November 2020 - ghc-devs

The future of Phabricator
by Ben Gamari 18 Sep '21

18 Sep '21

TL;DR. For several reasons I think we should consider alternatives to Phabricator. My view is that GitLab seems like the best option. Hello everyone, Over the past year I have been growing increasingly weary of our continued dependence on Phabricator. Without a doubt, its code review interface is the best I have used. However, for a myriad of reasons I am recently questioning whether it is still the best tool for GHC's needs. # The problem There are a number of reasons why I am currently uncertain about Phabricator. For one, at this point we have no options for support in the event that something goes wrong as the company responsible for Phabricator, Phacility, has closed their support channels to non-paying customers. Furthermore, in the past year or two Phacility has been placing their development resources in the parts their customers pay them for, which appear to be much different that the parts that we actively use. For this reason, some parts that we rely on seem oddly half-finished. This concern was recently underscored by some rather unfortunate misfeatures in Harbormaster which resulted in broken CI after the Hadrian merge and now apparent bugs which have made it difficult to migrate to the CircleCI integration we previously prepared. Perhaps most importantly, in our recent development priorities survey our use of Phabricator was the most common complaint by a fair margin, both in case of respondents who have contributed patches and those who have not. On the whole, past contributors and potential future contributors alike have strongly indicated that they want a more Git-like experience. Of course, this wasn't terribly surprising; this is just the most recent case where contributors have made this preference known. Frankly, in a sense it is hard to blame them. The fact that users need to install a PHP tool, Arcanist, to contribute anything but documentation patches has always seemed like unnecessary friction to me and I would be quite happy to be rid of it. Indeed we have had a quite healthy number of GitHub documentation patches since we started accepting them. This makes me thing that there may indeed be potential contributoes that we are leaving on the table. # What to do With Rackspace support ending at the end of year, now may be a good time to consider whether we really want to continue on this road. Phabricator is great at code review but I am less and less certain that it is worth the maintenance uncertainty and potential lost contributors that it costs. Moreover, good alternatives seem closer at-hand than they were when we deployed Phabricator. ## Move to GitHub When people complain about our infrastructure, they often use GitHub as the example of what they would like to see. However, realistically I have a hard time seeing GitHub as a viable option. Its feature set is simply insufficient enough to handle the needs of a larger project like GHC without significant external tooling (as seen in the case of Rust-lang). The concrete reasons have been well-documented in previous discussions but, to summarize, * its review functionality is extremely painful to use with larger patches * rebased patches lead to extreme confusion and lost review comments * it lacks support for pre-receive hooks, which serve as a last line of defense to prevent unintended submodule changes * its inability to deal with external tickets is problematic * there is essentially no possibility that we could eventually migrate GHC's tickets to GitHub's ticket tracker without considerable data loss (much less manage the ticket load that would result), meaning that we would forever be stuck with maintaining Trac. * on a personal note, its search functionality has often left me empty-handed On the whole, these issues seem pretty hard to surmount. ## Move to GitLab In using GitLab for another project over the past months I have been positively surprised by its quality. It handles rebased merge requests far better than GitHub, has functional search, and quite a usable review interface. Furthermore, upstream has been extremely responsive to suggestions for improvement [1]. Even out-of-the-box it seems to be flexible enough to accommodate our needs, supporting integration with external issue trackers, having reasonable release management features, and support for code owners to automate review triaging (replacing much of the functionality of Phabricator's Herald). Finally, other FOSS projects' [3] recent migrations from Phabrictor to GitLab have shown that GitLab-the-company is quite willing to offer help when needed. I took some time this weekend to try setting up a quick GHC instance [2] to play around with. Even after just a few hours of playing around I think the result is surprisingly usable. Out of curiosity I also played around with importing some tickets from Trac (building on Matt Pickering's Trac-to-Maniphest migration tool). With relatively little effort I was even able to get nearly all of our tickets (as of 9 months ago) imported while preserving ticket numbers (although there are naturally a few wrinkles that would need to be worked out). Naturally, I think we should treat the question of ticket tracker migration as an orthogonal one to code review, but it is good to know that this is possible. ## Continue with Phabricator Continuing with Phabricator is of course an option. Its review functionality is great and it has served us reasonably well. However, compared to GitLab and even GitHub of today its features seem less distinguished than they once did. Moreover, the prospect of having to maintain a largely stagnant product with no support strikes me as a slightly dangerous game to play. Working around the issues we have recently encountered has already cost a non-negligible amount of time. # The bottom line If it wasn't clear already, I think that we should strongly consider a move to GitLab. At this point it seems clear that it isn't going to vanish, has a steady pace of development, is featureful, and available. However, these are just my thoughts. What do you think? Cheers, - Ben [1] 11.4 will ship with a file tree view in the code review interface, which I reported (https://gitlab.com/gitlab-org/gitlab-ce/issues/46474) as being is one of the Phabricator features I missed the most during review [2] https://gitlab.home.smart-cactus.org/ghc/ghc/issues/14641 [3] The GNOME and freedesktop.org projects have recently migrated, the former from a hodge-podge of self-hosted services and the latter from Phabricator

