October 2024 - Haskell-Cafe

Best way to implement "scoped exceptions"?
by Tom Ellis 28 May '25

28 May '25

I'd like to implement "scoped exceptions"[1], that is, a combinator of type withScopedException :: ((forall a. e -> IO a) -> IO r) -> IO (Either e r) so I can use exceptions in a convenient way, without having to set up fragile "action at a distance" based on the type of the thing I'm throwing and catching, for example: scopedExceptionExample :: IO (Either String (Either Int Void)) scopedExceptionExample = do withScopedException $ \throw1 -> withScopedException $ \throw2 -> if (1 :: Int) < 0 then throw1 "Hello" else throw2 1234 -- ghci> scopedExceptionExample -- Right (Left 1234) Here's a hacky way of doing it, based on tagging each exception with a unique value, and then filtering when handling: data MyException where MyException :: e -> Data.Unique.Unique -> MyException instance Show MyException where show _ = "<MyException>" instance Exception MyException withScopedException :: ((forall a. e -> IO a) -> IO r) -> IO (Either e r) wiathScopedException f = do fresh <- Data.Unique.newUnique flip tryJust (f (\e -> throwIO (MyException e fresh))) $ \case MyException e tag -> -- unsafeCoerce is very unpleasant if tag == fresh then Just (unsafeCoerce e) else Nothing This is the approach taken by the effectful library[2]. But is there a better way? Can I persuade GHC's RTS to work like this directly? Tom [1] I don't know if these already have a name [2] https://hackage.haskell.org/package/effectful-core-2.2.1.0/docs/src/Effectf…

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[ANN] Darcs 2.18.4 release
by Ben Franksen 09 Jan '25

09 Jan '25

Hello Everyone On behalf of the Darcs [1] team, I would like to announce the release of Darcs 2.18.4 [2]. See [3] for the full release notes. The main reason for this release was to make it build with stack against stackage lts-22.34 on Linux. This in turn should hopefully make it re-appear in the Debian and Ubuntu GNU/Linux distros. Cheers Ben [1] https://darcs.net [2] https://hackage.haskell.org/package/darcs-2.18.4 [3] https://hackage.haskell.org/package/darcs-2.18.4/changelog

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Re: [Haskell-cafe] Configuring cabal to tell HPC to ignore certain functions
by Ryan Scott 30 Dec '24

30 Dec '24

Hi Ivan, Short answer: I'm not aware of a way to automate the process of excluding certain definitions from cabal-generated HPC coverage reports. Long answer: there is a way to manually modify HPC coverage reports using the hpc overlay command. Sadly, most the existing documentation on this command [1] is pretty sparse, so the only way I was able to discover this was by reading the original HPC paper [2] (in that paper, `hpc overlay` is called `hpc-makemtix`) and by finding an example of how to use `hpc overlay` in GHC's test suite [3]. To translate that example into prose: suppose you have the following program (hpc001.hs): main = print (const "Hello" "World") Here, the expression "World" is never evaluated, so by default HPC will claim that there is no coverage for it. This is pretty silly, however, so we'd like to instruct HPC not to warn about "World". To do so, first compile the example above with coverage enabled and run it: $ ghc -fhpc hpc001.hs $ ./hpc001 This should generate an hpc001.tix file. If you run `hpc markup hpc001.tix` and look at the resulting HTML report, you'll see that it warns about "World". Let's fix that. To do so, we have to define an HPC overlay file. The example given in GHC's test suite (sample_overlay.ovr) is: module "Main" { inside "main" { tick "\"World\"" on line 1; } } While I can't find a reference for the syntax that HPC overlay files use, this particular example is pretty self-explanatory: don't report about the expression "World" on line 1, which is found inside the `main` function in the `Main` module. In order to make use of this overlay file, we first have to convert it to a .tix file: $ hpc overlay sample_overlay.ovr > sample_overlay1.tix Then we have to combine it with our original .tix file: $ hpc combine hpc_sample.tix sample_overlay1.tix > total1.tix If we run `hpc markup total1.txt`, we now see that HPC no longer warns about "World", just as we'd hoped for. That is where my understanding of this feature ends, however. In particular, I'm not aware of a better way to generate overlay files (other than to write them by hand, which still feels somewhat laborious), and I'm not aware of any deeper integration with cabal so that these overlay files would be picked up and automatically applied (e.g., for reporting coverage on Hackage). Given that the HPC documentation for `hpc overlay` [1] labels this as an experimental feature, I'd be surprised if such tooling or integration existed. I agree that it would be cool if did, however. Best, Ryan ----- [1] https://hpc-bin.readthedocs.io/en/latest/hpc.html#hpc-overlay-and-hpc-draft [2] https://dl.acm.org/doi/abs/10.1145/1291201.1291203 [3] https://gitlab.haskell.org/ghc/ghc/-/blob/e39c8c993c1da534c5893ca418d1fa4cb…

