- Haskell-Cafe - Haskell.org

function composition and Rank2Types
by Olaf Klinke 04 Feb '23

04 Feb '23

Dear Cafe, I used to believe that f . g $ x is equivalent to f $ g x but apparently that is not the case when rank-2 types are involved. Counterexample: {-# LANGUAGE RankNTypes #-} type Cont = forall r. (() -> r) -> r data Foo = Foo Cont mkCont :: () -> Cont mkCont x = \f -> f x foo1 :: () -> Foo foo1 x = Foo $ mkCont x foo2 :: () -> Foo foo2 x = Foo . mkCont $ x The function foo1 type-checks, the function foo2 doesn't. And that is although ghci tells me that mkCont :: () -> Cont Foo :: Cont -> Foo so I should be able to compose? What is going on here? GHC 9.0.2 errors with • Couldn't match type ‘b0’ with ‘Cont’ Expected: b0 -> Foo Actual: Cont -> Foo Cannot instantiate unification variable ‘b0’ with a type involving polytypes: Cont • In the first argument of ‘(.)’, namely ‘Foo’ In the expression: Foo . mkCont and GHC 8.8.4 errors with • Couldn't match type ‘(() -> r0) -> r0’ with ‘Cont’ Expected type: ((() -> r0) -> r0) -> Foo Actual type: Cont -> Foo • In the first argument of ‘(.)’, namely ‘Foo’ In the expression: Foo . mkCont The latter suggests that r is unnecessarily specialized to some specific-yet-unknown r0 which destroys the universal quantification. Olaf

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2nd Call for Participation: BOB 2023 (Berlin, March 17)
by Stefan Wehr 02 Feb '23

02 Feb '23

========================================================================= BOB 2023 Conference “What happens if we simply use what’s best?” March 17, 2023, Berlin https://bobkonf.de/2023/ Program: https://bobkonf.de/2023/program.html Registration: https://bobkonf.de/2023/registration.html ========================================================================= BOB conference is a place for developers, architects, and decision-makers to explore technologies beyond the mainstream in software development and to find the best tools available to software developers today. Our goal is for all participants of BOB to return home with new insights that enable them to improve their own software development experience. The program features 14 talks and 8 tutorials on current topics: https://bobkonf.de/2023/program.html Talk subjects includes functional programming, software architecture, accessibility, digital transformation, version control, formal methods, and devops. BOB will feature tutorials on Elixir, Scheme, Kotlin, Agda, Domain Storytelling, Hexagonal Frontend Architecture, and other topics. Yulia Startsev will give the keynote talk on "Re-thinking Modules for the Web". Registration is open - tickets are all under 200€, and many discount options are available, as are grants for members of groups underrepresented in tech: https://bobkonf.de/2023/registration.html

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Haskell text
by Simon Thompson 02 Feb '23

02 Feb '23

I'm happy to announce that the PDF of Haskell, The Craft of Functional Programming (3rd edition) is available to view and download for free. Later in the year I'll revise it and make it available as an online text. But for now, here it is, uncorrected: https://simonjohnthompson.github.io/craft3e/craft3e.pdf Simon Sent from my iPhone

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how to write a pipes Consumer which return a final state
by PICCA Frederic-Emmanuel 01 Feb '23

01 Feb '23

Hello, I try to create a pipes which return at the end a Cube. the process is quite simple 1) extract data from a file 2) comput a cube of data from these data 3) accumulate these cube during the process 4) when there is no more cube return the final cube I am trying for now to write the part with the 'state' which is hiden in a recursion like this. accumulateP :: (MonadIO m, Shape sh) => Cube sh -> Consumer (DataFrameSpace sh) m (Cube sh) accumulateP cube = do s <- await accumulateP =<< (liftIO $ addSpace s cube) the add Space method return the modified Cube At some point I should see that await can not return a value and I would decide to return the value instead of doing another loop My question is how do I stop this recursion and return the final cube ? thanks for your help.

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multiline strings editor for emacs
by Markus Läll 01 Feb '23

01 Feb '23

Dear Cafe, searched, but this didn't turn up anything -- does there exist a "multiline string editor for emacs" in haskell? I.e something that would pop-out a new buffer to edit the string, and that would be inserted and slash-aligned back into the original file once done? This would be simple enough to implement, perhaps someone has already done it? (And just to be sure: there isn't a built-in multiline quasiquoter in base, right? I know that there are multitudes of packages that implement this, but would just like something that is built in.) Thanks! -- Markus Läll

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Re: [Haskell-cafe] [ANNOUNCE] GHC 9.6.1-alpha2 is now available
by Olaf Klinke 31 Jan '23

