http://blog.gmane.org/gmane.comp.lang.haskell.cafe hourly 1 1901-01-01T00:00+00:00 http://gmane.org/img/gmane-25t.png http://gmane.org http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98490 <pre>Hello list, As my first post.  I'd like to open a can of worms that I sure has been opened before.  That is record syntax. As we all know, there are some type safety problems with our current record syntax. The most obvious is that this compiles without even giving you a warning: This compiles, then crashes at run time.  Rediculus, why didn't the glorious GHC catch this?  This is not some ambigious scenario...  One way I found to improve type checking is this: This doesn't compile anymore because a :: A' and c :: B' -&gt; X It seems like the later method is simply better.  Better type checking = fewer runtime errors(I actually ran into a crash in a real project because of this lack of proper type checking...). Can we have a nice sugar for doing the later method? Thank you for your time, Timothy_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe&lt; at &gt;haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe </pre> timothyhobbs< at >seznam.cz 2012-05-26T22:28:29 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98489 <pre>Hi Haskellers, I've posted this question (and my code) to stack overflow as well ( http://stackoverflow.com/questions/10747079/what-are-the-key-differences-between-the-repa-2-and-3-apis), so if anyone here has the answer, I'll post it to that site for the world's reference. Using the Repa 2 API, I have written some simple image convolution tests which run more than fast enough. The trick to getting good performance was to call 'force' after every array transformation. I can't quite figure out the analogous thing to do with Repa 3 - at stackoverflow you can see my Repa 3 code, which runs correctly but very slowly. It is not clear to me how exactly the monadic "computeP" functions in Repa 3 are intended to be used - I have several calls to 'force' in my Repa 2 code, but only 1 call to computeP in the Repa 3 version. I've read the excellent "Numeric Haskell" Repa tutorial by Don S, but it doesn't cover Repa 3. My thanks in advance! _______________________________________________ Haskell-Cafe mailing list </pre> Michael Serra 2012-05-26T20:09:12 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98485 <pre>Hello,friends! I come from China, wishing to enjoy haskell with you. </pre> Dante.py 2012-05-26T00:58:59 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98479 <pre>Template Haskell supports antiquotation for built-in quasiquotes, e.g.: [| \x -&gt; x + $([|3 * 4|]) |] However, as far as I can tell, there is no way of supporting antiquotation in user-defined quasiquoters, because the only way to specify a new quasiquoter is through a quoteExp function of type String -&gt; Q Exp. Of course, it is perfectly possible to write a parser for some fragment of Haskell inside your quoteExp function, but that seems crazy given that Template Haskell or rather GHC already implements a parser for the whole language. I know about Language.Haskell.Exts.Parser in haskell-src-exts, which provides parseExp :: String -&gt; ParseResult Exp, but that Exp is a different type to the one provided by Template Haskell.&lt;http://hackage.haskell.org/packages/archive/haskell-src-exts/1.9.0/doc/html/Language-Haskell-Exts-Syntax.html#t:Exp&gt; I'm also aware of Dominic Orchard's syntax-trees package, which supports converting between the two representations using a cunning hack that pretty-prints </pre> Sam Lindley 2012-05-25T19:51:38 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98463 <pre>Hi cafe, I have a problem with haddock documentation created when installing new packages with cabal on windows. The generated html files have all links in the form j:\Users\...\doc\...\xxx.html, but firefox says, it cannot open that link. Actually all links should be prefixed by "file:///". I wonder if this is only on windows so and if there is a solution to this. Is there a cabal or haddock flag for this? Nicu _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe&lt; at &gt;haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe </pre> Nicu Ionita 2012-05-24T23:19:11 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98462 <pre>Hello Cafe, since I haven't found anything like that, I wrote a small library [1] to read from a webcam in Haskell, using V4L on Linux. It uses the v4l2 package and repa for images. Is anyone interested in contributing to that, or giving some hints on how it could be made faster (I am not sure if I use repa correctly for best performance at all times), or just in emitting any other constructive comments? If yes, please give me a shout. Cheers, Christian PS I would put the whole thing on hackage, but my cabal currently doesn't want to do an sdist. [1] https://github.com/cgo/hsimage </pre> . 