- Haskell-Cafe - Haskell.org

Mysterious complaint about .hi files
by Josef Svenningsson 07 Jun '11

07 Jun '11

Hi cafe! I'm hitting a very strange problem when using haskell-src-exts and haskell-src-exts-qq. Consider the following module: \begin{code} {-# Language QuasiQuotes #-} module TestBug where import Language.Haskell.Exts import Language.Haskell.Exts.QQ unit = TyTuple Boxed [] ty = [dec| quux :: (a,b) |] \end{code} This module doesn't load for me using ghc 7.0.3. I've pasted the full error message at the end of this email but the error message begins with the following lines: TestBug.hs:11:11: Can't find interface-file declaration for variable Language.Haskell.Exts.Syntax.Boxed Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error Using -ddump-if-trace didn't help me much. The funny thing is that if I comment out the last line (the definition of 'ty') then the module loads just fine even though it uses the Boxed type in the definition of 'unit'. So the problem only manifests itself when I use tuples from haskell-src-exts-qq. Everything else that I've used from haskell-src-exts-qq works fine, it's just when I try to use tuples that things go haywire. I've tried to remove the packages and reinstall them but it didn't help. Any clues? Josef TestBug.hs:11:11: Can't find interface-file declaration for variable Language.Haskell.Exts.Syntax.Boxed Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error In the first argument of `Language.Haskell.Exts.Syntax.TyTuple', namely `Language.Haskell.Exts.Syntax.Boxed' In the third argument of `Language.Haskell.Exts.Syntax.TypeSig', namely `Language.Haskell.Exts.Syntax.TyTuple Language.Haskell.Exts.Syntax.Boxed ((:) (Language.Haskell.Exts.Syntax.TyVar (Language.Haskell.Exts.Syntax.Ident ((:) 'a' []))) ((:) (Language.Haskell.Exts.Syntax.TyVar (Language.Haskell.Exts.Syntax.Ident ((:) 'b' []))) []))' In the expression: Language.Haskell.Exts.Syntax.TypeSig (SrcLoc ((:) '<' ((:) 'u' ((:) 'n' ((:) 'k' ((:) 'n' ((:) 'o' ((:) 'w' ((:) 'n' ((:) '>' ((:) '.' ((:) 'h' ((:) 's' [])))))))))))) 1 2) ((:) (Language.Haskell.Exts.Syntax.Ident ((:) 'q' ((:) 'u' ((:) 'u' ((:) 'x' []))))) []) (Language.Haskell.Exts.Syntax.TyTuple Language.Haskell.Exts.Syntax.Boxed ((:) (Language.Haskell.Exts.Syntax.TyVar (Language.Haskell.Exts.Syntax.Ident ((:) 'a' []))) ((:) (Language.Haskell.Exts.Syntax.TyVar (Language.Haskell.Exts.Syntax.Ident ((:) 'b' []))) []))) Failed, modules loaded: none.

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Re: [Haskell-cafe] Maybe use advice
by John Lato 07 Jun '11

07 Jun '11

Is it necessary (helpful) to use 'rewrite'? Nearly every time I've tried it, in the end 'transform' has been a better choice. Then you wouldn't need the 'Just's at all, and it should work fine. John > From: Lyndon Maydwell <maydwell(a)gmail.com> > > (missed including cafe) > > f :: [Modification] -> Maybe [Modification] > and > f _ = Just $ f ... > are incompatible > > I managed to get the behaviour I'm after with the use of Either, but > this really is messy: > > > -- Sets of changes > o (Modifier (Changes []) i) = Just $ i > o (Modifier (Changes [c]) i) = Just $ Modifier c i > o (Modifier (Changes l) i) = g (f (Left l)) > where > g (Right l) = Just $ Modifier (Changes l) i > g (Left l) = Nothing > > f (Left (Scale x y : Scale x' y' : l)) = > f $ Right $ Scale (x*x') (y*y') : h (f $ Left l) > f (Left (Translate x y : Translate x' y' : l)) = > f $ Right $ Translate (x+x') (y+y') : h (f $ Left l) > f (Left (Rotate x : Rotate x' : l)) = > f $ Right $ Rotate (x+x') : h (f $ Left l) > f x = x > > h (Left l) = l > h (Right l) = l > > > On Tue, Jun 7, 2011 at 3:11 AM, Maciej Marcin Piechotka > <uzytkownik2(a)gmail.com> wrote: > > On Mon, 2011-06-06 at 23:38 +0800, Lyndon Maydwell wrote: > >> I'm writing an optimisation routine using Uniplate. Unfortunately, a > >> sub-function I'm writing is getting caught in an infinite loop because > >> it doesn't return Nothing when there are no optimisations left. > >> > >> I'd like a way to move the last Just into f, but this makes recursion > >> very messy. I was wondering if there was a nice way to use something > >> like the Monad or Applicative instance to help here. > >> > >> -- Sets of changes > >> o (Modifier (Changes []) ?i) = Just $ i > >> o (Modifier (Changes [c]) i) = Just $ Modifier c i > >> o (Modifier (Changes l) ? i) = Just $ Modifier (Changes (f l)) i > >> ? where > >> ? ? f (Scale ? ? x y : Scale ? ? x' y' : l) = f $ Scale ? ? (x*x') > (y*y') : f l > >> ? ? f (Translate x y : Translate x' y' : l) = f $ Translate (x+x') > (y+y') : f l > >> ? ? f (Rotate ? ?x ? : Rotate ? ?x' ? ?: l) = f $ Rotate ? ?(x+x') ? ? ? > ?: f l > >> ? ? f l = l > >> > >> > >> Any ideas? > > > > Something like: > > > > ... > > f (Rotate ? ?x ? : Rotate ? ?x' ? ?: l) > > ? ?= Just $ f (Rotate (x+x') : fromMaybe l (f l)) > > f l = Nothing -- As far as I understend > > > > Regards > > > > _______________________________________________ >

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Can it be proven there are no intermediate "useful" type classes between Applicative Functors & Monads?
by KC 07 Jun '11

07 Jun '11

If new intermediate classes crop up then there would be no point in fixing class (Applicative m) => Monad m where since it would have to be changed if new intermediate classes are found. I realize non-existence proofs are hard. -- -- Regards, KC

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Regression moving to ghc7
by tsuraan 07 Jun '11

07 Jun '11

I'm trying to write a rolling window variation of the adler32 hash function (here: https://gist.github.com/1011151) and I got it to a nearly acceptable speed under ghc 6.12.3 (~35MB/s on my work machine), so I thought I'd see how the new ghc 7.0.3 treated it. The answer is "not good". I get about 270KB/s. The key difference that I'm seeing from looking at the ghc-core program is that ghc 7.0.3 isn't using primitive ops for anything (unless they are no longer annotated with "#"). Can anybody shed any light as to what's going on? I'd also welcome any optimization tips that people would like to share. I've been messing with the code so much over the past day or two that I'm not even sure if it computes an adler32 anymore, but I hope that any optimization tops that pertain to that code would also apply to a correct algorithm. Right now, I'd just like to have the code run quickly, and to figure out why I'm seeing such a terrible regression under ghc7.

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How to install GhC on a Mac without registering?
by John D. Ramsdell 07 Jun '11

07 Jun '11

I rarely use a Mac because it is too cute, but I bought one for my wife. I'd like to install GHC on her Mac just to test Haskell Platform and cabal install. (I rarely use Windows too, but testing GHC & cabal install is completely painless.) To do a quick test, do I really have to register as an Apple developer, or is there a way to test the platform anonymously? Someone who some times worries about privacy, John

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Maybe use advice
by Lyndon Maydwell 06 Jun '11

06 Jun '11

I'm writing an optimisation routine using Uniplate. Unfortunately, a sub-function I'm writing is getting caught in an infinite loop because it doesn't return Nothing when there are no optimisations left. I'd like a way to move the last Just into f, but this makes recursion very messy. I was wondering if there was a nice way to use something like the Monad or Applicative instance to help here. -- Sets of changes o (Modifier (Changes []) i) = Just $ i o (Modifier (Changes [c]) i) = Just $ Modifier c i o (Modifier (Changes l) i) = Just $ Modifier (Changes (f l)) i where f (Scale x y : Scale x' y' : l) = f $ Scale (x*x') (y*y') : f l f (Translate x y : Translate x' y' : l) = f $ Translate (x+x') (y+y') : f l f (Rotate x : Rotate x' : l) = f $ Rotate (x+x') : f l f l = l Any ideas?

