- Haskell-Cafe - Haskell.org

Metaprogramming in Haskell vs. Ocaml
by aditya siram 07 Apr '10

07 Apr '10

Hi all, I would like to learn a little bit more about metaprogramming in Haskell. And I'm also wondering why metaprogramming is used much more in Ocaml than in Haskell. Camlp4 (Ocaml's metaprogramming facility) doesn't seem to much more powerful that Template Haskell. The former is celebrated in its community as a killer feature of the language and its use is encouraged by numerous accessible tutorials. The latter however is down-played and not as well documented. Seeing that the languages are parallel in many ways is there some reason for this? Is metaprogramming less useful in Haskell? Is it worth learning? thanks ... -deech

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Established names for a couple of list functionals?
by Stephen Tetley 07 Apr '10

07 Apr '10

Hello all Having traversals with special behaviour for the first or last element is useful for my current work: -- first element special -- anacrusisMap :: (a -> b) -> (a -> b) -> [a] -> [b] anacrusisMap _ _ [] = [] anacrusisMap f g (a:as) = f a : map g as -- last element special -- cabooseMap :: (a -> b) -> (a -> b) -> [a] -> [b] cabooseMap _ _ [] = [] cabooseMap f g (a:as) = step a as where step x [] = [g x] step x (y:ys) = f x : step y ys *Overlay1> anacrusisMap (+1) id [1..10] [2,2,3,4,5,6,7,8,9,10] *Overlay1> cabooseMap id (+1) [1..10] [1,2,3,4,5,6,7,8,9,11] My question (trivial, but still...) - is there any prior art naming these functions? For the record, my name derivation is: + anacrusis - musical term, the pickup notes before the first bar in a score. + caboose - there was discussion on Haskell-Cafe a couple of months ago about a list data type with a distinguished terminal type. Caboose is the name I remember. Thanks Stephen

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Network: buffering troubles
by Yves Parès 07 Apr '10

07 Apr '10

Hello, I have isolated a problem when using (lazy) ByteStrings through the network (GHC 6.12.1, Ubuntu 9.10 32bits): Here is my client: http://old.nabble.com/file/p28162932/Client.hs Client.hs And my server: http://old.nabble.com/file/p28162932/Server.hs Server.hs The server holds until the client closes the connection, even if I flush the handle on the client side after writing or even if I turn off buffering on both sides. ----- Yves Parès Live long and prosper -- View this message in context: http://old.nabble.com/Network%3A-buffering-troubles-tp28162932p28162932.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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hsql repository
by Daniil Elovkov 07 Apr '10

07 Apr '10

Hello I've looked at hsql-* packages at hackage and found that they have progressed to 1.8.1. However, as always there is no hsql-oracle on hackage. It was usually found in the repository. But I can't find where the repository is. Places, like code.haskell.org and htoolkit.sourceforge.net seem to have the older 1.7 version. Can somebody point me to the repository? Is there any chance that hsql-oracle compiles with new hsql? Thanks Daniil Elovkov

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Break Function Using Lazy Pairs
by aditya siram 07 Apr '10

07 Apr '10

Hi all, For the past couple of weeks I've been trying to understand tying-the-knot style programming in Haskell. A couple of days ago, Heinrich Apfelmus posted the following 'break' function as part of an unrelated thread: break [] = ([],[]) break (x:xs) = if x == '\n' then ([],xs) else (x:ys,zs) where (ys,zs) = Main.break xs I've stepped through the code manually to see if I understand and I'm hoping someone will tell me if I'm on the right track: -- break "hi\nbye" = -- let f1 = (break "i\nbye") -- = let f2 = (break "\nbye") -- = ([],"bye") -- ('i' : fst f2, snd f2) => ('i' : [], "bye") -- ('h' : fst f1, snd f1) => ('h' : 'i' : [], "bye") -- => ("hi","bye") -deech

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Crypto: PC1 - Cipher?
by Günther Schmidt 07 Apr '10

07 Apr '10

Hello, does anyone know what the PC1-cipher is and if there is a Haskell implementation? Günther

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Re: HXT Namespaces and XPath
by Uwe Schmidt 07 Apr '10

