- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] formal treatment of module dependencies
by Johannes Waldmann 08 Mar '17

08 Mar '17

> specifying and solving module dependencies related: http://www.mancoosi.org/papers/ (not for Haskell modules, but for Debian-like package specs) PS: keep in mind that even the most elaborated versioned dependency mechanism is just a work-around - as long as we don't have *all* software properties in the types. Only then could we have a formal proof that a module A can use module B2 instead of module B1, etc. And, we do not "just" want correctness w.r.t. a specification, but also resource consumption. - J.W.

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reading and writing Data.Ratio.Rational
by Olaf Klinke 07 Mar '17

07 Mar '17

Dear cafe, when processing text files containing numbers of the form "xxxx.yyyy" I used to parse them into Double using that type's Read instance. Obviously, even with arithmetic no more complicated than the field operations the result might have ugly rounding errors like 12.000000000002 due to the fact that numbers like 0.7 are not dyadic rationals. The math in my program is not complicated and the numbers are not large, so I don't care about Rationals having potentially huge memory footprints. So here's my question. A literal like 0.7 has type Fractional a => a, but the Read instance of Rational rejects the string "0.7". Must it be this way? Do I have to go via toRational.(read :: String -> Data.Scientific.Scientific)? Note that the documentation of Data.Scientific explicitly states that using (/) is unsafe, so I'd rather stay with the field Rational. For the output as decimal expansion, there is of course long division as described here [1], but I wonder whether either this exists in some library or there is even a more efficient solution. Any pointers are appreciated. Thanks, Olaf [1] http://stackoverflow.com/questions/30931369/how-to-convert-a-rational-into-…

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Professor of Cyber Security at Kent
by Simon Thompson 07 Mar '17

07 Mar '17

We’re looking to appoint a professor of Cyber Security at Kent, to head up our existing group, with its Masters course and research activities. The Cyber Security group also sits alongside our Programming Languages and Systems group, and someone with strengths in PL aspects of security would fit very well. The advert for a Professor of Cyber Security is live at https://jobs.kent.ac.uk/fe/tpl_kent01.asp?s=4A515F4E5A565B1A&jobid=40739,21… <https://jobs.kent.ac.uk/fe/tpl_kent01.asp?s=4A515F4E5A565B1A&jobid=40739,21…> and post has been advertised with an associated lectureship in security, so the successful candidate will have a chance to shape that appointment too. If you’re interested, do think about applying, or get in touch with us. I’d be happy to answer emails, as would Richard Jones, head of school: cs-hos AT kent DOT ac DOT uk Simon Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk <mailto:s.j.thompson@kent.ac.uk> | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt <http://www.cs.kent.ac.uk/~sjt>

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Yesod app uses wrong port when connecting to PostgreSQL
by Geraldus 06 Mar '17

06 Mar '17

Hi folks! I'm struggling with following error: libpq: failed (could not connect to server: Connection refused Is the server running on host "localhost" (127.0.0.1) and accepting TCP/IP connections on port 5432? I see such messages in Keter's log (in app folder). The VPS where the app deployed running PostgreSQL 9.6 on port 5433. I have changed default port in: - config/settings - config/keter.yaml - /opt/keter/etc/postgres.yaml on server side None of these steps helped. I wasn't able to find other port configuration places. What else can I do?

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Maintaining fb, yesod-fb, yesod-auth-fb
by Sibi 06 Mar '17

06 Mar '17

Posting here to share my intent on taking the maintainership of fb, yesod-fb and yesod-auth-fb packages. The packages has not been updated for quite some time and are currently not building in GHC 8.0.2. Also, I use this package at my day job and we are planning to switch to GHC 8 soon. Regards, Sibi

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Blast: a simple distributed computing library
by jean-christophe mincke 06 Mar '17

06 Mar '17

Hi everyone. Over the past few months, I have written Blast, a distributed computing library, inspired by Apache Spark. It is now open source and can be found on github: https://github.com/jcmincke/Blast Regards J-C

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Mailing list request for DataHaskell
by Marco Zocca 06 Mar '17

06 Mar '17

Dear all, I write on behalf of DataHaskell [0, 1], an open source community devoted to data science, scientific computing and machine learning. We would like haskell.org to host a mailing list for our community, to archive discussions that might otherwise get lost on the Gitter chatroom [2]. With this email I'd also like to extend the invitation to join to all interested people and companies. Thank you and kind regards, Marco Zocca (https://github.com/ocramz) [0] datahaskell.org [1] github.com/DataHaskell [2] https://gitter.im/dataHaskell/Lobby

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Inlinable (>>>)
by David Sorokin 05 Mar '17

05 Mar '17

Hi Cafe, I offer to define the functions of the Control.Category module inlinable: -- | Right-to-left composition (<<<) :: Category cat => cat b c -> cat a b -> cat a c {-# INLINABLE (<<<) #-} (<<<) = (.) -- | Left-to-right composition (>>>) :: Category cat => cat a b -> cat b c -> cat a c {-# INLINABLE (>>>) #-} f >>> g = g . f Perhaps all functions from this module should be marked by this pragma as possible. I suppose that the current definition without the pragma is the cause why the execution slows down in my applications, when using monad parameters in the constraints. I noticed it yet two or three years ago, while playing with the monad transformers, but decided to write now. Best regards, David Sorokin

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Declaring Functors for Type constrained data types
by Guru Devanla 01 Mar '17

01 Mar '17

Hello All, I am working on a program that will define a bunch of tasks. Each task will have to implement certain methods as part of a type class. -- task 1 data UpdateAcctsTask = UpdateAccts -- task 2 data EmailConfig = EmaiConfig {someattrs::String} data SendEmailTask = SendEmailsTask EmailConfig -- task 3 data GeneralWriterTask a = GeneralWriterTask a Each of these tasks implement a class, Taskable. The return values are simplified for this example. class Taskable a where process :: a -> Bool can_run :: a -> Bool This works fine. I can expand on these tasks and execute them. Now, I wanted to be able to defined dependencies between these (Taskable's). I decided I could create a data type for this dependency and may be also get a FreeMonad around this structure for further processing using a graph of Tasks. But, before that I wanted to create an wrapper for these Taskables and create a functor for it as follows The first thing I did was, define a Task, which generalizes over all the above defined (and future Taskables) data Task a where Task :: (Taskable a) => a -> Task a instance Functor Task where fmap:: (Taskable a, Taskable b) -> (a -> b) -> Task a -> Task b --- THIS DOES NOT WORK fmap f (Task a) = Task $ f a But, I realized that I cannot define an fmap over a type constraint. My questions are: 1. Is there any way to do this. I see there is an answer of SO. I wanted to make sure if there were any improvements to this since that answer' was posted. http://stackoverflow.com/questions/17157579/functor-instance-for-a-gadt-wit… 2. Secondly, I would like to know why this is not possible. Is it a current limitation of GHC or if there is some fundamental category theory concepts that dis-allows such declarations that I need to grok! Appreciate any help on this. Thank you!

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Re: [Haskell-cafe] Looking for maintainers or comaintainers on my Haskell projects
by sighingnow＠163.com 01 Mar '17

01 Mar '17

Hi! I’m interested in co-maintaining the Hindent project. I use Hindent extensively and I have submitted several patches to Hindent on github during the last year. I would like to contribute to improving Hindent and making it better. Thanks. Tao He

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