- Haskell-Cafe - Haskell.org

Injective type families for GHC
by Jan Stolarek 10 Feb '15

10 Feb '15

Haskellers, I am finishing work on adding injective type families to GHC. I know that in the past many people have asked for this feature. If you have use cases for injective type families I would appreciate if you could show them to me. My implementation has some restrictions and I want to see how severe these restrictions are from a practical point of view. Janek --- Politechnika Łódzka Treść tej wiadomości zawiera informacje przeznaczone tylko dla adresata. Jeżeli nie jesteście Państwo jej adresatem, bądź otrzymaliście ją przez pomyłkę prosimy o powiadomienie o tym nadawcy oraz trwałe jej usunięcie.

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Use of STM in business?
by Tom Ellis 10 Feb '15

10 Feb '15

>From associate of mine: "I am quite curious if anyone actually uses Haskell (esp STM) successfully in business." Of course I can anwser the general question about usage of Haskell in business, but I don't know much about STM specifically. Can anyone provide me with any references to where STM has been used in production in a business setting? Thanks, Tom

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GHC 7.10 Prelude: we need your opinion
by Simon Peyton Jones 10 Feb '15

10 Feb '15

Haskell Friends This email asks for your help in deciding how to proceed with some Prelude changes in GHC 7.10. Please read on, but all the info is also at the survey link, here: http://goo.gl/forms/XP1W2JdfpX. Deadline is 21 Feb. The Core Libraries Committee (CLC) is responsible for developing the core libraries that ship with GHC. This is an important but painstaking task, and we owe the CLC a big vote of thanks for taking it on. For over a year the CLC has been working on integrating the Foldable and Traversable classes (shipped in base in GHC 7.8) into the core libraries, and into the Prelude in particular. Detailed planning for GHC 7.10 started in the autumn of 2014, and the CLC went ahead with this integration. Then we had a failure of communication. As these changes affect the Prelude, which is in scope for all users of Haskell, these changes should be held to a higher bar than the regular libraries@ review process. However, the Foldable/Traversable changes were not particularly well signposted. Many people have only recently woken up to them, and some have objected (both in principle and detail). This is an extremely unfortunate situation. On the one hand we are at RC2 for GHC 7.10, so library authors have invested effort in updating their libraries to the new Prelude. On the other, altering the Prelude is in effect altering the language, something we take pretty seriously. We should have had this debate back in 2014, but here we are, and it is unproductive to argue about whose fault it is. We all share responsibility. We need to decide what to do now. A small group of us met by Skype and we've decided to do this: · Push back GHC 7.10's release by at least a month, to late March. This delay also gives us breathing space to address an unrelated show-stopping bug, Trac #9858. · Invite input from the Haskell community on which of two approaches to adopt (this survey<http://goo.gl/forms/XP1W2JdfpX>). The main questions revolve around impact on the Haskell ecosystem (commercial applications, teaching, libraries, etc etc), so we want to ask your opinion rather than guess it. · Ask Simon Marlow and Simon Peyton Jones to decide which approach to follow for GHC 7.10. Wiki pages have been created summarizing these two primary alternatives, including many more points and counter-points and technical details: · Overall summary: https://ghc.haskell.org/trac/ghc/wiki/Prelude710<https://www.google.com/url?q=https%3A%2F%2Fghc.haskell.org%2Ftrac%2Fghc%2Fw…> · Details of Plan List: https://ghc.haskell.org/trac/ghc/wiki/Prelude710/List<https://www.google.com/url?q=https%3A%2F%2Fghc.haskell.org%2Ftrac%2Fghc%2Fw…> · Details of Plan FTP: https://ghc.haskell.org/trac/ghc/wiki/Prelude710/FTP<https://www.google.com/url?q=https%3A%2F%2Fghc.haskell.org%2Ftrac%2Fghc%2Fw…> This survey invites your input on which plan we should follow. Would you please · Read the details of the alternative plans on the three wiki pages above · Add your response to the survey<http://goo.gl/forms/XP1W2JdfpX> Please do read the background. Well-informed responses will help. Thank you! DEADLINE: 21 February 2015 Simon PJ

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How does RTS judge that a STM transaction is blocked?
by Magicloud Magiclouds 10 Feb '15

10 Feb '15

Hi, I met an issue while I was using xhb. When a certain property of root window being changed, it was supposed to give me an AccessError. But actually I got "thread blocked indefinitely in an STM transaction" when I waitForEvent. AFAIK, in xhb, it forks a thread to read what X server sends. Then parse and put it into a TChan. waitForEvent reads from the TChan. It is reasonable to me that if X server did not send anything, then waitForEvent would be blocked. But apparently there are some other policies in RTS. So what is that? And how should I debug it? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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GHC-7.8.4 for Mac OS X
by Ozgur Akgun 09 Feb '15

