August 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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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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WebUI for GHC/Haskell tooling (eventlog)
by Csaba Hruska 13 Sep '20

13 Sep '20

Hello, I've written a blog post <https://www.patreon.com/posts/41065262> about my WebUI based eventlog related tool. It is also related to eventlog2html and ghc-debug. I'm interested in your opinion and ideas related to ghc debug/profiling tooling. If you have time please read the post and it would be great to hear some positive or negative feedback from you. It would be great to discuss this topic with you. Thanks, Csaba

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Implicit reboxing of unboxed tuple in let-patterns
by Spiwack, Arnaud 07 Sep '20

07 Sep '20

Dear all, I discovered the hard way, yesterday, that lazy let pattern matching is allowed on unboxed tuples. And that it implicitly reboxes the pattern. Here is how the manual describes it, from the relevant section <https://downloads.haskell.org/ghc/latest/docs/html/users_guide/glasgow_exts…> : You can have an unboxed tuple in a pattern binding, thus f x = let (# p,q #) = h x in ..body.. If the types of p and q are not unboxed, the resulting binding is lazy like any other Haskell pattern binding. The above example desugars like this: f x = let t = case h x of { (# p,q #) -> (p,q) } p = fst t q = snd t in ..body.. Indeed, the bindings can even be recursive. Notice how h x is lazily bound, hence won’t necessarily be run when body is forced. as opposed to if I had written, for instance, let u = hxin ..body.. My question is: are we happy with this? I did find this extremely surprising. If I’m using unboxed tuples, it’s because I want to guarantee to myself a strict, unboxed behaviour. But a very subtle syntactic detail seems to break this expectation for me. My expectation would be that I would need to explicitly rebox things before they get lazy again. I find that this behaviour invites trouble. But you may disagree. Let me know!

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COMPLETE pragmas
by Richard Eisenberg 03 Sep '20

03 Sep '20

Hi Sebastian, I enjoyed your presentation last week at ICFP! This thread (https://ghc-devs.haskell.narkive.com/NXBBDXg1/suppressing-false-incomplete-…) played out before you became so interested in pattern-match coverage. I'd be curious for your thoughts there -- do you agree with the conclusions in the thread? My motivation for asking is https://github.com/conal/linalg/pull/54 (you don't need to read the whole thing), which can be boiled down to a request for a COMPLETE pragma that works at a polymorphic result type. (Or a COMPLETE pragma written in a module that is not the defining module for a pattern synonym.) https://gitlab.haskell.org/ghc/ghc/-/issues/14422 describes a similar, but even more challenging scenario. Do you see any ways forward here? Thanks! Richard

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GHC Logo
by Ben Gamari 02 Sep '20

02 Sep '20

Hi everyone, Recently a sponsor asked for a logo for our project. As far as I know, GHC doesn't really have a consistent logo; the closest that we have had is the stylized "GHC" on the top of ghc.haskell.org. To accomodate the request, I took a few minutes and reworked the typography of the Thompson-Wheeler Haskell logo for use by GHC. I couldn't positively identify the typeface used for the "Haskell" text, but I believe that the extra-bold Cantarell face that I chose in the GHC variant has a similar feel to the Haskell logo and is free to use. I've posted the logo on the Wiki for future reference [1]. Feedback is very much welcome. Cheers, - Ben [1] https://gitlab.haskell.org/ghc/ghc/-/wikis/logo

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Creative ideas on how to debug heap corruption
by Moritz Angermann 02 Sep '20

02 Sep '20

Hi there! as some of you may know, I've been working on an aarch64 native code generator. Now I've hit a situation where my stage2 compiler somehow corrupts my heap. Initially I thought this would likely be missing memory barriers, however they are emitted. This doesn't mean it can't be, but at least it's not as simple as "they are just missing". The crashes I see are non deterministic, in fact I sometimes even manage to compile a Hello World module, without crashes. Other times it crashes with unknown closure errors or it just crashes. But it always crashes during GC. Changing the nursery size make it crasha bit more frequent, but nothing obvious sticks out yet. If anyone has some create ideas, I'd love to hear them. I've been wondering if just logging allocations (offset, range, type) would help figuring out what we expected to be there; and then maybe try to break on the allocation, (and subsequent writes). I'm sure some have been down this road before. Cheers, Moritz

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HsPragTick
by Alan & Kim Zimmerman 02 Sep '20

02 Sep '20

I am working through the API Annotations, and have come across HsPragTick. In source it appears as c = {-# GENERATED "foobar" 1 : 2 - 3 : 4 #-} 0.00 But it does not seem to be used anywhere. It is passed through for renaming and type checking, and Coverage.hs uses it as addTickHsExpr (HsPragE _ HsPragTick{} (L pos e0)) = do e2 <- allocTickBox (ExpBox False) False False (locA pos) $ addTickHsExpr e0 return $ unLoc e2 So if it is used at all, the contents are ignored. Can it be removed? Alan

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Re: HsPragTick
by Ryan Scott 31 Aug '20

31 Aug '20

I think that HsPragTick is unused as of [1]. In fact, I was under the impression that [1] removed HsPragTick entirely (as the commit message) would suggest, but upon further inspection, that doesn't appear to be the case. Vlad, do you recall why HsPragTick was kept around? Ryan S. ----- [1] https://gitlab.haskell.org/ghc/ghc/-/merge_requests/2154

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