August 2021 - ghc-devs

How to use Lexer.lexer to produce closing braces as well?
by Kwanghoon Choi 26 Oct '21

26 Oct '21

Hi, I have recently been playing with GHC's Lexer.lexer in the ghc-parser-lib package. Given module HelloWorld where main = putStrLn "Hello World!\n" it produces stack exec -- lexer-exe ./examples/HelloWorld.hs Lexing&Parsing: ./examples/HelloWorld.hs module at (1, 1): module CONID at (1, 8): CONID where at (1, 19): where vocurly at (3, 1): vocurly <==== { is inserted automatically!! VARID at (3, 1): VARID = at (3, 6): = VARID at (3, 8): VARID STRING at (3, 17): STRING ; at (4, 1): ; By the example above, the lexer automatically inserts an opening brace (i.e. vocurly) right after 'where'. But it does not insert a matching closing brace (i.e., vccurly), which would lead to a failure in parsing a list of tokens produced by the lexer. My question is how to use the GHC lexer to produce closing braces as well. All my code is available - https://github.com/kwanghoon/hslexer To save your time, the relevant part of the code is as follows: In app/HaskellLexer.hs, singleHaskellToken :: P (Located Token) singleHaskellToken = Lexer.lexer False (\locatedToken -> P (\pstate -> POk pstate locatedToken)) tokInfos :: [Terminal Token] -> P (Line, Column, [Terminal Token]) tokInfos s = do locatedToken <- singleHaskellToken case locatedToken of L srcspan ITeof -> let (start_line, start_col, end_line, end_col) = srcSpanToLineCol srcspan in return (end_line, end_col, s) L srcspan tok -> let (start_line, start_col, end_line, end_col) = srcSpanToLineCol srcspan in tokInfos (Terminal (fromToken tok) start_line start_col (Just tok) : s) Thanks in advance Best regards, Kwanghoon

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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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How to set idScope properly when converting LhsExpr GhcPs to CoreExpr?
by Yiyun Liu 10 Sep '21

10 Sep '21

Hi all, We have a function in Liquidhaskell's code base for converting refinement expressions into GHC's core expressions using the GHC API. Its implementation is based on tcRnExpr <https://gitlab.haskell.org/ghc/ghc/-/blob/ghc-8.10.6-release/compiler/typec…>except we keep the typechecked expression and desugar it before returning. However, our function fails to set the idScope information properly when LiquidHaskell is invoked as a plugin (installed as a typeCheckResultAction). This repo <https://github.com/yiyunliu/ghc-elaboration-test>contains a minimal example that demonstrates the issue. In the CoreExpr returned by elabRnExpr <https://github.com/yiyunliu/ghc-elaboration-test/blob/cb7397884b7949e6b7fb6…>, every symbol from the current module that's being compiled is labeled as local. The README.md file shows how the exported symbol testPlus is marked as local in QQQ.hs, the module where it's defined. Is there a function we should call to set the idScope information properly? Thanks, -Yiyun

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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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Trying to create a regression test for #20261
by Alexis Praga 01 Sep '21

01 Sep '21

Hi, As a intermediate beginner in Haskell, I want to try to tackle a small issue for contributing to GHC. I've looked into https://gitlab.haskell.org/ghc/ghc/-/issues/20261, but it's not clear how to integrate a running time in a regression test. >From what I understand, regression tests should be compared against sucess/failure or a given output. Should there be a hard-coded limit ? If that's the case, how could I create such a test ? I could not find a similar test in the source code. Any help is appreciated, thanks ! -- Alexis Praga

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GHC plugin authors?
by Richard Eisenberg 01 Sep '21

01 Sep '21

Hi all, I have seen a few posts from Sam Derbyshire here asking for feedback about plugin API design, and the responses have been minimal. This poses a design challenge, because the GHC folk who design the interface are sometimes distinct from the people who use the interface. We're trying to be good, seeking feedback from real, live clients. Is there a better way to do so than this mailing list? Example: we could create a Category on discourse.haskell.org, if that would reach the audience better. Or we could make a repo with issue trackers somewhere simply to track plugin design. What would work? (I recognize that I'm asking in a perhaps-ineffective channel for advice, but I really don't have a better idea right now. Maybe some of you plugin authors are here and will point us in a better direction.) Thanks, Richard

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Re: Failed to build latest stable GHC on FreeBSD with Hadrian
by Jeffrey Young 28 Aug '21