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New implementation for `ImpredicativeTypes`
by Alejandro Serrano Mena 08 Sep '21

08 Sep '21

Hi all, As I mentioned some time ago, we have been busy working on a new implementation of `ImpredicativeTypes` for GHC. I am very thankful to everybody who back then sent us examples of impredicativity which would be nice to support, as far as we know this branch supports all of them! :) If you want to try it, at https://gitlab.haskell.org/trupill/ghc/commit/a3f95a0fe0f647702fd7225fa719a… you can find the result of the pipeline, which includes builds for several platforms (click on the "Artifacts" button, the one which looks like a cloud, to get them). The code is being developed at https://gitlab.haskell.org/trupill/ghc. Any code should run *unchanged* except for some eta-expansion required for some specific usage patterns of higher-rank types. Please don't hesitate to ask any questions or clarifications about it. A merge request for tracking this can be found at https://gitlab.haskell.org/ghc/ghc/merge_requests/1659 Kind regards, Alejandro

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Fat interface files?
by Ben Gamari 21 Dec '20

21 Dec '20

Hi Edward, While chatting with the ghc-ide folks recently I realized that it would be useful to be able to preserve Core such that compilation can be restarted (e.g. to be pushed down the bytecode pipeline to evaluate TH splices). As I recall this is precisely what you implemented in your "fat interface file" work. Do you recall what the state of this work was? Do you have a branch with last-known-good work? Do you recall any tricky questions that remained outstanding? Cheers, - Ben

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[ANNOUNCE] Glasgow Haskell Compiler 9.0.1-alpha1 released
by Ben Gamari 06 Dec '20

06 Dec '20

Hello all, The GHC team is very pleased to announce the availability of the first alpha release in the GHC 9.0 series. Source and binary distributions are available at the usual place: https://downloads.haskell.org/ghc/9.0.1-alpha1/ This first alpha comes quite a bit later than expected. However, we have done a significant amount of testing on this pre-release and therefore hope to be able to move forward quickly with a release candidate next week and with a final release in mid-October. GHC 9.0.1 will bring a number of new features: * A first cut of the new LinearTypes language extension [1], allowing use of linear function syntax and linear record fields. * A new bignum library (ghc-bignum), allowing GHC to be more easily used with integer libraries other than GMP. * Improvements in code generation, resulting in considerable performance improvements in some programs. * Improvements in pattern-match checking, allowing more precise detection of redundant cases and reduced compilation time. * Implementation of the "simplified subsumption" proposal [2] simplifying the type system and paving the way for QuickLook impredicativity in GHC 9.2. * Implementation of the QualifiedDo extension [3], allowing more convenient overloading of `do` syntax. * Improvements in compilation time. And many more. See the release notes [4] for a full accounting of the changes in this release. Do note that there are a few things that we expect will change before the final release: * We expect to sort out a notarization workflow for Apple Darwin, allowing our binary distributions to be used on macOS Catalina without hassle. Until this has been sorted out Catalina users can exempt the current macOS binary distribution from the notarization requirement themselves by running `xattr -cr .` on the unpacked tree before running `make install`. * We will likely transition the Alpine binary distribution to be fully statically-linked, providing a convenient, distribution-independent packaging option for Linux users. * We will be merging a robust solution for #17760 which will introduce a new primitive, `keepAlive#`, to the `base` library, subsuming most uses of `touch#`. As always, do test this release and open tickets for whatever issues you encounter. To help with this, we will be publishing a blog post describing use of our new `head.hackage` infrastructure to ease testing of larger projects with Hackage dependencies later this week. Cheers, - Ben [1] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0111-l… [2] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-s… [3] https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0216-q… [4] https://downloads.haskell.org/ghc/9.0.1-alpha1/docs/html/users_guide/9.0.1-…

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Using a development snapshot of happy
by Vladislav Zavialov 04 Dec '20