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ANN: dtd-text DTD parser, V0.1.0.0
by Yitzchak Gale 06 Jun '11

06 Jun '11

The dtd-text package[1] provides a parser for XML DTDs. It implements most of the parts of the W3C XML specification relating to DTDs, and is compatible with versions 1.0 and 1.1 of the specification.[2] The result of the parse is a Haskell DTD object from the dtd-types[3] package. This first preliminary version of dtd-text, version 0.1.0.0, requires at least version 0.3.0.1 of dtd-types. Synopsis: -- Parse a DTD from a Data.Text.Lazy: dtdParse :: L.Text -> DTD That should usually be all you need. -- Or, for advanced users, if the DTD contains external -- parameter entities and you want to supply their values: dtdParseWithExtern :: SymTable -> L.Text -> DTD -- where type SymTable = M.Map Text L.Text I really should have edited the Cabal description of this package before I uploaded it. It promises an attoparsec-text parser and blaze-builder renderer for DTDs. First of all, the renderer is vaporware - I haven't written it yet. Just the parser was quite a bit of work, so I decided to release it before even starting on the renderer. Second, although dtd-text does use attoparsec-text, and does export parsers for all of the significant components of a DTD, those parsers are of limited usefulness on their own. It turns out that in order to support the full algorithm specified in the spec for parameter entity resolution, which is rather imperative in nature, two layers of parsing are necessary. So the dtd-text package also has some internal plumbing so that it can present a simple interface. This is a very preliminary alpha release. All I can say so far is that it compiles on my machine (GHC 7.0.2 on 64 bit Linux), and that I tested it against a huge, extremely complicated DTD, and it seems to have done the RIght Thing. Since there are likely to be bugs that I will need to fix soon, I will wait until then to fix the package description. More about external parameter entities, for advanced users: As mentioned above, this parser does not attempt to go out and fetch the values of external references for you from files and URLs. If you need to extract information from the DTD before you fetch them yourself, such as system IDs and public IDs, you might be able to get them by applying parseDTD to all or part of the DTD as an initial parse. The parser tries very hard to give partial results when things are missing, while still doing its best to avoid problems like looping references. So if your DTD has many deeply intertwined external parameter entities, this parser may not be very practical for you; on the other hand, I personally have never seen such a DTD in the wild. A final caveat: this version of dtd-text does not yet support conditional sections. Enjoy, Yitz [1] http://hackage.haskell.org/package/dtd-text [2] http://www.w3.org/TR/2008/REC-xml-20081126/ [3] http://hackage.haskell.org/package/dtd-types

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ANN: dtd-text DTD parser and renderer, V0.1.1.0
by Yitzchak Gale 06 Jun '11

06 Jun '11

The dtd-text package[1] provides a parser and renderer for XML DTDs. It implements most of the parts of the W3C XML specification relating to DTDs, and is compatible with versions 1.0 and 1.1 of the specification.[2] The parser and renderer operate on Haskell DTD objects from the dtd-types[3] package. The parser is based on the attoparsec-text[4] parser library. Version 0.1.1.0 adds the first preliminary version of the renderer, based on the blaze-builder[5] package. Synopsis: -- Parse a DTD without parameter entity resolution. -- See the attoparsec-text package for information -- about how to use this parser. dtd :: Parse DTD -- Parse a DTD from a Data.Text.Lazy while resolving -- references to internal parameter entities. If you -- are not sure which interface to use, use this one. dtdParse :: L.Text -> DTD -- Parse a DTD from a Data.Text.Lazy while resolving -- references to internal and external parameter entities. -- Supply the values of the external parameter references -- using a SymTable, where type SymTable = M.Map Text L.Text dtdParseWithExtern :: SymTable -> L.Text -> DTD -- Create a builder to render a DTD. -- See the blaze-builder package for information -- about how to use this builder. buildDTD :: DTD -> Builder Enjoy, Yitz [1] http://hackage.haskell.org/package/dtd-text [2] http://www.w3.org/TR/2008/REC-xml-20081126/ [3] http://hackage.haskell.org/package/dtd-types [4] http://hackage.haskell.org/package/attoparsec-text [5] http://hackage.haskell.org/package/blaze-builder

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Re: [Haskell-cafe] Are casts required?
by Ryan Ingram 06 Jun '11

06 Jun '11

I always forget to reply all. Silly gmail. On Mon, Jun 6, 2011 at 2:07 AM, Ryan Ingram <ryani.spam(a)gmail.com> wrote: > Hi Pat. There aren't any casts in that code. There are type annotations, > but this is different than the idea of a cast like in C. > > For example > ((3 :: Integer) :: Int) > is a compile error. > > What you are seeing is that 3 has the type (forall a. Num a => a); that is, > the literal '3' gets converted by the compiler into > > fromInteger (I# 3#) > > where 3# represents the machine word '3' and I# is the internal constructor > Word# -> Integer. > > class Num a where > ... > fromInteger :: Integer -> a > > So by 'casting', or rather, providing a type annotation, you are specifying > what instance of Num gets the call to 'fromInteger'. > > As to whether you *need* a type annotation: it depends. For example: > foo () = sameId newId 3 > the compiler will infer the type of 'foo' to be > foo :: forall a. IDs a => () -> a > > If you declare foo as a value, though, you run into the dreaded > monomorphism restriction, and you might get a complaint from the compiler > about ambiguity. > foo2 = sameId newId 3 > > > The monomorphism restriction forces values to be values; otherwise consider > this > > > -- the usual 'expensive' computation > fib :: Num a => a -> a > fib 0 = 1 > fib n = fib (n-1) + fib (n-2) > > x = fib 100000 > > What's the type of x? Most generally, it's > x :: Num a => a > > But this means that x will be recalculated every time it's used; the value > can't be saved since x doesn't represent a single value but rather a > separate value for each instance of Num. You are allowed to manually > specify this type, but without it, the compiler says 'You meant this to be a > value!' and forces it to a particular type if it can, or complains about > ambiguity if it can't. As to how it does so, look up the rules for > defaulting and monomorphism in the Haskell report. > > -- ryan > > > > On Mon, Jun 6, 2011 at 12:45 AM, Patrick Browne <patrick.browne(a)dit.ie>wrote: > >> Are casts required to run the code below? >> If so why? >> Thanks, >> Pat >> >> >> -- Idetifiers for objects >> class (Integral i) => IDs i where >> startId :: i >> newId :: i -> i >> newId i = succ i >> sameId, notSameId :: i -> i -> Bool >> -- Assertion is not easily expressible in Haskell >> -- notSameId i newId i = True >> sameId i j = i == j >> notSameId i j = not (sameId i j) >> startId = 1 >> >> >> instance IDs Integer where >> >> >> >> -- are casts need here? >> sameId (newId startId::Integer) 3 >> sameId (3::Integer) (4::Integer) >> notSameId (3::Integer) (newId (3::Integer)) >> >> This message has been scanned for content and viruses by the DIT >> Information Services E-Mail Scanning Service, and is believed to be clean. >> http://www.dit.ie >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Are casts required?
by Patrick Browne 06 Jun '11

06 Jun '11

Are casts required to run the code below? If so why? Thanks, Pat -- Idetifiers for objects class (Integral i) => IDs i where startId :: i newId :: i -> i newId i = succ i sameId, notSameId :: i -> i -> Bool -- Assertion is not easily expressible in Haskell -- notSameId i newId i = True sameId i j = i == j notSameId i j = not (sameId i j) startId = 1 instance IDs Integer where -- are casts need here? sameId (newId startId::Integer) 3 sameId (3::Integer) (4::Integer) notSameId (3::Integer) (newId (3::Integer)) This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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ANN: mecha-0.0.5
by Tom Hawkins 06 Jun '11

06 Jun '11

Mecha [1,2,3] is a constructive solid modeling DSL. I haven't worked on Mecha in awhile, so this release just contains some minor cleanup and reorganization. My next step is to build an OpenGL interactive visualization tool, something like a basic CAD window to rotate and zoom around 3D models. What is the easiest way to generate polygon meshes from constructive solid geometry? Marching cubes [4] seems pretty involved. -Tom [1] http://hackage.haskell.org/package/mecha [2] https://github.com/tomahawkins/mecha [3] http://tomahawkins.org/ [4] http://en.wikipedia.org/wiki/Marching_cubes

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lambda abstraction
by Patrick Browne 06 Jun '11

06 Jun '11

Are the following two functions equivalent? (i.e. do they describe the same computation) let add1 a = a + 2 let add2 = \ a -> a + 2 :t add1 add1 :: forall a. (Num a) => a -> a :t add2 add2 :: forall a. (Num a) => a -> a Does Haskell interpreter convert all functions in the form of add1 to the lambda form of add2? Does this lambda form represent the operational semantics of Haskell? How should I translate the type information into English? Thanks Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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OK! I have a Mac with Snow Leopard 10.6.7?, Xcode 3.2.6, Haskell Platform 2011.2.0.1; What are 2or 3 ways so far to get a GUI & graphics?
by KC 06 Jun '11

06 Jun '11

-- -- Regards, KC

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I'm trying to install the Haskell Platform on a Mac and ...
by KC 05 Jun '11

05 Jun '11

I'm trying to install the Haskell Platform on a Mac and I've already installed Xcode 3.2.6 but the platform says that the tools are not installed. Do I need to reboot? -- -- Regards, KC

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Subcategories on Hackage
by Tillmann Vogt 05 Jun '11