2012-05-24T22:20:40 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98461 <pre>Hello, I think the above book by Reyes, Reyes and Zolfaghari is quite intriguing. At the same time I can't get my mind around the notions of "generic figures" and "change of figures"! The first seems to be objects in the base category and the latter morphisms in the base category. Whoever has read this book and understands it please give me some pointers. Sorry the vague call for help. Kind regards, Vasili </pre> Vasili I. Galchin 2012-05-24T19:03:20 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98451 <pre>Hi, I am writing a small program using LDAP hackage. A weird problem occured. When the code was run by runhaskell, things were fine, worked as expected. But when ghc compiled (no any args), and ran, I got this: LDAP error: ldapSimpleBind: LDAPException LdapServerDown(-1): Can't contact LDAP server. There is sure no problem with the server. So I am confused. I thought the two supposed to be the same. </pre> Magicloud Magiclouds 2012-05-24T09:36:26 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98437 <pre>Hi, I made a cabal package that use HDBC-sqlite3 on Windows. When cabal install it, I got an error from ld.exe: cannot find -lsqlite3. How can I tell ld.exe where sqlite3.dll is plcaed? Thanks. </pre> Canhua 2012-05-24T02:04:11 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98429 <pre>Dear all, our local hackathon in Munich on the 12th of May was a real success. Thanks to all those who joined! Look at the site for a picture: http://www.haskell-munich.de/dates Our next regular meeting is on Monday, the 28th of May at Cafe Puck at 19h30. This is a holiday in Germany. We hope that many of you will come! If you plan to do so, please click the button on the above mentioned site. Until then, I wish all of you a nice time, Heinrich -- hoerdegen&lt; at &gt;funktional.info www.funktional.info </pre> Heinrich Hördegen 2012-05-23T14:28:27 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98426 <pre>Hello Haskell world. One well-known missfeature of TemplateHaskell is it's «untypedness». I mean, that if you have say an expression (Exp or Q Exp), you do not know it's type (does it represent String or Int or whatever). And GHC does not know too. So, one could easily construct bad-typed expressions using TH, and GHC will complain only on generated code, not on TH code. Such a missfeature was a reasoned decision, because adding some typing into TH will add much complexity to compiler and to language. As far as can I see, using features of last GHC one could write typed TH library relatively easily, and saving backwards compatibility. For example, now we have Q monad and Exp type in "template-haskell" package. Let's imagine some new package, say "typed-template-haskell", with new TQ monad and new polymorphic type Exp :: * -&gt; *. Using last GHC's features, one will easily write something like "expr :: Exp String", which will mean that "expr" represents a string expression. And we will need a n</pre> Ilya Portnov 2012-05-23T11:45:01 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98420 <pre>Dear Haskell Cafe, I've been playing around with the -fhpc option to GHC and have come across some things being highlighted yellow in the HTML markup for which I can't work out the right way to 'cover' them. A cut down program that shows what I'm talking about is at http://hpaste.org/68888 Looking at the output of hpc show, I see: 47 0 Main 15:6-15:9 ExpBox False 48 2 Main 15:6-15:9 ExpBox False 49 0 Main 15:6-15:9 ExpBox False 50 0 Main 15:6-15:9 ExpBox False 51 1 Main 22:13-22:14 TopLevelBox ["=="] 52 0 Main 22:13-22:14 TopLevelBox ["/="] 53 1 Main 22:17-22:20 TopLevelBox ["showsPrec"] 54 0 Main 22:17-22:20 TopLevelBox ["showList"] I.e. each of those bits whose highlighting I don't unders</pre> David Turner 2012-05-23T09:40:00 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98415 <pre>So I wanted to find the first index in a vector whose running sum is greater than a given number. The straightforward way is to create the running sum and then search: Vector.findIndex (&gt;=target) (Vector.scanl' (+) 0 vector) But vectors are strict so it could do extra work, and what if I don't want to generate garbage? I could do it with a fold, but it would have to have the ability to abort early. Of course I could write such a fold myself using indexing: import qualified Data.Vector.Generic as Vector fold_abort :: (Vector.Vector v a) =&gt; (accum -&gt; a -&gt; Maybe accum) -&gt; accum -&gt; v a -&gt; accum fold_abort f accum vec = go 0 accum where go i accum = maybe accum (go (i+1)) $ f accum =&lt;&lt; vec Vector.!? i find_before :: (Vector.Vector v a, Num a, Ord a) =&gt; a -&gt; v a -&gt; Int find_before n = fst . fold_abort go (0, 0) where go (i, total) a | total + a &gt;= n = Nothing | otherwise = Just (i+1, total+a) So it's bigger and clunkier, but I would think it would be much more efficient (pr</pre> Evan Laforge 2012-05-23T04:33:36 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98407 <pre>Hello Cafe, This is just FYI. We Hasekell community in Japan will have an event called Haskell Day 2012 on 27th May to cerebrate the publication of the Japanese version of "Learn you". 180+ people have already registered: # This page is written in Japanese. http://partake.in/events/ab7f77b4-7541-47a4-867d-21a096ca883c P.S. The Japanese version of "Learn you" was on sale yesterday and already sold out in many bookstores. :-) --Kazu </pre> Kazu Yamamoto 2012-05-23T02:20:59 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98403 <pre>In ghci 7.4.1: Prelude&gt; :t (+1) (+1) :: Num a =&gt; a -&gt; a Prelude&gt; let inc=(+1) Prelude&gt; :t inc inc :: Integer -&gt; Integer Why the difference? Thanks, Greg _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe&lt; at &gt;haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe </pre> Greg Fitzgerald 2012-05-22T19:27:54 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98393 <pre>Hej, I'm using the LDAP package by John Goerzen to retrieve some information from an Active Directory database. Part of this information are the full names of my company's employees. Many of these names contain characters which aren't part of the