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Re: [Haskell-cafe] attpoarsec and recursive parsing
by Olaf Klinke 24 Nov '24

24 Nov '24

Hi Frederic, Essentially, the '|' character acts like a binary operator in parsing arithmetical expression trees. To that end, the most ergonomic module I know of is provided by parsec [1], which has a port [2] to attoparsec. You have to decide whether the '|' operator should be left- or right- associative. That question vanishes if you decide to re-design your data type as data MaskLocation = MaskLocation Text | MaskLocation'Tmpl Text | MaskLocation'Or (NonEmpty MaskLocation) and use the sepBy1 combinator. Also you should think about whether and how to escape the separator Char inside ordinary MaskLocation strings. Olaf [1] https://hackage.haskell.org/package/parsec-3.1.17.0/docs/Text-ParserCombina… [2] https://hackage.haskell.org/package/attoparsec-expr

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[ANNOUNCE] GHC 9.12.1-alpha1 is now available
by Zubin Duggal 05 Nov '24

05 Nov '24

The GHC developers are very pleased to announce the availability of the first alpha release of GHC 9.12.1. Binary distributions, source distributions, and documentation are available at [downloads.haskell.org][]. We hope to have this release available via ghcup shortly. GHC 9.12 will bring a number of new features and improvements, including: * The new language extension [OrPatterns] allowing you to combine multiple pattern clauses into one. * The [MultilineStrings] language extension to allow you to more easily write strings spanning multiple lines in your source code. * Improvements to the OverloadedRecordDot extension, allowing the built-in `HasField` class to be used for records with fields of non lifted representations. * The [NamedDefaults] language extension has been introduced allowing you to define defaults for typeclasses other than `Num`. * More deterministic object code output, controlled by the `-fobject-determinism` flag, which improves determinism of builds a lot (though does not fully do so) at the cost of some compiler performance (1-2%). See #12935 for the details * GHC now accepts type syntax in expressions as part of [GHC Proposal #281]. * ... and many more A full accounting of changes can be found in the [release notes][]. As always, GHC's release status, including planned future releases, can be found on the GHC Wiki [status][]. We would like to thank GitHub, IOG, the Zw3rk stake pool, Well-Typed, Tweag I/O, Serokell, Equinix, SimSpace, the 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. [release notes]: https://downloads.haskell.org/ghc/9.12.1-alpha1/docs/users_guide/9.12.1-not… [status]: https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-status [downloads.haskell.org] https://downloads.haskell.org/ghc/9.12.1-alpha1 [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new [OrPatterns]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0522-o… [MultilineStrings]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0569-m… [GHC Proposal #281]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0281-v… [NamedDefaults]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0409-e… Cheers, Zubin

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Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 04 Nov '24

04 Nov '24

Dear all, If you or one of your students recently completed a PhD (or Habilitation) in the area of functional programming, please submit the dissertation abstract for publication in JFP: simple process, no refereeing, open access, 200+ published to date, deadline 31st May 2024. Please share! Best wishes, Graham Hutton ============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 31st May 2024 http://tinyurl.com/jfp-phd-abstracts ============================================================ PREAMBLE: Many students complete PhDs in functional programming each year. As a service to the community, twice per year the Journal of Functional Programming publishes the abstracts from PhD dissertations completed during the previous year. The abstracts are made freely available on the JFP website, i.e. not behind any paywall. They do not require any transfer of copyright, merely a license from the author. A dissertation is eligible for inclusion if parts of it have or could have appeared in JFP, that is, if it is in the general area of functional programming. The abstracts are not reviewed. Please submit dissertation abstracts according to the instructions below. We welcome submissions from both the student and the advisor/supervisor although we encourage them to coordinate. Habilitation dissertations are also eligible for inclusion. ============================================================ SUBMISSION: Please submit the following information to Graham Hutton <graham.hutton(a)nottingham.ac.uk> by 31st May 2024. o Dissertation title: (including any subtitle) o Student: (full name) o Awarding institution: (full name and country) o Date of award: (month and year; depending on the institution, this may be the date of the viva, corrections being approved, graduation ceremony, or otherwise) o Advisor/supervisor: (full names) o Dissertation URL: (please provide a permanently accessible link to the dissertation if you have one, such as to an institutional repository or other public archive; links to personal web pages should be considered a last resort) o Dissertation abstract: (plain text, maximum 350 words; you may use \emph{...} for emphasis, but we prefer no other markup or formatting; if your original abstract exceeds the word limit, please submit an abridged version within the limit) Please do not submit a copy of the dissertation itself, as this is not required. JFP reserves the right to decline to publish abstracts that are not deemed appropriate. ============================================================ PHD ABSTRACT EDITOR: Graham Hutton School of Computer Science University of Nottingham Nottingham NG8 1BB United Kingdom ============================================================ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