05 Jun '11

Hi, There are some categories on Hackage that have become so large that it is hard to find something, i.e. Data(414 packages) and Graphics (191). Thats why I suggest to use subcategories separated from the category with a dot. To show that this makes sense I made subcategories for graphics libraries at the end of this email. Whatever happens to hackage2 this would be an immediate improvement. How ------ I would volunteer for the manual categorization and let the community look over it. I could upload the changes with a script but the version number has to increase even if only the category has changed. I also don't want to be responsible for a massive spike in the upload statistics. Shouldn't the cabal file be excluded from the versioning policy? => It is allowed to upload a library with the same version number if only the cabal file has changed. One should write a notifiaction mail to all owners to reply if they don't agree. Then after a week executing the script that applies the changes. Categories for Graphics: Graphics.2d bacteria program: braindead utility to compose Xinerama backgrounds barchart library and program: Creating Bar Charts in Haskell chalkboard library and programs: Combinators for building and processing 2D images. chalkboard-viewer library: OpenGL based viewer for chalkboard rendered images. Chart library: A library for generating 2D Charts and Plots dia-base library: An EDSL for teaching Haskell with diagrams - data types dia-functions library: An EDSL for teaching Haskell with diagrams - functions diagrams library: Embedded domain-specific language for declarative vector graphics diagrams-cairo library: Cairo backend for diagrams drawing EDSL diagrams-core library: Core libraries for diagrams EDSL diagrams-lib library: Embedded domain-specific language for declarative graphics explore program: Experimental Plot data Reconstructor funcmp library: Functional MetaPost gloss library: Painless 2D vector graphics, animations and simulations. gloss-examples programs: Examples using the gloss library GoogleChart library: Generate web-based charts using the Google Chart API graphics-drawingcombinators library: A functional interface to 2D drawing in OpenGL haha library and program: A simple library for creating animated ascii art on ANSI terminals. HDRUtils library: Utilities for reading, manipulating, and writing HDR images hevolisa program: Genetic Mona Lisa problem in Haskell hevolisa-dph program: Genetic Mona Lisa problem in Haskell - using Data Parallel Haskell Hieroglyph library: Purely functional 2D graphics for visualization. HPlot library and program: A minimal monadic PLplot interface for Haskell hs-captcha library: Generate images suitable for use as CAPTCHAs in online web-form security. hs-gchart library: Haskell wrapper for the Google Chart API hsparklines library: Sparklines for Haskell internetmarke program: Shell command for constructing custom stamps for German Post plot library: A plotting library, exportable as eps/pdf/svg/png or renderable with gtk plot-gtk library: GTK plots and interaction with GHCi printxosd program: Simple tool to display some text on an on-screen display scaleimage program: Scale an image to a new geometry testpattern program: Display a monitor test pattern wumpus-basic library: Basic objects and system code built on Wumpus-Core. wumpus-core library: Pure Haskell PostScript and SVG generation. wumpus-drawing library: High-level drawing objects built on Wumpus-Basic. wumpus-microprint library: Microprints - "greek-text" pictures. wumpus-tree library: Drawing trees zsh-battery program: Ascii bars representing battery status Graphics.3d Attrac program: Visualisation of Strange Attractors in 3-Dimensions cal3d library: Haskell binding to the Cal3D animation library. cal3d-examples programs: Examples for the Cal3d animation library. cal3d-opengl library: OpenGL rendering for the Cal3D animation library FieldTrip library: Functional 3D gnuplot library and program: 2D and 3D plots using gnuplot HGL library: A simple graphics library based on X11 or Win32 hgl-example program: Various animations generated using HGL IcoGrid library: Library for generating grids of hexagons and pentagons mapped to a sphere. nymphaea program: An interactive GUI for manipulating L-systems reactive-fieldtrip library: Connect Reactive and FieldTrip reactive-glut library: Connects Reactive and GLUT Graphics.Fractal fractal program: Draw Newton, Julia and Mandelbrot fractals gmndl program: Mandelbrot Set explorer using GTK hfractal program: OpenGL fractal renderer mandulia program: A zooming visualisation of the Mandelbrot Set as many Julia Sets. Graphics.Raytracing glintcollider program: A simple ray tracer in an early stage of development. glome-hs library and program: ray tracer GlomeTrace library: Ray Tracing Library GlomeVec library: Simple 3D vector library HRay program: Haskell raytracer jort program: JP's own ray tracer Graphics.Layout graph-rewriting-layout library: Force-directed node placement intended for incremental graph drawing pango library: Binding to the Pango text rendering engine. Graphics.Algorithm triangulation library: triangulation of polygons splines library: B-Splines, other splines, and NURBS. Graphics.RasterFormats AC-PPM library: Trivial package for writing PPM images. bitmap library: A library for handling and manipulating bitmaps. bitmap-opengl library: OpenGL support for Data.Bitmap. exif library: A Haskell binding to a subset of libexif fig library: Manipulation of FIG files graphicsFormats library: Classes for renderable objects. hsmagick library: FFI bindings for the GraphicsMagick library Imlib library jpeg library: A library for decoding JPEG files written in pure Haskell pgm library: Pure Haskell implementation of PGM image format SDL-image library: Binding to libSDL_image stb-image library: A wrapper around Sean Barrett's JPEG/PNG decoder tga library: Reading and writing of tga image files Graphics.VectorFormats cairo library: Binding to the Cairo library. craftwerk library and programs: 2D graphics library with integrated TikZ output. craftwerk-cairo library: Cairo backend for Craftwerk. craftwerk-gtk library and program: Gtk UI for Craftwerk. HPDF library: Generation of PDF documents hps library: Haskell Postscript hps-cairo library: Cairo rendering for the haskell postscript library HsHaruPDF library: Haskell binding to libharu (http://libharu.sourceforge.net/) poppler library: Binding to the Poppler. svgcairo library: Binding to the libsvg-cairo library. SVGPath library: Parsing the path command from SVG svgutils library and program: Helper functions for dealing with SVG files Graphics.3dFormats collada-output library and program: Generate animated 3d objects in COLLADA collada-types library: data exchange between graphic applications graphics-formats-collada library: Load 3D geometry in the COLLADA format obj library: Reads and writes obj models. Graphics.Engine (OpenGL and libraries that make it easier to use OpenGL) GLURaw library: A raw binding for the OpenGL graphics system GLUT library: A binding for the OpenGL Utility Toolkit GLFW library: A Haskell binding for GLFW GLFW-b library: GLFW bindings GLFW-b-demo program: GLFW-b test\/example\/demo GLFW-OGL library: A binding for GLFW (OGL) GPipe library: A functional graphics API for programmable GPUs GPipe-Collada library: Load GPipe meshes from Collada files GPipe-Examples programs: Examples for the GPipes package GPipe-TextureLoad library: Load GPipe textures from filesystem hogre library: Haskell binding to a subset of OGRE hogre-examples programs: Examples for using Hogre. haskgame library: Haskell game library. IrrHaskell library: Haskell FRP binding to the Irrlicht game engine. lambdacube-bullet program: Example for combining LambdaCube and Bullet lambdacube-engine library: 3D rendering engine entirely written in Haskell lambdacube-examples programs: Examples for LambdaCube nehe-tuts programs: Port of the NeHe OpenGL tutorials to Haskell. OGL library: A context aware binding for the OpenGL graphics system OpenGL library: A binding for the OpenGL graphics system OpenGLCheck library: Quickcheck instances for various data structures. OpenGLRaw library: A raw binding for the OpenGL graphics system OpenGLRaw21 library: The intersection of OpenGL 2.1 and OpenGL 3.1 Core rsagl library: The RogueStar Animation and Graphics Library scenegraph library: Scene Graph Graphics.Fonts font-opengl-basic4x6 library and program: Basic4x6 font for OpenGL freetype2 library and program: Haskell binding for FreeType 2 library FTGL library: Portable TrueType font rendering for OpenGL using the Freetype2 library SDL-ttf library: Binding to libSDL_ttf SFont library: SFont SDL Bitmap Fonts stb-truetype library: A wrapper around Sean Barrett's TrueType rasterizer library. SVGFonts library and program: Fonts from the SVG-Font format Graphics.OSspecific GLHUI library: Open OpenGL context windows in X11 with libX11 ige-mac-integration library: Bindings for the Gtk/OS X integration library. minimung program: Shows how to run grabber on Mac OS X Win32 library: A binding to part of the Win32 library X11 library: A binding to the X11 graphics library X11-extras library: Missing bindings to the X11 graphics library X11-rm library: A binding to the resource management functions missing from X11. X11-xdamage library: A binding to the Xdamage X11 extension library X11-xfixes library: A binding to the Xfixes X11 extension library X11-xft library: Bindings to the Xft, X Free Type interface library, and some Xrender parts x11-xim library: A binding to the xim of X11 graphics library X11-xshape library: A binding to the Xshape X11 extension library Xauth library: A binding to the X11 authentication library xhb library: X Haskell Bindings xosd library: A binding to the X on-screen display Graphics.UserInterfaces glade library: Binding to the glade library. gtk library: Binding to the Gtk+ graphical user interface library. gtk-serialized-event library: GTK+ Serialized event. gtkglext library: Binding to the GTK+ OpenGL Extension gtkimageview library: Binding to the GtkImageView library. gtksourceview2 library: Binding to the GtkSourceView library. lui library: Purely FunctionaL User Interface soegtk library: GUI functions as used in the book "The Haskell School of Expression". Graphics.Vector AC-Vector library: Efficient geometric vectors and transformations. AC-Vector-Fancy library: Fancy type-system stuff for AC-Vector Vec-OpenGLRaw library: Instances and functions to interoperate Vec and OpenGL. Vec-Transform library: Extends the Vec package with some 4x4 transform matrices vect library: A low-dimensional linear algebra library, tailored to computer graphics. vect-opengl library: OpenGL support for the `vect' low-dimensional linear algebra library. Graphics.Visualization graphviz library and program: Graphviz bindings for Haskell. hubigraph library: A haskell wrap for Ubigraph splot program: A tool for visualizing the lifecycle of many concurrent multi-staged processes. timeplot program: A tool for visualizing time series from log files. Games HTicTacToe program: An SDL tic-tac-toe game. Graphics (not subcategorized yet) AC-Colour library: Efficient RGB colour types. AC-EasyRaster-GTK library: GTK+ pixel plotting. assimp library: The Assimp asset import library Chitra program: A platform independent mechanism to render graphics using vnc. colour library: A model for human colour/color perception CV library: OpenCV based machine vision library cv-combinators library and programs: Functional Combinators for Computer Vision derivation-trees library: Typeset Derivation Trees via MetaPost gd library: A Haskell binding to a subset of the GD graphics library ghci-diagrams library: Display simple diagrams from ghci graph-rewriting-gl library: OpenGL interface for interactive port graph rewriting HOpenCV library and program: A binding for the OpenCV computer vision library IFS library and program: Iterated Function System generation for Haskell Noise library: A Haskell coherent noise generator based on libnoise ObjectIO library OpenCLRaw library: The OpenCL Standard for heterogenous data-parallel computing OpenVG library: OpenVG (ShivaVG-0.2.1) binding OpenVGRaw library: Raw binding to OpenVG (ShivaVG-0.2.1 implementation). probability library: Probabilistic Functional Programming SGdemo program: An example of using the SG and OpenGL libraries SDL library: Binding to libSDL SDL-gfx library: Binding to libSDL_gfx SDL-mixer library: Binding to libSDL_mixer SDL-mpeg library: Binding to the SMPEG library smallpt-hs program: A Haskell port of the smallpt path tracer. Sprig library: Binding to Sprig treemap-html library: Generates HTML for Data.Tree as TreeMap treemap-html-tools library and programs: Treemap related commands for producing foldable TreeMap HTML. vte library: Binding to the VTE library. webkit library: Binding to the Webkit library. wordcloud library: Library for generating word/tag clouds.

