- Haskell-Cafe - Haskell.org

Best way to implement "scoped exceptions"?
by Tom Ellis 16 Jun '23

16 Jun '23

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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Multicolumn aggregate in Beam
by Daneel Yaitskov 06 Jun '23

06 Jun '23

Hi, I am using beam library in my project and trying to express a query with a several aggregating subqueries: select (select * from t order by ts desc limit 1), (select * from t order by ts asc limit 1), (select count(*) from t) as nnn I figured out how to do multiple columns: userDataTableDigestQ :: UserTable -> SqlSelect Postgres (UserDataTable, UserDataTable) userDataTableDigestQ ut = select ag where al = all_ (userDataTableRef $ usertableName ut) ag = subselect_ $ aggregate_ (BF.bimap group_ group_) ((,) <$> limit_ 1 (orderBy_ (asc_ . userdatatableTs) al) <*> limit_ 1 (orderBy_ (desc_ . userdatatableTs) al)) but mixing group_ and count_ produces a very long type error Is there other sql-backend agnostic Haskell library which could express the query above? -- Best regards, Daniil Iaitskov

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Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 05 Jun '23

05 Jun '23

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 30th June 2023. Please share! Best wishes, Graham Hutton ============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 30th June 2023 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 30th June 2023. 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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Course materials to help me teach Software Engineering with Haskell?
by Ignat Insarov 04 Jun '23

04 Jun '23

Hello café! I am looking for some kind of lesson plans, lecture notes or other inspiring materials that would help me construct an advanced course in Software Engineering with Haskell — or other pure functional technologies — with the aim to teach this art to myself and others to the highest level of excellence. I know there is a choice of published books with «Haskell» in the title, and I shall draw from them. However, I am not super happy about any of the books I looked into so far, because they are so concrete. They would pick a bunch of best practices and some well known libraries and write about that in excruciating detail. It seems a published book cannot afford to say anything abstract or outlandish. Assorti lecture notes I could find on the Internet are more diverse and fresh, but I have not found many and whichever I found are all somewhat beginner level, they talk about Haskell's syntax and evaluation more than about Software Engineering overall. The stuff I wish to see would be like: * Continuation passing style. * Recursion schemes. * Polynomial functors. * The nature of `IO`. * Evaluation of Haskell on a graph machine. * The expression problem. In short, stuff that transcends specific libraries and concrete code. In the ideal, I wish there was a literature review that refers to foundational research. I do not expect ready-made perfection; please send me whatever you think would help, even your brief thoughts and wishes.

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Re: [Haskell-cafe] [hledger] ANN: hledger 1.30
by Simon Michael 02 Jun '23

02 Jun '23

> On Jun 2, 2023, at 00:55, Henning Thielemann <googlegroups(a)henning-thielemann.de> wrote: > > I can build with GHC-8.8.4, but with GHC-8.10 and later I get: > > > Hledger/Cli/Commands/Demo.hs:63:3: error: > • Exception when trying to run compile-time code: > /ram/cabal/tmp/src-191030/hledger-1.30/embeddedfiles/add.cast: openFile: does not exist (No such file or directory) > Code: embedFileRelative "embeddedfiles/add.cast" > • In the untyped splice: > $(embedFileRelative "embeddedfiles/add.cast") > | > 63 | $(embedFileRelative "embeddedfiles/add.cast"), -- https://asciinema.org/a/567935 The easiest way to start a journal (add) > | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Thanks for the report; I have released hledger-1.30.1 on Hackage to fix this.

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could not deduce Show compile error
by Zoran Bošnjak 02 Jun '23

02 Jun '23

Dear haskell cafe members, I would appreciate a suggestion how to fix compile error on this simple test program (I am using ghc 9.0.2). The idea is to have 'data Flow a b' unrestricted and create necessary constraints only when running/interpreting the flow. The problem is obviously an intermediate type 'b' in 'Compose', where the 'Show' instance is not deduced. I have a vague clue that some type families might be necessary to propagate Show constraint, or a type class with associated type family, but I don't know exactly how. --- import Prelude hiding ((.), id) import Control.Category data Flow a b where Id :: Flow a a Compose :: Flow a b -> Flow b c -> Flow a c instance Category Flow where id = Id (.) = flip Compose runFlow :: Show a => Flow a b -> a -> IO b runFlow f x = case f of Id -> print x >> pure x Compose f1 f2 -> runFlow f1 x >>= runFlow f2 main :: IO () main = runFlow (Id >>> Id) () --- The error is: • Could not deduce (Show b1) arising from a use of ‘runFlow’ from the context: Show a bound by the type signature for: runFlow :: forall a b. Show a => Flow a b -> a -> IO b at a02.hs:13:1-42 Possible fix: add (Show b1) to the context of the data constructor ‘Compose’ • In the second argument of ‘(>>=)’, namely ‘runFlow f2’ In the expression: runFlow f1 x >>= runFlow f2 In a case alternative: Compose f1 f2 -> runFlow f1 x >>= runFlow f2 | 16 | Compose f1 f2 -> runFlow f1 x >>= runFlow f2 | ^^^^^^^^^^ kind regards, Zoran

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type error when specializing lens zooms in ghc >= 9.0
by Olaf Klinke 02 Jun '23