09 Feb '15

Hi, Does anyone plan to put a binary release of GHC-7.8.4 for OS X? There isn't one here: https://www.haskell.org/ghc/download_ghc_7_8_4 Cheers! Ozgur

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Installing Package Candidates With Cabal ...
by aditya siram 09 Feb '15

09 Feb '15

Hi all, Is there a way to 'cabal install' a package candidate? The description at https://hackage.haskell.org/upload says it "let others install it before publishing it to the main index" which seems to indicate it can. Am I missing something? Thanks! -deech

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Building with some dependencies pre-installed in a sandbox
by Jakub Arnold 09 Feb '15

09 Feb '15

Hey guys, sorry for re-posting, but I feel that my original question is largely different from this one. I've managed to get my broken dependency to build (the mysql package), but unfortunately I had to manually change its source code to make this happen, (original post https://www.haskell.org/pipermail/haskell-cafe/2015-February/118064.html) but I do have the package installed locally (or in the sandbox of my app). The problem is that when I try to `cabal install` the whole application, it will re-download the `mysql` package from Hackage and try to build it again, instead of using the version I custom built and installed myself. Is it normal that cabal won't re-use the already existing packages, or did I somehow change it so that it won't recognize it? Also, is it possible to explicitly say where should cabal resolve some of its dependencies? Such as this case when one package is in a local path, and other should be built from hackage. I'm not really sure what the best approach is here. again sorry for posting twice in a row, but I just can't seem to find this information anywhere in the docs Thanks!

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MySQL + Windows + Haskell = ???
by Jakub Arnold 09 Feb '15

09 Feb '15

Hey guys, I've been trying to get my Yesod app working on Windows 8.1 using the Haskell Platform 2014.2.0.0, and everything seemes to work well, except for MySQL. The `mysql` package doesn't build with the following error setup.exe: The program mysql_config is required but it could not be found I've found a related GitHub issue (https://github.com/bos/mysql/issues/3) but there doesn't seem to be any solution. Is there anyone who's successfully running using Haskell and MySQL on Windows? I've also tried `hdbc-mysql` and `mysql-simple`, but they both depend on the `mysql` package, so that didn't really help. I can't really use a different database, or Linux. Thanks for any tips, Jakub

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Cabal: Missing field in record construction configProf
by Henk-Jan van Tuyl 09 Feb '15

09 Feb '15

L.S., I am trying to install a package with the new GHC 7.10 RC 2 (I recompiled cabal.exe with it, and now "cabal update" finally works again). The install fails, even after renaming the cabal config file: > cabal install Config file path source is default config file. Config file C:\Users\X\AppData\Roaming\cabal\config not found. Writing default configuration to C:\Users\X\AppData\Roaming\cabal\config Resolving dependencies... Configuring random-1.1... cabal: Distribution\Client\Config.hs:(246,37)-(299,9): Missing field in record construction configProf What does this message mean? How can I correct this? Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Officially supported Android backend, GSoC?
by Ivan Perez 09 Feb '15

09 Feb '15

Hi Café For the past few years I have done quite a lot of work in functional languages targeting mobile platforms. I've tried every functional language I could find. GHC targets Android via Neurocyte's (and JoeyH's) backend [1], although, afaik, this is not officially supported. There have been recent discussions on strategies to improve the current state, especially wrt. TH. Nevertheless, the progress is amazing and the backend works very well. I haven't found any major bugs. We've successfully used this at Keera Studios to write multiple games [2,3,4], and I've also written small applications for Google Glass (yes, Haskell works on Glass!). Users are running the games on different Android devices (with different hardware and OS version), and none of them has reported any (ghc-related) bugs so far. Haskell's ability to 'write once, run anywhere' could be a major selling point. Soon Haskell might become one of the very few functional languages that can target any platform, be it web, desktop or mobile, with minimal changes to the codebase (F# can do all of this; but, apart from language differences, developing and deploying software for Android is not free as in beer). So, I'd like to ask: 1) Does officially supporting the Android backend sound like a (strategically) reasonable and viable idea? 2) What would it take? Could a Google Summer of Code project help get things up to speed? (The project proposal deadline is probably very soon.) Cheers Ivan [1] https://github.com/neurocyte/ghc-android [2] http://tiny.cc/w53rtx (Link to Facebook photo, visible without a FB account). [3] http://tiny.cc/l73rtx (Link to Facebook photo, visible without a FB account). [4] http://tiny.cc/e83rtx (Link to Facebook photo, visible without a FB account).

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