February 2022 - ghc-devs

Question about ambiguous predicates in pattern bindings
by Benjamin Redelings 16 Aug '22

16 Aug '22

Hi, 1. I'm reading "A Static semantics for Haskell" and trying to code it up. I came across some odd behavior with pattern bindings, and I was wondering if someone could explain or point me in the right direction. Suppose you have the declaration (x,y) = ('a',2) My current code is yielding: x :: Num a => Char y :: Num a => a However, I notice that ghci gives x the type Char, with no constraint, which is what I would expect. It also gives y the type 'Num b => b', so I don't think it is defaulting a to Int here. The weird results from my code stem from rule BIND-PRED in Figure 15 of https://homepages.inf.ed.ac.uk/wadler/papers/staticsemantics/static-semanti… E |- bind ~~> \dicts : theta -> monobinds in bind : (LIE_{encl}, theta => LVE) Here theta = ( Num a ) and LVE = { x :: Char, y :: a }. So, theta => LVE is { x :: Num a => Char, y :: Num a => a } The obvious thing to do is avoid changing a type T to Num a => T if T does not contain a. Also I'm not totally sure if that trick gets to the bottom of the issue. However, the paper doesn't mention define theta => LVE that way. Is there something else I should read on this? 2. If we just chop out predicates which mention variables not in the type ( == ambiguous predicates?) then I'm not totally sure how to create code for this. I would imagine that we have something like tup dn = ('a', fromInteger dn 2) x = case (tup dn) of (x,y) -> x y dn case (tup dn) of (x,y) -> y In this case its not clear where to get the `dn` argument of `tup` from, in the definition of `x`. Can we pass in `undefined`? Should we do something else? If anyone can shed light on this, I would be grateful :-) -BenRI

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9.4 release planning (& GHCJS merge process)
by Matthew Pickering 24 Feb '22

24 Feb '22

Hi all, Firstly we are anticipating branching 9.4 in about 6 weeks time (approx start of April). Most of the major features originally milestoned[1] for this release have landed and the main branch is currently in a nice state. This timeline will be solidified once the 9.2.2 release has been completed. The major outstanding work that I am aware of is Windows toolchain work (#21019) (Ben Gamari) Partial Register Stall (#20405) (Andreas Klebinger) hi-haddock (!6224) (Zubin Duggal / Matthew Pickering ) Directed Coercions (!6476) (Sam Derbyshire) Secondly, we are anticipating adding a javascript backend inspired by GHCJS as the highlight of the 9.6 release. This work is led by the team at IOHK. * The planning and progress of this process is tracked on this wiki page - https://gitlab.haskell.org/ghc/ghc/-/wikis/javascript-backend * We anticipate reviewing and merging patches incrementally into a feature branch until the point where test coverage is suitable and the patches can be merged into the main branch. Cheers, Matt [1]: https://gitlab.haskell.org/ghc/ghc/-/milestones/370

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Well-Typed GHC team update
by Matthew Pickering 23 Feb '22

23 Feb '22

Hi all, We have recently reorganised the structure of the Well-Typed team working on the GHC project. In short, I am now responsible for more of the management tasks that Ben has traditionally carried out. In particular I have responsibility for organising the work between us and keeping track of the tasks that need to be completed. With that in mind, if you need something doing, please email/ping me and not Ben with your request and I will make sure someone deals with it. On gitlab you can also notify me directly by using the @hq-shepard tag, which provides the level of indirection for when this changes again in the future. The team working at Well-Typed on GHC currently consists of Myself (Matthew Pickering) Ben Gamari Andreas Klebinger Sam Derbyshire Zubin Duggal Cheers, Matt

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struggling to get master to build
by Norman Ramsey 18 Feb '22

