August 2022 - ghc-devs

Implementing a compilation server
by Domínguez, Facundo 11 Oct '22

11 Oct '22

Dear ghc devs, I'm using the ghc API to write a compilation server (a.k.a. persistent worker). The idea is to serve requests to compile individual modules. In this fashion, we can compile modules with different compilation flags and yet pay only once for the startup overheads of the compiler. One challenge of this approach is to reuse as much as possible from the ghc API session/environment from one compilation request to the next, so we save the trouble of reconstructing it each time. This message is to ask for advise on how to better accomplish this reuse. I tried reusing the whole environment for multiple requests, but I'm conjecturing that this might cause troubles when the requests require building modules with different values of -this-unit-id. Another problem that streams from this is that recompiling a module which defines a type class instance fails because it encounters in the environment the type class instance from the previous compilation. My work-in-progress implementation is here [1]. There appears to be multiple ways to compile a module in the API, so far I have been trying DriverPipeline.compileFile. My best lead right now is to look for inspiration in how GHCi implements the load command, but this does a sort of --make compilation while I'm going here for the one-shot style. Thanks in advance, Facundo [1] https://github.com/tweag/rules_haskell/blob/16ba422457ea4daa5dbf40d46327ebc…

2 3

Benchmarking lexical scan during the downsweep for the ImplicitQualifiedImport proposal
by Tristan Cacqueray 29 Aug '22

29 Aug '22

Hello all, In the context of https://github.com/ghc-proposals/ghc-proposals/pull/500, I am looking for a strategy to measure the impact of resolving modules dependencies using a lexical scan during the downsweep phase. For example, in this branch: https://gitlab.haskell.org/TristanCacqueray/ghc/-/tree/make-lexical-analysis I've added a tokenizer pass in the GHC.Driver.Make.getPreprocessedImports function, and using `/usr/bin/time -o /dev/stdout -v _build/stage1/bin/ghc compiler/GHC/Tc/TyCl.hs 2> /dev/null` I measure: LexicalAnalysis found 91 qualified name, and lasted: 150msec Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.72 Maximum resident set size (kbytes): 142396 Using a clean build I get: Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.68 Maximum resident set size (kbytes): 140012 Now my question is, how would you measure the time and space cost of such change. For example, what profiling tool would you recommend and how can I test a modified ghc to build ghc itself? Thanks in advance, -Tristan

2 2

Instructions to build GHC
by Harendra Kumar 26 Aug '22

26 Aug '22

Hi, I used to follow the instructions in INSTALL.md and README.md in the root of the repo, and they always worked until recently. I am aware that the make based system has been replaced by Hadrian. But it seems these files are not updated with the new build instructions. 1. Either these files should be removed from the source or they should be updated. 2. Where are the new build instructions? Is it hadrian/README.md? If so can we put a pointer to that in INSTALL.md? Thanks, Harendra

1 0

The end of the road for the `make` build system
by Ben Gamari 24 Aug '22

24 Aug '22

Hello all, With the 9.4.2 release being out, we are turning our attention to preparations for 9.6. Among these tasks is the removal of the `make`-based build system, allowing us to finally close #17527. The branch removing `make` can be found in !7094, which we hope to send to Marge by the end of this week. For those of you still using `make`, this is your call to switch. Hadrian's documentation can be found in the tree (`hadrian/doc` [1]), the Wiki [2], and our packagers' migration guide [3]. If you have any remaining issues with Hadrian or its documentation do let us know so that we can address them. Cheers, - Ben [1] https://gitlab.haskell.org/ghc/ghc/-/tree/master/hadrian/doc [2] https://gitlab.haskell.org/ghc/ghc/-/wikis/building/hadrian [3] https://www.haskell.org/ghc/blog/20220805-make-to-hadrian.html