28 Aug '21

Hi Alexis, This line shows the error: > | Configure package 'ghc-bin' > # cabal-configure (for _build/stage0/utils/haddock/setup-config) > # cabal-configure (for _build/stage0/ghc/setup-config) > hadrian: Encountered missing or private dependencies: > ghc ==9.3.* A package had a constraint on the ghc api to only accept ghc == 9.3.*. Since you are trying to build ghc 9.2 this fails. The package in question is printed by hadrian in this bit of your log: > src/Development/Shake/Internal/Rules/Oracle.hs:159:32 in > shake-0.19.4-9f51719be46f95213676da51d6a6626f69f0cfa15957aab44867ff78b757df38:Development.Shake.Internal.Rules.Oracle > * Depends on: OracleQ (ContextDataKey (Context {stage = Stage0, package = > Package {pkgType = Program, pkgName = "haddock", pkgPath = > "utils/haddock"}, way = v})) > at need, called at src/Hadrian/Oracles/Cabal/Rules.hs:54:9 in Notice the `pkgName` field in the `Package` record in the error that hadrian produced. It shows us that haddock was the cuplrit. Not sure why/how that cabal file got mixed up but I bet if you went to `${GHC_HOME}/utils/haddock/haddock.cabal` you'll find the constraint in question and should be able to switch it to `ghc==9.2.*` and rebuild according to the wiki. Hope this helps, Jeff On 8/27/21 10:08 AM, ghc-devs-request(a)haskell.org wrote: > Send ghc-devs mailing list submissions to > ghc-devs(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > or, via email, send a message with subject or body 'help' to > ghc-devs-request(a)haskell.org > > You can reach the person managing the list at > ghc-devs-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of ghc-devs digest..." > > > Today's Topics: > > 1. Failed to build latest stable GHC on FreeBSD with Hadrian > (Alexis Praga) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 27 Aug 2021 19:15:26 +0200 > From: Alexis Praga <alexis.praga(a)gmail.com> > To: ghc-devs(a)haskell.org > Subject: Failed to build latest stable GHC on FreeBSD with Hadrian > Message-ID: > <CAH3o9gzh99J8SWPebkKsykEf+UD59S7-bSZi9OPQY1WxC_tE=w(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > Hi, > > As a complete beginner in regards to GHC, I tried to build GHC 9.2 as it > looked like the latest stable from git.I failed to build 9.0.1 before that. > > After checking out the ghc-9.2 branch, I ran (following the wiki): > >> ./boot >> set LOCALBASE=/usr/local >> ./configure --with-gmp-includes=$LOCALBASE/include > --with-gmp-libraries=$LOCALBASE/lib --disable-large-address-space >> hadrian/build -j > This failed with : > >> | Configure package 'rts' >> | Configure package 'haddock' >> | Configure package 'ghc-bin' >> # cabal-configure (for _build/stage0/utils/haddock/setup-config) >> # cabal-configure (for _build/stage0/ghc/setup-config) >> hadrian: Encountered missing or private dependencies: >> ghc ==9.3.* >> >> Error when running Shake build system: >> at action, called at src/Rules.hs:40:19 in main:Rules >> at need, called at src/Rules.hs:62:5 in main:Rules >> * Depends on: _build/stage0/bin/haddock >> at apply1, called at > src/Development/Shake/Internal/Rules/Oracle.hs:159:32 in > shake-0.19.4-9f51719be46f95213676da51d6a6626f69f0cfa15957aab44867ff78b757df38:Development.Shake.Internal.Rules.Oracle >> * Depends on: OracleQ (ContextDataKey (Context {stage = Stage0, package = > Package {pkgType = Program, pkgName = "haddock", pkgPath = > "utils/haddock"}, way = v})) >> at need, called at src/Hadrian/Oracles/Cabal/Rules.hs:54:9 in > main:Hadrian.Oracles.Cabal.Rules >> * Depends on: _build/stage0/utils/haddock/setup-config >> * Raised the exception: >> ExitFailure 1 > The config.log is attached. > > Any tips on how to proceed would be appreciated, thanks ! >

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Failed to build latest stable GHC on FreeBSD with Hadrian
by Alexis Praga 27 Aug '21

27 Aug '21

Hi, As a complete beginner in regards to GHC, I tried to build GHC 9.2 as it looked like the latest stable from git.I failed to build 9.0.1 before that. After checking out the ghc-9.2 branch, I ran (following the wiki): > ./boot > set LOCALBASE=/usr/local > ./configure --with-gmp-includes=$LOCALBASE/include --with-gmp-libraries=$LOCALBASE/lib --disable-large-address-space > hadrian/build -j This failed with : > | Configure package 'rts' > | Configure package 'haddock' > | Configure package 'ghc-bin' > # cabal-configure (for _build/stage0/utils/haddock/setup-config) > # cabal-configure (for _build/stage0/ghc/setup-config) > hadrian: Encountered missing or private dependencies: > ghc ==9.3.* > > Error when running Shake build system: > at action, called at src/Rules.hs:40:19 in main:Rules > at need, called at src/Rules.hs:62:5 in main:Rules > * Depends on: _build/stage0/bin/haddock > at apply1, called at src/Development/Shake/Internal/Rules/Oracle.hs:159:32 in shake-0.19.4-9f51719be46f95213676da51d6a6626f69f0cfa15957aab44867ff78b757df38:Development.Shake.Internal.Rules.Oracle > * Depends on: OracleQ (ContextDataKey (Context {stage = Stage0, package = Package {pkgType = Program, pkgName = "haddock", pkgPath = "utils/haddock"}, way = v})) > at need, called at src/Hadrian/Oracles/Cabal/Rules.hs:54:9 in main:Hadrian.Oracles.Cabal.Rules > * Depends on: _build/stage0/utils/haddock/setup-config > * Raised the exception: > ExitFailure 1 The config.log is attached. Any tips on how to proceed would be appreciated, thanks ! -- Alexis Praga

