January 2011 - Haskell-Cafe

Freeglut
by Eric 08 Jan '11

08 Jan '11

Hi all, I would like to use freeglut instead of GLUT for my Haskell OpenGL program, but when I place the freeglut dll in the program's directory and try to run the program on Windows XP, I get the following error message: user error (unknown GLUT call glutSetOption, check for freeglut) Can anyone help? Eric M.

3 2

Newbie type question for wxHaskell learner
by b1g3ar5 07 Jan '11

07 Jan '11

I've tried to solve this but I am failing. I can do this: p0<-panel nb [] e0<-textCtrl p [text:=my_list!!0] but I want to do this on all of my_list, so I tried: let es = map (\x-> textCtrl (panel nb []) [text:=x]) my_list Now, this won't work because the panel nb [] is IO (Panel()) and the parameter to textCtrl needs to be a Panel() How do I get out of IO? Thanks.

2 6

Exportable and importable instances
by Marco Túlio Gontijo e Silva 07 Jan '11

07 Jan '11

Hi. I'm planning to propose a language extension for Haskell to make it possible to control the visibility of type class instances in module export and import lists as a research project. I'm planning to implement this in GHC. I've already mentioned it in #ghc(a)irc.freenode.net and got interesting comments from tibbe and quicksilver. I'd be glad to receive more feedback on the idea. If you have any comments, please reply to this thread or mail me individually, at your will. Greetings. -- marcot http://marcot.eti.br/ [Flattr=54498]

2 1

UTF-8 BOM
by Tony Morris 07 Jan '11

07 Jan '11

I am reading files with System.IO.readFile. Some of these files start with a UTF-8 Byte Order Marker (0xef 0xbb 0xbf). For some functions that process this String, this causes choking so I drop the BOM as shown below. This feels particularly hacky, but I am not in control of many of these functions (that perhaps could use ByteString with a better solution). I'm wondering if there is a better way of achieving this goal. Thanks for any tips. dropBOM :: String -> String dropBOM [] = [] dropBOM s@(x:xs) = let unicodeMarker = '\65279' -- UTF-8 BOM in if x == unicodeMarker then xs else s readBOMFile :: FilePath -> IO String readBOMFile p = dropBOM `fmap` readFile p -- Tony Morris http://tmorris.net/

6 5

Problem on overlapping instances
by Magicloud Magiclouds 07 Jan '11

07 Jan '11

Hi, I am using Data.Binary which defined "instance Binary a => Binary [a]". Now I need to define "instance Binary [String]" to make something special for string list. How to make it work? I looked into the chapter of overlappinginstances, nothing works. -- 竹密岂妨流水过山高哪阻野云飞

7 8

Unknown symbol `__dso_handle' with Template Haskell and wxHaskell
by Alexander Bau 07 Jan '11

07 Jan '11

Hi, I am using Template Haskell and wxHaskell [1]. When Graphics.UI.WX is loaded during THs code generation, I get the following error: > ... > Loading package wxdirect-0.12.1.3 ... linking ... done. > ghc: /usr/local/lib/wxcore-0.12.1.6/ghc-6.12.3/HSwxcore-0.12.1.6.o: > unknown symbol `__dso_handle' > Loading package wxcore-0.12.1.6 ... > linking ... ghc: unable to load package `wxcore-0.12.1.6' System stats: ghc-6.12.3 template-haskell-2.4.0.1 wx-0.12.1.6 Regards, Alex [1] Example: -- Test.hs ---------------------- {-# LANGUAGE TemplateHaskell #-} module Test2 where import Language.Haskell.TH import qualified Graphics.UI.WX as WX foo :: Q [Dec] foo = do info <- reify ''WX.Color runIO $ print info return [] -- Main.hs ---------------------- {-# LANGUAGE TemplateHaskell #-} module Main where import Test $(foo)

3 3

ghc -O2 and HUnit weirdness
by Joey Hess 06 Jan '11

06 Jan '11

Suppose I'm playing with HUnit for nearly the first time. I might write a test program like this, to see what a test failure looks like in practice: import Test.HUnit import Test.HUnit.Tools main = runVerboseTests $ TestList [ TestCase $ assertBool "should fail" False , TestCase $ assertEqual "should also fail" "foo" "bar"] # runghc test.hs Testing 0 ### Failure in: 0 should fail Testing 1 ### Failure in: 1 should also fail expected: "foo" but got: "bar" Cases: 2 Tried: 2 Errors: 0 Failures: 2 (Counts {cases = 2, tried = 2, errors = 0, failures = 2},0) Good, that works. Then I drop it into a Makefile that already knows how to build haskell programs: # make test ghc -odir build -hidir build -O2 --make test [1 of 1] Compiling Main ( test.hs, build/Main.o ) Linking test ... ./test Testing 0 Testing 1 ### Failure in: 1 should also fail expected: "foo" but got: "bar" Cases: 2 Tried: 2 Errors: 0 Failures: 1 Wait, what?! One of my test cases is impossibly, passing. So, the problem seems to be that ghc -O2 somehow optimises the static assertBool _ True away, in what seems to be a bad way. Remove the -O2 and the test fails as expected. Presumably, although I have not verified, less static boolean values would not trigger the optimisation. Is this a ghc or HUnit bug? (Versions: 6.12.1, 1.2.2.1) -- see shy jo

