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Posted code snippet for Ex. 12 of Ch. 5 of "Fun w/ Phantom Types".
by David Banas 05 Jul '15

05 Jul '15

Hi all, I’ve spent the last few days working on Exercise 12, at the end of Chapter 5 of Ralf Hinze’s paper, “Fun with Phantom Types”. (Thanks, Conal, for all the help!) I thought I’d share my code, in case anyone else happens to be in the same Haskell space-time as me, right now. :) https://github.com/capn-freako/Haskell_Misc/blob/master/norm_by_eval.hs I hope everyone is enjoying their weekend. Cheers, -db

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List-based ini parser?
by Mike Meyer 05 Jul '15

05 Jul '15

Ok,I've looked at the packages google and hackage found (ini, hsini & ConfigFile), and can't use any of them for dealing with the ini files I'm being handed. The problem is they all parse the config file into Maps, and that doesn't seem to be an option. I need lists, because I have multiple sections with the same name that turn into a list of objects, as well as sections that can have multiple options with the same name that turn into multiple objects. Any chance I overlooked a parser? Or maybe some parsing options in ConfigFile? Any other advice on a library to do this?

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MVars and locks
by Tom Ellis 04 Jul '15

04 Jul '15

In the following section of the book "Parallel and Concurrent Programming in Haskell" http://chimera.labs.oreilly.com/books/1230000000929/ch07.html#sec_conc-phon… Simon Marlow explains that MVars can be used to implement something like locks on shared functional state: "To acquire the lock, we take the MVar, whereas, to update the variable and release the lock, we put the MVar." Am I right in thinking this only holds if we are careful to ensure both those operations happen in the given order? For example, if I take the MVar (lock) and I am about to put something back in (unlock) but someone else puts something in before I do, then the lock system becomes broken, doesn't it? So if we want to be sure we have a robust lock system we have to wrap up MVars in another abstraction? Tom

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ANNOUNCE: SubHask-0.1
by Mike Izbicki 02 Jul '15

02 Jul '15

SubHask is an alternative prelude for programming in subcategories of Hask. The documentation includes examples on: * the category of polynomials * how to give set a monad instance in two different categories * basic linear algebra using the category of linear functions More information at: http://github.com/mikeizbicki/subhask I'd like to emphasize that SubHask still has a number of bugs that need to be fixed. This release should be seen as just a technology preview.

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A datatype with a single constructor and forall to multiple constructors without forall
by Hemanth Kapila 01 Jul '15

01 Jul '15

Hi, Occassionally, in (*good*) haskell libraries, I came across data types that are defined with a single constructor and a forall. For instance, Iteratee[0] is defined as below *newtype Iteratee s m a =* * Iteratee {runIter :: forall r. (a -> Stream s -> m r) -> ((Stream s -> Iteratee s m a) -> Maybe SomeException -> m r) -> m r}* I *think* it is equivalent to the following. But am not sure. *data Iteratee s m a = IOne (m (a , Stream s))* * | ITwo (Stream s -> m (Iteratee s m a)) (Maybe SomeException)* Can some one please explain what are the advantages of using the forall version? Does it significantly improve performance because it is newtype? Or does it help in reasoning about code (in some proof tools or such)? And can some one kindly point me to some references that help me convert from one form to another reliably? Many Thanks, Hemanth

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Re: [Haskell-cafe] haskell-src-exts maintenance
by Roman Cheplyaka 01 Jul '15

01 Jul '15

Thanks for the update, Peter. Forwarding it to cafe. On 01/07/15 10:36, Peter A Jonsson wrote: > Hi Roman, > > I’ve inserted what I replied to Matthew a couple of days ago below. > Unfortunately my reply bounced from both libraries and haskell-cafe > since I’m not a subscriber. Matthew should have gotten the reply > though. > > Kind Regards, > > Peter > > > Dear Matthew, > >> I think it is important to get this work merged and update the >> parser to support modern language features introduced in GHC 7.8 >> and 7.10. > > We all agree about the importance of HSE supporting modern language > features that people actually use. The simple reason for my > inactivity lately is that I’ve been swamped at work so HSE has been > on the backburner. I just started my vacation though so I’m slowly > working through my backlog of things that I haven’t had time to do > previously. Without having looked at any patches in the bug tracker I > can say that the current state of HSE is stable so we should be able > to merge the simple things quickly and the more involved things > should be doable. Once that is done it’s probably time to make a new > yearly major release.

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Advice gfx library
by Nicholls, Mark 01 Jul '15

01 Jul '15

I am interested in writing some real code now! What i could do with is a gfx library that Can render mpeg/mov files Scale Transform etc Is simple! Runs on windows in some form Ive got hudaks book about multimedia, and understand the general reactive idea behind it....i want to build on top of that Ive nosed around gloss and grapefruit, but the mpeg piece seems missing from these Any ideas? Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage. 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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How to Hijack stdin,err,out, thread safely?
by Hiromi ISHII 01 Jul '15

01 Jul '15

Hi, I'm currently writing a web app to check the spec of input code usging QuickCheck. quickCheck* functions writes results to stderr & stdout, but I don't want them written to the app's original stdout/err. (I'm using SafeHaskell features to avoid malicious code executed, so don't worry about that :-)) So I want to execute I/O action hijacking stdout/err in a thread-safe manner. For example, I need function like below: censorHandles :: IO a -> IO (a, String, String) censorHandles = ... or withStd :: Handle -> Handle -> Handle -> IO a -> IO a withStd = ... -- Hiromi ISHII konn.jinro(a)gmail.com

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Munich Haskell Meeting, 2015-06-30 @ 19:30
by Christian Neukirchen 30 Jun '15

30 Jun '15

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, June 30 at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates 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-jun-2015/ **Please note that I cannot make it tomorrow, but I'll reserve the table for you.** Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Dealing with invertible functions
by Clinton Mead 29 Jun '15

29 Jun '15

I was trying to think of a way to deal with invertible functions, say if I want to set up a one-to-one mapping from A<->B without having to maintain two sets of functions (and worry about them getting out of sync). So I thought about making an "invertible" function. This is a function that knows it's own inverse, and you can compose them and get the inverses for free. Of course you need to set up the base functions manually, but then after that the composed functions don't have to be maintained separately both ways. Below I'm going to include some code, and I have a few questions: 1) Am I (badly) reinventing the wheel. 2) Is there otherwise something terrible with this approach. 3) I ended up wanting a function with signature "f a b -> a -> b". This feels strangly like applicative but not exactly. Am I reinventing the wheel here also or should I be doing this differently? Any advise appreciated, the ideone link is here: https://ideone.com/HlUHUd But I've also copied the code below: --- import Prelude hiding ((.)) import Control.Category ((.), (<<<), (>>>), Category) data InvertibleFunction a b = InvertibleFunction (a -> b) (b -> a) instance Category InvertibleFunction where (InvertibleFunction b_c c_b) . (InvertibleFunction a_b b_a) = InvertibleFunction (b_c . a_b) (b_a . c_b) inv (InvertibleFunction x y) = InvertibleFunction y x add :: (Num n) => n -> InvertibleFunction n n add x = InvertibleFunction (+x) (\y -> y - x) class KindaApplicative f where (<$>) :: f a b -> a -> b instance KindaApplicative InvertibleFunction where (InvertibleFunction f _) <$> x = f x main = print $ ((inv (add 2)) <$> 5)

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