07 Apr '10

Hi Mads, > In HXT, namespace prefixes bound by an XML document are valid in the > context of an XPath. How do avoid that? > > An example program will clarify: > > simpleXml :: String > simpleXml = "<soap:Body xmlns:soap=\"http://www.w3.org/2003/05/soap-envelope\"/>" > > nsEnv :: [(String, String)] > nsEnv = [ ("s" , "http://www.w3.org/2003/05/soap-envelope") ] > > evalXPath :: String -> String -> [XmlTree] > evalXPath xpath xml = > runLA ( xread > >>> propagateNamespaces > >>> getXPathTreesWithNsEnv nsEnv xpath > ) xml > > Here: > > evalXPath "//s:Body" simpleXml == > evalXPath "//soap:Body" simpleXml > > Even though I only mentions the prefix "s" (and not "soap") in the > function nsEnv. When working with namespaces in XML, the prefixes are not longer significant. After namespace propagation every name in the XML document is identified by a qualified name. This is a pair consisting of the namespace URI and the local part. The prefixes become irrelevant. A namespace aware XPath expression needs a namespace environment, as given in the example, to construct these qualified names for the names in the XPath expression. So the results of both evalXPath calls in your example must be the same. > I do not want the XPath to see prefixes declared in the xml-document, as > it means that two semantically similar XML documents can get different > results when applied to the same XPath. If you intend, that the prefixes are significant, then you should not work with namespace propagation. If namespaces are not propagated, all names occurring in the document are taken as they are. That means "s:Body" is different from "soap:Body". For solving your tasks there could be 2 ways: 1. don't use namespaces, no propagateNamespaces, no getXPathTreesWithNsEnv, use the prefixes instead. Disadvantage: the prefixes become significant. If prefixes change, the semantics changes. 2. Use propagateNamespaces and getXPathTreesWithNsEnv, but select all nodes via the namespace URIs, not via the prefixes. If you want to output the manipulated XML parts and use standard prefixes for the namespaces, then use the namespace manipulation functions in http://hackage.haskell.org/packages/archive/hxt/latest/doc/html/Text-XML-HX… Cheers, Uwe

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[haskell.org Google Summer of Code] Student Applications Due Friday!
by Edward Kmett 07 Apr '10

07 Apr '10

This is a friendly reminder that student applications for the summer of code are due to Google by Friday, April 9th. http://socghop.appspot.com/document/show/gsoc_program/google/gsoc2010/timel… That is just *3 days* from now! Keep in mind that while we've been tracking general interest in different projects through the trac, the application doesn't count unless you submit it to Google at http://socghop.appspot.com/gsoc/program/home/google/gsoc2010 While all details of the application do not need to be finalized, as much of the information from http://hackage.haskell.org/trac/summer-of-code/wiki/StudApply2010 as possible should probably be included in your application. If you are stuck and having a hard time thinking of ideas, I highly recommend the excellent guide that Don Stewart posted recently: http://donsbot.wordpress.com/2010/04/01/the-8-most-important-haskell-org-gs… I'd also recommend taking a look through the trac at http://hackage.haskell.org/trac/summer-of-code/report/1 to gauge interest. Please email me if you have any questions or concerns. -Edward Kmett

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Why does Haskell not infer most general type?
by Job Vranish 07 Apr '10

07 Apr '10

Is haskell supposed to always infer the most general type (barring extensions)? I found a simple case where this is not true: f _ = undefined where _ = y :: Int -> Int y x = undefined where _ = f x Haskell infers the types of 'y' and 'f' as: f :: Int -> a y :: Int -> Int This confused me at first, but after thinking about it a while it seemed to make sense. But then my friend John pointed out that you can add type sigs for 'f' and 'y': f :: a -> b y :: a -> b and have it still typecheck! This thoroughly confused me. Why does haskell not infer the most general type for these functions? Is it a limitation of the algorithm? a limitation of the recursive let binding? Any insight would be appreciated :) - Job

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GSOC Haskell Project
by Mihai Maruseac 06 Apr '10

06 Apr '10

Hi, I'd like to introduce my idea for the Haskell GSOC of this year. In fact, you already know about it, since I've talked about it here on the haskell-cafe, on my blog and on reddit (even on #haskell one day). Basically, what I'm trying to do is a new debugger for Haskell, one that would be very intuitive for beginners, a graphical one. I've given some examples and more details on my blog [0], [1], also linked on reditt and other places. This is not the application, I'm posting this only to receive some kind of feedback before writing it. I know that it seems to be a little too ambitious but I do think that I can divide the work into sessions and finish what I'll start this summer during the next year and following. [0]: http://pgraycode.wordpress.com/2010/03/20/haskell-project-idea/ [1]: http://pgraycode.wordpress.com/2010/03/24/visual-haskell-debugger-part-2/ Thanks for your attention, -- Mihai Maruseac

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