02 Jun '23

Dear Cafe, I'm an infrequent lens user so please forgive me if the below problem has a trivial solution. For a long time the documentation of Control.Lens.Zoom has been claiming that 'zoom' and 'magnify' can be specialized to the following types. {-# LANGUAGE RankNTypes #-} -- Type error with GHC 9.0.2 or 9.2.7, okay with GHC 8.8.4 import Control.Lens.Zoom (zoom,magnify) import Control.Lens.Type (Lens') import Control.Monad.RWS (RWS) zoomRWS :: Monoid w => Lens' s s' -> RWS r w s' a -> RWS r w s a zoomRWS = zoom magnifyRWS :: Monoid w => Lens' r r' -> RWS r' w s a -> RWS r w s a magnifyRWS = magnify Indeed GHC 8.8.4 compiles this happily (on lens-4.18.1), but 9.0.2 (on lens-5.0.1) throws nearly undecipherable errors, claiming that Lens' can not be matched with some more specialized type involving Magnified and LensLike. The source of Control.Lens.Zoom does not differ substantially between 4.18.1 and 5.0.1 so my guess is it is a type checker issue. Can anyone explain? On which bug tracker should I raise this issue, if it is indeed a valid one? (I pulled the above versions from stackage lts-16.31, -19.5 and -20.20, respectively and compiled with stack.) Thanks Olaf

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[CfP][CONFLANG23] 2nd Workshop on Configuration Languages
by Yann Hamdaoui 01 Jun '23

01 Jun '23

Dear Haskellers, and maybe more generally programming language enthusiasts, I am happy to announce the 2nd Workshop on Configuration Languages, co-located at SPLASH 23, and focused on the quite peculiar (and thus, exciting!) space of designing configuration languages, and more general configuration and infrastructure management. Cheers, Yann CALL FOR PRESENTATIONS - 2nd Workshop on Configuration Languages 2023 (CONFLANG23) EVENT DETAILS Event: CONFLANG23, colocated at SPLASH 2023 Date: Tuesday 24 October 2023 Submission deadline: Friday 12 July 2023 Location: Cascais, Portugal Website: https://2023.splashcon.org/home/conflang-2023 OVERVIEW CONFLANG is a new workshop on the design, the usage and the tooling of configuration programming languages. CONFLANG aims at uniting language designers, industry practitioners and passionate hobbyists to share knowledge in any form. Topics of interest include, but are not limited to: - Infrastructure and configuration code maintenance and evolution - Specification learning and mining for configurations - Infrastructure and Configuration testing and verification - Infrastructure as Code and configuration repair - New languages for configuration - The application of language security and type theory to program configuration CALL FOR PRESENTATIONS The committee welcomes proposals for presentations: - Traditional talks on any theoretical or practical aspect of the usage, the tooling and the design of configuration languages - Experience and case study talks on the real world usage and deployment of configuration languages - Explorative talks and/or demos on experimenting with configuration languages and related tools SUBMISSION GUIDELINES Please submit an abstract (up to 600 words, excluding title, author names, and bibliography) of your proposed talk using the submission link provided below. - **Format**: 600 words maximum abstract (estimated between 1 and 1,5 pages) as a PDF, excluding title, author names, and bibliography. Any additional material will be considered at the discretion of the PC. - **URL** : https://conflang23.hotcrp.com/ PROGRAM CHAIRS - Yann Hamdaoui (Tweag) - Jürgen Cito (TU Wien and Facebook) - Mark Santolucito (Columbia University) - Marcel van Lohuizen (CUE Lang)

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ANN: hledger 1.30
by Simon Michael 01 Jun '23

01 Jun '23

I'm pleased to announce hledger 1.30 ! Highlights: - Boolean queries - with the new `expr:` query type you can freely combined other queries with AND, OR, NOT and parentheses. - Easier CSV file management - you can now read a CSV .rules file, and let it fetch the data file from a downloads directory, simplifying CSV file management. - Built-in demos - a new `demo` command plays several asciinema casts to demonstrate features. We'll add more of these in future. - hledger-ui has a new Cash accounts screen, and now starts on the Menu screen. - The usual miscellaneous fixes. Thank you release contributors Chris Lemaire and Yehoshua Pesach Wallach. - https://github.com/simonmichael/hledger/releases/1.30 - https://hledger.org/release-notes.html#hledger-1-30 - https://hledger.org/install What is hledger ? - Fast, reliable, free, multicurrency, double-entry, plain text accounting software for unix, mac, windows, and the web - Built around human-readable, version-controllable plain text files - Inspired by and largely compatible with Ledger CLI, convertible to and from Beancount - Written in Haskell for reliability and longevity. For help getting started or more info, see https://hledger.org and join our Matrix/IRC chat or mail list: https://hledger.org/support . Newcomers, experts, contributors, sponsors, feedback are welcome! For more about plain text accounting, see https://plaintextaccounting.org . Live well and prosper, -Simon

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property testing, point-free style
by Olaf Klinke 31 May '23

31 May '23

Dear Cafe, The expression \x -> f x == g x is a testable property, as long as values for x can be randomly generated. For clarity I'd prefer a point-free style, e.g. f ≡ g Are there extensions to QuickCheck that let me write this? The QuickCheck package itself does not seem to contain such an operator. My current work-around is a newtype ExtensionalEquality a b that holds two functions of type (a -> b) and a Testable instance for it. But I've got a hunch that I re-invented some wheel here. (My ExtensionalEquality is isomorphic to Refl (a -> b) (a -> b) but Refl ist conceptually about type equality, not term equality.) Thanks Olaf

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