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Re: [Haskell-cafe] ANN: dtd-types 0.3.0.1
by Yitzchak Gale 05 Jun '11

05 Jun '11

The dtd-types[1] package provides types for processing XML DTDs in Haskell. These types are intended to be compatible with and extend the set of types provided by John Millikin's xml-types package[2]. This version, 0.3.0.1, was released in support of the dtd-text package[3]. It includes some major changes to some of the names, as well as some bug fixes. In particular, all names relating to XML attributes which used to begin with the prefix "Attr" or "attr" have been changed to begin with "att" or "Att" instead. The "EntityContent" type has been renamed to "EntityValue". These are much more consistent with nomenclature in the XML specs. At this point, the dtd-types package should start becoming a bit more stable, since it is now being used by other packages. Enjoy, Yitz [1] http://hackage.haskell.org/package/dtd-types/ [2] http://hackage.haskell.org/package/xml-types/ [3] http://hackage.haskell.org/package/dtd-text/

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Fwd: Abnormal behaviors when Using ghci
by 吴兴博 05 Jun '11

05 Jun '11

---------- Forwarded message ---------- From: 吴兴博 <wuxb45(a)gmail.com> Date: 2011/6/5 Subject: Abnormal behaviors when Using ghci To: cvs-ghc(a)haskell.org 1) I'm using Haskell platform 2011.2 on windows (7). Every several days, ghci will crash with no messages. even when I'm just typing with text buffer, without an 'enter'. I got nothing after the crash, not even an exception code, don't even mention the core-dump. Should I report bugs with nothing to give, only "it crashes."? I think I should be careful with sending messages. 2) do that: open ghci. > :set -fbreak-on-exception > :trace [undefined] then you catch a _exception > :print _exception Then it prints a chaos... that's because the printing of the exception contains too many white-spaces because of indent. I think this can be some kind of malformed features. since It isn't a bug, what 'type' should the problem be? :-) regards! -- ---------------- 吴兴博 Wu Xingbo

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Re: [Haskell-cafe] [iteratee] how to do nothing .. properly
by John Lato 05 Jun '11

05 Jun '11

Hi Sergey, I can't explain this; maybe it's a bug in enumWith? I'll look into it. Thanks, John > Message: 20 Date: Thu, 2 Jun 2011 02:46:32 +0400 > From: Sergey Mironov <ierton(a)gmail.com> > Subject: [Haskell-cafe] [iteratee] how to do nothing .. properly > To: haskell-cafe(a)haskell.org > Message-ID: <BANLkTimMFRWgH9Nopt-eua+L7jQcGq+u=g(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > Hi. Would anybody explain a situation with iter6 and iter7 below? > Strange thing - first one consumes no intput, while second consumes it > all, while all the difference is peek which should do no processing > (just copy next item in stream and return to user). > What I am trying to do - is to write an iteratee consuing no input, > but returning a constant I give to it. I thought (return a) should do > it, but it seems I was wrong as return actually consumes all unparsed > stream. iter6 experience tells me that (peek>>return a) is what I > need, but it's completely confusing and not what I expected. > > Thanks, > Sergey > > import Data.Iteratee as I > import Data.Iteratee.IO > import Control.Monad > import Control.Exception > import Data.ByteString > import Data.Char > import Data.String > > -- countBytes :: (..., Num b) => Iteratee s m a -> Iteratee s m (a, b) > countBytes i = enumWith i I.length > > iter6 = do > h <- countBytes $ (peek >> return 0) > s <- I.stream2list > return (h,s) > > iter7 = do > h <- countBytes $ (return 0) > s <- I.stream2list > return (h,s) > > print6 = enumPure1Chunk [1..10] (iter6) >>= run >>= print > print7 = enumPure1Chunk [1..10] (iter7) >>= run >>= print > > > Here is example ghci session > > *Main> print6 > ((0,0),[1,2,3,4,5,6,7,8,9,10]) > -- read 0 items, returns 0 > *Main> print7 > ((0,10),[]) > -- read 10 items (???) returns 0 > *Main> > > > > ------------------------------ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > End of Haskell-Cafe Digest, Vol 94, Issue 3 > ******************************************* >

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Fwd: [GeekUp] Functional Programming Night at GeekUp Liverpool May
by Hakim Cassimally 05 Jun '11

05 Jun '11

If anyone's in Northwest UK on Tuesday 31st May, why not come to Geekup's first ever "Functional Programming Night"? http://lanyrd.com/2011/geekup-liverpool-may/ It'll mostly consist of 3 short talks on Haskell, Lisp, and Clojure: * Haskell in the Real World (Hakim @osfameron) * Implenting a Lisp interpreter in .Net (Simon Johnson) * Clojure: why you should be interested in a 50 year old language (Tom Mortimer-Jones @morty_uk) There will also be beer! The talks are likely to be more "general interest" than assuming deep knowledge of FP, but it would be great to meet anyone around UK NW to gauge interest in more in-depth sessions sometime in the future! Hakim ---------- Forwarded message ---------- From: John McKerrell <mcknut(a)gmail.com> Date: 6 May 2011 15:10 Subject: [GeekUp] Functional Programming Night at GeekUp Liverpool May To: geekup(a)googlegroups.com Just wanted to announce May's GeekUp properly. We'll be having a Functional Programming Extravaganza with three (count em', 3!) talks from Hakim Cassimally, Simon Johnson and Tom Mortimer-Jones. Meeting in 3345 on Parr Street as ever, talks starting at 7pm but turn up from 6:30 and we'll be chatting over beer in the bar. Turn-out has been great for the recent events but we're always happy to welcome more people, there's more information on lanyrd: http://lanyrd.com/2011/geekup-liverpool-may/ Let us know you're coming by signing up on that link but feel free to turn up anyway, attendance is free and open to all comers of course. John -- http://geekup.org/ | http://geekup.org/wiki/ | http://jobboard.geekup.org/ To post e-mail: geekup(a)googlegroups.com Or go online: http://groups.google.com/group/geekup/ To unsubscribe e-mail: geekup+unsubscribe(a)googlegroups.com

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hmatrix-repa: hmatrix and Repa interoperability
by Alexander McPhail 05 Jun '11

05 Jun '11

Hi, I have uploaded a module to hackage in the package hmatrix-repa. It provides conversion functions between hmatrix vectors/matrices and repa arrays. I don't know whether it will be of much use, but even the Repa documentation suggests using LAPACK for performance critical tasks, such as matrix multiplication. These are reference implementations and might benefit from some optimisations. Cheers, VIvian DISCLAIMER This transmission contains information that may be confidential. It is intended for the named addressee only. Unless you are the named addressee you may not copy or use it or disclose it to anyone else.

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*GROUP HUG*
by Gregory Crosswhite 05 Jun '11

05 Jun '11

Hey everyone, Okay, this will sound silly, but I ventured into the Scala mailing list recently and asked an ignorant question on it, and I was shocked when people reacted not by enlightening me but by jumping on me and reacting with hostility. I bring this up not to badmouth the Scala community (they are apparently going through growing pains and will hopefully mature with time!) but just because it made me appreciate just how awesome you guys are, so I just feel the need to publicly express my admiration and thank to everyone on this list for having fostered such an incredibly professional, fanatically nonhostile, and generally pleasant place to talk about Haskell!!! *GROUP HUG* Okay, I'm done now. :-) Cheers, Greg

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ANN: unordered-container 0.1.3.0
by Johan Tibell 04 Jun '11

04 Jun '11

Hi all, I've just uploaded a new version of the unordered-containers package, a package of fast hashing-based container types. Version 0.1.3.0 [1] adds: * the ability to take the union of two maps, * lower memory overhead per key/value pair (contributed by Jan-Willem Maessen), and * the beginning of a `Data.HashSet` module (contributed by Bryan O'Sullivan). If you want to contribute, you can get the source from the git repository: git clone https://github.com/tibbe/unordered-containers.git Alternatively, just fork the project on GitHub [2]. 1. http://http://hackage.haskell.org/package/unordered-containers-0.1.3.0 2. https://github.com/tibbe/unordered-containers

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Problem with linker
by Alejandro Serrano Mena 04 Jun '11

04 Jun '11

Hi, I'm trying to use Hoogle as a library in my Summer of Code project. But when I try to compile using Cabal, I get the following error: Linking dist/build/scion-browser/scion-browser ... /home/serras/.cabal/lib/hoogle-4.2.4/ghc-7.0.2/libHShoogle-4.2.4.a(Serialise.o): In function `s1ePR_info': (.text+0x2015): undefined reference to `hooglezm4zi2zi4_Pathszuhoogle_version1_closure' /home/serras/.cabal/lib/hoogle-4.2.4/ghc-7.0.2/libHShoogle-4.2.4.a(Serialise.o): In function `s1f8a_info': (.text+0x338f): undefined reference to `__stginit_hooglezm4zi2zi4_Pathszuhoogle_' I've been looking for some information and it seems to be related with the fact that Hoogle first builds a library and then build an executable using the library, but all files get recompiled in that second pass. Is there any way to workaround this problem?

