January 2015 - Haskell-Cafe

typeclass woes...how to constain a typeclass to be "closed" under an operation....
by Nicholls, Mark 06 Jan '15

06 Jan '15

This is reposted from the beginnners….I’ve not done Haskell for a while and was struggling to get anything to work, I’ve hopefully refound my feet. Its quite common in maths to have operations in a theory that are (set) closed, i just want to translate that notion to a typeclass I have a suggestion that does work class Foo m where op :: m a -> m (S a) That is closed, but now were working on types of kind • -> • (S can be a type family or a data type…lets say it’s a type family….probably an associated type (if I know what that means)) Is this the idiom/pattern i should follow? Or can the closure contraint be expressed directly in a typeclass any other way? From: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Brandon Allbery Sent: 06 January 2015 1:59 PM To: Rob Leslie Cc: Haskell Cafe Subject: Re: [Haskell-cafe] Haskeline and asynchronous messages On Tue, Jan 6, 2015 at 3:27 AM, Rob Leslie <rob(a)mars.org<mailto:rob@mars.org>> wrote: Does anyone have a suggestion on using Haskeline in an environment where another thread may be writing messages to the terminal -- is it possible to somehow print incoming messages above the input line without disturbing the input line? Terminals don't really work that way; you need to be looking at something like curses or vty. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com<mailto:allbery.b@gmail.com> ballbery(a)sinenomine.net<mailto:ballbery@sinenomine.net> unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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Haskeline and asynchronous messages
by Rob Leslie 06 Jan '15

06 Jan '15

Greetings, Does anyone have a suggestion on using Haskeline in an environment where another thread may be writing messages to the terminal -- is it possible to somehow print incoming messages above the input line without disturbing the input line? Thanks for any advice. -- Rob Leslie rob(a)mars.org

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Postgresql Intervals with Opaleye
by info＠rotnetix.com 05 Jan '15

05 Jan '15

Another opaleye question: What type should I use for the postgresql interval type? Thanks Riaan

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Installing grapefruit: "lexical error at character 'i'"
by Jeffrey Brown 05 Jan '15

05 Jan '15

Dear list, I am trying to install grapefruit on Kubuntu 14.04. Most of the dependencies seemed to install without incident, but I am running into the following error: jeff@jeff-Lenovo:~$ cabal install grapefruit-ui-gtk grapefruit-examples Resolving dependencies... Configuring grapefruit-ui-0.1.0.5... Building grapefruit-ui-0.1.0.5... Preprocessing library grapefruit-ui-0.1.0.5... src/Internal/UIItem.hs-boot:12:2: lexical error at character 'i' Failed to install grapefruit-ui-0.1.0.5 cabal: Error: some packages failed to install: grapefruit-examples-0.1.0.5 depends on grapefruit-ui-0.1.0.5 which failed to install. grapefruit-ui-0.1.0.5 failed during the building phase. The exception was: ExitFailure 1 grapefruit-ui-gtk-0.1.0.5 depends on grapefruit-ui-0.1.0.5 which failed to install. jeff@jeff-Lenovo:~$ I wanted to find the offending 'i' character, but the file "src/Internal/UIItem.hs-boot" I was unable to find anywhere: jeff@jeff-Lenovo:/$ pwd / jeff@jeff-Lenovo:/$ sudo find . -name *UIItem.hs* [sudo] password for jeff: jeff@jeff-Lenovo:/$ Any ideas? Thanks, Jeff

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Concurrency problems with mysql bindings?
by Elliot Cameron 05 Jan '15

05 Jan '15

Hello Fellow Haskellers, I am not experienced with the FFI, but I have suspicions that Haskell's "mysql" package falls short in terms of its support for concurrency. A tiny bit of discussion has occurred here: https://github.com/bos/mysql/issues/11 Since it appears that "mysql" is the de-facto binding for MySQL (see reverse dependencies at http://www.stackage.org/package/mysql ), it seems that this lack has far-reaching effects on the Haskell ecosystem at large. What attracts many to Haskell is it's strengths in writing parallel /concurrent code, but it seems we lose this benefit when interacting with MySQL. I for one have a beautiful Haskell program using MySQL which I would love to push into production, but it's performance pales in comparison to a Python script that builds up calls to bash and actually achieves concurrency. At this point I have not been able to gain traction for this problem, and since I'm not an experienced Haskell developer, my contributions are slow and tedious. I'm hoping that I can garner some help in the following: 1) Confirm or deny my suspicions as represented in the issue linked above 2) If they are confirmed to any degree, I hope for some help in working through the solution Please let me know if you would like to help in either of these ways! Even the smallest comments will be highly valued! Elliot Cameron Covenant Eyes Software Developer elliot.cameron(a)covenanteyes.com 810-771-8322