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Polymorphic equality in LiquidHaskell
by Henning Thielemann 24 Oct '24

24 Oct '24

I want to assert statically assert equality of polymorphic parameters using LiquidHaskell. See for instance the following simplified Array type containing an array shape and for demonstration purposes only a single element: data Array sh a = Array {shape :: sh, element :: a} {-@ lift2 :: (Eq sh) => (a -> b -> c) -> arrA : Array sh a -> {arrB : Array sh b | shape arrA == shape arrB} -> Array sh c @-} lift2 :: (Eq sh) => (a -> b -> c) -> Array sh a -> Array sh b -> Array sh c lift2 f (Array sha a) (Array shb b) = if sha == shb then Array sha $ f a b else error "shapes mismatch" Running liquidhaskell yields Illegal type specification ... Sort Error in Refinement: {arrB : (Example.Equality.Array sh##a31s b##a31u) | Example.Equality.shape arrA == Example.Equality.shape arrB} Unbound symbol Example.Equality.shape I think the Haskell function (==) is not automatically lifted to LiquidHaskell's logic language. Is there another way to assert a kind of equality in LiquidHaskell?

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Configuring cabal to tell HPC to ignore certain functions
by Ivan Perez 23 Oct '24

23 Oct '24

Hi Café I want to configure a cabal package (specifically, copilot-core) so that, when HPC is executed during testing with coverage enabled, it ignores certain definitions, for example: - Anything that is auto-generated (e.g., Show). - Data constructors and field accessors, for which GHC also defines functions automatically, are all assumed to work. - Proxy doesn't need to be WHNFed. I think there's a way to extract the tix file and modify it, but I'm trying to make this automated, and configure the cabal package so that the right options are passed by cabal to HPC. That way, even hackage will pick it up when it reports the coverage of our library. Any pointers? Thanks! Ivan

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[ANNOUNCE] GHC 9.8.3 is now available
by Ben Gamari 21 Oct '24

21 Oct '24

The GHC developers are happy to announce the availability of GHC 9.8.3. Binary distributions, source distributions, and documentation are available on the [release] page. This release is primarily a bugfix release the 9.8 series. These include: * Significantly improve performance of code loading via dynamic linking (#23415) * Fix a variety of miscompilations involving sub-word-size FFI arguments (#25018, #24314) * Fix a rare miscompilation by the x86 native code generator (#24507) * Improve runtime performance of some applications of `runRW#` (#25055) * Reduce fragmentation when using the non-moving garbage collector (#23340) * Fix source links in Haddock's hyperlinked sources output (#24086) A full accounting of changes can be found in the [release notes]. As some of the fixed issues do affect correctness users are encouraged to upgrade promptly. 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 compiling! - Ben [release]: https://www.haskell.org/ghc/download_ghc_9_8_3.html [ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new [release notes]: https://downloads.haskell.org/~ghc/9.8.3/docs/users_guide/9.8.3-notes.html

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attpoarsec and recursive parsing
by PICCA Frederic-Emmanuel 18 Oct '24

18 Oct '24

Hello I have this type data MaskLocation = MaskLocation Text | MaskLocation'Tmpl Text | MaskLocation'Or MaskLocation MaskLocation deriving (Eq, Generic, Show) deriving anyclass (FromJSON, ToJSON) instance FieldEmitter MaskLocation where fieldEmitter (MaskLocation t) = t fieldEmitter (MaskLocation'Tmpl t) = t fieldEmitter (MaskLocation'Or l r) = fieldEmitter l <> " | " <> fieldEmitter r I just added the MaskLocation'Or constructor in order to allow writing "mymask.npy | mymask2.npy | ..." So my question how should I write the new parser here the parser for the MaskLocation without the MaskLocation'Or instance FieldParsable MaskLocation where fieldParser = do t ← takeText pure $ if "{scannumber:" `Data.Text.isInfixOf` t then MaskLocation'Tmpl t else MaskLocation t It seems to me that I need to consume all text until I find a '|', call the parser on the collected text and start again the process. But I do not have a clear idea of how to collect all the values once the process is over. If someone could help me design this parser, It would be great. thanks a lot Frederic

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