04 Dec '20

Hi ghc-devs, I’m working on the unification of parsers for terms and types, and one of the things I’d really like to make use of is a feature I implemented in ‘happy’ in October 2019 (9 months ago): https://github.com/simonmar/happy/pull/153 It’s been merged upstream, but there has been no release of ‘happy’, despite repeated requests: 1. I asked for a release in December: https://github.com/simonmar/happy/issues/164 2. Ben asked for a release a month ago: https://github.com/simonmar/happy/issues/168 I see two solutions here: a) Find a co-maintainer for ‘happy’ who could make releases more frequently (I understand the current maintainers probably don’t have the time to do it). b) Use a development snapshot of ‘happy’ in GHC Maybe we need to do both, but one reason I’d like to see (b) in particular happen is that I can imagine introducing more features to ‘happy’ for use in GHC, and it’d be nice not to wait for a release every time. For instance, there are some changes I’d like to make to happy/alex in order to implement #17750 So here are two questions I have: 1. Are there any objections to this idea? 2. If not, could someone more familiar with the build process guide me as to how this should be implemented? Do I add ‘happy’ as a submodule and change something in the ./configure script, or is there more to it? Do I need to modify make/hadrian, and if so, then how? Thanks, - Vlad

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Use of forall as a sigil
by Bryan Richter 03 Dec '20

03 Dec '20

Dear forall ghc-devs. ghc-devs, As I read through the "Visible 'forall' in types of terms" proposal[1], I stumbled over something that isn't relevant to the proposal itself, so I thought I would bring it here. Given f :: forall a. a -> a (1) I intuitively understand the 'forall' in (1) to represent the phrase "for all". I would read (1) as "For all objects a in Hask, f is some relation from a to a." After reading the proposal, I think my intuition may be wrong, since I discovered `forall a ->`. This means something similar to, but practically different from, `forall a.`. Thus it seems like 'forall' is now simply used as a sigil to represent "here is where some special parameter goes". It could as well be an emoji. What's more, the practical difference between the two forms is *only* distinguished by ` ->` versus `.`. That's putting quite a lot of meaning into a rather small number of pixels, and further reduces any original connection to meaning that 'forall' might have had. I won't object to #281 based only on existing syntax, but I *do* object to the existing syntax. :) Is this a hopeless situation, or is there any possibility of bringing back meaning to the syntax of "dependent quantifiers"? -Bryan [1]: https://github.com/ghc-proposals/ghc-proposals/pull/281

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PSA: type checker plugins to be affected by upcoming changes to GHC
by Richard Eisenberg 30 Nov '20

30 Nov '20

Hi all, I'm hard at work on two significant refactorings within GHC's constraint solver. The first is at https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4149. It removes flattening meta-variables and flattening skolems. This is a very nice simplification. Instead, it just reduces type families directly. My other patch (held up by the first) is at https://gitlab.haskell.org/ghc/ghc/-/tree/wip/derived-refactor and will remove Derived constraints, to be replaced by a little bit of cleverness in suppressing certain confusing error messages. My guess is that either or both of these will invalidate the current behavior of type-checker plugins. Sadly, this is not just a case of finding a new function that replicates the old behavior -- enough is changing under the hood that you might actually have to rewrite chunks of your code. I have never written a type-checker plugin, and so I don't currently have advice for you. But if you are a plugin author affected by this change and want help, please reach out -- I would be happy to walk you through the changes, and then hopefully make a little video explaining the process to other plugin authors. Neither patch will make it for 9.0, but I expect both to be in 9.2. There may be more where this came from (https://gitlab.haskell.org/ghc/ghc/-/issues/18965) in the future, but it's all for a good cause. (I have bcc'd plugin authors that I'm aware of. Just adding this in case you're surprised at receiving this email.) Thanks, Richard

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Hadrian: Error "cannot find -lHSrts-1.0_thr_l" when linking ghc executable.
by Roland Senn 25 Nov '20

