July 2015 - Haskell-Cafe

SPJ's Venn diagram on type-correctness
by Mathijs Kwik 06 Jul '15

06 Jul '15

Hi all, Sometime in the past, I came across a presentation from SPJ in which he showed a Venn diagram showing "programs that compile" and "programs that are correct". Unfortunately, I cannot remember the exact wording and I'm unable to find the slides/talk on google/youtube. Does anyone remember the exact details of the diagram? Or the title of the talk? Is there still a link to it? Thanks, Mathijs

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ANN: stack-0.1.2.0
by Emanuel Borsboom 05 Jul '15

05 Jul '15

New release of stack, a build tool. Changes: https://github.com/commercialhaskell/stack/releases/tag/v0.1.2.0 General information and download links (now with package repositories for CentOS, Fedora, and Debian): https://github.com/commercialhaskell/stack#readme If you already have stack-0.1.1.0, you can upgrade using: stack upgrade --git && stack upgrade [normally just 'stack upgrade' will suffice, but due to my flub in the last release we ended up with a stack-9.9.9 on hackage which can't be removed (see https://github.com/haskell/hackage-server/issues/382) and you have to get the stack version that knows to ignore it first]

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ANN: zoom-refs - zoom and pairing for mutable references
by Marcin Mrotek 05 Jul '15

05 Jul '15

Hello, I've just uploaded my package zoom-refs: http://hackage.haskell.org/package/zoom-refs-0.0.0.0 It's a "port" of State monad zoom (from lens) to mutable references: zoomTVar :: Lens' a b -> TVar a -> TVar b These TVars aren't actually the raw TVars from STM, but wrappers that provide the same functionality. Similar functions are provided for STRefs and IORefs. Additionally, TVars and STRefs can be paired to create composite references: pairTVars :: TVar a -> TVar b -> TVar (a,b) No such functionality is provided for IORefs, as there would be no way to guarantee atomicity of operations on the underlying references. Together. mutable references can be used sort of like Functors and Applicatives, though one needs to use lenses rather than plain functons to map them. Finally, there are multiple references that use traversals instead of lenses to zoom: readMultiTVar :: Monoid a -> MultiTVar a -> STM a readMultiTVarList :: MultiTVar a -> STM [a] readMultiTVarHead :: MultiTVar a -> STM (Maybe a) Best regards, Marcin Mrotek

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Extensible states
by Alberto G. Corona 05 Jul '15

05 Jul '15

Hi, Anyone used some of the extensible record packages to create a kind of extensible state monad? I mean something that besides having "get", "gets" and "put" would have some kind of "add" and "gets": add :: a -> State () gets :: State (Maybe a) or add :: LabelName -> a -> State () gets :: LabelName -> State (Maybe a) So that I can extend the state without using additional monad transformers. Monad transformers are very hard for beginners and scramble error messages I did the first option for MFlow, hplayground and Transient packages (setSData and getSData). But my solution uses a map indexed by type and this requires a lookup for each access. I would like to know if there is an alternative with no lookups. I´m not obsessed with speed but In some applications the speed may be important.... Anyone? -- Alberto.

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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 04 Jul '15

04 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 01 Jul '15

01 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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