- Haskell-Cafe - Haskell.org

Conceptualizing Functor vs Performance
by Yuri Lensky 30 Jun '14

30 Jun '14

I am having trouble consolidating a performance-based data representation decision I would like to make against the concept of a "functor". The question can be reduced to the following: Conceptually I think that Data.Vector /should/ be a functor. More specifically, I see no CONCEPTUAL reason that it shouldn't be possible to define some "efficient" container-like object (for example for fast Matrix operations for unboxable numbers, but something that still works for a symbolic variable type) that chooses to represent its data as a Data.Vector.Unboxed if possible, but chooses some more generic structure (perhaps a strict list or a generic Data.Vector) if that is not the case. I haven't found such a functor, and can't seem to implement it on my own. The point is that in theory there is no true constraint on the type of the object being contained so it SHOULD be a Functor/Traversable/etc., the representation simply changes to something more efficient only if possible, and decays to a more generic type gracefully if necessary. Perhaps there is some type hackery that can be done with TypeFamilies? I.E. define some container type "data C a = (ListType a) a", but I haven't found a generic way to do this. Finally, I understand that "efficient" data structure can be problem-dependent, but I have no problem defining many different such "generic" types for different applications. The simplest example would be something like (invalid Haskell): (Data.Vector.Unboxed.Vector v a) => data instance (ListType a) = v (Data.Vector.Generic.Vector v a) => data instance (ListType a) = v data instance (ListType a) = ([]) Of course implementing fmap is a whole separate issue. The point is I do not think this is the right way/possible, but am wondering what the "true" solution is. Thanks.

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Smallcheck Depth
by Omari Norman 30 Jun '14

30 Jun '14

The docs for smallcheck, and the original paper, say that the depth is the depth of nested constructor applications. For some reason this behavior has changed in newer versions of Smallcheck and maybe someone can shed some light on why. For instance, I would think the depth of True and False is 0, as there are no nested constructors. Sure enough, in Smallcheck 0.2.1, (series 0) :: [Bool] gives [True, False]. Yet in smallcheck 1.1.1, (list 0 series) :: [Bool] gives []. Thanks. Omari

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Implementing parental using blood type
by Rafael Almeida 29 Jun '14

29 Jun '14

Hello, I'm trying to practice my Haskell. So I had the idea of making a program which, given a mother's and a child blood type, it can determine whether a certain father is possible or not. Please take a look on this gist with my implementation: https://gist.github.com/aflag/14429dfb2e89791a44e2#file-parentaltesting Would you care to comment on it? It looks a bit cumbersome to my eyes. I'm trying to find the most intuitive and elegant way of do it. I'm not so much worried with performance. []'s Rafael

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Requesting maintainership: bzlib-conduit and process-conduit
by Michael Snoyman 29 Jun '14

29 Jun '14

I'd like to request to be granted maintainership of two conduit-related packages: bzlib-conduit and process-conduit. Both packages are maintained by the same author. I opened up a pull request[1] on bzlib-conduit back in January, and haven't heard back yet. There's also an issue with process-conduit[2] which I'd like to address, but given the lack of response of bzlib-conduit, haven't sent a pull request yet. I also emailed the author directly five days ago regarding the bzlib-conduit pull request, and have not received a response. [1] https://github.com/tanakh/bzlib-conduit/pull/7 [2] https://github.com/tanakh/process-conduit/issues/14

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HSpec vs Doctest for TDD
by Gautier DI FOLCO 29 Jun '14

29 Jun '14

Hi all, I'm a huge fan of TDD (Test-Driven Development) et I used to use tools such as RSpec (Ruby). So naturally, I looked to HSpec, but it seems not idiomatic in Haskell. I have a bunch of questions: - Do you do TDD? - Which tools do you use? - Is doctest "better" (in some senses) than HSpec? Why? - Are HSpec and Doctest complementary? If so, in which case do you use one or another? - Is there some Haskell-specific good practices do to TDD? Thanks in advance for your lights.

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Empty case alternative
by Frantisek Farka 28 Jun '14

28 Jun '14

Hello cafe, I was looking into Haskell Report 2010 and notice something in the definition of case statement: https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-460003.… Apparently an empty alternative within case statement is a plausible production. ('alt' production rule, third option). If I get it correctly, following example shows the empty alternative: > > cStmt e = case e of { 'a' -> True ; {-- empty here --} ; 'b' -> False } > What I miss is any ratio behind this. What is the empty alternative good for? Any idea? Franta

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Moderators, please block zhangliu529@sina.com
by Nikita Volkov 28 Jun '14

28 Jun '14

Every time I send a message to Cafe I get an autoreply from this user with the following text: 您发的邮件我已收到，我将尽快给您答复。 It's extremely annoying. Please, exclude this user from the mailing list.