25 Nov '20

Hi there! I have two GHC trees. One is called ghcdebug and is a little bit outdated (16. September 2020). In this tree, I can work normally: Change code, compile and run the resulting GHC compiler. The second tree is up-to-date! Compiling works fine, however the linker steps for the ghc executable fails. When I restart the failing link step, I get the following error: $ hadrian/build --flavour=devel2 --freeze1 -j2 stage2:exe:ghc-bin Up to date | Run Ghc LinkHs Stage1: _build/stage1/ghc/build/c/hschooks.o (and 7 more) => _build/stage1/bin/ghc /usr/bin/ld.gold: error: cannot find -lHSrts-1.0_thr_l _build/stage1/ghc/build/GHCi/Leak.o(.text+0xd5): error: undefined reference to 'stg_upd_frame_info' _build/stage1/ghc/build/GHCi/Leak.o(.text+0x1c2): error: undefined reference to 'stg_upd_frame_info' _build/stage1/ghc/build/GHCi/Leak.o(.text+0x1e6): error: undefined reference to 'stg_ap_ppp_info' ... and many many more lines with "error: undefined reference to ..." I get the same error when I use a freshly cloned ghc tree. I also get the error independently of the GHC version used to compile (8.8.3 or 8.10.2). I didn't modify any settings files. I do: hadrian/build clean hadrian/build --flavour=devel2 -j2 I'm using a normal x68_64 box with plain vanilla Linux Debian 10. No Docker and no Nix. The directory _build/stage/rts/build contains: roland@goms:~/Projekte/ghc$ ls -all _build/stage1/rts/build/ insgesamt 19396 drwxr-xr-x 5 roland roland 4096 Nov 24 17:58 . drwxr-xr-x 3 roland roland 4096 Nov 24 17:56 .. drwxr-xr-x 2 roland roland 4096 Nov 24 17:56 autogen drwxr-xr-x 7 roland roland 12288 Nov 24 17:58 c drwxr-xr-x 2 roland roland 4096 Nov 24 17:58 cmm -rw-r--r-- 1 roland roland 31349 Nov 24 17:58 DerivedConstants.h -rw-r--r-- 1 roland roland 13764 Nov 24 17:57 ffi.h -rw-r--r-- 1 roland roland 4343 Nov 24 17:57 ffitarget.h -rw-r--r-- 1 roland roland 15128 Nov 24 17:58 ghcautoconf.h -rw-r--r-- 1 roland roland 618 Nov 24 17:58 ghcplatform.h -rw-r--r-- 1 roland roland 732 Nov 24 17:58 ghcversion.h -rw-r--r-- 1 roland roland 82486 Nov 24 17:57 libCffi.a lrwxrwxrwx 1 roland roland 9 Nov 24 17:57 libCffi_thr.a -> libCffi.a -rw-r--r-- 1 roland roland 9166966 Nov 24 17:58 libHSrts-1.0.a -rw-r--r-- 1 roland roland 10490730 Nov 24 17:58 libHSrts-1.0_thr.a Any ideas what's wrong or how to fix? Many thanks and Cheers, Roland

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Questions about System F and data types
by Jan van Brügge 24 Nov '20

24 Nov '20

Hi, I know that this is probably not the correct place to ask as my question is not directly related to GHC itself, but I thought here are the people that can most likely answer this. To better understand PL papers, especially those involving System Fc and its extensions, I started to write a formal proof of type safety including alpha-equivalence. Currently I have a complete proof for System F <https://github.com/jvanbruegge/isabelle-lambda-calculus> (without coercions and data types yet). I mainly used the System Fc paper <https://www.microsoft.com/en-us/research/wp-content/uploads/2007/01/tldi22-…> as reference, ignoring all the parts about coercions. 1. In the Fc paper, the rule `AppT` has a judgement delta as assumption, which does not exist. I assumed the `ty` judgement was meant there by looking at the arguments. Is this correct? 2. In the `Abs` rule, the type of the variable is required to be valid and of kind star by the judgement `ty`. In the `Let` rule, this assumption is missing. I tried it like that, but was not able to complete the proofs. Is such an assumption missing there or should I be able to proof it without? 3. In the Fc paper, the types and kinds of datatype declarations are added to the context with a separate judgement that interprets the datatype declarations. In the System Fc pro paper <https://www.microsoft.com/en-us/research/wp-content/uploads/2016/02/p53-yor…> (from what I can tell) those types and kinds are assumed to be already part of the initial context. At the moment I prove progress against the empty context, so I guess I would have to relax that to an initial context that only contains types and kinds of type constants and nothing else. Is that correct? What is here the "best practice" in terms of PL research? Thank you all Jan

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HEAD user manual
by Simon Peyton Jones 24 Nov '20

24 Nov '20

Where can I find the GHC user manual for HEAD these days? I tried https://ghc.gitlab.haskell.org/ghc/doc/, but that takes me to 8.11. The user documentation page https://gitlab.haskell.org/ghc/ghc/-/wikis/ghc-users-guide#ghc-users-docume… has the same link. Thanks Simon

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