July 2021 - ghc-devs - Haskell.org

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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Potential improvement in compacting GC
by Ömer Sinan Ağacan 02 Sep '21

02 Sep '21

Hello, I recently implemented the algorithm used by GHC's compacting GC in another project. The algorithm (after marking) makes two passes over the heap /generation. In GHC, these passes are implemented in [1] and in the next function. In my implementation I tried 3 ways of implementing these passes, one of which is the same as GHC's code, and benchmarked each version. I found that the fastest implementation is not what's used in GHC, but it could easily be used. I should mention that my code targets Wasm, and I benchmarked Wasm instructions executed. Not CPU cycles, CPU instructions, or anything like that. It's very likely that the results will be different when benchmarking code running on actual hardware. Secondly, in my case the heap is mostly dead (residency is low). In GHC, compaction for the oldest generation is enabled when residency passes a threshold, so the assumption is the heap is mostly live. I'm guessing this should also make some difference. Anyway, the first implementation I tried was similar to GHC's scan, but I did scan += object_size(scan); instead of bumping scan by one, as GHC does in [2]. This was the slowest version. Second implementation did the same as GHC (bumped scan by one). This was faster, but still slower than the next version. What I found to be the best is scanning the bitmap, not the heap. The bitmap iterator reads one word at a time. In each iteration it checks if the bitmap word is 0. In GHC, in the best case this can skip 16 words on heap on 32-bit systems, and 32 words on 64-bit. Reminder: we need two bits per object in the bitmap, see [3]. (this is not the case in my implementation so the payoff is better) When the bitmap word is not 0 I use "count trailing zeros" (or "count leading zeros" depending on the bitmap implementation) to get the number of words to skip. This is a single instruction on Wasm and x86 (TZCNT or LZCNT, available via __builtin_ctzl and __builtin_clzl in gcc). So instead of skipping one word at a time, this can potentially skip 16 words (or 32 on 64-bit architectures). When that's not possible, it can still skip multiple words by using ctz/clz. Ömer [1]: https://github.com/ghc/ghc/blob/bd877edd9499a351db947cd51ed583872b2facdf/rt… [2]: https://github.com/ghc/ghc/blob/bd877edd9499a351db947cd51ed583872b2facdf/rt… [3]: https://github.com/ghc/ghc/blob/bd877edd9499a351db947cd51ed583872b2facdf/rt…

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How to Collect Type Errors?
by Guanda Yuan 06 Aug '21

06 Aug '21

Hello all, I want to write a plugin for Haskell Language Server, to collect type errors in the code. I'm new to GHC API, so I need some help. My current thought is, I can collect diagnostics or the error messages and extract the type error infos. I have tried to referred to the Haddock documentation for GHC API, but I'm still not sure which function should I use. Thanks, Guanda

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Install WIP app in haskell org
by Phyx 31 Jul '21

31 Jul '21

Hi Ben, Is it possible to install the WIP app into the github Haskell org? https://github.com/apps/wip It's quite handy to mark things as in progress. Regards, Tamar

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CI build failures
by ÉRDI Gergő 27 Jul '21

27 Jul '21

Hi, I'm seeing three build failures in CI: 1. On perf-nofib, it fails with: == make boot -j --jobserver-fds=3,4 --no-print-directory; in /builds/cactus/ghc/nofib/real/smallpt ------------------------------------------------------------------------ /builds/cactus/ghc/ghc/bin/ghc -M -dep-suffix "" -dep-makefile .depend -osuf o -O2 -Wno-tabs -Rghc-timing -H32m -hisuf hi -packageunboxed-ref -rtsopts smallpt.hs <command line>: cannot satisfy -package unboxed-ref (use -v for more information) (e.g. https://gitlab.haskell.org/cactus/ghc/-/jobs/743141#L1465) 2. On validate-x86_64-darwin, pretty much every test fails because of the following extra stderr output: + +<no location info>: + warning: Couldn't figure out C compiler information! + Make sure you're using GNU gcc, or clang (e.g. https://gitlab.haskell.org/cactus/ghc/-/jobs/743129#L3655) 3. On validate-x86_64-linux-deb9-integer-simple, T11545 fails on memory consumption: Unexpected stat failures: perf/compiler/T11545.run T11545 [stat decreased from x86_64-linux-deb9-integer-simple-validate baseline @ 5f3991c7cab8ccc9ab8daeebbfce57afbd9acc33] (normal) This one is interesting because there is already a commit that is supposed to fix this: commit efaad7add092c88eab46e00a9f349d4675bbee06 Author: Matthew Pickering <matthewtpickering(a)gmail.com> Date: Wed Jul 21 10:03:42 2021 +0100 Stop ug_boring_info retaining a chain of old CoreExpr [...] ------------------------- Metric Decrease: T11545 ------------------------- But still, it's failing. Can someone kick these build setups please? -- .--= ULLA! =-----------------. \ http://gergo.erdi.hu \ `---= gergo(a)erdi.hu =-------'

