- Haskell-Cafe - Haskell.org

ghc-8.2.2 downloads: missing checksums
by Jeremy Henty 03 Dec '17

03 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 01 Dec '17

01 Dec '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 30 Nov '17

30 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 29 Nov '17

29 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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emacs problem : haskell mode keeps hanging to report types, renders emacs useless
by Sean Matthews 27 Nov '17

27 Nov '17

I have just reinstalled everything. Newest release of emacs, Haskell-pakage and Haskell-mode and Haskell. I have tried to do this as cleanly as possible (via the customise and package install interfaces). Now when I try to edit haskell files, the system keeps freezing to report types. Nothing that I try (setting configuration variables, etc.) has any effect. I know that there is a problem with parsing the command line with the newest version of GHCi, but I think I can avoid that by using :load instead of C-c C-l. This freezing problem, however, is another level of problem. It renders my emacs basically useless for editing haskell. Anybody recognise the problem, and have a fix? Thanks, Sean Matthews -- Sean Matthews seanmatthews1(a)gmail.com / +49 1515 800 1901

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Munich Haskell Meeting, 2017-11-29 @ 19:30
by Christian Neukirchen 27 Nov '17

27 Nov '17

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Wednesday, November 29 at Cafe Puck at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-nov-2017/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Running an action periodically
by Tristan Seligmann 26 Nov '17

26 Nov '17

I just wrote this helper: > import Control.Concurrent > import Control.Concurrent.STM > import Control.Exception.Safe > > pollT :: Int -> IO a -> IO (STM (Maybe a), Async b) > pollT delay act = do > tv <- atomically (newTVar Nothing) > as <- > async . forever $ do > r <- tryAny act > case r of > Left _ -> pure () > Right r' -> atomically (writeTVar tv (Just r')) > threadDelay delay > pure (readTVar tv, as) I was sort of surprised not to find something like this in an existing library. 1. Did I miss an existing implementation? 2. Any problems with this one? 3. Any suggestions for a better name? 4. Any thoughts on the general idea? That is, "run an action periodically, updating a TVar with the result". There's a couple of obvious variations that can be built on top of this, like retrying if the TVar is Nothing when reading from it, or writing Nothing to the TVar when the action fails rather than keeping the old value. Maybe passing in the old value to the action?

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Can reallyUnsafePtrEquality give false positives?
by Michael Walker 25 Nov '17

25 Nov '17

Hello, Can reallyUnsafePtrEquality give false positives? I can see how it can give false negatives (eg, compiler optimisations increasing or decreasing sharing), but I'm not so sure if it can give false positives. I don't see how in a garbage collected language two live values could compare reference equal. Unless the implementation is something like: reallyUnsafePtrEquality a b = getptr a == getptr b ...as that then means that if the GC moves things after `getptr a` is evaluated but before `getptr b` is, then you could get a false positive. But that doesn't seem like a sensible implementation to me, because then reallyUnsafePtrEquality would surely be totally useless. -- Michael Walker (http://www.barrucadu.co.uk)

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