- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] 3 level hierarchy of Haskell objects
by Patrick Browne 10 Aug '12

10 Aug '12

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resource post on Concurrent and Parallel Haskell
by Obscaenvs 10 Aug '12

10 Aug '12

Hi! I have recently gone through some tutorials on the subject of Concurrent and Parallel Haskell, most notably chapter 24 in Real World Haskell. In doing so I have collected some thoughts and resources on the subject in a post on my blog: http://monoid.se/haskell/a-resource-post-on-concurrent-and-parallel-haskell/ Is there some other resource that should really be included in such a list, or is there in your opinion anything else that should be pointed out for somebody starting out in this particular corner of the Haskell ecosystem? I would appreciate pointers and suggestions, even pertaining to English grammar :) /F

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unix package licensing
by Oleg Sidorkin 10 Aug '12

10 Aug '12

System/Posix/DynamicLinker/Module/ByteString.hsc and System/Posix/DynamicLinker/Prim.hsc sources in unix-2.5.1.0 package contains the following reference to GPL-2 c2hs package: -- Derived from GModule.chs by M.Weber & M.Chakravarty which is part of c2hs -- I left the API more or less the same, mostly the flags are different. Does it mean that GPL license should be applied to unix package and all dependent packages? unix package is included into Haskel Platform so does it mean that Haskell Platform should be GPL also?

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MIN_VERSION_base
by Kazu Yamamoto 10 Aug '12

10 Aug '12

Hello, The stable branch of the network library includes the following code: #if !(MIN_VERSION_base(4,6,0)) I cannot compile the stable branch with HP 2011.4.0.0 on Linux 2: Preprocessing library network-2.3.0.14... BSD.hsc:115:23: error: missing binary operator before token "(" BSD.hsc:115:23: error: missing binary operator before token "(" BSD.hsc:115:23: error: missing binary operator before token "(" compiling dist/build/Network/BSDhsc_make.c failed (exit code 1) command was: /usr/bin/gcc -c dist/build/Network/BSD_hsc_make.c -o dist/build/Network/BSD_hsc_make.o -fno-stack-protector -fno-stack-protector -DGLASGOW_HASKELL_=700 -Dlinux_BUILD_OS -Dlinux_HOST_OS -Di386_BUILD_ARCH -Di386_HOST_ARCH -Iinclude -DCALLCONV=ccall -I/ghc7.0.4/lib/ghc-7.0.4/unix-2.4.2.0/include -I/ghc7.0.4/lib/ghc-7.0.4/bytestring-0.9.1.10/include -I/ghc7.0.4/lib/ghc-7.0.4/base-4.3.1.0/include -I/ghc7.0.4/lib/ghc-7.0.4/include -I/ghc7.0.4/lib/ghc-7.0.4/include -I/ghc7.0.4/lib/ghc-7.0.4/include/ "dist/build/autogen/cabal_macros.h" defines MIN_VERSION_base but "dist/build/Network/BSD_hsc_make.c" does not include it. If you guys know why this happens, please let me know. For more information, please refer to: https://github.com/haskell/network/issues/43 --Kazu

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[Wadler 89] Philip Wadler. "Theorems for free!": Can't get the same results in Haskell
by Stayvoid 09 Aug '12

09 Aug '12

Hi there, I was reading "Real World Haskell" and found this paper. [1] On the first page of the paper there is an example: inc* (odds_Int [1,2,3]) = [2,4] /= [4] = odds_Int (inc* [1,2,3]) I tried to implement it in Haskell: (I'm a newbie. I guess it's possible to write a better version.) module Param where import Prelude odds :: [Int] -> [Int] odds [] = [] odds [x] = if odd x then [x] else [] odds (x:xs) = if odds [x] == [] then odds xs else [x] ++ odds xs inc :: [Int] -> [Int] inc [] = error "Empty list" inc [x] = [succ x] inc (x:xs) = inc [x] ++ inc xs Looks fine: *Param> odds [1,2,3] [1,3] *Param> inc [1,2,3] [2,3,4] But my results differ from the paper's: *Param> inc (odds [1,2,3]) [2,4] *Param> odds (inc [1,2,3]) [3] I doubt that there is an error in the paper. So it seems that something is wrong with my code. But I can't find the error. Could you help me? (Or there is no error at all and I misunderstood something.) [1] http://ttic.uchicago.edu/~dreyer/course/papers/wadler.pdf Thanks