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Where else do I need to register fixity declarations?
by Erdi, Gergo 27 Jul '21

27 Jul '21

PUBLIC Hi, In the attached program, I am typechecking two Haskell modules with GHC 9.0.1: `Imported.hs` defines some infix operators, and `Importer.hs` uses them. After typechecking the first one, I put it in the moduleNameProvidersMap and the HPT. However, when I am typechecking the second one, the fixity declarations of the infix operators aren't picked up correctly. I know they aren't because I have defined operators that only typecheck if they are parsed right-associatively. In contrast, if I put both the definitions and the usage into the same file (`Standalone.hs`), typechecking succeeds (as I would expect it to). What am I doing wrong? Is filling the `mi_fixities` field of the `ModIface` not enough to let importers see the correct fixities? Thanks, Gergo This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at https: //www.sc.com/en/our-locations Where you have a Financial Markets relationship with Standard Chartered PLC, Standard Chartered Bank and their subsidiaries (the "Group"), information on the regulatory standards we adhere to and how it may affect you can be found in our Regulatory Compliance Statement at https: //www.sc.com/rcs/ and Regulatory Compliance Disclosures at http: //www.sc.com/rcs/fm Insofar as this communication is not sent by the Global Research team and contains any market commentary, the market commentary has been prepared by the sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied on for any other purpose and is subject to the relevant disclaimers available at https: //www.sc.com/en/regulatory-disclosures/#market-disclaimer. Insofar as this communication is sent by the Global Research team and contains any research materials prepared by members of the team, the research material is for information purpose only and shall not be relied on for any other purpose, and is subject to the relevant disclaimers available at https: //research.sc.com/research/api/application/static/terms-and-conditions. Insofar as this e-mail contains the term sheet for a proposed transaction, by responding affirmatively to this e-mail, you agree that you have understood the terms and conditions in the attached term sheet and evaluated the merits and risks of the transaction. We may at times also request you to sign the term sheet to acknowledge the same. Please visit https: //www.sc.com/en/regulatory-disclosures/dodd-frank/ for important information with respect to derivative products.

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Is simplified subsumption really necessary?
by Chris Smith 23 Jul '21

23 Jul '21

This might be in the "ship has sailed" territory, but I'd like to bring it up anyway. https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-s… says: Suppose GHC lacked all four features, and someone proposed adding them. > That proposal would never leave the launchpad. > Let's test that hypothesis. I've been spending increasing amounts of time fighting against simplified subsumption while porting Haskell code to GHC 9.0. It's not that any specific instance of this problem is hard to fix; rather, it's that it completely screws up my intuition about what should be valid Haskell. It doesn't help that HLS still requires 8.10.4, so I usually don't find out I've broken my libraries for GHC 9.0 until continuous integration kicks in. At this point, it's become fairly routine that my code that works fine with 8.10.4 is broken with 9.0, and this makes me sad. Understandably, eta expansion reducing the strictness of terms is bad. But wouldn't it be possible to choose a desugaring with seq that doesn't do so?

