November 2017 - Haskell-Cafe

Fwd: Safe TCP accept loop
by Sumit Raja 30 May '18

30 May '18

Hi, I sent this to beginners but got no responses so I'm hoping someone here can help. I’ve looked at https://wiki.haskell.org/Concurrency_demos/Graceful_exit and used(2) Using throwTo without the use of block and unblock. It runs on 8.2.1 with my limited testing. I can’t find out much about what the > 7.x GHC replacement for block/unblock is other than mask. What does the unblock do in acceptConnections'? Does this method of handling an accept loop still need masking of async exceptions ? If so where does this need to be done? I've got a version with async running for my specific application (https://bitbucket.org/sumitraja/hvrr/src/6927216597a35a9a0d7f2e55cca83fa5b7…) but I don't know if I need to unmask at any point so was hoping for some guidance. Thanks Sumit

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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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ghc-8.2.2 downloads: missing checksums
by Jeremy Henty 02 Dec '17

02 Dec '17

The SHA1SUMS and SHA256SUMS files in https://downloads.haskell.org/~ghc/8.2.2/ do not have checksums for ghc-8.2.2-x86_64-unknown-linux.tar.xz . Regards, Jeremy Henty

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out-commented code vs. case with redundant pattern matches
by Henning Thielemann 30 Nov '17

30 Nov '17

Occasionally I have multiple implementations of the same task and want to choose one quickly but statically. I do not want to out-comment unused branches because they shall still be type checked. So far I used this scheme: case 0::Int of 0 -> putStrLn "A" 1 -> putStrLn "B" _ -> putStrLn "C" With ghc-8.0.2 and ghc-8.2.2 I get these warnings: RedundantCase.hs:4:7: warning: [-Woverlapping-patterns] Pattern match is redundant In a case alternative: 0 -> ... RedundantCase.hs:5:7: warning: [-Woverlapping-patterns] Pattern match is redundant In a case alternative: 1 -> ... I thought that "redundant" means that the first two cases overlap with '_'. But if I replace '_' by '2' I get not only the non-exhaustive patterns warning but an additional redundancy warning on pattern '2'. Is there a nice way to tell GHC that the unused branches are intended, without generally disabling overlapping patterns warning? I mean, this one does not provoke any warnings: if True then putStrLn "X" else putStrLn "Y" but is limited to two branches.

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Re: [Haskell-cafe] out-commented code vs. case with redundant pattern matches
by Olaf Klinke 29 Nov '17

29 Nov '17

Henning, how about putting the branches into their own variables. Admittedly this pollutes the namespace but is much more readable IMHO. workflow = new_workflow where new_workflow = putStrLn "A" old_workflow = putStrLn "B" I suppose GHC complains neither about unused where clauses nor about unused top-level bindings. At least with GHCi 7.8.3 the above code produces no warnings. Olaf

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Re: [Haskell-cafe] out-commented code vs. case with redundant pattern matches
by Doug McIlroy 29 Nov '17

29 Nov '17

Surely GHC is too clever by half in asserting that just 1 of two impossible branches is redundant in the code > case 0::Int of > 0 -> putStrLn "A" > 1 -> putStrLn "B" > _ -> putStrLn "C" The assertion suggests that GHC code optimization would leave some useless vestige of the third case, The long-range solution to Thielmann's quandry is a bug report. Doug

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Looking for opportunity to present Clash (Haskell-to-Hardware compiler) in San Francisco area
by Christiaan Baaij 28 Nov '17

28 Nov '17

Hello Cafe!, I'm on a business trip in San Francisco from 2nd until 8th of December, but only have meetings on 4th and 6th. I was wondering if there are any people out in the SF area interested in a presentation about Clash, a Haskell-to-Hardware compiler ( http://clash-lang.org) and could host me in some way (i.e. provide a room to present); or just talk Haskell. It could be on the 3rd, 5th, or 7th. Please contact me if you're interested, or know of a place/meetup I could contact. Cheers! Christiaan

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New Haskell Meetup in San Diego, California
by erik 27 Nov '17

27 Nov '17

Hello Haskell Cafe, We have decided to start a new Haskell meetup in San Diego, California: https://www.meetup.com/Haskell-Hangout/ Our initial goals are to cover both theoretical and practical aspects of the language. More broadly, we'd like to provide a space for people to discuss, learn, and further their Haskell understanding and to foster awareness of and interest in the language in the community of professional developers here in San Diego. For our first meeting on January 10th, we're going to discuss Philip Wadler's 1992 paper "Monads for Functional Programming". If you're interested and nearby, feel free to join us. All are welcome to attend, no Haskell experience required. Best, -- Erik Aker

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