standard ASCII set, for example ä å ü ê and so on. When I retrieve those names from the directory (the LDAP package returns them as Strings) the encoding breaks and I get results like "R\195\188diger" instead of "Rüdiger". The Active Directory server supports LDAP v2 and v3. I assume the OpenLDAP C API, which is the backend behind the LDAP package, automatically chooses v3 to connect if available (this is speculation, correct me if I'm wrong). Since LDAP v3 only speaks UTF8 and ASCII I also assume that the server returns UTF8. Is this a known problem in the LDAP package? Or is this related to the OpenLDAP C API? Or even something on the server side? Any information would be helpful! Best regards, Vincent </pre> Vincent Ambo 2012-05-22T14:30:47 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98392 <pre>Has anyone ever worked on implementing something like this in Haskell? http://www.cs.hmc.edu/~stone/papers/ocm-unpublished.pdf The outline of the idea: - Concurrent programming is really hard with the popular frameworks today. - For most purposes parallel programming is hard, in some part because it requires concurrent programming. Of course there are attempts to do non-concurrent parallel programming, but I hope it's not too controversial to say that such frameworks are still on the fringe. - Cooperative concurrency is way easier than preemptive concurrency because between invocations of pause/yield/wait, sequential reasoning works. - Historically, cooperative concurrency only worked on a single processors, because running threads in parallel would break the atomicity of sequential blocks (between invocations of p/y/w). - Researchers have been poring tons of effort into efficiently running blocks of code atomically. - Hey, we can do parallel cooperative multithreading! The paper discusses implementatio</pre> Benjamin Ylvisaker 2012-05-22T13:55:41 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98389 <pre>Dear cafe, I have just upgraded to GHC 7.4, and now I run into some compilation problems. Specifically, I have a package that needs both Data.Array.Unboxed and System.IO.Unsafe. But I cannot enable both base and haskell98 in a cabal file. is there any way out of this? If I enable both haskell98 and base, I get Query.hs:1:1: Ambiguous module name `Prelude': it was found in multiple packages: base haskell98-2.0.0.1 cabal: Error: some packages failed to install: as documented elsewhere (http://hackage.haskell.org/trac/ghc/ticket/4958) With haskell98 replacing base, I get Query.hs:55:8: Could not find module `System.IO.Unsafe' It is a member of the hidden package `base'. Perhaps you need to add `base' to the build-depends in your .cabal file. Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: and without haskell98 or array, cabal tells me to add those to .cabal build-depends. So how can I use the Data.Array.Unboxed package together wit</pre> Tom Nielsen 2012-05-22T09:41:57 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98387 <pre>Hi all, After using zippers for a while, I wanted to dig a bit deeper into them. I found there is some relation between Zipper and Comonad, but this confuses me somewhat. After reading a bit more about Comonads [1] and [2], I think I understand them somewhat, and I see how they too are useful for navigating a data structures and giving functions the ability to "look around". What confuses me though, is the relation between these 2. This source [3] mentions all zippers can be made instances of Comonad, and demonstrates how to do this for simple, 1-dimensional (list) zippers. But a comment on [4] says a Zipper by itself is already an application of Comonad. I want to find out which is the case. Looking at the types does not yield me a solution yet. This is Zipper from LYAH: data Tree a = Empty | Node a (Tree a) (Tree a) data Crumb a = LeftCrumb a (Tree a) | RightCrumb a (Tree a) type Breadcrumbs a = [Crumb a] type Zipper a = (Tree a, Breadcrumbs a) This is Comonad from the comonad package (flattened to 1</pre> Mathijs Kwik 2012-05-22T06:42:46 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98369 <pre>When explaining monads to beginners (having an imperative background), I found myself to say that there is *roughly* three "groups" of monads (because they're always worried about their cost, i.e. their incidental complexity): - Function-oriented monads (e.g. State, Reader, Cont) - Reductible data-oriented monads (e.g. Maybe, Identity, Writer) - Irreductible data-oriented monads (e.g. List, free monads) (which, as I understood, have a (&gt;&gt;=) operation that has a quadratic complexity if not used properly) Are there others "groups" I'm missing and is there terms to formalize them? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe&lt; at &gt;haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe </pre> Yves Parès 2012-05-21T13:25:40 http://comments.gmane.org/gmane.comp.lang.haskell.cafe/98366 <pre>Hello cafe, I have released Mighttpd 2.6.0: http://mew.org/~kazu/proj/mighttpd/en/ Some users started using Mighttpd 2 and I was requested to implement missing features for real world operation. So, I implemented the following features in Mighttpd 2.6.0: - Route file reloading - Graceful shutdown for upgrading - URL rewriting (with HTTP redirection) For more information, please read the homepage above. I would like to thank Michael Snoyman for merging the necessary patch to warp. Regards, --Kazu </pre> Kazu Yamamoto 2012-05-21T02:49:34 Search the mailing list at Gmane query http://search.gmane.org/?group=$group=gmane.comp.lang.haskell.cafe