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Using overloaded syntax to avoid `base` dependency (RE: Marking ParsedModule fragments as non-user-originating)
by Erdi, Gergo 23 Jul '21

23 Jul '21

PUBLIC OK so I tried out OverloadedStrings and it basically went as bad as I expected. The documentation on `HsOverLit` is very promising: it points to the Note [Overloaded literal witnesses], which states: Note [Overloaded literal witnesses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ *Before* type checking, the HsExpr in an HsOverLit is the name of the coercion function, 'fromInteger' or 'fromRational'. So that sounds great, right? It sounds like just before renaming, I should be able to replace `HsLit _ (HsString _ fs)` with `HsOverLit _ (OverLit _ (HsIsString _ fs) unpack` with my own `unpack` function coming from my own package, and everything would work out. Unfortunately, this is not what happens: if I try getting this through the renamer, I get this error: Failed to load interface for 'Data.String' no unit id matching 'base' was found Can't find interface-file declaration for variable fromString Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error So even though I am specifying my own coercion function, it is still looking for `Data.String.fromString` which is not going to work, since I don't have `base`. So either I am misunderstanding that Note, or it is simply out of date, but in either case, this isn't going to be a viable route to going base-less. Gergo From: Erdi, Gergo Sent: Tuesday, July 6, 2021 5:39 PM To: Simon Peyton Jones <simonpj(a)microsoft.com> Cc: GHC <ghc-devs(a)haskell.org> Subject: RE: Marking ParsedModule fragments as non-user-originating PUBLIC Thanks Simon! Of course, you're right, it's the renamer, not the typechecker - I didn't really check, just saw that "it happens during `typecheckModule`. I'll look at the rebindable syntax stuff in detail, but at least for OverloadedStrings, I already know that the problem will be that ultimately they do go through the `String` type from `base`, and I need to use GHC baselessly. This is a problem for two reasons: * I can't implement `IsString` for `MyString`, because `IsString` is in `base` * Even if I made my own fake `base` with a fake `IsString` class, there is nothing to put in the codomain of `fromString`: I *only* have `MyString`, not `String`. And renaming `MyString to `String` in my fake `base` is not going to cut it, since `String` is wired into GHC to be a type synonym for `[Char]` (which `MyString` is not). I foresee similar problems for OverloadedLists :/ Thanks, Gergo From: Simon Peyton Jones <simonpj(a)microsoft.com<mailto:simonpj@microsoft.com>> Sent: Tuesday, July 6, 2021 5:08 PM To: Erdi, Gergo <Gergo.Erdi(a)sc.com<mailto:Gergo.Erdi@sc.com>> Cc: GHC <ghc-devs(a)haskell.org<mailto:ghc-devs@haskell.org>> Subject: [External] RE: Marking ParsedModule fragments as non-user-originating The typechecker now complains that the `ViewPatterns` language extension is not turned on I think it's the renamer: rnPatAndThen mk p@(ViewPat _ expr pat) = do { liftCps $ do { vp_flag <- xoptM LangExt.ViewPatterns ; checkErr vp_flag (badViewPat p) } More generally, don't you just want OverloadedStrings or OverloadedLists? You might want to read Note [Handling overloaded and rebindable constructs] in GHC.Rename.Expr, and Note [Rebindable syntax and HsExpansion] in GCH.Hs.Expr. These Notes describe how GHC already does something similar to what you want. Maybe you can use the same mechanism in your plugin. Simon From: ghc-devs <ghc-devs-bounces(a)haskell.org<mailto:ghc-devs-bounces@haskell.org>> On Behalf Of Erdi, Gergo via ghc-devs Sent: 06 July 2021 09:08 To: ghc-devs(a)haskell.org<mailto:ghc-devs@haskell.org> Subject: Marking ParsedModule fragments as non-user-originating PUBLIC Hi, I'd like to hijack some syntax (like string literals or list patterns) for my own use, and I thought a low-tech way of doing that is to transform the ParsedModule before typechecking. For example, if I have a function `uncons :: Array a -> Maybe (a, Array a)`, I can rewrite the pattern `[x1, x2, x3]` into the view pattern `(uncons -> Just (x1, (uncons -> Just (x2, (uncons -> Just (x3, (uncons -> Nothing)))))))` and let the normal GHC type checker take over from here. This is working for me so far, except for one problem: the typechecker now complains that the `ViewPatterns` language extension is not turned on. I would like to make the view patterns coming from my ParsedModule rewriter to be exempt from this check (but of course still require the `ViewPatterns` extension for user-originating code). Is there a way to do that? Or would I be better off checking for user-originating view patterns myself before the rewrite and then changing the `DynFlags` to always enable view patterns for typechecking? Thanks, Gergo This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at https: //www.sc.com/en/our-locations Where you have a Financial Markets relationship with Standard Chartered PLC, Standard Chartered Bank and their subsidiaries (the "Group"), information on the regulatory standards we adhere to and how it may affect you can be found in our Regulatory Compliance Statement at https: //www.sc.com/rcs/ and Regulatory Compliance Disclosures at http: //www.sc.com/rcs/fm Insofar as this communication is not sent by the Global Research team and contains any market commentary, the market commentary has been prepared by the sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied on for any other purpose and is subject to the relevant disclaimers available at https: //www.sc.com/en/regulatory-disclosures/#market-disclaimer. Insofar as this communication is sent by the Global Research team and contains any research materials prepared by members of the team, the research material is for information purpose only and shall not be relied on for any other purpose, and is subject to the relevant disclaimers available at https: //research.sc.com/research/api/application/static/terms-and-conditions. Insofar as this e-mail contains the term sheet for a proposed transaction, by responding affirmatively to this e-mail, you agree that you have understood the terms and conditions in the attached term sheet and evaluated the merits and risks of the transaction. We may at times also request you to sign the term sheet to acknowledge the same. Please visit https: //www.sc.com/en/regulatory-disclosures/dodd-frank/ for important information with respect to derivative products.