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haskellwiki slow/unresponsive
by Greg Weber 04 Jun '11

04 Jun '11

I have been trying to make a few edits to the haskell wiki and find it an excruciating process when I press save and then have to wait a long time to see if the save will go through. I just clicked on the introduction page and it may have took an entire minute to load. Can we put at the top: this wiki written in php, not haskell! Seriously though, the wiki has performed poorly for a long time now. It gives a bad impression to newcomers and deters contributions. Can I (and others!) donate money so someone can make the wiki responsive? Thanks, Greg Weber

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How on Earth Do You Reason about Space?
by Aleksandar Dimitrov 03 Jun '11

03 Jun '11

Dear Cafe, (Excuse the probably very ranty email; I am, unfortunately, at the end of my wits, and I hope that as fellow programmers, you will understand that this is among the most dreadful situations for our kind to be in.) Say, we have an input file that contains a word per line. I want to find all unigrams (unique words) in that file, and associate with them the amount of times they occurred in the file. This would allow me, for example, to make a list of word frequencies in a given text. Simple enough task. Here's an implementation using iteratees (lazy IO is evil) and unordered-containers' Data.HashMap.Strict, which enforces WHNF in values and keys: > import qualified Data.ByteString.Char8 as S > import qualified Data.Iteratee as I > import Data.Iteratee.IO > > import qualified Data.HashMap.Strict as T > > import Data.Iteratee.Char > import Data.List (foldl') > import Data.Char (toLower) > > import Data.Ord (comparing) > import Data.List (sortBy) > import System.Environment (getArgs) > > type Wordcounts = T.HashMap S.ByteString Int > > f' :: Monad m => I.Iteratee S.ByteString m Wordcounts > f' = I.joinI $ enumLinesBS (I.liftI $ step T.empty) > where step t (I.Chunk str) = I.liftI (step $! foldl' maybeIncrement t str) > step t stream = I.idone t stream > maybeIncrement t s > | s == S.empty = t > | otherwise = {-# SCC "m-I-other" #-} T.insertWith (+) s 1 t > > main :: IO () > main = getArgs >>= fileDriverVBuf 65536 f'.head >>= print.prettyList > where prettyList = -- sortBy (comparing snd) . T.toList > T.keys Some lines are empty, and I don't want them to be recorded, so that's why maybeIncrement is necessary. hpaste of this code: http://hpaste.org/47300/spaceleak (ignore convert, that's yet another issue.) Now, here's some observations: on a 75M input file (minuscule, compared to what I actually need) this program will eat 30M of heap space (says profiling) and return in 14 secs. I have two problems with that: a) that's too much heap space, b) the actual memory residency is *much* worse. ad b) average memory residency is at 38MB (this is OK, given heap consumption) but max residency is at 130MB, which is unacceptable to me (remember that I need to run this on files *much* bigger than just 75M.) <<ghc: 9013546200 bytes, 16876 GCs, 38232524/130572448 avg/max bytes residency (58 samples), 306M in use, 0.00 INIT (0.00 elapsed), 6.12 MUT (6.28 elapsed), 7.45 GC (7.50 elapsed) :ghc>> In fact, htop reports total memory residency of the program at around 320 MB at the end of its life-cycle (compiled and ran without profiling options.) I tried running this program on a 1.4GB file, and had to kill it when at 3.5GB memory consumption, my computer started paging. The actual hash-map, however, shouldn't be much bigger than in the 75MB case (I expect about twice the size,) since the input is natural language. I redirected the output of the program (showing a list of assoc pairs that were in the hash map) to a file, and that file measured 11MB in the case of a 75MB corpus, and 40MB when I ran the program on a 1.4GB corpus (I had to use a machine with 24GB of RAM to be able to run this.) ad a) heap consumption is too high for two reasons: firstly, the actual data I care about is much less than there's data on the heap. Secondly, about half the heap space is in LAG state. Here are profiles that will illustrate this: http://imgur.com/wBWmJ&XN1mW&ltYNR. - The first image shows 50% of the heap space being gobbled up with data that shouldn't be there anymore (LAG) - The second image shows the types that are in LAG state: ByteString and HashMap. So, it seems I'm keeping around hash maps? - The third image blames the cost centre m-i-other for the LAG. - Then there's the retainer profile: I have no idea how to read it. I thought it had something to do with iteratee reading in chunks of data, but after setting the chunk size to 4096 bytes and actually getting *less* spikes than before, I doubt that really is the case. It seems like half of the data this program puts on the heap is not cleaned up properly, and the otherwise-branch of maybeIncrement (i.e. the only really important line is this program) is to blame. I'm thinking there's too little strictness somewhere in the T.insertWith clause, but I have no idea how to make it better. Honestly, sprinkling around magic seq-dust every time something goes wrong doesn't feel good. I cannot reason about Haskell's space-performance even a little bit, and I have no idea how to change that. Some more remarks: - I tried using lazy IO first, but that was a total disaster. Iteratee is nice, and works very well, but maybe I don't know how to use it? - In terms of map-implementation, I observed that unordered-containers < Data.Map < bytestring-trie. It surprised me to see that Data.Map would perform better than bytestring-trie for this task (usually, Data.Map would be around twice as fast.) - I should probably look into using a mutable data structure as a map. I would find it a bit disenchanting to have to use a mutable data structure this way, but maybe that's just reality for you. RANT I have tried and tried again to avoid writing programs in Haskell that would leak space like BP likes to leak oil. However, I have yet to produce a single instance of a program that would do anything at all and at the same time consume less memory than there is actual data in the input file. It is very disconcerting to me that I seem to be unable, even after quite some practice, to identify space leaks in trivial programs like the above. I know of no good resource to educate myself in that regard. I have read the GHC manual, RWH's chapter on profiling, also "Inside T5"'s recent series on the Haskell heap, but no dice. Even if I can clearly see the exact line where at least some of the leaking happens (as I can in this case,) it seems impossible for me to prevent it. *thank you very much* for reading this far. This is probably a mostly useless email anyhow, I just had to get it off my chest. Maybe, just maybe, someone among you will have a crucial insight that will save Haskell for me :-) But currently, I see no justification to not start my next project in Lua, Python or Java. Sure, Haskell's code is pretty, and it's fun, but if I can't actually *run* it, why bother? (Yes, this isn't the first time I've ran into this problem …) Thanks, and kindest regards to you all, Aleks

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[JOB] NYU's Center for Genomics and Systems Biology seeks Functional Programmer
by Ashish Agarwal 03 Jun '11

03 Jun '11

<Relevance to this list: The posting below is for an OCaml programmer but welcomes applications from Haskell programmers.> Dear Functional Programmers, I am pleased to announce a job opportunity at The Center for Genomics and Systems Biology (CGSB) at New York University (NYU), located in the heart of Manhattan. The position's main function will be to develop software in the OCaml language to manage, analyze, and display the vast amounts of data generated by next-generation sequencing technologies. NYU's strong commitment to this field is represented by its $100M investment in the brand new CGSB building, which houses the latest sequencing platforms and excellent high performance computing facilities. The position will support the computational needs of several experimental labs by designing and building: o A database for tracking samples, very large quantities of raw data, and complex analysis results o A website for users to submit new samples, monitor progress of their workflow, and visualize data o A system for distributing batch jobs to a cluster, accounting for dependencies between jobs and cached results All components are expected to follow good functional programming design. There are no formal experience or education requirements. Although the software will be written in OCaml, we welcome applicants with experience in other functional languages, especially Haskell, F#, and SML. There is flexibility in the exact nature of the position; you may expand your work to machine learning, parallel programming, or any of the many topics relevant to bioinformatics, so long as the core requirements of the position are fulfilled. Experience in the following areas is a plus but not required: bioinformatics, statistics, type theory, distributed computing, and UNIX systems administration. NYU researchers are using sequencing technologies to investigate basic questions about the nature of life and to address fundamental problems in human health. The very large datasets generated by these technologies pose significant computational challenges for which the robust principles of functional programming are ideally suited. I hope you will find this an exciting opportunity. If you are interested, please contact me with a CV and brief description of your background. Thank you. __________________________________________ Ashish Agarwal New York University http://ashishagarwal.org

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License of hslogger, HDBC, etc.
by John Goerzen 03 Jun '11

03 Jun '11

Hi Jon & all, I've decided that I'm OK with re-licensing hslogger, HDBC, and well all of my Haskell libraries (not end programs) under 3-clause BSD. My schedule is extremely tight right now but if someone wants to send me patches for these things I will try to apply them within the week. -- John

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Re: [Haskell-cafe] Status of Haskell + Mac + GUIs & graphics
by John Lato 03 Jun '11

03 Jun '11

> > Message: 17 > Date: Fri, 20 May 2011 15:59:51 -0700 > From: Evan Laforge <qdunkan(a)gmail.com> > Subject: Re: [Haskell-cafe] Status of Haskell + Mac + GUIs & graphics > To: Erik Hesselink <hesselink(a)gmail.com> > Cc: haskell-cafe(a)haskell.org > Message-ID: <BANLkTi=74mM6ORtU2e192jtoOT9g49fzTQ(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > > Note that it is supposed to be possible to build gtk2hs with gtk+osx, > > which will not use X11 but use the native OS X GUI. I've not been able > > to get this to work, but it's been a while since I tried. The Haskell > > wiki mentions it doesn't support Glade, but does support Cairo. If > > this were to work, gtk2hs would be a serious option as well. > > I've tried this 3 or 4 times, and failed every time. It's crazy > complicated. > I used to use this combination regularly. IME the difficulties are primarily with getting a working gtk+osx build. Once that was accomplished, gtk2hs was a straightforward install, provided you build from the src repo with the -quartz flag. Recently I've switched to using macports gtk2 with the quartz, no-x11 variant, which also uses native cocoa. This is much more reliable than gtk+osx. I don't know if it supports Glade though. My biggest problem with wx is that there's no support for building 64-bit wx applications on OS X. If that were fixed I might prefer it. John Lato

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Re: [Haskell-cafe] Names for pretty-printing combinators
by Ivan Lazar Miljenovic 02 Jun '11