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"Generating" type synonyms without Template Haskell
by Tom Ellis 27 Jun '14

27 Jun '14

I'd like to know how to conveniently generate specialisations of a product type without resorting to Template Haskell. Concretely, I have a type like data MyProduct a b c = MyProduct { foo :: a , bar :: b , baz :: c } and I use it in type signatures in various different specialisations. Generally the "base" type of each component stays fixed but it is wrapped in zero or more type constructors. For example a. MyProduct Int Bool String b. MyProduct (Maybe Int) (Maybe Bool) (Maybe String) c. MyProduct [IO Int] [IO Bool] [IO String] d. MyProduct (P Int) (P Bool) (P String) for various P that are not always functors in my uses. I thought I might be able to reduce this duplication by parametrisation, that is data MyProduct f a b c = MyProduct { foo :: f a , bar :: f b , baz :: f c } However, I can't always substitute a type constructor for `f` because that doesn't work for cases a. or c.. I thought a type family might work but it seems they have to be fully applied, like type synonyms. Is there an approach to this problem that will allow me to avoid Template Haskell here? Thanks, Tom

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POSIX capabilities support
by Vasili I. Galchin 27 Jun '14

27 Jun '14

Hello Haskellers, I have looked through the various POSIX hack age modules to see if there is capabilities support. ??? Regards, Vasya

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ICFP 2014 Call for Participation
by David Van Horn 26 Jun '14

26 Jun '14

[ Please note that much of the block reservation of hotel rooms currently being held for ICFP participants will be released next week. The beginning of September is a very busy conference week in Göteborg, so there is high pressure on hotel rooms in that period. If you plan to attend ICFP 2014, it would be best to make your hotel reservations now before the block reservation expires. ] ===================================================================== Call for Participation ICFP 2014 19th ACM SIGPLAN International Conference on Functional Programming and affiliated events August 31 - September 6, 2013 Gothenburg, Sweden http://icfpconference.org/icfp2014/ ===================================================================== ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. A full week dedicated to functional programming: 1 conference, 1 symposium, 10 workshops, tutorials, programming contest results, student research competition * Accepted Papers: http://www.icfpconference.org/icfp2014/accepted.html * Local arrangements (including travel and accommodation): http://icfpconference.org/icfp2014/local.html * Registration is available via: https://regmaster4.com/2014conf/ICFP14/register.php Early registration is due 3 August, 2014. * Programming contest, 25-28 July, 2014: http://icfpcontest.org/ * Follow @icfp_conference on twitter for the latest news: http://twitter.com/#!/icfp_conference There are several events affiliated with ICFP: Sunday, August 31 ACM SIGPLAN Workshop on Generic Programming ACM SIGPLAN Workshop on Higher-order Programming with Effects Monday, September 1 – Wednesday, September 3 ICFP Thursday, September 4 ACM SIGPLAN Commercial Users of Functional Programming: Day 1, Tutorials ACM SIGPLAN Haskell Symposium: Day 1 ACM SIGPLAN Workshop on Functional High-Performance Computing ACM SIGPLAN ML Family Workshop Friday, September 5 ACM SIGPLAN Commercial Users of Functional Programming: Day 2, Tutorials ACM SIGPLAN Haskell Symposium: Day 2 ACM SIGPLAN OCaml Workshop ACM SIGPLAN Erlang Workshop ACM SIGPLAN Workshop on Haskell and Rewriting Techniques Saturday, September 6 ACM SIGPLAN Commercial Users of Functional Programming: Day 3, Talks ACM SIGPLAN Haskell Implementors Workshop ACM SIGPLAN Workshop on Functional Art, Music, Modeling and Design Conference Organizers General Chair: Johan Jeuring, Utrecht University Program Chair: Manuel Chakravarty, University of New South Wales Local Arrangements Chair: Björn von Sydow, Chalmers University Industrial Relations Chair: Anil Madhavapeddy, University of Cambridge Workshop Co-Chairs: Tom Schrijvers, Ghent University Sam Tobin-Hochstadt, Indiana University Programming Contest Co-Chairs: Duncan Coutts, Well Typed LLP Nicolas Wu, University of Oxford Student Research Competition Chair: Meng Wang, Chalmers University Publicity Chair: David Van Horn, University of Maryland Video Chair: Iavor Diatchki, Galois Malcolm Wallace, Standard Chartered Bank Industrial partners: Platinum partners Jane Street Capital Gold partners Google Microsoft Research Mozilla Oracle Labs Standard Chartered Bank Silver partners Bloomberg Credit Suisse CyberPoint Erlang Solutions Facebook Galois Klarna Lexifi Twitter Bronze partners IntelliFactory Opera Software QuviQ =====================================================================

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