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parsec: parserFail & multiple error messages
by silly8888 09 Aug '12

09 Aug '12

I am trying to create a parsec parser that parses an integer and then checks if that integer has the right size. If not, it generates an error. I tried the following: 8<--------------------------------------------------------------- import Text.Parsec import Text.Parsec.String integer :: Parser Int integer = do s <- many1 digit let n = read s if n > 65535 then parserFail "integer overflow" else return n 8<--------------------------------------------------------------- The problem is that when I try this parse integer "" "70000" I get the following error: Left (line 1, column 6): unexpected end of input expecting digit integer overflow ie there are three error messages but I only want the last one. Is there something I can do about this?

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Parsec type error with Flexible Contexts
by Marco Túlio Gontijo e Silva 09 Aug '12

09 Aug '12

Hi. I wanted to generalize the definitions in Text.Parsec.Language to be able to use them with Parsec Text instead of Parsec String. I'm getting a type error with the following code snippet: http://hpaste.org/72844 I don't really understand the error, specially with regard to the fact that the if I comment one of the marked lines on the code pasted the program build. Any help is welcome. Greetings. -- marcot http://marcot.eti.br/

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NSF funding available for U.S. graduate students to attend ICFP 2012
by Andy Adams-Moran 09 Aug '12

09 Aug '12

U.S. Graduate Students and Advisors! Funding is now available for U.S. graduate students to attend ICFP 2012, the International Conference on Functional Programming, to be held in Copenhagen, September 9-15 2012. You must be full-time in a U.S. graduate program to apply, but need not have a paper in the conference. Apply here: http://tinyurl.com/8wkh4ww In the comments field, note that you're applying for NSF funding, and provide your supervisor's name and your school. Not attending a U.S. graduate school? Check out the other possibilities for funding your travel at: http://www.icfpconference.org/icfp2012/students.html Looking forward to seeing you in Copenhagen! Cheers, The ICFP 2012 Organizing Team -- Andy Adams-Moran Phone: 503.626.6616, x113 Galois Fax: 503.214.8120 421 SW 6th Ave, Suite #300 http://www.galois.com Portland, OR 97204 adams-moran(a)galois.com

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NSF funding available for U.S. graduate students to attend ICFP 2012
by Andy Adams-Moran 09 Aug '12

09 Aug '12

U.S. Graduate Students and Advisors! Funding is now available for U.S. graduate students to attend ICFP 2012, the International Conference on Functional Programming, to be held in Copenhagen, September 9-15 2012. You must be full-time in a U.S. graduate program to apply, but need not have a paper in the conference. Apply here: http://tinyurl.com/8wkh4ww In the comments field, note that you're applying for NSF funding, and provide your supervisor's name and your school. Not attending a U.S. graduate school? Check out the other possibilities for funding your travel at: http://www.icfpconference.org/icfp2012/students.html Looking forward to seeing you in Copenhagen! Cheers, The ICFP 2012 Organizing Team -- Andy Adams-Moran Phone: 503.626.6616, x113 Galois Fax: 503.214.8120 421 SW 6th Ave, Suite #300 http://www.galois.com Portland, OR 97204 adams-moran(a)galois.com

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Data.Data and OverlappingInstances
by Timo von Holtz 09 Aug '12

09 Aug '12

Hi cafe, in my code I use Data.Data and OverlappingInstances. My problem now is, that I can't use functions using these instances as arguments for fromConstrM. This is what my problem looks like: {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE UndecidableInstances #-} import Data.Data class Test a where foo :: Monad m => m a instance Num a => Test a where foo = return 1 instance Test Int where foo = return 2 test constr = fromConstrM foo constr but when I compile I get: test.hs:15:26: Overlapping instances for Test d arising from a use of `foo' at test.hs:15:26-28 Matching instances: instance [overlap ok] (Num a) => Test a -- Defined at test.hs:9:9-23 instance [overlap ok] Test Int -- Defined at test.hs:12:9-16 (The choice depends on the instantiation of `d' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) In the first argument of `fromConstrM', namely `foo' In the expression: fromConstrM foo constr In the definition of `test': test constr = fromConstrM foo constr Failed, modules loaded: none. Is there a way out? Right now I use a "case (typeOf x) of" kind of construct, but it gets pretty messy and I even have to unsafeCoerce at one point. - Timo

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