02 Jun '11

(Changing the subject as it's going off-topic from the original email :p) On 25 May 2011 22:45, Simon Meier <iridcode(a)gmail.com> wrote: > 2011/5/25 Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com>: >> >> Also, by clashes with Applicative, are you referring to empty and <$> >> ? I'm not sure if a better name than "empty" can be found; as for >> <$>, maybe using pretty's notation of $$ and $+$ rather than <$> and >> <$$> ? > > What about 'emptyDoc'? It's a bit more of a mouthful, and getting a bit close to Hungarian Notation, isn't it? :p Also, empty is already used a fair amount in other libraries as it stands (admittedly, most of those are in modules that are typically imported qualified...). > Moreover, if you are changing the names of > combinators, then moving them away from Applicative and Arrow would be > a good idea; i.e., don't use <+>, as it already used by ArrowPlus. *sigh* We really need to get a list of used operators somewhere so that libraries can stake a claim :p > Moreover, if you can make a Monoid instance such that `mappend` equals > <>, you would also make the library compatible to a future > introduction of (<>) = mappend. I was thinking about that actually... -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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ANN: fix-imports-0.1.0
by Evan Laforge 02 Jun '11

02 Jun '11

I just uploaded a new package called fix-imports. Maintaining the import block is one of the less fun things I do while writing haskell. So I wrote a little program to do that for me. Basically, I edit source, then hit ,a in vim, and it figures out which modules need to be imported, which imports are not needed, and sorts and formats the import list. I've been using it for the last week or so and it seems to be working ok, so I decided to upload it to hackage in case anyone else doesn't like maintaining import lists manually. The catch: it only works for qualified imports. Here's copy paste from the package description: A small standalone program to manage the import block of a haskell program. It will try to add import lines for qualified names with no corresponding import, remove unused import lines, and sort the import block according to some heuristic you can define. This only works for qualified imports! Unqualified imports are left untouched. It's most convenient if bound to an editor key.

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Oracle Sessions in Takusen
by Dmitry Olshansky 02 Jun '11

02 Jun '11

Hello, Could anyone explain strange behavior of Takusen with OracleDB (OraClient 11.x)? Several sequential sessions give "Seqmentation Fault" error. In case of nested sessions it works well. {-# LANGUAGE ScopedTypeVariables #-} module Main where import Database.Oracle.Enumerator import Control.Monad(replicateM) import Control.Monad.Trans(liftIO) main = do {- -- This gives an Segmentation Fault for the second session replicateM 2 (do res <- withSession (connect "x" "x" "x") (do doQuery (sql "SELECT dummy FROM dual") (\(d::String) (_::Maybe String) -> result' $ Just d) Nothing ) print res ) -} -- This is works well withSession (connect "x" "x" "x") (do r1 <- doQuery (sql "SELECT dummy FROM dual") (\(d::String) (_::Maybe String) -> result' $ Just d) Nothing liftIO $ print r1 liftIO $ withSession (connect "x" "x" "x") (do r2 <- doQuery (sql "SELECT dummy FROM dual") (\(d::String) (_::Maybe String) -> result' $ Just d) Nothing liftIO $ print r2 ) ) Best regards, Dmitry

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Haskell Weekly News: Issue 184
by Daniel Santa Cruz 02 Jun '11

02 Jun '11

Welcome to issue 184 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of May 22 to 28, 2011. Announcements The newsletter has not been posting new library announcements, but Ivan Lazar's announcement of his new wl-pprint-text library had to be an exception. Way to be creative Ivan! http://goo.gl/C6tu0 Simon Marlow wrote in to announce CamHac: (Haskell Hackathon in Cambridge, UK) during August 12 to 14, 2011. Registration is free, but they are limited to 50 people, so sign up soon! http://goo.gl/BySLB Quotes of the Week * Henning Thielemann: [On a slow solution to Euler's totien function] It's declarative and may help to verify more efficient implementations. * KC: Librarians have been struggling for years with classifying topics; I don't imagine classifying coding libraries as any easier. * max ulidtko: [on the *group hug* thread] Wow. I just subscribed to the list just an hour ago or so, and already receiving hugs! * Jacek Generowicz: What would be the point of asking non-ignorant questions, unless it is a rhethorical one? * Ketil Malde: Being brilliant doesn't mean the absence of mental bad hair days, but merely that they happen more rarely than for the rest of us. :-) * AndrewMorton: I was attacked by dselect as a small child and have since avoided Debian. * Cale: The hardest part of writing any binding is doing something to make up for the fact that libraries written for imperative languages are typically badly designed to begin with. * <lambdabot> *Exception: *Exception: *Exception: <Cale> E X C E P T I O N * Jafet: In strict languages, you implement Haskell, then use fix as usual. Top Reddit Stories * Multicore Garbage Collection with Local Heaps :: PDF Domain: community.haskell.org, Score: 45, Comments: 4 On Reddit: http://goo.gl/OcOqI Original: http://goo.gl/Zdkxv * Node.js creator first tried with haskell, but says he wasn't smart enough to hack GHC Domain: bostinnovation.com, Score: 41, Comments: 39 On Reddit: http://goo.gl/cDQWT Original: http://goo.gl/p8VpL * ANNOUNCEMENT: HaNS, the Haskell Network Stack. HaNS is a lightweight, pure Haskell network stack that can be used for Haskell networking in the context of the HaLVM, or with a Linux tap device. Domain: haskell.org, Score: 40, Comments: 12 On Reddit: http://goo.gl/hBlRB Original: http://goo.gl/H62UL * Haskell + FFI + Java + SWT: crazy, maybe, but working! Domain: jpmoresmau.blogspot.com, Score: 36, Comments: 17 On Reddit: http://goo.gl/9l45y Original: http://goo.gl/B6jO7 * Isomorphism lenses Domain: twanvl.nl, Score: 32, Comments: 10 On Reddit: http://goo.gl/rlBfs Original: http://goo.gl/BHrBh * Tail recursion makes your loops cleaner Domain: blog.ezyang.com, Score: 29, Comments: 14 On Reddit: http://goo.gl/7VD9g Original: http://goo.gl/YMqok * An insufficiently lazy map: the dangers of spine-strict data structures Domain: blog.ezyang.com, Score: 25, Comments: 0 On Reddit: http://goo.gl/DTBwg Original: http://goo.gl/dMEoI * From a 13M executable to an 84k one: use -dynamic with GHC for linking excellence Domain: stackoverflow.com, Score: 23, Comments: 13 On Reddit: http://goo.gl/tsN5H Original: http://goo.gl/poXbW * This is how a new package should be announced: Pretty-printing package for lazy Text values Domain: haskell.org, Score: 21, Comments: 3 On Reddit: http://goo.gl/iduqP Original: http://goo.gl/qyhrj * Happstack + JMacro + HSX Domain: happstack.blogspot.com, Score: 15, Comments: 2 On Reddit: http://goo.gl/33J1S Original: http://goo.gl/9mk8h Top StackOverflow Questions * Reading GHC Core votes: 27, answers: 4 Read on SO: http://goo.gl/wb6In * Small Haskell program compiled with GHC into huge binary votes: 26, answers: 2 Read on SO: http://goo.gl/poXbW * Force pre-computation of a constant votes: 8, answers: 1 Read on SO: http://goo.gl/Ri16N * Optimization of Function Calls in Haskell votes: 6, answers: 2 Read on SO: http://goo.gl/gPshb * How to make a CAF not a CAF in Haskell? votes: 6, answers: 6 Read on SO: http://goo.gl/KYTo7 * Concise if-then-else notation in do-blocks in Haskell votes: 6, answers: 4 Read on SO: http://goo.gl/dRlGY * Haskell (haskeline) word completion votes: 6, answers: 1 Read on SO: http://goo.gl/3TbFo * Why doesn't Haskell have symbols (a la ruby) / atoms (a la erlang)? votes: 6, answers: 5 Read on SO: http://goo.gl/4KxP8 * Why does Haskell appear to default to reading Int when reading Num? votes: 6, answers: 2 Read on SO: http://goo.gl/alCkY About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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[iteratee] how to do nothing .. properly
by Sergey Mironov 02 Jun '11

02 Jun '11

Hi. Would anybody explain a situation with iter6 and iter7 below? Strange thing - first one consumes no intput, while second consumes it all, while all the difference is peek which should do no processing (just copy next item in stream and return to user). What I am trying to do - is to write an iteratee consuing no input, but returning a constant I give to it. I thought (return a) should do it, but it seems I was wrong as return actually consumes all unparsed stream. iter6 experience tells me that (peek>>return a) is what I need, but it's completely confusing and not what I expected. Thanks, Sergey import Data.Iteratee as I import Data.Iteratee.IO import Control.Monad import Control.Exception import Data.ByteString import Data.Char import Data.String -- countBytes :: (..., Num b) => Iteratee s m a -> Iteratee s m (a, b) countBytes i = enumWith i I.length iter6 = do h <- countBytes $ (peek >> return 0) s <- I.stream2list return (h,s) iter7 = do h <- countBytes $ (return 0) s <- I.stream2list return (h,s) print6 = enumPure1Chunk [1..10] (iter6) >>= run >>= print print7 = enumPure1Chunk [1..10] (iter7) >>= run >>= print Here is example ghci session *Main> print6 ((0,0),[1,2,3,4,5,6,7,8,9,10]) -- read 0 items, returns 0 *Main> print7 ((0,10),[]) -- read 10 items (???) returns 0 *Main>

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Haskell School of Expression (graphics)
by michael rice 01 Jun '11