31 Jan '23

This may be a naive question and I may be completely misunderstanding what WebAssembly is all about: What can the Haskell developer expect once the WebAssembly and JavaScript backends of GHC have matured? I dream of a commutative square like follows. f x -----> y Haskell source | | |GHC |GHC | | v v b -----> b HTML w GHC would become a functor from (a sub-category of) Hask to a category with one object and a rich hom-set of (serializable) endomorphisms. Cloud Haskell [0] seems to provide a functor of this kind. In detail, for a pure Haskell function between Haskell types x and y, the compiler would be able to map values to a web representation b and compile f to a web representation w :: b -> b. The latter should be embeddable in static HTML [1], so that a client-server architecture is not needed. There are obvious limitations, e.g. when b = JSON and w is a JavaScript function, then x and y may be limited to Generic types. One application of this functor would be a web version of Haskintex [2], where one can ensure that certain elements of a document have a relationship y = f(x) defined by a Haskell function. But unlike Haskintex, the computation would take place not at create-time but at display-time so that a limited amount of interaction is possible. Olaf [0] https://hackage.haskell.org/package/distributed-static [1] According to [3], WebAssembly is currently designed to be fetched, not embedded. This appears to be no fundamental restriction, however, as binary images can also be embedded in static HTML. [2] https://hackage.haskell.org/package/haskintex [3] https://developer.mozilla.org/en-US/docs/WebAssembly/Loading_and_running

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[ANNOUNCE] GHC 9.6.1-alpha2 is now available
by Ben Gamari 31 Jan '23

31 Jan '23

The GHC team is very pleased to announce the availability of GHC 9.6.1-alpha2. As usual, binaries and source distributions are available at downloads.haskell.org: https://downloads.haskell.org/ghc/9.6.1-alpha2/ Beginning with GHC 9.6.1, GHC can be built as a cross-compiler to WebAssembly and JavaScript. This is an important step towards robust support for compiling Haskell to the Web, but there are a few caveats to be aware of in the 9.6 series: - Both the Javascript and WebAssembly backends are still at an early stage of development and are present in this release as a technology preview - Using GHC as a cross-compiler is not as easy as we would like it to be; in particular, there are challenges related to Template Haskell. - GHC is not yet run-time retargetable; a given GHC binary targets exactly one platform, and both WebAssembly and JavaScript are considered platforms for this purpose. Cross-compilers must be built from source by their users We hope to lift all of these limitations in future releases. Additionally, 9.6.1 will include: - Significant latency improvements in the non-moving garbage collector - Efficient runtime support for delimited continuations - Improvements in compiler error messages - Numerous improvements in the compiler's memory usage See the [release notes] for a comprehensive accounting of changes in this release. As always, one can find a [migration guide] to aid in transitioning from older releases on the GHC Wiki. We have also recently started extending our release process to cover a wider set of Linux distributions. In particular, we now offer Rocky 8 and Ubuntu 20.04 binary distributions which cover RedHat-derivative and distributions using older `glibc` releases (namely 2.27), respectively. Please do give this release a try and open a [ticket] if you see anything amiss. Cheers, - Ben [ticket]: https://gitlab.haskell.org/ghc/ghc/issues/ [migration-guide]: https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.6 [release notes]: https://downloads.haskell.org/ghc/9.6.1-alpha2/docs/users_guide/9.6.1-notes…

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Introducing a pure EBML (WebM) parser
by Tristan Cacqueray 29 Jan '23

29 Jan '23

Dear Café, I've created a new package to parse the Extensible Binary Meta Language format using the binary library: https://hackage.haskell.org/package/ebml And I just published a motivating example, an audio broadcasting server for WebSocket clients: https://tristancacqueray.github.io/blog/broadcasting-webm Please let me know if you are interested, contributions and bug reports are welcome! Cheers, -Tristan

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Munich Haskell Meeting, 2023-01-30 @ 19:30
by Leah Neukirchen 27 Jan '23

27 Jan '23

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, January 30 at Augustiner-Gaststätte Rumpler at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this nuudel so we can reserve enough seats! It is OK to add yourself to the nuudel anonymously or pseudonymously. https://nuudel.digitalcourage.de/mjxWpyCCGJY7r6uA Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

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incomplete-uni-patterns
by Kazu Yamamoto 27 Jan '23

27 Jan '23

Hello, The recent GHC added incomplete-uni-patterns to the -Wall option. So, we have a waning with the following code: let [addr,port] = args To avoid this, I changed the code to: let addr = head args port = head $ tail args In my opinion, this seems Lisp rather than Haskell. Also, I need to avoid: let Just val = mval Rahter, I use: let val = fromJust mval This is annoying to me. How do you get along with incomplete-uni-patterns? I would like to know the best current practice. --Kazu

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