1 0

[ANNOUNCE] GHC 9.4.2 is now available
by Ben Gamari 22 Aug '22

22 Aug '22

The GHC developers are happy to announce the availability of GHC 9.4.2. Binary distributions, source distributions, and documentation are available at downloads.haskell.org: https://downloads.haskell.org/ghc/9.4.2 This release is primarily a bugfix release addressing a few packaging issues found in 9.4.1. See the [release notes] for a full accounting. Note that, as GHC 9.4 is the first release series where the release artifacts are all generated by our new Hadrian build system, it is possible that there will be packaging issues. If you enounter trouble while using a binary distribution, please open a [ticket]. Likewise, if you are a downstream packager, do consider migrating to [Hadrian] to run your build; the Hadrian build system can be built using `cabal-install`, `stack`, or the in-tree [bootstrap script]. See the accompanying [blog post] for details on migrating packaging to Hadrian. We would also like to emphasize that GHC 9.4 must be used in conjunction with Cabal-3.8 or later. This is particularly important for Windows users due to changes in GHC's Windows toolchain. We would like to thank Microsoft Azure, GitHub, IOG, the Zw3rk stake pool, Well-Typed, Tweag I/O, Serokell, Equinix, SimSpace, Haskell Foundation, and other anonymous contributors whose on-going financial and in-kind support has facilitated GHC maintenance and release management over the years. Finally, this release would not have been possible without the hundreds of open-source contributors whose work comprise this release. As always, do give this release a try and open a [ticket] if you see anything amiss. Happy Haskelling, - Ben [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new [bootstrap script]: https://gitlab.haskell.org/ghc/ghc/-/blob/e2520df3fffa0cf22fb19c5fb872832d1… [Hadrian]: https://gitlab.haskell.org/ghc/ghc/-/blob/e2520df3fffa0cf22fb19c5fb872832d1… [release notes]: https://downloads.haskell.org/~ghc/9.4.2/docs/users_guide/9.4.2-notes.html [blog post]: https://www.haskell.org/ghc/blog/20220805-make-to-hadrian.html

1 0

Partial type synonyms -- first-class!
by ÉRDI Gergő 17 Aug '22

17 Aug '22

As I mentioned at the end of the original thread (but probably no one was interested in this enough to read that far), after a breakthrough idea I have now started working on partial type synonyms that are NOT just defined using macro expansion semantics, and indeed can be a first-class abstraction. I think if I can iron out the wrinkles detailed at the bottom of this message, this would be good for a GHC proposal. 1. Defining partial type synonyms The idea is to typecheck the type synonym declaration type Syn a = MyType _ [a] Int into a type synonym that has some implicit (invisible) type parameters: type Syn {w1} a = MyType w1 [a] Int We don't need to come up with any new design for "what this means": this means exactly the same as type Syn w1 a = MyType w1 [a] Int which is something you can already write today. The only difference is how `w1` is given at use sites. 2. Using partial type synonyms When a partial type synonym is applied, its argument list is expanded by adding wildcard arguments for the invisible parameters. So this use site: Syn Double is typececked into the type application Syn {_} Double Again, we don't need to come up with any new design for "what this means": this is the same as writing `Syn _ Double` into the same position, it's just the `_` is not written in the surface syntax. In particular: * The level of the type variable conjured for the `_` is determined fully by where the `Syn Double` occurs, and not at all by the definition of `Syn` * Using the type synonym `Syn` is only valid where a wildcard occurrence would be valid. In particular, using `Syn` in a type signature causes the signature itself to be regarded as partial (`isCompleteHsSig` returns `False` for it). 3. Implementation I have a proof-of-concept implementation at https://gitlab.haskell.org/cactus/ghc/-/compare/master...cactus%2Fpartial-t… I tried to make it as small a change as I could. Its key points are: * In the renamer, we allow wildcards in `TySynCtx` context * In `tcTySynRhs`, we allow wildcards in the right-hand side, retrieve them from the result of typechecking, and intersperse them with the "normal" type parameters to get a well-scoped telescope. * About a third of the lines changed is just a boring refactoring of `isCompleteHsSig` to be able to look up type synonym `TyCon`s. This is needed because a type signature referring to partial type synonyms will be partial itself once those type synonym applications are elaborated. * When typechecking a type application, implicit arguments get filled with the result of `tcAnonWildCardOcc`. Now, my questions about the problems I've encountered along the way. I hope these questions are concrete enough that I can get concrete answers to them from this list: 1. How do I find the wildcard-originating tyvars robustly? Currently, I am using `candidateQTyVarsWithBinders` but that picks up "regular" out-of-scope tyvars as well. I believe this causes a regression of #17567; furthermore, it means if the user writes `type Syn a = Foo b` it is handled the same as if they wrote `type Syn a = Foo _`, but it would be better to reject it instead. 2. What should the `Role` of these implicit type parameters be? For now, I've went with `Nominal` for lack of a better answer. 3. When typechecking a type application, I need to insert wildcards for these implicit arguments. I am currently using `AnonTCB InvisArg` binders to represent these implicit type parameters, which means by the time I get to `tcInferTyApps`, they would get passed through `instantiate` to `tcInstInvisibleTyBinder`. I currently have a horrible kuldge here to check, before calling `tcInstInivisibleTyBinder`, that the invisible binder's kind doesn't meet the preconditions of it; and if it doesn't, I create a wildcard instead. But of course, it would be nicer if I could put something in the `TcTyConBinder` that identifies these binders properly. Thanks in advance, Gergo -- .--= ULLA! =-----------------. \ http://gergo.erdi.hu \ `---= gergo(a)erdi.hu =-------'