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Rewriting plugins: request for feedback
by Sam Derbyshire 22 Jul '21

22 Jul '21

Hi everyone, Recent refactors to the constraint solver have complicated the story for authors of type-checking plugins. Following a suggestion by Christiaan Baaij, and aided by Richard Eisenberg, I'm working on adding the ability for type-checking plugins to rewrite type family applications directly (hooking straight into GHC's type-family rewriting code). The goal is to ease migration for the plugin authors, hopefully simplifying the common operation of rewriting type family applications. Let me now describe the current API for this new functionality, for which I would like feedback. -------------------------------------------------------------------------- A new field is added to the TcPlugin data type, namely tcPluginRewrite :: s -> UniqFM TyCon TcPluginRewriter Here s is the existential plugin state that users can choose as they wish for their own plugin. Plugins thus register which type family TyCons they are interested in rewriting, like so: myTcPluginRewrite :: MyPluginState -> UniqFM TyCon TcPluginRewriter myTcPluginRewrite s@(MyPluginState {myFam1TyCon, myFam2TyCon}) = listToUFM [ (myFam1TyCon, myFam1Rewriter s), (myFam2TyCon, myFam2Rewriter s) ] For each type family TyCon, the plugin should provide a rewriting function (corresponding to myFam1Rewriter and myFam2Rewriter above), of type type TcPluginRewriter = [Ct] -> [TcType] -> TcPluginM TcPluginRewriteResult This means that a rewriting function is supplied with the current Given constraints and the arguments of the (guaranteed to be saturated) type-family application, and should return a result of type: data TcPluginRewriteResult where TcPluginRewriteError :: (Diagnostic a, Typeable a) => a -> TcPluginRewriteResult TcPluginNoRewrite :: TcPluginRewriteResult TcPluginRewriteTo :: { rewriteTo :: TcType , rewriteEvidence :: TcCoercion } -> TcPluginRewriteResult That is, the plugin can specify what the type-family application should be rewritten to, throw an error, or do nothing. -------------------------------------------------------------------------- I'm hoping that the components of existing type-checking plugins concerned with type-family applications can instead use this mechanism and achieve more robust results by doing so, with less effort. I would like to hear from plugin authors whether this API suits their needs. If you know of someone not on this list that might be interested, please invite them to join the conversation. The current state of this patch can be found in the GHC MR 5909: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5909 Thanks, Sam

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