March 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

17 40

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

8 23

[ANNOUNCE] Glasgow Haskell Compiler 8.10.1 released
by Ben Gamari 18 May '20

18 May '20

Hello all, The GHC team is happy to announce the availability of GHC 8.10.1. Source and binary distributions are available at the usual place: https://downloads.haskell.org/ghc/8.10.1/ GHC 8.10.1 brings a number of new features including: * The new UnliftedNewtypes extension allowing newtypes around unlifted types. * The new StandaloneKindSignatures extension allows users to give top-level kind signatures to type, type family, and class declarations. * A new warning, -Wderiving-defaults, to draw attention to ambiguous deriving clauses * A number of improvements in code generation, including changes * A new GHCi command, :instances, for listing the class instances available for a type. * An upgraded Windows toolchain lifting the MAX_PATH limitation * A new, low-latency garbage collector. * Improved support profiling, including support for sending profiler samples to the eventlog, allowing correlation between the profile and other program events Note that at the moment we still require that macOS Catalina users exempt the binary distribution from the notarization requirement by running `xattr -cr .` on the unpacked tree before running `make install`. This situation will hopefully be improved for GHC 8.10.2 with the resolution of #17418 [1]. Cheers, - Ben [1] https://gitlab.haskell.org/ghc/ghc/issues/17418

2 3

[HiW'20] Call for Talks
by Ben Gamari 14 May '20

14 May '20

Hello everyone, Haskell Implementors Workshop is calling for talk proposals. Co-located with ICFP, HiW is an ideal place to describe a Haskell library, a Haskell extension, compiler, works-in-progress, demo a new Haskell-related tool, or even propose future lines of Haskell development. The deadline for submissions is July 2nd 2020. Call for Talks ============== The 12th Haskell Implementors’ Workshop is to be held alongside ICFP 2020 this year in New Jersey. It is a forum for people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure, to share their work and discuss future directions and collaborations with others. Talks and/or demos are proposed by submitting an abstract, and selected by a small program committee. There will be no published proceedings. The workshop will be informal and interactive, with open spaces in the timetable and room for ad-hoc discussion, demos and lightning talks. Scope and Target Audience ------------------------- It is important to distinguish the Haskell Implementors’ Workshop from the Haskell Symposium which is also co-located with ICFP 2020. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors’ Workshop will have no proceedings – although we will aim to make talk videos, slides and presented data available with the consent of the speakers. The Implementors’ Workshop is an ideal place to describe a Haskell extension, describe works-in-progress, demo a new Haskell-related tool, or even propose future lines of Haskell development. Members of the wider Haskell community encouraged to attend the workshop – we need your feedback to keep the Haskell ecosystem thriving. Students working with Haskell are specially encouraged to share their work. The scope covers any of the following topics. There may be some topics that people feel we’ve missed, so by all means submit a proposal even if it doesn’t fit exactly into one of these buckets: - Compilation techniques - Language features and extensions - Type system implementation - Concurrency and parallelism: language design and implementation - Performance, optimization and benchmarking - Virtual machines and run-time systems - Libraries and tools for development or deployment Talks ----- We invite proposals from potential speakers for talks and demonstrations. We are aiming for 20-minute talks with 5 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 300 words. Submissions can be made via HotCRP at https://icfp-hiw20.hotcrp.com/ until July 2nd (anywhere on earth). We will also have lightning talks session. These have been very well received in recent years, and we aim to increase the time available to them. Lightning talks be ~7mins and are scheduled on the day of the workshop. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Logistics --------- We recognize the on-going threat that COVID-19 poses to our participants' safety. While August is nearly half a year away, we must account for the possibility that the virus continues to pose a significant threat into the summer. For this reason, we are investigating options that would allow remote presentations this year. In light of this, we urge potential presenters not to be discouraged from submitting and encourage participants to keep time open in their calendars, regardless of the on-going COVID situation. Rest assured that the conference organizers are working to ensure that the Implementors Workshop can be held safely and productively, regardless of how the COVID-19 situation evolves. Program Committee ----------------- - Andrey Mokhov (Newcastle University) - Ben Gamari (Well-Typed LLP) - Christian Baaij (QBayLogic) - George Karachalias (Tweag I/O) - Klara Marntirosian (KU Leuven) - Matthew Pickering (Univeristy of Bristol) - Ryan Scott (Indiana University Bloomington) Best wishes, Ben

1 1

Fusing loops by specializing on functions with SpecConstr?
by Alexis King 06 Apr '20

