October 2011 - Haskell-Cafe

Installing hledger-web
by Arnaud Bailly 04 Oct '11

04 Oct '11

Hello, I installed hledger and tried installing hledger-web but got the following error: [2 of 8] Compiling Hledger.Web.Settings.StaticFiles ( Hledger/Web/Settings/StaticFiles.hs, dist/build/hledger-web/hledger-web-tmp/Hledger/Web/Settings/StaticFiles.o ) Hledger/Web/Settings/StaticFiles.hs:1:1: Exception when trying to run compile-time code: ./static: getDirectoryContents: does not exist (No such file or directory) Code: staticFiles staticDir cabal: Error: some packages failed to install: hledger-web-0.16 failed during the building phase. The exception was: ExitFailure 1 I install on xubuntu 11.04 with ghc 7.0.3 and latest haskell platform. The installation is quite new without much other libs. Thanks for helping, Arnaud

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Trying to experiment with tangible values using GuiTV
by Martin Baldan 03 Oct '11

03 Oct '11

Hello all, I'm very new to Haskell, but still I'd like to play with "tangible values", which I find really intriguing. After getting some kind help in haskell-beginners, I managed to compile and run a small example. Which is as follows: ------------------ {-# LANGUAGE OverlappingInstances, UndecidableInstances , IncoherentInstances, FlexibleContexts , TypeSynonymInstances, FlexibleInstances , MultiParamTypeClasses #-} -- For ghc 6.6 compatibility {-# OPTIONS -fglasgow-exts -fallow-overlapping-instances -fallow-incoherent-instances #-} ---- Some GuiTV examples. See also the examples in TV. import Data.List (sort) -- my addition import Data.Monoid -- end my addition import Interface.TV.UI import Control.Arrow.DeepArrow import Data.FunArr -- TypeCompose import Data.Title -- To pick up the FunArr instance for OFun. import Interface.TV.OFun() reverseT :: CTV (String -> String) reverseT = tv (oTitle "reverse" defaultOut) reverse main = runUI reverseT ----------------------- There were some warnings during compilation: ----- [1 of 1] Compiling Main ( myexample.hs, myexample.o ) myexample.hs:7:11: Warning: -fallow-overlapping-instances is deprecated: use -XOverlappingInstances or pragma {-# LANGUAGE OverlappingInstances #-} instead myexample.hs:7:11: Warning: -fallow-incoherent-instances is deprecated: use -XIncoherentInstances or pragma {-# LANGUAGE IncoherentInstances #-} instead Linking myexample ... martin@martin-desktop:/media/ ext4logicaUnTera/cosas_linux/programs/haskell/TV/examples/src/miejemplo$ ghc --make myexample.hs myexample.hs:7:11: Warning: -fallow-overlapping-instances is deprecated: use -XOverlappingInstances or pragma {-# LANGUAGE OverlappingInstances #-} instead myexample.hs:7:11: Warning: -fallow-incoherent-instances is deprecated: use -XIncoherentInstances or pragma {-# LANGUAGE IncoherentInstances #-} instead ----- When I run it, I get a little window with the "reverse" title and two textboxes. However, when I write something in the upper box, nothing happens. I must say that when I use runIO instead of runUI in this little program, the interactive program does work. Only the GUI version doesn't. So, the person who helped me (Brandon Allbery) said it's probably because of a GuiTV being a bit too old. He recommended me to ask about the issue here, at haskell-cafe. Any suggestions? Maybe I should use GtkTV istead of GuiTV? Thanks in advance! Martin O.B.

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ghc with -fPIC support
by Phyx 03 Oct '11

03 Oct '11

Hi All, I'm trying to install GHC with -fPIC support in Fedora. I've grabbed a source tarball since it seems no binary one has this. In Build.mk i've changed the quick build type to ifeq "$(BuildFlavour)" "quick" SRC_HC_OPTS = -H64m -O0 -fasm -fPIC GhcStage1HcOpts = -O -fasm -fPIC GhcStage2HcOpts = -O0 -fasm -fPIC GhcLibHcOpts = -O -fasm -fPIC SplitObjs = NO HADDOCK_DOCS = NO BUILD_DOCBOOK_HTML = NO BUILD_DOCBOOK_PS = NO BUILD_DOCBOOK_PDF = NO endif unfortunately, when compiling i still get the ld error *** *** ghc -fglasgow-exts --make -shared -oHs2lib.a /tmp/Hs2lib924498/Hs2lib.hs dllmain.o -static -fno-warn-deprecated-flags -O2 -package ghc -package Hs2lib -i/home/phyx/Documents/Haskell/Hs2lib -optl-Wl,-s -funfolding-use-threshold=16 -optc-O3 -optc-ffast-math Linking a.out ... *** /usr/bin/ld: /tmp/Hs2lib924498/Hs2lib.o: relocation R_X86_64_32 against `ghczmprim_GHCziUnit_Z0T_closure' can not be used when making a shared object; recompile with -fPIC *** /tmp/Hs2lib924498/Hs2lib.o: could not read symbols: Bad value I've also told cabal to build any packages with -fPIC and shared. Anyone have any ideas? Regards, Phyx

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How to compile this example code?
by Magicloud Magiclouds 03 Oct '11

03 Oct '11

Hi, I am learning to use data-flags, and got this example code: import Data.Flags newtype MyFlags = MyFlags CInt deriving (Eq, Flags) #{enum MyFlags, MyFlags , myFlag1 = C_FLAG1 , myFlag2 = C_FLAG2 , myFlag3 = C_FLAG3 } I modified it trying to compile it. Well, I got illegal syntax at "#{e". In fact, I am not quite know the syntax here. Any clue? -- 竹密岂妨流水过山高哪阻野云飞