18 Feb '22

I recently pulled to bring my local copy of GHC HEAD up to 67dd5724297094af93be1887ef000845722c6f2b. But I cannot seem to get it to build. I'm following these steps: git pull --ff-only --prune --recurse-submodules --force rm -r _build ./boot ./configure ./hadrian build -j The build is failing with this error message: Warning: -rtsopts and -with-rtsopts have no effect with -shared. Call hs_init_ghc() from your main() function to set these options. ld.lld: error: can't create dynamic relocation R_X86_64_64 against local symbol in readonly segment; recompile object files with -fPIC or pass '-Wl,-z,notext' to allow text relocations in the output >>> defined in _build/stage1/rts/build/S/adjustor/NativeAmd64Asm.thr_dyn_o >>> referenced by _build/stage1/rts/build/S/adjustor/NativeAmd64Asm.thr_dyn_o:(.text+0x50) collect2: error: ld returned 1 exit status `cc' failed in phase `Linker'. (Exit code: 1) Command failed Build failed. The offending command is very long indeed; it is appended to this message. What am I doing wrong, and what do I need to change? Norman Command line: _build/stage0/bin/ghc -Wall -Wcompat -hisuf thr_dyn_hi -osuf thr_dyn_o -hcsuf thr_dyn_hc -fPIC -dynamic -optc-DTHREADED_RTS -hide-all-packages -no-user-package-db '-package-env -' '-package-db _build/stage1/lib/package.conf.d' '-this-unit-id rts-1.0.2' -i -i/home/nr/asterius/ghc/_build/stage1/rts/build -i/home/nr/asterius/ghc/_build/stage1/rts/build/autogen -i/home/nr/asterius/ghc/rts -Irts/include -I_build/stage1/rts/build -I_build/stage1/rts/build/include -Irts/include -optP-include -optP_build/stage1/rts/build/autogen/cabal_macros.h -ghcversion-file=rts/include/ghcversion.h -outputdir _build/stage1/rts/build -fdiagnostics-color=always -dynamic -shared -dynload deploy -optl-Wl,-rpath,$ORIGIN -optl-Wl,-zorigin -fno-use-rpaths -no-auto-link-packages -rtsopts -lc -lm -lrt -ldl -lpthread -lnuma -L_build/stage1/rts/build -lffi -Wnoncanonical-monad-instances -optc-Wno-error=inline -optP-Wno-nonportable-include-path 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-ghcversion-file=rts/include/ghcversion.h -haddock -Irts -I_build/stage1/rts/build '-DRtsWay="rts_thr_dyn"' -DFS_NAMESPACE=rts -DCOMPILING_RTS -DTHREADED_RTS -Wno-deprecated-flags -Wcpp-undef ===> Command failed with error code: 1

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RE: Where do I put the *definition* of a DFunId?
by Erdi, Gergo 15 Feb '22

15 Feb '22

PUBLIC > I'm not surprised that eventually it crashes and burns, because, > again, I have only declared my DFunId ('$fShowOrderPolicy' in this > case), but never defined it. Its definition would be a CoreExpr, > right? So where would I put the pair of '(dfun, > myCoreExprOfTheRightType)' for GHC to pick it up? Or is it the case > that GHC would only need an 'Id's definition if it is trying to > inline/specialize it, i.e. should I just attach the definition to the > DFunId as an unfolding? Or is 'registerModule' already incomplete and > it should put the 'CoreProgram' of the module to somewhere deep in the > GHC state? I tried adding a reasonable-looking unfolding to the DFun, but I am still getting the same error when compiling code that uses the instance: $fShowOrderPolicy [InlPrag=CONLIKE] :: Show OrderPolicy [LclIdX[DFunId], Unf=DFun: \ -> C:Show \ _ [Occ=Dead] -> unpackCStringUtf8# "foo"# \ _ [Occ=Dead] _ [Occ=Dead] _ [Occ=Dead] -> unpackCStringUtf8# "bar"# \ _ [Occ=Dead] _ [Occ=Dead] -> unpackCStringUtf8# "baz"#] panic! (the 'impossible' happened) GHC version 9.3.20211130: lookupIdSubst $fShowOrderPolicy InScope {foo mapM_} Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Utils.Panic pprPanic, called at compiler/GHC/Core/Subst.hs:260:17 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Core.Subst I have added a minimal working reproducer to https://gitlab.haskell.org/ghc/ghc/-/issues/21080 So the question remains, where do I need to put the DFun's definition (a CoreExpr of the right type) for it to be usable as an instance? 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.

1 0

install stage0 libraries?
by Norman Ramsey 11 Feb '22

11 Feb '22

If I understand correctly, the way Hadrian works is that all of the .conf files, binaries, and so on in _build/stage0 are built with the bootstrap compiler. I need to write client code that uses the ghc library built from GHC HEAD (so ghc-9.3), but I'd prefer to use the bootstrap compiler. (The HLS works with the bootstrap compiler.) Is there a way to get Hadrian to install the stage0 libraries in a way that cabal-install can find it using standard options? I tried to do it myself using `ghc-pkg register`, and all I accomplished was to make an unrecoverable mess of my system. (The problem I'm trying to solve is to get my client code to compile against the latest library, while also enabling the HLS to find all the things.) Norman