01 Jun '11

I installed the Haskell Platform (7.0.2) earlier, and I just downloaded package soegtk with cabal, apparently unsuccessfully. Any suggestions? Michael [michael@sabal ~]$ cabal updateConfig file path source is default config file.Config file /home/michael/.cabal/config not found.Writing default configuration to /home/michael/.cabal/configDownloading the latest package list from hackage.haskell.org[michael@sabal ~]$ cabal list SOE* soegtk Synopsis: GUI functions as used in the book "The Haskell School of Expression". Default available version: 0.11.1 Installed versions: [ Not installed ] Homepage: http://www.haskell.org/gtk2hs/ License: LGPL-2.1 [michael@sabal ~]$ cabal install soegtkResolving dependencies...Downloading cairo-0.11.1... /tmp/cairo-0.11.12906/cairo-0.11.1/Gtk2HsSetup.hs:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [-Wcpp][1 of 2] Compiling Gtk2HsSetup ( /tmp/cairo-0.11.12906/cairo-0.11.1/Gtk2HsSetup.hs, /tmp/cairo-0.11.12906/cairo-0.11.1/dist/setup/Gtk2HsSetup.o ) /tmp/cairo-0.11.12906/cairo-0.11.1/Gtk2HsSetup.hs:190:70: Couldn't match expected type `[PackageDB]' with actual type `PackageDB' Expected type: PackageDBStack Actual type: PackageDB In the sixth argument of `registerPackage', namely `packageDb' In the expression: registerPackage verbosity installedPkgInfo pkg lbi inplace packageDbDownloading glib-0.11.2... /tmp/glib-0.11.22906/glib-0.11.2/Gtk2HsSetup.hs:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [-Wcpp][1 of 2] Compiling Gtk2HsSetup ( /tmp/glib-0.11.22906/glib-0.11.2/Gtk2HsSetup.hs, /tmp/glib-0.11.22906/glib-0.11.2/dist/setup/Gtk2HsSetup.o ) /tmp/glib-0.11.22906/glib-0.11.2/Gtk2HsSetup.hs:190:70: Couldn't match expected type `[PackageDB]' with actual type `PackageDB' Expected type: PackageDBStack Actual type: PackageDB In the sixth argument of `registerPackage', namely `packageDb' In the expression: registerPackage verbosity installedPkgInfo pkg lbi inplace packageDbcabal: Error: some packages failed to install:cairo-0.11.1 failed during the configure step. The exception was:ExitFailure 1gio-0.11.1 depends on glib-0.11.2 which failed to install.glib-0.11.2 failed during the configure step. The exception was:ExitFailure 1gtk-0.11.2 depends on glib-0.11.2 which failed to install.pango-0.11.2 depends on glib-0.11.2 which failed to install.soegtk-0.11.1 depends on glib-0.11.2 which failed to install.[michael@sabal ~]$

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Re: [Haskell-cafe] How on Earth Do You Reason about Space?
by John Lato 01 Jun '11

01 Jun '11

From: "Edward Z. Yang" <ezyang(a)MIT.EDU> > > Hello Aleksandar, > > It is possible that the iteratees library is space leaking; I recall some > recent discussion to this effect. Your example seems simple enough that > you might recompile with a version of iteratees that has -auto-all enabled. > Unfortunately, it's not really a safe bet to assume your libraries are > leak free, and if you've pinpointed it down to a single line, and there > doesn't seem a way to squash the leak, I'd bet it's the library's fault. > > Edward > I can't reproduce the space leak here. I tried Aleksander's original code, my iteratee version, the Ngrams version posted by Johan Tibell, and a lazy bytestring version. my iteratee version (only f' has changed from Aleksander's code): f' :: Monad m => I.Iteratee S.ByteString m Wordcounts f' = I.joinI $ (enumLinesBS I.><> I.filter (not . S.null)) $ I.foldl' (\t s -> T.insertWith (+) s 1 t) T.empty my lazy bytestring version > import Data.Iteratee.Char > import Data.List (foldl')import Data.Char (toLower) > > import Data.Ord (comparing) > import Data.List (sortBy) > import System.Environment (getArgs) > import qualified Data.ByteString.Lazy.Char8 as L > import qualified Data.HashMap.Strict as T > > f'2 = foldl' (\t s -> T.insertWith (+) s 1 t) T.empty . filter (not . L.null) . L.lines > > main2 :: IO () > main2 = getArgs >>= L.readFile .head >>= print . T.keys . f'2 None of these leak space for me (all compiled with ghc-7.0.3 -O2). Performance was pretty comparable for every version, although Aleksander's original did seem to have a very small edge. As someone already pointed out, keep in mind that this will use a lot of memory anyway, unless there's a lot of repetition of words. I'd be happy to help track down a space leak in iteratee, but for now I'm not seeing one. Best, John Lato

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maybe a GHC bug but not sure
by Edward Amsden 01 Jun '11

01 Jun '11

When building the Haskell Objective C bindings tool from <http://code.google.com/p/hoc> Using GHC 7.0.3 for Haskell Platform 2011.2.0.1, OS X 64 bit HOC compiles HOC_cbits.o before trying to build it into the rest of the program (I don't say link because this came up while trying to compile: > [18 of 31] Compiling HOC.StdArgumentTypes ( HOC/HOC/StdArgumentTypes.hs, dist/build/HOC/StdArgumentTypes.o ) > [loading packages, like you'd see from ghci...] > Loading object (static) dist/build/HOC_cbits.o ... ghc: dist/build/HOC_cbits.o: Bad magic. Expected: feedfacf, got: feedface. GHC says "the impossible happened" and tells me to report it as a GHC bug, but based on the odd behavior I think the HOC build script might be doing something odd. I just don't have any idea where to start looking. For instance, why the heck is it looking at the magic bits for an object file while compiling a source file? Any insight appreciated. -- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com

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Re: [Haskell-cafe] How on Earth Do You Reason about Space?
by John Lato 01 Jun '11

01 Jun '11

> > From: Brandon Moore <brandon_m_moore(a)yahoo.com> > > > I was worried data sharing might mean your keys > retain entire 64K chunks of the input. However, it > seems enumLines depends on the StringLike ByteString > instance, which just converts to and from String. > That can't be efficient, but I suppose it avoids excessive sharing. That's true for 'enumLines', however the OP is using 'enumLinesBS', which operates on bytestrings directly. Data sharing certainly could be an issue here. I tried performing Data.ByteString.copy before inserting the key into the map, but that used more memory. I don't have an explanation for this; it's not what I would expect. The other parameter which affects sharing is the chunk size. I got a much better memory profile when using a chunksize of 1024 instead of 65536. Oddly enough, when using the large chunksize I saw lower memory usage from Data.Map, but with the small chunksize Data.HashMap has a significant advantage. John Lato

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ANN: dtd-types 0.2.0.0
by Yitzchak Gale 01 Jun '11

01 Jun '11

The dtd-types package provides types for processing XML DTDs in Haskell. These types are intended to be compatible with and extend the set of types provided by John Millikin's xml-types package. The consensus seems to be to leave this as a separate package and not to merge it with xml-types. This version fixes a number of issues with the preliminary version. Among those issues were: omission of processing instructions, pointed out by John Millikin, and Haddock problems pointed out by Michael Snoyman. Thanks! Hackage: http://hackage.haskell.org/package/dtd-types/ Darcs: http://code.haskell.org/dtd/dtd-types Enjoy, Yitz

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Re: [Haskell-cafe] ANNOUNCE: CamHac: Haskell Hackathon in Cambridge, UK, 12-14 August 2011
by Simon Marlow 01 Jun '11

01 Jun '11

On 26/05/2011 15:35, Simon Marlow wrote: > CamHac is happening - come and spend a long weekend in Cambridge hacking > Haskell code in great surroundings with fantastic company! > > Full details on the wiki page: > > http://www.haskell.org/haskellwiki/CamHac > > When: Friday-Sunday 12-14 August 2011 > Where: the Horobin Room of Homerton Conference Centre [1] > > Registration is free! See the wiki page above for details. We're limited > to 50 people, and registration is on a first-come first-served basis, so > sign up soon. We've reached 50 registrations already! Luckily the venue has a larger room available, so we've changed our booking and we can now fit 72 haskellers in. We're now in the Leah-Manning room: http://www.homertonconference.com/Leah-Manning.html Cheers, Simon

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Enabling GADTs breaks Rank2Types code compilation - Why?
by dm-list-haskell-cafe＠scs.stanford.edu 01 Jun '11

01 Jun '11

I'm using GHC 7.0.2 and running into a compiler error that I cannot understand. Can anyone shed light on the issue for me? The code does not make use of GADTs and compiles just fine without them. But when I add a {-# LANGUAGE GADTs #-} pragma, it fails to compile. Here is the code: ==== {-# LANGUAGE Rank2Types #-} {-# LANGUAGE GADTs #-} mkUnit :: (forall t. t -> m t) -> m () mkUnit f = f () data Const b a = Const { unConst :: b } sillyId :: r -> r sillyId r = unConst (mkUnit mkConst_r) -- This line can't compile with GADTs where mkConst_r t = mkConst r t mkConst :: b -> t -> Const b t mkConst b _ = Const b ==== And the error I get is: Couldn't match type `t0' with `t' because type variable `t' would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: t -> Const r t The following variables have types that mention t0 mkConst_r :: t0 -> Const r t0 (bound at /u/dm/hack/hs/gadt.hs:11:11) In the first argument of `mkUnit', namely `mkConst_r' In the first argument of `unConst', namely `(mkUnit mkConst_r)' In the expression: unConst (mkUnit mkConst_r) I've found several workarounds for the issue, but I'd like to understand what the error message means and why it is caused by GADTs. Thanks in advance for any help. David

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Lazy Evaluation in Monads
by Scott Lawrence 01 Jun '11

01 Jun '11

I was under the impression that operations performed in monads (in this case, the IO monad) were lazy. (Certainly, every time I make the opposite assumption, my code fails :P .) Which doesn't explain why the following code fails to terminate: iRecurse :: (Num a) => IO a iRecurse = do recurse <- iRecurse return 1 main = (putStrLn . show) =<< iRecurse Any pointers to a good explanation of when the IO monad is lazy? === The long story === I wrote a function unfold with type signature (([a] -> a) -> [a]), for generating a list in which each element can be calculated from all of the previous elements. unfold :: ([a] -> a) -> [a] unfold f = unfold1 f [] unfold1 :: ([a] -> a) -> [a] -> [a] unfold1 f l = f l : unfold1 f (f l : l) Now I'm attempting to do the same thing, except where f returns a monad. (My use case is when f randomly selects the next element, i.e. text generation from markov chains.) So I want unfoldM1 :: (Monad m) => ([a] -> m a) -> [a] -> m [a] My instinct, then, would be to do something like: unfoldM1 f l = do next <- f l rest <- unfoldM1 f (next : l) return (next : rest) But that, like iRecurse above, doesn't work.