06 Apr '20

Hi all, I have recently been toying with FRP, and I’ve noticed that traditional formulations generate a lot of tiny loops that GHC does a very poor job optimizing. Here’s a simplified example: newtype SF a b = SF { runSF :: a -> (b, SF a b) } add1_snd :: SF (String, Int) (String, Int) add1_snd = second add1 where add1 = SF $ \a -> let !b = a + 1 in (b, add1) second f = SF $ \(a, b) -> let !(c, f') = runSF f b in ((a, c), second f') Here, `add1_snd` is defined in terms of two recursive bindings, `add1` and `second`. Because they’re both recursive, GHC doesn’t know what to do with them, and the optimized program still has two separate recursive knots. But this is a missed optimization, as `add1_snd` is equivalent to the following definition, which fuses the two loops together and consequently has just one recursive knot: add1_snd_fused :: SF (String, Int) (String, Int) add1_snd_fused = SF $ \(a, b) -> let !c = b + 1 in ((a, c), add1_snd_fused) How could GHC get from `add1_snd` to `add1_snd_fused`? In theory, SpecConstr could do it! Suppose we specialize `second` at the call pattern `second add1`: {-# RULE "second/add1" second add1 = second_add1 #-} second_add1 = SF $ \(a, b) -> let !(c, f') = runSF add1 b in ((a, c), second f') This doesn’t immediately look like an improvement, but we’re actually almost there. If we unroll `add1` once on the RHS of `second_add1`, the simplifier will get us the rest of the way. We’ll end up with let !b1 = b + 1 !(c, f') = (b1, add1) in ((a, c), second f') and after substituting f' to get `second add1`, the RULE will tie the knot for us. This may look like small potatoes in isolation, but real programs can generate hundreds of these tiny, tiny loops, and fusing them together would be a big win. The only problem is SpecConstr doesn’t currently specialize on functions! The original paper, “Call-pattern Specialisation for Haskell Programs,” mentions this as a possibility in Section 6.2, but it points out that actually doing this in practice would be pretty tricky: > Specialising for function arguments is more slippery than for > constructor arguments. In the example above the argument was a > simple variable, but what if it was instead a lambda term? [...] > > The trouble is that lambda abstractions are much more fragile than > constructor applications, in the sense that simple transformations > may make two abstractions look different although they have the > same value. Still, the difference this could make in a program of mine is so large that I am interested in exploring it anyway. I am wondering if anyone has investigated this possibility any further since the paper was published, or if anyone knows of other use cases that would benefit from this capability. Thanks, Alexis

5 16

Advice implementing new constraint entailment rules
by Matthew Pickering 02 Apr '20

02 Apr '20

Hello, I am attempting to implement two new constraint entailment rules which dictate how to implement a new constraint form "CodeC" can be used to satisfy constraints. The main idea is that all constraints store the level they they are introduced and required (in the Template Haskell sense of level) and that only constraints of the right level can be used. The "CodeC" constraint form allows the level of constraints to be manipulated. Therefore the two rules In order to implement this I want to add two constraint rewriting rules in the following way: 1. If in a given, `CodeC C @ n` ~> `C @ n+1` 2. If in a wanted `CodeC C @ n` -> `C @ n - 1` Can someone give me some pointers about the specific part of the constraint solver where I should add these rules? I am unsure if this rewriting of wanted constraints already occurs or not. Cheers, Matt

2 4

License for grammar
by Евгений Слободкин 01 Apr '20

01 Apr '20

Hi all! I implemented Haskell grammar for ANTLRv4 based on HaskellReport 2010 and GHC source (Parser.y and Lexer.x files). Link: https://github.com/antlr/grammars-v4/blob/master/haskell/Haskell.g4 Could someone please help me figuring out which license this grammar should be published on?

3 2

Re: intent to enable 2fa requirement for github.com/haskell org
by Carter Schonwald 25 Mar '20

25 Mar '20

Duncan, David, please figure out 2fa tools that work for you and enable them, https://github.com/tadfisher/pass-otp https://github.com/solokeys/solo https://github.com/herrjemand/awesome-webauthn#hardware-authenticators https://1password.com/ https://keepass.info/download.html if you are having trouble figuring out tools you're comfortable using, please share with us those constraints we can help you! im here to help (and i'm delaying enabling another day or two to provide help to some active contributors who are having their own difficulties setitng up this stuff) On Wed, Mar 18, 2020 at 6:16 PM Duncan Coutts <duncan(a)dcoutts.me.uk> wrote: > On Wed, 2020-03-18 at 14:46 -0400, Carter Schonwald wrote: > > hey everyone, because so much important stuff for the community, it > > makes sense to add 2fa required for the org, are there any good > > reasons to either wait to do this, or not do it? Feedback welcome! > > I think I might get cut off. > > Is it not still the case that github's 2fa needs a program running on a > mobile phone, or an SMS-capable mobile phone? Is there any support for > normal tools running on a normal Linux machine? > > (I think last time I tried to use the SMS route, it refused to send SMS > messages to my landline, despite the fact that I can receive them) > > > Duncan > >

2 2

Roadmap to compacting ModIface
by Matthew Pickering 25 Mar '20

25 Mar '20

Hello all, I have written down the remaining steps which need to be taken in order to compact a ModIface, which we hope will be useful for applications such as IDEs to reduce GC time. https://gitlab.haskell.org/ghc/ghc/issues/17097#roadmap-to-compacting-a-mod… If there is anyone who wishes to help with this project then please ping me on IRC. So far this is joint work between myself and Daniel G. The first step we need to take is to get 1675 merged which replaces the type backing a FastString from a ByteString to a ShortByteString (and hence from a pinned ByteArray to an unpinned ByteArray). https://gitlab.haskell.org/ghc/ghc/-/merge_requests/1675 Cheers, Matt

3 4

Brief GitLab downtime
by Ben Gamari 21 Mar '20

21 Mar '20

Hi everyone, I'll be rebooting gitlab.haskell.org for a short upgrade. Shouldn't be more than a few minutes. As usual, I'll email when things are back up. Cheers, - Ben

1 3