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Registering orphan instances and filling the `deps` field of ModIface [RE: Where (else) do I need to register instances from loaded modules?]
by Erdi, Gergo 27 Aug '21

27 Aug '21

PUBLIC Re: #1., I've simplified the code by removing all the Unit shuffling: everything is now in mainUnit. Unfortunately, this doesn't change anything other than cutting a cool 100 lines from the MWE code; I'm attaching the new smaller & simpler version. It is based on GHC 9.0.1 with an extra patch to export the first half of mkIfaceTc, just before calling mkFullIface. With this change, it really should be filling everything in ModIface the same way as the real GHC (so e.g. `deps` should be properly filled), modulo the fingerprints. Unfortunately, the `deps` field of the `ModIface`s created this way still doesn't contain any orphan instance modules: `Class.src`'s deps are empty (which is what I'd expect), `Instance.src` has a dependency in dep_mods (but not in dep_orphs) on `Class {-# SORUCE #-}` (I'm not sure why it's marked as a source import when `Class` module was already loaded into the module providers map before typechecking `Instance`...), and of course `Use.src` still fails so I can't get to its dependencies. It is confusing to me that the problem should be because of the dep_orphs, because `Instance.src` does NOT use any orphan instances from `Class.src; merely, it provides a new orphan instance for other modules to depend on. Can someone explain this part to me? Thanks, Gergo -----Original Message----- From: Erdi, Gergo Sent: Wednesday, August 25, 2021 10:37 AM To: Matthew Pickering <matthewtpickering(a)gmail.com> Cc: GHC <ghc-devs(a)haskell.org>; gergo(a)erdi.hu; Montelatici, Raphael Laurent <Raphael.Montelatici(a)sc.com> Subject: Re: Where (else) do I need to register instances from loaded modules? Hi, Thanks Matt! Unfortunately, I couldn't get it working even with these suggestions. 1. I changed registerModule so that it only changes the HPT when toUnitId (moduleUnit mod) == homeUnitId (hsc_dflags env). But I'm pretty sure this will be vacuously true, since the unit of the module comes from prepareModule setting the home unit before the call to `downsweep`. TBH I don't understand the whole idea behind the home package, and maybe all my pain comes from misusing packages in the first place. As you can see in the original code I've sent, I'm loading modules from different units; in my real use case, I'd even like to do things like load module1 from unit1, then module2 from unit2, then module3 from unit1 again -- basically just using the units as a cheap way to classify modules so that later downstream, I can make decisions based on the unit of the module of the source of a given definition. Should I give up on this idea, is this going to lead to problems later on? 2. I also changed mkModIface to fill in the `deps` and while I'm at it, the `usages` field, by copy-pasting more code from mkIfaceTc. I'm really starting to think that I'd be better off just changing mkIfaceTc to return a PartialModIface... Anyway, the new definitions of `deps` and `usages` are: let pluginModules = map lpModule (cachedPlugins (hsc_dflags hsc_env)) unit_id = homeUnitId (hsc_dflags hsc_env) deps <- mkDependencies unit_id (map mi_module pluginModules) tcg dep_files <- readIORef dependent_files let used_names = mkUsedNames tcg usages <- mkUsageInfo hsc_env this_mod (imp_mods imports) used_names dep_files merged pluginModules With these changes applied, I am still seeing the same error. Printing shows that `dep_orphs deps` is empty for Instance.src; further tracing, in turn, shows that `imp_orphs . tcg_imports $ tcg` is also empty! I have also found the wording in the documentation regarding dep_orphs confusing: it sounds like, in my case, this would mean that Use has a dependency on Instance, because it is using an orphan instance from there. But my problems are way before I am making a ModIface for Use, since it fails already during typechecking. But you saying the problem is the incomplete deps in ModIface must mean that dep_orphs needs to be filled in the ModIface for Instance (or, oh god I hope not, Class), right? If I sound somewhat confused, rest assured that in reality I am even MORE confused! Any further advice welcome, Gergo -----Original Message----- From: Matthew Pickering <matthewtpickering(a)gmail.com> Sent: Tuesday, August 24, 2021 5:16 PM To: Erdi, Gergo <Gergo.Erdi(a)sc.com> Cc: GHC <ghc-devs(a)haskell.org>; gergo(a)erdi.hu; Montelatici, Raphael Laurent <Raphael.Montelatici(a)sc.com> Subject: [External] Re: Where (else) do I need to register instances from loaded modules? Hi Gergo, Some things I noticed: * The HPT only contains modules from the current home unit. It seems that registerModule ignores this so you break this invariant. * It seems that `Instance` contains an orphan, but you don't fill in the `deps` field when making a `ModIface`, which should contain all the orphan modules below the module you are compiling. I think that if you fill in `deps` correctly then things may work out. `Dependencies` eventually becomes `ImportAvail`, which gets consulted by `tcVisibleOrphanMods` which is called in `tcGetInstEnvs` which is called by `matchInstEnv`. Matt 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. 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