August 2017 - Haskell-Cafe

Archival of community.haskell.org
by Mario Rogic 19 May '18

19 May '18

Hi all As announced back in 2015 <http://blog.haskell.org/post/7/the_future_of_community.haskell.org/> we have started archiving community.haskell.org services. We've created https://archives.haskell.org/, and the old projects mailing lists have been the first to be archived. Next on the list is the remainder of projects.haskell.org, and the code.haskell.org site. As always, you can email admin(a)h.o or hop on the #haskell-infrastructure freenode channel to provide feedback / concerns. Thanks - Mario

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Join a transformer
by Michael Snoyman 11 Apr '18

11 Apr '18

I'm wondering if this pattern exists and has a name. We have the concept of joining a Monad: join :: Monad m => m (m a) -> ma How about joining a monad transformer? joinT :: (Monad m, MonadTrans t) => t (t m) a -> t m a I believe implementing this in terms of MonadTransControl[1] might be possible, but I was wondering if there's an already existing idiom for this. Michael [1] http://haddocks.fpcomplete.com/fp/7.4.2/20130301-40/monad-control/Control-M…

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PatternSignatures (rant)
by Johannes Waldmann 06 Apr '18

06 Apr '18

Dear all, (this is a rant - please ignore) why has "PatternSignatures" been renamed to "ScopedTypeVariables" (in ghc-6.10)? show me the alleged "type variable" in this code: do x :: Int <- [ 1 .. 10 ] ; return $ x^2 and why on earth do I need to use a language extension at all to get to the most basic thing of declarative programming: to declare the type of an identifier? This seriously hurts (me, at least) when teaching Haskell. Thanks - J.W.

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[hs-bibutils] intention to take over package
by Václav Haisman 28 Sep '17

28 Sep '17

Hi. I would like to take over the hs-bibutils Hackage package. It has not been updated since 2014, the listed maintainer is not responsive to emails, and listed home page is 404 page. -- VH

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How to import hidden modules
by Ricardo Peña 02 Sep '17

02 Sep '17

Hi, I need to define a class instance for the algebraic datatype Data.Map.Map in the Hierarchical GCH libraries, but I do not find the way to import Data.Map.Internal which it is assumed to be hidden. Is there any way to access to this definition? Thanks -Ricardo

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To merge package documentation with the platform doc tree
by John Beattie 01 Sep '17

01 Sep '17

Hi, Suppose I download a package and install it. I would like to be able to have the html documentation for that package and I would like it to be integrated with the local doc tree in the haskell platform. Is this a recognised usecase and is there a recommended way to do that? My first thought was to use haddock but I haven't managed to work out if there is a combination of haddock options which would get me there. As an alternative, I was wondering about cloning the GHC platform source and pasting the new library into the source tree alongside the other libraries and building the documentation. I might have to tweak a makefile or somesuch but would that work? Thanks, John Beattie

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megaparsec 6 migration
by ☂Josh Chia (謝任中) 01 Sep '17

01 Sep '17

I'm trying to migrate my codebase from megaparsec 5 to megaparsec 6. There appears to be major changes. For example, Text.Megaparsec.String.Parser & Text.Megaparsec.Error.Dec have both disappeared. I'm sure I could slowly plod through the new documentation, figure out the types and figure out how to migrate the code on my own, but examples & tutorials would be more time-efficient. Are there examples or tutorials compatible with megaparsec 6? For a start, what should I do with the above two things that have 'disappeared'? What do I replace them with? Josh

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Crossreferenced codebase for Stackage LTS 9.2 available
by Robin Palotai 01 Sep '17

01 Sep '17

To find out who else uses your favorite functions and how: http://stuff.codereview.me/lts/9.2 . For example, click 'async' at http://stuff.codereview.me/lts/9.2/#async-2.1.1.1/Control/Concurrent/Async.… to get all the callsites of that method. There are additional files available at http://stuff.codereview.me/ if you want to serve the xrefs locally, or merge your own packages into the index (assuming you use the same stackage lts version). A request: the crossreferencing is pretty usable, but far from complete. If you find any issues (missing crossreferences or bad ones), please have a look at the buglist at https://github.com/google/haskell-indexer/issues, and thumbs-up any that you find hurting, or open new ones! Thank you & happy code browsing, Robin

