January 2008 - Haskell-Cafe

QuickCheck properties: export or not?
by Paul Johnson 15 Jan '08

15 Jan '08

I was recently sent a patch for my Ranged Sets library which exported all the QuickCheck properties. I'd left them unexported on the grounds that they clutter up the documentation (although simplified versions are included in the documentation) and you can easily run them with the standard "quickcheck" script. The contributor, gwern0(a)gmail.com suggested an explicit test harness instead. I'm not unduly bothered one way or the other, but I thought I'd ask the community: what is the best practice here? Paul.

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Displaying # of reductions after each computation in ghci?
by Fernando Rodriguez 15 Jan '08

15 Jan '08

Hi, Is there a way to configure ghci to display the number of reuctions after each compution (as in winhugs)? Thanks!

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Getting frantic with FranTk
by Torsten Otto 14 Jan '08

14 Jan '08

Howdy, with a just-in-time-learning approach I managed to teach my class of advanced high schoolers the basics of functional programming using Haskell (I had only used Scheme before). Now to show them that Haskell is not a weirdo esoteric command line only language, I thought I'd end the class with spoken output (easy enough with the System module and "say" in Mac OS) and a GUI for input using FranTk. There is however something very basic I'm missing: I have Hugs installed at /opt/local/bin/hugs in MacOS 10.4/10.5. What do I do to get Hugs to recognize FranTk? I have downloaded it, but I can't even get the demo to work, none of the imports can be found: module Main where import FranTk import qualified StaticTypes as S import IOExts For Windows, there's a .bat to use, but I can't figure out how to get it to work on a Unix-system like Mac OS. I seems to be more than just to include the path when starting Hugs... Thanks in advance for any hints, regards, Torsten Otto

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Strange GLFW linker problem on Linux
by Peter Verswyvelen 14 Jan '08

14 Jan '08

Because apparently GHC works much better on Linux than Windows, I switched to that platform. I'm currently trying Fedora 8. So now I'm completely alone in the dark: no experience with Haskell, no experience with Linux; I left decades of experience with Microsoft products behind me ;-) I tried compiling GFLW (used by SOE) and a Haskell test app on my laptop, and that worked fine. Then I installed Fedora on my desktop, installed all development tools, and GLFW (both the C library and Haskell wrapper) also compiled and linked fine, but when trying to link the test app to GLFW, I got linker errors (see below). It seems these symbols are defined in libXrandr, but I have both the binary as the devel package installed: [root@localhost dev]# yum list installed | grep Xrandr libXrandr.i386 1.2.2-1.fc8 installed libXrandr-devel.i386 1.2.2-1.fc8 installed So I have no idea what to do. The main difference between the laptop and desktop is that I installed the NVidia drivers on the desktop manually (first installed the kernel-devel package, and then downloaded and installed the linux driver from the nvidia website) Any hints? Thanks, Peter Linker errors reported by ghc --make Main.hs: /usr/local/lib/GLFW-0.1/ghc-6.8.2/libHSGLFW-0.1.a(x11_window.o): In function `_glfwPlatformRefreshWindowParams': x11_window.c:(.text+0x803): undefined reference to `XRRGetScreenInfo' x11_window.c:(.text+0x80d): undefined reference to `XRRConfigCurrentRate' x11_window.c:(.text+0x81b): undefined reference to `XRRFreeScreenConfigInfo' /usr/local/lib/GLFW-0.1/ghc-6.8.2/libHSGLFW-0.1.a(x11_window.o): In function `_glfwPlatformCloseWindow': x11_window.c:(.text+0xb7a): undefined reference to `XRRGetScreenInfo' x11_window.c:(.text+0xb9d): undefined reference to `XRRSetScreenConfig' x11_window.c:(.text+0xba6): undefined reference to `XRRFreeScreenConfigInfo' /usr/local/lib/GLFW-0.1/ghc-6.8.2/libHSGLFW-0.1.a(x11_window.o): In function `_glfwPlatformPollEvents': x11_window.c:(.text+0x115b): undefined reference to `XRRUpdateConfiguration' /usr/local/lib/GLFW-0.1/ghc-6.8.2/libHSGLFW-0.1.a(x11_window.o): In function `_glfwPlatformOpenWindow': x11_window.c:(.text+0x1fa5): undefined reference to `XRRSelectInput' /usr/local/lib/GLFW-0.1/ghc-6.8.2/libHSGLFW-0.1.a(x11_fullscreen.o): In function `_glfwPlatformGetVideoModes': x11_fullscreen.c:(.text+0x1ca): undefined reference to `XRRGetScreenInfo' x11_fullscreen.c:(.text+0x1db): undefined reference to `XRRConfigSizes' x11_fullscreen.c:(.text+0x219): undefined reference to `XRRFreeScreenConfigInfo' /usr/local/lib/GLFW-0.1/ghc-6.8.2/libHSGLFW-0.1.a(x11_fullscreen.o): In function `_glfwSetVideoModeMODE': x11_fullscreen.c:(.text+0x329): undefined reference to `XRRGetScreenInfo' x11_fullscreen.c:(.text+0x344): undefined reference to `XRRConfigCurrentConfiguration' x11_fullscreen.c:(.text+0x394): undefined reference to `XRRSetScreenConfigAndRate' x11_fullscreen.c:(.text+0x3ac): undefined reference to `XRRSetScreenConfig' /usr/local/lib/GLFW-0.1/ghc-6.8.2/libHSGLFW-0.1.a(x11_fullscreen.o): In function `_glfwGetClosestVideoMode': x11_fullscreen.c:(.text+0x3fa): undefined reference to `XRRGetScreenInfo' x11_fullscreen.c:(.text+0x408): undefined reference to `XRRConfigSizes' x11_fullscreen.c:(.text+0x48f): undefined reference to `XRRConfigRates' x11_fullscreen.c:(.text+0x4dc): undefined reference to `XRRFreeScreenConfigInfo' x11_fullscreen.c:(.text+0x512): undefined reference to `XRRFreeScreenConfigInfo' /usr/local/lib/GLFW-0.1/ghc-6.8.2/libHSGLFW-0.1.a(x11_fullscreen.o): In function `_glfwSetVideoModeMODE': x11_fullscreen.c:(.text+0x3be): undefined reference to `XRRFreeScreenConfigInfo' /usr/local/lib/GLFW-0.1/ghc-6.8.2/libHSGLFW-0.1.a(x11_init.o): In function `_glfwPlatformInit': x11_init.c:(.text+0xea): undefined reference to `XRRQueryExtension' collect2: ld returned 1 exit status