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Decimal type-level arithmetic.
by Serguey Zefirov 31 May '11

31 May '11

I would like to present my version of type arithmetic with decimal encoding: http://thesz.mskhug.ru/svn/hhdl/TyleA.hs It is not worth Cabal package in its current state, but I hope it would be useful for someone. It is easy to use, just say Plus (D1 :. D2 :. D0) D8 to get a type of 128. Or you can say $(tySize 128) if you're not afraid to use Template Haskell. I tested it on my current project, it works quite well with numbers around 128..512, about 10-20 times faster than Peano numbers. As operations over this representation are not lazy (Peano numbers oeprations are lazy for one argument of Plus and for part of one argument of Max), it is not well suited for big number of operations done at once. I encountered it when I autogenerated a big arithmetic expression for big polymorphic data type. I think it is quite rare situation.

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Thoughts about currying and optimisations
by Yves Parès 31 May '11

31 May '11

Hello Café, An idea came to me: unless the compiler notices that stuffA and stuffB are equivalent, would it be correct to suppose that A is better than B? stuffA x = if someComputationallyExpensiveTest x then doSomething else doSomethingElse stuffB x y = if someComputationallyExpensiveTest x then doSomething y else doSomethingElse y I explain: in stuffA, the function only depends on x, so when doing: a = stuffA xxx runs the expensive test once and for all, and a can directly be bound to doSomething or doSomethingElse so calling after: a foo a bar won't run the test Whereas: b = stuffB xxx is valid due to curryfication, but: b foo b bar will both run the expensive test

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Can't figure out cmap in hmatrix
by Mats Klingberg 31 May '11

31 May '11

Hello, I'm playing around a bit with the hmatrix package (http://hackage.haskell.org/package/hmatrix) but can't quite figure out how to make the cmap function in Numeric.Container work. An example: ghci> import Numeric.LinearAlgebra ghci> let v = fromList [1.0,2.0,3.0] ghci> v fromList [1.0,2.0,3.0] :: Data.Vector.Storable.Vector ghci> :t v v :: Vector Double ghci> cmap sqrt v <interactive>:1:1: No instance for (Container Vector e0) arising from a use of `cmap' Possible fix: add an instance declaration for (Container Vector e0) In the expression: cmap sqrt v In an equation for `it': it = cmap sqrt v ghci> :t cmap cmap :: (Container c e, Element b, Element a) => (a -> b) -> c a -> c b There is an instance for (Container Vector Double) but I assume that since the signature of cmap doesn't mention the type variable 'e' GHCi can't infer it. Googling hasn't helped me so far, except for digging up another post to this list with the same (?) problem, but no answer: http://www.haskell.org/pipermail/haskell-cafe/2011-April/091390.html Is there a way to tell GHC what instance to use, or how should cmap be used? Thanks! Mats

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Sub class and model expansion
by Patrick Browne 31 May '11

31 May '11

Hi, I'm not sure if this is possible but I am trying to use Haskell’s type class to specify *model expansion*. Model expansion allows the specification of new symbols (known as enrichment) or the specification further properties that should hold on old symbols. I am trying to enrich simplified Monoids to Groups. The code below is not intended to do anything other than to specify simplified Groups as a subclass of Monoid. The main problem is that I cannot use ! on the LHS of equations in Group. Is it possible to expand the specification of Monoid to Group using Haskell type classes? Pat data M = M deriving Show -- Monoid with one operation (!) and identity e (no associativity) class Monoid a where (!) :: a -> a -> a e :: a a ! e = a -- A Group is a Monoid with an inverse, every Group is a Monoid -- (a necessary condition in logic: Group implies Monoid) -- The inverse property could be expressed as: -- 1) forAll a thereExists b such that a ! b = e -- 2) (inv a) ! a = e class Monoid a => Group a where inverse :: a -> a -- Haskell does not like ! on the LHS of equation for inverse in Group. -- (inverse a) ! a = e This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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ANN: quickcheck-properties
by Alexey Khudyakov 31 May '11

31 May '11

Hello It always puzzled me why there are no packages for for testing general type classes laws. (Monoid laws, monad laws etc). It looks like ideal case for quickcheck and smallcheck. I'm glad to present package quickcheck-properties[1]. It containt set of generic properties for semiroups, monoids, groups and unsatisfactory imlementatation of functor's properties. Despite its name package do not depend on quickcheck and could be used with smallcheck as well. ==================== Examples ==================== Here are few examples: >>> quickCheck $ eq $ prop_Monoid (T :: T [Int]) +++ OK, passed 100 tests. >>> quickCheck $ eq $ prop_Monoid (T :: T (Maybe [Int])) +++ OK, passed 100 tests. prop_Monoid tests all monoid properties: associativity, left and right identity. T is used to fix type of monoid to test. It could be done with type signature for prop_Monoid but writing signature for three parameter function is just too cumbersome. eq compares expression in properties using == operator. Many (most?) of properties have form > expression = another expression Easiest solution is to compare them with ==. But is's not satisfactory. Data type may not have Eq instance. Eq instance may provide structural equality but some form of equivalince is needed. etc. It's left to library user to decide how to compare values. In this contrived example lists are compared by length. > eqOn length $ prop_Monoid (T :: T [Int]) ==================== Problems ==================== There are unsolved problems. So it's announcement with question. Functor/Applicative/Monads laws involve arbitrary functions. It would be natural to generate them using quickcheck but I don't know how to generate random functions. Any suggestions? [1] http://hackage.haskell.org/package/quickcheck-properties -- Aleksey Khudyakov

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PhD studentship in Nottingham
by Graham Hutton 31 May '11

31 May '11

Dear all, I are currently advertising a PhD studentship in functional programming -- the advert is copied below, and attached. If you know of any good candidates who many be interested in this, or there is a local mailing list for advertising such things, I would be much obliged if you could pass on the advert. Best wishes, Graham +------------------------------------------------------------------+ PhD Studentship in Functional Programming School of Computer Science University of Nottingham, UK Applications are invited for a fully-funded PhD studentship in the Functional Programming Lab in the School of Computer Science at the University of Nottingham, starting on 1st October 2011, under the supervision of Prof Graham Hutton. The suggested topic for the studentship is to develop a practical implementation of the worker/wrapper transformation, a simple but powerful unifying paradigm for program optimisation, but other ideas for possible topics are welcome too. The studentship is for three and a half years, and includes a maintenance grant of 13,590 UK pounds per year, and tuition fees. Applicants for the position will require a first-class Honours degree (or equivalent) in Computer Science and/or Mathematics, some experience in functional programming, and an aptitude for mathematical subjects. Additional desirable attributes include a Masters degree, and/or some experience in programming language semantics, implementation and optimisation. Due to the nature of the funding, the position is only open to UK citizens, or EU citizens who have resided in the UK for the last three years. The successful applicant will work under the supervision of Prof Graham Hutton in the FP Lab in Nottingham, a leading centre for research on functional programming. The group currently comprises 5 academic staff, 1 research fellow, and 10 PhD students. In order to apply, please submit the following to Prof Graham Hutton (graham.hutton(a)nottingham.ac.uk) by 24th June 2011: an up-to-date copy of your CV (including the results of all your University examinations to date) along with a brief covering letter that describes your experience in functional programming, your reasons for wishing to pursue a PhD in this area, and any ideas you have regarding possible research directions. Closing date for applications: 24th June 2011. +------------------------------------------------------------------+ -- Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

1 0

What's the advantage of writing Haskell this way?
by John Ky 31 May '11

31 May '11

Hi all, I'm trying to learn about enumerators by reading this paper<https://john-millikin.com/downloads/enumerator_0.4.10.pdf>and came across some code on page 2 that I found hard to digest, but I think I finally got it: import Data.Monoid data Stream a = Chunks [a] | EOF deriving (Show, Eq) instance Monad Stream where return = Chunks . return Chunks xs >>= f = mconcat (fmap f xs) EOF >>= _ = EOF instance Monoid (Stream a) where mempty = Chunks mempty mappend (Chunks xs) (Chunks ys) = Chunks (xs ++ ys) mappend _ _ = EOF I guess, it shows my lack of experience in Haskell, but my question is, why is writing the code this way preferred over say writing it like this: import Data.Monoid data Stream a = Chunks [a] | EOF deriving (Show, Eq) instance Monad Stream where return x = Chunks [x] Chunks xs >>= f = mconcat (fmap f xs) EOF >>= _ = EOF instance Monoid (Stream a) where mempty = Chunks [] mappend (Chunks xs) (Chunks ys) = Chunks (xs ++ ys) mappend _ _ = EOF Cheers, -John

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Please add a method for optimized concat to the Semigroup class
by Yitzchak Gale 31 May '11

31 May '11

Hi Edward, Thanks much for the very useful semigroups package. When using it in practice, it would be very useful to have an analogue to the mconcat method of Monoid. It has the obvious default implementation, but allows for an optimized implementation for specific instances. That turns out to be something that comes up all the time (at least for me) in real life. Thanks, Yitz

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Erlang's module discussion
by Tom Murphy 31 May '11

31 May '11

Hi All, I sure love Hackage, but there's a very interesting discussion going on, on the Erlang mailing list, about completely restructuring the module-model. Before you dismiss it as crazy, know that the topic was brought up by Joe Armstrong, one of the creators of the language. Here's the archive: http://erlang.org/pipermail/erlang-questions/2011-May/058769.html Food for thought... Tom

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