. The current migration >> plan is to perform the final import on gitlab.haskell.org on >Saturday, 9 >> March 2019. >> >> This will involve both gitlab.haskell.org and ghc.haskell.org being >down >> for likely the entirety of the day Saturday and likely some of Sunday >> (EST time zone). Read-only access will be available to >> gitlab.staging.haskell.org for ticket lookup while the import is >> underway. >> >> After the import we will wait at least a week or so before we begin >the >> process of decommissioning Trac, which will be kept in read-only mode >> for the duration. >> >> Do let me know if the 9 March timing is problematic. >> >> Cheers, >> >> - Ben >> >> >> [1] https://gitlab.staging.haskell.org/ghc/ghc >> [2] >https://github.com/bgamari/trac-to-remarkup/blob/master/TicketImport.hs#L227 >> [3] https://gitlab.staging.haskell.org/ghc/ghc/wikis/index >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.

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Final steps in GHC's Trac-to-GitLab migration
by Ben Gamari 06 Mar '19

06 Mar '19

Hi everyone, Over the past few weeks we have been hard at work sorting out the last batch of issues in GHC's Trac-to-GitLab import [1]. At this point I believe we have sorted out the issues which are necessary to perform the final migration: * We are missing only two tickets (#1436 and #2074 which will require a bit of manual intervention to import due to extremely large description lengths) * A variety of markup issues have been resolved * More metadata is now preserved via labels. We may choose to reorganize or eliminate some of these labels in time but it's easier to remove metadata after import than it is to reintroduce it. The logic which maps Trac metadata to GitLab labels can be found here [2] * We now generate a Wiki table of contents [3] which is significantly more readable than GitLab's default page list. This will be updated by a cron job until underlying GitLab pages list becomes more readable. * We now generate redirects for Trac ticket and Wiki links (although this isn't visible in the staging instance) * Milestones are now properly closed when closed in Trac * Mapping between Trac and GitLab usernames is now a bit more robust As in previous test imports, we would appreciate it if you could have a look over the import and let us know of any problems your encounter. If no serious issues are identified with the staging site we plan to proceed with the migration this coming weekend. The current migration plan is to perform the final import on gitlab.haskell.org on Saturday, 9 March 2019. This will involve both gitlab.haskell.org and ghc.haskell.org being down for likely the entirety of the day Saturday and likely some of Sunday (EST time zone). Read-only access will be available to gitlab.staging.haskell.org for ticket lookup while the import is underway. After the import we will wait at least a week or so before we begin the process of decommissioning Trac, which will be kept in read-only mode for the duration. Do let me know if the 9 March timing is problematic. Cheers, - Ben [1] https://gitlab.staging.haskell.org/ghc/ghc [2] https://github.com/bgamari/trac-to-remarkup/blob/master/TicketImport.hs#L227 [3] https://gitlab.staging.haskell.org/ghc/ghc/wikis/index

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[ANNOUNCE] GHC 8.6.4 is now available
by Ben Gamari 05 Mar '19

05 Mar '19

Hello everyone, The GHC team is very happy to announce the availability of GHC 8.6.4, 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/8.6.4 The 8.6.4 release fixes several regressions present in 8.6.3 including: - A regression resulting in segmentation faults on Windows introduced by the fix for #16071 backported in 8.6.3. This fix has been reverted, meaning that 8.6.4 is once again susceptible to #16071. #16071 will be fixed in GHC 8.8.1. - A bug resulting in incorrect locking on Darwin, potentially resulting in hangs at shutdown (#16150) - A few bugs in the profiled runtime resulting in the potential for memory unsafety has been fixed (#15508). - The `process` and `transformers` libraries shipped now properly reflect released Hackage releases (#16199) - A bug where TH name resolution would break in a plugin context has been fixed (#16104) - Compilers that do not support TemplateHaskell no longer advertise such support in `--supported-languages` (#16331) 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. Note also that this is the first release cut from our (yet again) revamped continuous integration infrastructure. While we have done a great deal of checking to ensure that the build configuration reflects that of previous releases, do let us know if something looks off. Happy compiling! Cheers, - Ben [1] https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.6.4&col=id…

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