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First Israel Haskell Event
by B K 14 Jan '08

14 Jan '08

Hello all, A Haskell gathering will take place for the first time in Israel: Friday, January 18 2008 at 11:00 AM Village Green, 33 Jaffa Rd, (corner of Rivlin St), Jerusalem This will be an informal event where everyone can introduce themselves and talk about their thoughts on Haskell. Everyone and anyone are invited. If you are in the area then please stop by, and email me with any questions. Looking forward to an enlightening time, Benny

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Re: [Haskell-cafe] Yi editor tutorial
by gwern0＠gmail.com 14 Jan '08

14 Jan '08

On 2008.01.14 13:34:42 +0000, Andrew Birkett <andy(a)nobugs.org> scribbled 0.8K characters: > gwern0(a)gmail.com wrote: >> I'm going through them now, and I like them a lot. (Maybe I'll finally >> begin doing stuff with Yi!) >> Is there any particular reason you didn't put your tutorials on the >> Haskell wiki? I'd think they'd be good there. > > Cool, thanks. :-) > > I agree it'll be a good idea to move it to the wiki, and I'll do that soon. > On my own site, I can make lots of little changes locally and then > atomically (well, quickly at least!) rsync the new version up onto the > server. It makes life easier when I'm likely to change around the top > level structure. The wiki makes a lot of sense though, because it's > central and other people can extend the tutorial and fix bugs. I'd say editing a wiki is even easier. :) But if you want to wait until you've gotten it to a state where you're happy with it, that's good too. > It's also nice to be able to check the server logs and gauge interest too > (about 5000 hits since yesterday). > > Andrew True, but the interest won't last forever; so maybe after the Reddit links go quiescent? -- gwern rico Glock telex million propellants Warfare Ortega CNN 8182 Vauxhall

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cabal and hpc?
by Greg Fitzgerald 14 Jan '08

14 Jan '08

Has the Haskell Program Coverage tool been integrated into Cabal? That is, is there anything like "runhaskell Setup.hs coverage" to generate a coverage report? Thanks, Greg

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All equations must have the same arity - why?
by Neil Mitchell 14 Jan '08

14 Jan '08

Hi, It's nice to write functions in point free style: f = sort . nub But sometimes I have to add an extra case, on a certain value: f [] = [1] f = sort . nub But now these equations have different arities, and its rejected by Haskell. Why does this not simply desugar to: f [] = [1] f x = (sort . nub) x i.e. lift the arities to the longest argument list. Is there a reason this isn't done? Thanks Neil

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Code folding in Emacs
by Johan Tibell 14 Jan '08