4 4

implementing RealFloat decodeFloat
by Claude Heiland-Allen 06 Jan '11

06 Jan '11

Hi everyone, I've been working on [0] Haskell bindings for [1] libqd for [2] double-double and quad-double arithmetic, and have been struggling to implement [3] RealFloat, in particular [4] decodeFloat, mostly because of its postcondition but also some issues relating to variable precision: ---8<--- If decodeFloat x yields (m,n), then x is equal in value to m*b^^n, where b is the floating-point radix, and furthermore, either m and n are both zero or else b^(d-1) <= m < b^d, where d is the value of floatDigits x. In particular, decodeFloat 0 = (0,0). ---8<--- (BTW: that should probably really be "... <= abs m < ..."; perhaps this code could be added to the report and/or documentation, if it is correct: validDecode :: RealFloat f => f -> Bool validDecode f = case decodeFloat f of (0,0) -> True (m, e) -> b^(d-1) <= abs m && abs m < b^d where b = floatRadix f d = floatDigits f ) The double-double and quad-double types do not have a fixed precision, though they do have a fixed minimum precision: this means that decodeFloat will (in general) lose some precision. This is because for example in: data DoubleDouble = DoubleDouble !CDouble !CDouble dd = DoubleDouble a b the semantics are that "dd = a + b", with |a|>>|b|; coupled with the IEEE implicit leading 1 bit, this means that there may be large gaps between exponents: for example: "1 + 0.5**100 :: DoubleDouble". So far I've got a "mostly working" implementation thus: decodeFloat !(DoubleDouble a b) = case (decodeFloat a, decodeFloat b) of ((0, 0), (0, 0)) -> (0, 0) ((0, 0), (m, e)) -> (m `shiftL` f, e - f) ((m, e), (0, 0)) -> (m `shiftL` f, e - f) ((m1, e1), (m2, e2)) -> let fixup m e = if m < mMin then fixup (m `shiftL` 1) (e - 1) else if m >= mMax then fixup (m `shiftR` 1) (e + 1) else (m, e) mMin = 1 `shiftL` (ff - 1) mMax = 1 `shiftL` ff ff = floatDigits (0 :: DoubleDouble) g = e1 - e2 - f in fixup ((m1 `shiftL` f) + (m2 `shiftR` g)) (e1 - f) where f = floatDigits (0 :: CDouble) This does meet the postcondition as specified (which leads to breakage in other RealFloat methods), but has a recursion with no termination proof (so far), and is lossy in general: > let check f = uncurry encodeFloat (decodeFloat f) == f > check (1 + 0.5 ** 120 :: DoubleDouble) False It does however seem to meet a weaker condition: > let check2 f = (decodeFloat . (`asTypeOf` f) . uncurry encodeFloat . decodeFloat $ f) == decodeFloat f > check2 (1 + 0.5 ** 120 :: DoubleDouble) True Questions: 1. Is this weaker condition likely to be good enough in practice? 2. Can my implementation of decodeFloat be simplified? Thanks for any insights, Claude [0] http://hackage.haskell.org/package/qd [1] http://crd.lbl.gov/~dhbailey/mpdist/ [2] http://crd.lbl.gov/~dhbailey/dhbpapers/arith15.pdf [3] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht… [4] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht… -- http://claudiusmaximus.goto10.org

2 1

Haddock docs don't list instances
by Omari Norman 06 Jan '11

06 Jan '11

Web versions of Haddock documentation, even this old one, list a class's instances. http://www.haskell.org/ghc/docs/6.2.2/html/libraries/base/Prelude.html#5 My Haddock documentation does not have any class instances at all. This is true both of the documentation that comes with my Linux system (I use Debian) and of the documentation I generate locally using cabal-install or the Setup.hs program in my homemade packages. How do I get Haddock to make the instance lists? Anybody else have this problem--maybe it is a Debian bug? Thanks. --Omari -- Pay a little now, or pay a lot later.

1 0

Polymorphic function over pairs of maybes.
by Edward Amsden 06 Jan '11

06 Jan '11

Hello all: I'd like to right a function that could take a structure with type (random example): (Int, (String, (Int, Int))) and another where each individual value is a Maybe of the corresponding type, for example: (Maybe Int, (Maybe String, (Maybe Int, Maybe Int))) and perform a fromMaybe using the values to produce a new value with the original types. The catch is, I'd like to be able to do that for an n-deep structure with any type, without having to write a separate function for them. I'm not even sure that this is possible, but perhaps someone could explain how it is or isn't? -- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

7 8