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HEADS-UP: security fix, please upgrade clientsession to >= 0.7.3.1
by Felipe Almeida Lessa 03 Oct '11

03 Oct '11

Hello! Please be advised that clientsession < 0.7.3.1 is vulnerable to timing attacks [1]. We have just released a fix and it's already on Hackage [2]. We advise all users of clientsession (and, consequently, Yesod) to upgrade as soon as possible to a version >= 0.7.3.1. With a timing attack a malicious user may be able to construct a valid MAC for his message. However, the attacker is not able to recover the MAC key or the encryption key. So you don't need to change your keys, just upgrade ASAP. Cheers, =) [1] https://github.com/snoyberg/clientsession/pull/4 [2] http://hackage.haskell.org/package/clientsession-0.7.3.1 -- Felipe.

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ghc 7.2.1 and super simple DPH
by Peter Braam 03 Oct '11

03 Oct '11

Hi - I'm trying to compile DotP.hs from http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell#A_simple_examp… (see below) The compiler complains and says (twice in fact): DotP.hs:17:33: Not in scope: `fromPArrayP' Could someone help me out please? Thanks a lot! Peter {-# LANGUAGE ParallelArrays #-}{-# OPTIONS_GHC -fvectorise #-} module DotP (dotp_wrapper)where import qualified Preludeimport Data.Array.Parallel.Preludeimport Data.Array.Parallel.Prelude.Double dotp_double :: [:Double:] -> [:Double:] -> Double dotp_double xs ys = sumP [:x * y | x <- xs | y <- ys:] dotp_wrapper :: PArray Double -> PArray Double -> Double{-# NOINLINE dotp_wrapper #-} dotp_wrapper v w = dotp_double (fromPArrayP v) (fromPArrayP w)

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Problem on using template haskell.
by Magicloud Magiclouds 03 Oct '11

03 Oct '11

Hi, I am trying to use data-flags library. And failed on compiling the test code. The code is like following, and the compiling error is "test.hs:4:24: parse error on input `{'" {-# LANGUAGE TemplateHaskell #-} import Data.Flags.TH $(bitmaskWrapper "Severity" ''Int [] False [ ("NotClassified", #{const (2 ^ 0)}) , ("Information", #{const (2 ^ 1)}) , ("Warning", #{const (2 ^ 2)}) , ("Average", #{const (2 ^ 3)}) , ("High", #{const (2 ^ 4)}) , ("Disaster", #{const (2 ^ 5)}) What should I do? -- 竹密岂妨流水过山高哪阻野云飞

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Parameters and patterns
by José Romildo Malaquias 03 Oct '11

03 Oct '11

Hello. When studing programming languages I have learned that parameter is a variable (name) that appears in a function definition and denotes the value to which the function is applied when the function is called. Argument is the value to which the function is applied. The parameter allows the manipulation of the argument in the body of the funtion definition in order to produce the result. Now I am not sure how to apply these concepts to Haskell, as Haskell uses pattern matching to deal with argument passing to functions. For instance, in the definition f x = 2 * x + 1 x is a parameter, and in the application f 34 34 is an argument. But in the definition g (_:xs) = xs what is the parameter of the function g? Is it the pattern (_:xs)? If so then a parameter is not necessarily a variable anymore, and that seems very strange. And what is xs? Is it a parameter, although it does not denote the value to which the function is aplied, but just part of it? I am writing some slides to use in my functional programming classes, but I am not sure how to deal with these terms. Any comments? Romildo -- DECOM - ICEB - UFOP

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Re: [Haskell-cafe] Is it possible to represent such polymorphism?
by Tom Murphy 02 Oct '11

02 Oct '11

Assuming that z :: Int, you can declare an algebraic datatype data TwoOrThree a b = Three (a, b, Int) | Two (a, b) deriving(Show, Eq) -- so you can experiment And then define expand as expand :: TwoOrThree a b -> (a, b, Int) expand (Three tuple) = tuple expand (Two (a, b)) = (a, b, 1) Tom (amindfv) On Oct 2, 2011 6:04 AM, "Du Xi" <sdiyazg(a)sjtu.edu.cn> wrote:

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ANN: Peggy 0.2.1
by Hideyuki Tanaka 02 Oct '11

02 Oct '11

Hello, all. I have released 'Peggy' a new parser generator . It is based on Parsing Expression Grammer (PEG) [1], and generates efficient packrat parsers. # Where to get it * Hackage page (http://hackage.haskell.org/package/peggy) * github repository (https://github.com/tanakh/peggy) * Some documents are at (http://tanakh.github.com/Peggy/) # Advantage * Simple and Powerful syntax * No shift/reduce conflict * Unlimited look-ahead * You don't need to prepare separated Scanner * Linear time complexity * Based on modern Haskell ecosystem (bytestring, text, ListLike, Monads, etc...) * Support to use and generate Quasi Quoters # Examples Here are few example of parsers: * http://tanakh.github.com/Peggy/example.html * https://github.com/tanakh/Peggy/blob/master/example/Json.hs There is a self defined parser of peggy syntax, used for bootstrapping: * https://github.com/tanakh/Peggy/blob/master/bootstrap/peggy.peggy Please try it and give me feedbacks! Thanks, [1]: http://en.wikipedia.org/wiki/Parsing_expression_grammar -- Hideyuki Tanaka

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