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New Functional Programming Job Opportunities
by Functional Jobs 05 Jan '15

05 Jan '15

Here are some functional programming job opportunities that were posted recently: Senior Software Engineer at McGraw-Hill Education http://functionaljobs.com/jobs/8775-senior-software-engineer-at-mcgraw-hill… Cheers, Sean Murphy FunctionalJobs.com

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Class constraints on associated types ...
by aditya siram 05 Jan '15

05 Jan '15

Hi all, I'd like to be able to constrain an associated type. I used to have an instance that looked like: class Dispatch a b c | a b -> c where runF :: a -> b -> c instance (C a) => Dispatch D1 D2 ( a -> IO ()) where runF d1 d2 = (\_ -> return ()) Since I upgraded to 7.8 from 7.5 that instance declaration is no longer accepted is no longer accepted since it violates FD's. I have been updating my code to use type families like so: class Dispatch a b where type Impl a b :: * runF :: a -> b -> Impl a b instance (C a) => Dispatch D1 D2 where type Impl D1 D2 = a -> IO () runF d1 d2 = (\_ return ()) Unfortunately the `type Impl ...` line in the instance is rejected because it uses `a` on the RHS. In this one case I could just package it up into a newtype or something but I would ideally like to be able to constrain any number of arguments like: instance (C a, C b ... C z) => Dispatch D1 D2 where type Impl D1 D2 = a -> b -> ... -> z -> IO () ... This was something I could do in 7.6 (although I realize this is way safer). How do I go about getting that constraint back? Thanks! -deech

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Small language with parser in Haskell and Faker
by Alex Gaziev 05 Jan '15

05 Jan '15

Hi all! As for beginner – it was pretty hard to me to parse YAML or JSON. Too much information, too much functions for such a simple task (in Ruby it is really simple, because of duck typing). So I wrote mine super simple language for data files – Giml – and two parsers for it – in Ruby and in Haskell. Aim of this language - to be fast and so simple that you need only couple pages of code of any language without heavy dependencies to implement parser. Here is the manifest: https://github.com/gazay/giml. Parser in Haskell: https://github.com/gazay/gimlh It was written mostly for my library Faker, which was nicely discussed on reddit ( http://www.reddit.com/r/haskell/comments/2rb8ym/haskell_library_for_generat…) so I have a lot of plans what to do in future with Faker. But I didn't present Giml anywhere and it will be really great if someone can take a look (there only couple pages of code) and tell me what can be improved, what can be rewritten in better way and so on. I didn't use Parsec for a reason in this library as I wanted to write my parser as clean of dependencies as possible, but maybe it will improve performance or something else?

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ANN: git-vogue - a pre-commit check framework for haskell code quality
by Christian Marie 05 Jan '15

05 Jan '15

There are no standards in the Haskell community when it comes to IDEs and editors. As such we've found ourselves without a common benchmark for linting, formatting and code quality checks. Within our development team at Anchor, we wanted a tool that would: * Install in one command * Require nothing to be learned for a new developer * Tell that developer what they need to fix in the code that they modify. We wanted to to satisfy these needs, whilst saving developers from the drudgery of installing, configuring and running a suite of tools independently. Let's face it, if it's difficult to run the formatters and linters and not a part of your established work flow, some developers just aren't going to bother. We built such a tool: http://anchor.github.io/git-vogue/ To install and configure, just: cabal install git-vogue # Wait 15 minutes cd your-project git vogue init All staged files will now be checked on pre-commit. If you wish to check the whole repo: git vogue check --all You can automatically apply stylish-haskell fixes to staged files: git vouge fix Or the whole repo: git vogue fix --all Thank you to thsutton, groberts, oswynb, glasnt, tranma, barneydesmond and sio for your development efforts, and everyone who has taken the time to test and provide feedback.

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hxt: How to Unpickle Multiple Adjacent Elements
by Jimmy Lu 04 Jan '15

04 Jan '15

Hi, With xml element having DTD like following: <!ELEMENT entry ((art*, formula?, table?), hw, hsl?, (pr | altpr)?, (ahw, hsl?, (pr | altpr)?)*, vr?, fl?, lb*, in*, ((dx) | (cx?, def?))?, dro*, dxnl*, uro*, syns*)> I want to unpickle the sequence hw, hsl?, (pr | altpr)?, (ahw, hsl?, (pr | altpr)?)* into Haskell data type [Headword] where data Headword = Hw { hwText :: String , hwSl :: Maybe String , hwPr :: Maybe Pronunciation } However in the hxt pickle API I cannot found a way to extract a sequence of adjacent elements. Is there any one knowing how to do it? Thanks, Jimmy Lu

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