5 6

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

4 8

Test failures on `master`
by Erdi, Gergo 16 Aug '22

16 Aug '22

PUBLIC Hi, As of dca43a04fb, I am seeing the following test failures locally. Is this due to some configuration problem in my development environment, or did something slip through CI? Thanks, Gergo Unexpected failures: /tmp/ghctest-2v5a6979/test spaces/testsuite/tests/driver/j-space/jspace.run jspace [bad exit code (2)] (normal) /tmp/ghctest-2v5a6979/test spaces/testsuite/tests/plugins/plugins-external.run plugins-external [bad exit code (2)] (normal) /tmp/ghctest-2v5a6979/test spaces/testsuite/tests/quasiquotation/qq005/qq005.run qq005 [exit code non-0] (normal) /tmp/ghctest-2v5a6979/test spaces/testsuite/tests/quasiquotation/qq006/qq006.run qq006 [stderr mismatch] (normal) /tmp/ghctest-2v5a6979/test spaces/libraries/unix/tests/T3816.run T3816 [bad exit code (1)] (normal) /tmp/ghctest-2v5a6979/test spaces/testsuite/tests/plugins/test-echo-in-line.run test-echo-in-line [bad stdout] (normal) /tmp/ghctest-2v5a6979/test spaces/testsuite/tests/plugins/test-echo-in-line-many-args.run test-echo-in-line-many-args [bad stdout] (normal) /tmp/ghctest-2v5a6979/test spaces/testsuite/tests/plugins/test-echo-in-turn.run test-echo-in-turn [bad stdout] (normal) /tmp/ghctest-2v5a6979/test spaces/testsuite/tests/plugins/test-echo-in-turn-many-args.run test-echo-in-turn-many-args [bad stdout] (normal) /tmp/ghctest-2v5a6979/test spaces/libraries/unix/tests/user001.run user001 [bad stdout] (normal) 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

9.4.2 release plans
by Matthew Pickering 15 Aug '22

15 Aug '22

Hi all, Within the next week we are planning to release 9.4.2 to fix the critical issues found in 9.4.1. The 9.4.2 milestone is up-to-date with all the issues which are planned to be fixed in this release. https://gitlab.haskell.org/ghc/ghc/-/milestones/378#tab-issues Amongst these are: * -x option broken on command line (#22044 ) * Darwin bindists can't be installed (#21974) * Bytecode interpreter incorrect result when inlining constructor wrappers (#22042) * Some fixes to the make build system (#22047) Cheers, Matt

1 0

Re: Partial type synonyms -- first-class!
by Erdi, Gergo 12 Aug '22

12 Aug '22

PUBLIC For anyone interested, I've added MRs https://gitlab.haskell.org/ghc/ghc/-/merge_requests/8818 and https://gitlab.haskell.org/ghc/ghc/-/merge_requests/8819 to track my progress. From: ghc-devs <ghc-devs-bounces(a)haskell.org> On Behalf Of Edward Kmett Sent: Friday, August 12, 2022 10:43 AM To: ÉRDI Gergő <gergo(a)erdi.hu> Cc: GHC Devs <ghc-devs(a)haskell.org> Subject: [External] Re: Partial type synonyms -- first-class! FWIW, Gergo, I've been following what you've been doing pretty closely, so there's at least two of us tracking it in the background. =) I might have some clever(?) (ab)uses for it in the future in my linear haskell code. -Edward 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