1 0

Where do I put the *definition* of a DFunId?
by Erdi, Gergo 11 Feb '22

11 Feb '22

PUBLIC PUBLIC Hi, I'm trying to make a module out of thin air and register it to GHC so that other modules can import it. So far, I have had success with making a ModIface and a ModDetails, and then registering them using the following function: registerModule :: (GhcMonad m) => ModIface -> ModDetails -> m () registerModule iface details = modifySession $ extendHpt . addModule where mod_info = HomeModInfo iface details Nothing mod = mi_module iface modOrig = ModOrigin (Just True) [] [] True addModule = modifyUnitState $ \us -> us { moduleNameProvidersMap = M.insert (moduleName mod) (M.singleton mod modOrig) $ moduleNameProvidersMap us } extendHpt env = env { hsc_unit_env = let ue = hsc_unit_env env in ue { ue_hpt = hpt } } where hpt = addToHpt (hsc_HPT env) (moduleName mod) mod_info This worked when I was only _declaring_ functions and TyCons in these modules. However, now I would also like to add instances. And I'm hitting a problem here, because I don't know where to put the actual definitions of the instance. I'm completely lost here. Here's what I'm doing in detail. First, I make a fresh DFunId from a fresh Unique: let tag = occNameString . getDFunTyKey $ ty occ = mkDFunOcc (occNameString (getOccName showClass) <> tag) False emptyOccSet name = mkExternalName uniq mod occ loc dfun = mkDictFunId name [] [] showClass [ty] Then, I make a ClsInst that describes my instance: return ClsInst { is_cls_nm = getName showClass , is_tcs = [KnownTc $ getName tycon] , is_dfun_name = getName dfun , is_tvs = [] , is_cls = showClass , is_tys = [ty] , is_dfun = dfun , is_flag = OverlapFlag (NoOverlap NoSourceText) False , is_orphan = NotOrphan (getOccName $ getName tycon) } And then I add 'AnId dfun' to my ModDetails's type env in 'md_types',, add the instance to the 'md_insts', and fill the 'mi_decls' and 'mi_insts' of the ModIface accordingly. This gives me a ModIface/ModDetails pair just like before - but I never said what the definition of 'dfun' is! When I try compiling a real source module that imports this synthetic module and tries to use the instance, it gets as far as the "Desugar (after optimization)" step, and then fails with: ghc-mu-core-to-exp: panic! (the 'impossible' happened) GHC version 9.3.20211130: lookupIdSubst $fShowOrderPolicy InScope {foo mapM_} Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Utils.Panic pprPanic, called at compiler/GHC/Core/Subst.hs:260:17 in ghc-lib-0.20211130-7QA7vLTw0OYJmMsraoHe3v:GHC.Core.Subst I'm not surprised that eventually it crashes and burns, because, again, I have only declared my DFunId ('$fShowOrderPolicy' in this case), but never defined it. Its definition would be a CoreExpr, right? So where would I put the pair of '(dfun, myCoreExprOfTheRightType)' for GHC to pick it up? Or is it the case that GHC would only need an 'Id's definition if it is trying to inline/specialize it, i.e. should I just attach the definition to the DFunId as an unfolding? Or is 'registerModule' already incomplete and it should put the 'CoreProgram' of the module to somewhere deep in the GHC state? 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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the linters are killing me slowly
by Richard Eisenberg 10 Feb '22

10 Feb '22

Hi devs, Can we please, please not have the linters stop more useful output during CI? Over the past few months, I've lost several days of productivity due to the current design. Why several days? Because I typically end up with only 1.5-2 hours for GHC work in a day, and when I have to spend half of that recreating test results (which, sometimes, don't work, especially if I use Hadrian, as recommended), I often decide to prioritize other tasks that I have a more reasonable chance of actually finishing. It was floated some time ago that "Draft" MRs could skip linters. But I actually have a non-Draft MR now, and it failed the new Notes linter. (Why, actually, is that even a separate pass? I thought there was a test case for that, which I'm thrilled with.) It just feels to me that the current workflow is optimized for those of us who work on GHC close to 100% of the time. This is not the way to get new contributors. Frustrated, Richard

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how to complete a review on gitlab?
by Norman Ramsey 09 Feb '22

09 Feb '22

I've been assigned a few merge requests to review on gitlab. One I've completed and approved. One I've completed and have decided that somebody else needs to approve it; I'm not qualified. Two I've not yet started. Is there any way to tell gitlab, "I'm done with these two"? Norman

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Runnig a compiler plugin over ALL code
by Tillmann Vogt 09 Feb '22

09 Feb '22

Hi, I am running a compiler plugin over hundreds of libraries to extract statistical data. I use the plugin like it is done here: https://github.com/coot/ghc-tags-plugin So I just have to add a cabal.project.local to a library with: ignore-project: False package test ghc-options: -package-db=/home/till/.cabal/store/ghc-8.10.7/package.db -plugin-package=ghc-core-graph -fplugin=GhcPlugins.Optimind But unfortunately the plugin only runs over code that is used in the main function or that is used in other functions. Example: main = putStrLn (show (f 0)) f x = sin x g = h + i Then the compiler plugin only runs over f, h, i but not not g (unused top level functions). Can the plugin be forced to run over all code? -Tillmann

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