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ThEdu'17 Post-Proceedings: Call for Papers
by Achim D. Brucker 31 Aug '17

31 Aug '17

(Apologies for duplicates) Call for Papers - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Post-Proceedings --- ThEdu'17 - Electronic Proceedings in Theoretical Computer Science ThEdu'17 --- Theorem proving components for Educational software 6 August 2017, Gothenburg, Sweden, at CADE26 http://www.uc.pt/en/congressos/thedu/thedu17 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Post-Proceedings THedu'17 Scope: Computer Theorem Proving is becoming a paradigm as well as a technological base for a new generation of educational software in science, technology, engineering and mathematics. The workshop brought together experts in automated deduction with experts in education in order to further clarify the shape of the new software generation and to discuss existing systems. This call is open for everyone, also those who did not participate in the workshop. Important Dates (EPTCS Post-Proceedings) * Call for papers: 08 Sep 2017 * Submission (full papers): 17 Nov 2017 * Notification of acceptance: 15 Dec 2017 * Revised papers due: 19 Jan 2018 Topics of interest include: - methods of automated deduction applied to checking students' input; - methods of automated deduction applied to prove post-conditions for particular problem solutions; - combinations of deduction and computation enabling systems to propose next steps; - automated provers specific for dynamic geometry systems; - proof and proving in mathematics education. Submission We welcome submission of papers presenting original unpublished work which is not been submitted for publication elsewhere. The authors should comply with the "instructions for authors", LaTeX style files and accept the "Non-exclusive license to distribute" of EPTCS: Instructions for authors (http://info.eptcs.org/) LaTeX style file and formatting instructions (http://style.eptcs.org/) Copyright (http://copyright.eptcs.org/) Papers should be submitted via easychair, https://easychair.org/conferences/?conf=thedu17 . Program Committee Francisco Botana, University of Vigo at Pontevedra, Spain Achim Brucker, University of Sheffield, United Kingdom Roman Hašek, University of South Bohemia, Czech Republic Filip Maric, University of Belgrade, Serbia Walther Neuper, Graz University of Technology, Austria (co-chair) Pavel Pech, University of South Bohemia, Czech Republic Pedro Quaresma, University of Coimbra, Portugal (co-chair) Vanda Santos, CISUC, Portugal Wolfgang Schreiner, Johannes Kepler University, Austria Burkhart Wolff, University Paris-Sud, France -- Dr. Achim D. Brucker | Software Assurance & Security | University of Sheffield https://www.brucker.ch | https://logicalhacking.com/blog @adbrucker | @logicalhacking

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cabal file build-depends list sensitive to ordering
by Olaf Klinke 30 Aug '17

30 Aug '17

Dear cafe, I discovered that stack/cabal is sensitive to permutations in the dependencies list. Is that desired behaviour and documented anywhere? #File stack.yaml resolver: lts-9.2 packages: - '.' extra-deps: [iCalendar-0.4.0.3,mime-0.4.0.2,wx-0.92.3.0,wxcore-0.92.3.0,wxc-0.92.3.0,wxdirect-0.92.3.0] flags: {} extra-package-dbs: [] system-ghc: false # In file myproject.cabal build-depends: base >= 4.7 && < 5 , HaXml >= 1.13.3 , iCalendar >= 0.3.7.3 , containers , time >= 1.6.0.1 , text , case-insensitive , xslx , xlsx-tabular , wx "stack build" generates the rather uninformative error message In the dependencies for myproject-0.2.1.0: xslx must match -any, but the stack configuration has no specified version # In file myproject.cabal build-depends: base >= 4.7 && < 5 , xlsx , xlsx-tabular , HaXml >= 1.13.3 , iCalendar >= 0.3.7.3 , containers , time >= 1.6.0.1 , text , case-insensitive , wx This version builds just fine. -- Olaf

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