June 2014 - Haskell-Cafe

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

3 2

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.

1 0

"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

3 3

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

1 1

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 =====================================================================

1 0

ANN: psqueues
by Jasper Van der Jeugt 26 Jun '14

26 Jun '14

Here at Better, we have just a released a package which implements faster priority search queues for Haskell. We hope it proves to be useful to the community! More information can be found in this blogpost: http://medium.com/@bttr/announcing-psqueues-8a0fe9fe939 Grab it from Hackage or GitHub: http://hackage.haskell.org/package/psqueues https://github.com/bttr/psqueues On behalf of the engineering team at Better, Jasper Van der Jeugt

3 2

Quantification in Instance Contexts
by Julian K. Arni 26 Jun '14

26 Jun '14

Hi Cafe, I'm playing around with simple logic-programming at the type level. For instance, encoding trees: >> {-# LANGUAGE MultiParamTypeClasses >> , FunctionalDependencies >> , FlexibleInstances >> , UndecidableInstances >> , FlexibleContexts >> , OverlappingInstances #-} >> {-# OPTIONS_GHC -fcontext-stack=100 #-} >> >> >> -- *A >> -- / \ >> -- B* *C >> -- | >> -- D* >> >> >> data A >> data B >> data C >> data D >> class Child a b bool | a b -> bool >> instance Child A B TrueT >> instance Child B D TrueT >> instance Child B C TrueT >> class Path a b bool | a b -> bool Now the following obviously doesn't work (never mind for now that 'Path' needs a recursive definition, and that this is really just 'Grandchild'): >> instance (Child a b TrueT, Child b c TrueT) => Path a c TrueT Because 'b' is ambiguous. Fair enough. But I can't directly use a fundep, because 'b' *is* in fact ambiguous. What I want to tell the compiler is that it's really okay, since the RHS side (and any possible 'where' expressions) doesn't depend on what is instantiated for 'b'. I know I could switch to a class 'Children a bs bool' and encode all children of as a type-level list, and then have a fundep between 'a' and 'bs'. That's not a *bad* solution: it does give a good sense of the intention. But it's not very extensible; really I'd rather be using something like 'forall' - something, I would guess, along the lines of: >> instance forall a c. (forall b. (Child a b TrueT, Child b c TrueT)) => Path >> a c TrueT That, however, fails with: Malformed instance head: (forall b. (Child a b TrueT, Child b c TrueT)) -> Path a c TrueT In the instance declaration for ‘Path a c TrueT’ So: is there a way (presumably with 'forall') of telling the compiler that the ambiguous type 'b' actually won't "leak through" to the RHS, so stop bugging me about ambiguity? Thanks, Julian

4 7

direct-fastcgi and logging
by Denis Shevchenko 26 Jun '14

26 Jun '14

Hi all! I write a FastCGI-server based on excellent package 'direct-fastcgi' (http://hackage.haskell.org/package/direct-fastcgi) It works perfectly, but I want to inject a logging facility into a handler of requests. For example: import Control.Concurrent (forkIO) import Network.FastCGI main :: IO () main = acceptLoop forkIO handleRequest handleRequest :: FastCGI () handleRequest = do -- >>> In this place I want to log a message <<< setResponseStatus 400 setResponseHeader HttpContentType "application/json" fPutStr "{ "my": "problem_description" }" Yes, I know about this function in 'Network.FastCGI': fLog :: MonadFastCGI m => String -> m () but it "logs a message using the web server's logging facility", and I want to log into _my_own_ file. So which solution I can use here? I'd be very grateful for any help. ---------- Sincerely, Denis Shevchenko haskell(a)dshevchenko.biz

2 1

BNFC-meta being maintained? Is there public src repo?
by Ki Yung Ahn 25 Jun '14

25 Jun '14

Dear BNFC-meta developers and users, I've been using BNFC-meta for the parser of my experimental language Nax language implementation ( https://github.com/kyagrd/mininax ). It's very cool 'cause you don't need to write makefile or cabal entries to call the command line tools (bnfc, alex, happy), and you get quasiquoters for free, which really makes the early development pleasant. But due to the bug fixed in recent versions of BNFC, probably after latest BNFC-meta release, I'll have to switch back to using plain BNFC at some point. (FYI,the bug is that block comments arn't working). If there is a source repository that has ported more recent versions of BNFC into BNFC-meta, it'll be helpful for people like me. Or, if the source repository has open access and easy to participate (e.g., putting on github or something like that), we can draw community support maintaining BNFC-meta to keep up with the recent BNFC (and also happy and alex) versions. -- Ki Yung Ahn

2 1

determine hoolge database directory
by Tobias Pflug 25 Jun '14

25 Jun '14

Hi haskellers, i'm new to haskell and am currently working on some tool that is supposed to fiddle with hoogle database files. Now I'm looking for a sane way to determine the directory where hoogle stores its database directories (when you perform `hoogle data`). In my case this is: .cabal/share/x86_64-linux-ghc-7.8.2/hoogle-4.2.33/databases So this is [home directory]/.cabal/[ghc-version]/[hoogle-version]/databases Could someone suggest me a sane way how I can determine this path at runtime ? Maybe there is a way to extract this from somewhere ? I'm out of ideas on this. I could of course take the easy way out and let users declare the path as config parameter but it would be nice if there was a way ... thanks in advance. regards, Tobi

2 1