14 Jan '08

It would be pretty neat for Haskell hacking if the Emacs Haskell mode could do the following. Imagine you have written some code like so: -- | The parse state. data S = S {-# UNPACK #-} !B.ByteString -- current chunk L.ByteString -- rest of the input {-# UNPACK #-} !Int64 -- bytes read -- | The 'Parser' monad is just a State monad carrying around the -- input ByteString. newtype Parser a = Parser { unParser :: S -> Consumed (Reply a) } -- | Match byte that fulfills predicate. satisfy :: (Word8 -> Bool) -- ^ Predicate that byte must fulfill. -> Parser Word8 -- ^ Matched byte. satisfy p = Parser $ \(S s ss pos) -> if B.null s then (if L.null ss then Empty (Error (ParseError pos)) else case L.splitAt 1 ss of (consuming,rest) -> let now = B.concat . L.toChunks $ consuming b = B.head now in if p b then Consumed $ Ok b (S (B.tail now) rest (pos + 1)) else Empty $ Error $ ParseError pos ) else let b = B.head s in if p b then Consumed $ Ok (B.head s) (S (B.tail s) ss (pos + 1)) else Empty $ Error $ ParseError pos Now you want an overview of your source file. One way would be to replace all the implementation parts with ellipses, like so: -- | The parse state. data S = S {-# UNPACK #-} !B.ByteString -- current chunk L.ByteString -- rest of the input {-# UNPACK #-} !Int64 -- bytes read -- | The 'Parser' monad is just a State monad carrying around the -- input ByteString. newtype Parser a = Parser { unParser :: S -> Consumed (Reply a) } -- | Match byte that fulfills predicate. satisfy :: (Word8 -> Bool) -- ^ Predicate that byte must fulfill. -> Parser Word8 -- ^ Matched byte. satisfy p = ... Binding a haskell-fold-source function to a key chain would enable you to get a quick overview of your module showing only the comments and type signatures. I've used the little function from http://emacs.wordpress.com/2007/01/16/quick-and-dirty-code-folding/ but it doesn't work well together with indented type signatures like in the example above. Anyone with strong Emacs-fu that knows how one could implement such a function? -- Johan

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[IETF Apps meeting] A Theory of Templating Languages
by Stephane Bortzmeyer 14 Jan '08

14 Jan '08

The IETF (http://www.ietf.org/) holds a meeting of its Application Area and is looking for papers. In a position paper, Joe Gregorio asked for information about the theory of templating languages. Giving the interest here in DSLs and conceptualization, he may find on this list the help he wants and the references he searches: >From: "Joe Gregorio" <joe(a)bitworking.org> >Date: December 14, 2007 8:20:17 AM PST >Subject: Re: Position papers due Dec 14 > >Here is my brief position paper: > >Working on the URI Templating specification has made me realize that >there is a pretty substantial hole >in computer science theory: a lack of a theory of templating >languages. For example, the current version of >URI Templates is not Turing-complete, which excludes a whole bunch of >possible attacks. In the specification I state: > > On the balance, the template processing is not Turing complete, >thus avoiding a number of > security issues, ala the billion-laughs attack of XML DTDs. > >I was rightly called out on this on the W3C URI mailing list: > > This reads a little odd, as not being Turing-complete is not >sufficient > to avoid the attack. (And DTDs are not Turing-complete either.) > >The criticism is correct. The problem is that I don't know of any >finer grained levels of classifications >of templating languages than Turing/non-Turing, and not only for >security reasons, but for general capabilities. > >For example, if there were classes of templating languages, I could >say that URI Templates fell into 'class X', and >if that class had a known set of limitations and capabilities then I >could say that URI Templates thus had those >limitations and capabilities. The weakness to the billion laughs >attack comes from two facets of DTD usage, the >first being that templates can be defined in terms of other templates, >and the second is that the depth of template >definition, in terms of other templates, isn't limited. But the >converse isn't true, that is, I don't have a general >theory of templating to lean on that says since URI Template >expansions are never defined in terms of other >expansions then URI Templates are immune to such resource >exhaustion attacks. > >I did find one paper that makes a start at such work, "Enforcing >Strict Model-View Separation in Template Engines", >but the theory is a little weak and it focuses on the nebulous idea of >separation of model and view, as opposed to >a classification of capabilities and limitations. In addition there >seems to not be a lot of work on sub-turing languages, >and most interestingly the contemporary work that is being done is on >Membrane Computing Systems, which >is in turn motivated by studying cell evolutions and chemical >reactions. I am bringing this topic forward in the >hopes of learning of other pointers into the literature, and also >learning if this problem applies to others >in the Apps area, of if I'm all alone with this problem in URI >Templates. ----- End forwarded message -----

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