March 2010 - Haskell-Cafe

phonetic dictionnary
by Dupont Corentin 25 Mar '10

25 Mar '10

Hello, sorry if i ask a lot of questions these days! Do you know of a phonetic dictionnary? i'm very fond of enigmas. To solve certain enigmas, i've made a little program that search into a french dictionnary. But it would be much better if i could search into a phonetic dictionnary... The goal is to answer questions like "Wich word would rhyme with aligator?" Corentin

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explicit big lambdas
by Paul Brauner 25 Mar '10

25 Mar '10

Hi again, is there a way in some haskell extension to explicit (system F's) big lambdas and (term Type) applications in order to help type inference? If not: is there a way to provide ghc directly with core code before the type checking phase? Paul

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Re[Haskell-cafe] mote invocations in Haskell?
by Yves Parès 25 Mar '10

25 Mar '10

Hello, Is there a way to perform some kind of remote method invocation in haskell? (Or, "remote object", but I prefer not to use this term, as there are no "objects" strictly speaking in Haskell) I would like to use a higher level API than sockets for network programing. ----- Yves Parès Live long and prosper -- View this message in context: http://old.nabble.com/Remote-invocations-in-Haskell--tp28006793p28006793.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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breadth first search one-liner?
by Johannes Waldmann 25 Mar '10

25 Mar '10

Dear all, there is this neat "one-line" BFS implementation bfs :: Eq a => ( a -> [a] ) -> a -> [a] bfs next start = let xs = nub $ start : ( xs >>= next ) in xs but it has a problem: it only works for infinite graphs. This is fine: take 20 $ bfs ( \ x -> [2*x, x+1] ) 1 but this is not: take 20 $ bfs ( \ x -> filter (>0) [ div x 2, x - 1 ] ) 10 Is there a nice way to repair this? (I know how to code a BFS but here I'm asking for a one-liner.) J. W.

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ANN: Salvia-1.0.0
by Sebastiaan Visser 25 Mar '10

25 Mar '10

Hi all, Straight from Zurihac: I'm very pleased to announce the 1.0.0 release of the Salvia web server. Salvia is a feature rich web server and web application framework that can be used to write dynamic websites in Haskell. From the lower level protocol code up to the high level application code, everything is written as a Salvia handler. This approach makes the framework extremely modular and extensible. This release include the following stack of packages: * salvia-protocol: Protocol stack containing URI, HTTP, Cookie and Mime. * salvia: Basic server interface and implementation. * salvia-extras: Additional server handlers and back end implementation. * salvia-sessions: Session and user management support. * salvia-websocket: Cutting edge HTML5 web socket support. * salvia-demo: Demo servers showing how to use Salvia. All the code is on Hackage[1] and the source repositories can be found on GitHub[2]. Most of the basic ideas of the previous Salvia release are still in these package, but all the code has been cleaned up considerably. There is now a very strong separation between interface and implementation making it more easy to plug-in new back-ends for your web application. To do a full install first run `cabal update' followed by `cabal install salvia-demo'. Now you can run the salvia-demo command and point your browser to http://localhost:8080/ . Thanks to the people that helped me with coding, suggestions and bug-reports. Any comments/suggestions are welcome! Have fun, -- Sebatiaan Visser [1] http://hackage.haskell.org/package/salvia (-protocol, -demo, etc) [2] http://github.com/sebastiaanvisser/salvia (-protocol, -demo, etc)

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Using Get monad to efficiently parse byte-stuffed data
by Pom Monico 25 Mar '10

25 Mar '10

Hello all, I'm struggling to use the Get monad to efficiently parse the some binary data of the format below. I simply can't find a way to use the monad to parse it efficiently. Binary data is terminated by a 0xFF marker. If the data itself contains an 0xFF, a 0x00 is byte-stuffed after it to indicate it is part of the data. An 0xFF followed by a non-zero byte indicates that it is a real marker and terminates the data and the 0xFF is not part of the data. Going through the data using getWord8 is a no-go. It is just too slow. My solution so far has been to get the underlying bytestring and work with that, but obviously defeats the purpose of using the Get monad. What might be a better solution? Pom.

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Fwd: [Haskell-cafe] Graphical representation of Haskell code
by Miguel Vilaca 24 Mar '10

24 Mar '10

Hi all, Concerning INblobs, it's again online; a fire damaged the cable that links the university to the world!! I don't update the tool for a long time... but I'll take a look on that. Concerning visual functional programming, see this small chapter of my thesis about some of the existing languages. There are more subtleties on the visual side than those expected!! If you also consider debugging tools, take a look on Ghood http://hackage.haskell.org/package/GHood best regards Miguel Vilaça A 2010/03/23, às 05:31, Ronald Guida escreveu: > On Mon, Mar 22, 2010 at 7:02 PM, Dupont Corentin > <corentin.dupont(a)gmail.com> wrote: >> Hello, I’m relatively new to Haskell. >> I’m wondering if it exist a tool to graphically represent Haskell code. > ... >> Let’s try to do it on a simple example, as an exercise: >> f = Map (+1) > > Your graphic for "f = map (+1)" seems much more complex than the > corresponding code. I would agree with Ivan Miljenovic: >> I'm of the opinion that unless you just use it on small snippets, >> the generated images will be too large and unwieldy. > > The first question I would ask is /why/ would you like to visualize > some Haskell code? If you want to see the high-level structure of > a complex program, try SourceGraph. (I have never used it though.) > > On the other hand, if you are trying to visualize Haskell as part of > your efforts to learn the language, then I believe it would be best to > draw diagrams by hand, rather than relying on an automated tool. > The kinds of things that you'll want to depict are probably going to > vary considerably, depending on what you're trying to understand. > > Consider a few different implementations of the "map" function: > > -- version 1: recursion > map1 :: (a -> b) -> [a] -> [b] > map1 f [] = [] > map1 f (x:xs) = (f x) : map1 f xs > > -- version 2: fold > map2 :: (a -> b) -> [a] -> [b] > map2 f = foldr ((:) . f) [] > > -- version 3: continuation passing style > map3 :: (a -> b) -> [a] -> [b] > map3 f xs = map' (\x y -> f x : y) xs > where > map' k [] = [] > map' k (y:ys) = k y (map' k ys) > > -- version 4: list comprehension > map4 :: (a -> b) -> [a] -> [b] > map4 f xs = [f x | x <- xs] > > -- version 5: list monad > map5 :: (a -> b) -> [a] -> [b] > map5 f xs = xs >>= (return . f) > > These all do exactly the same thing, but each one uses different > techniques. If I'm trying to learn (or teach) Haskell, I would > probably need a slightly different visual "language" for each one > in order to capture the most relevant concepts in a useful way. > How would you visualize them? > > @Mihai Maruseac: > I think a visual debugger would be a wonderful idea. You may want > to consider how a visual debugger would work with each of these > versions of map. > > :-) You might also consider several versions of factorial :-) > http://www.willamette.edu/~fruehr/haskell/evolution.html > > -- Ron > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Fwd: [Haskell-cafe] Graphical representation of Haskell code
by Miguel Vilaca 24 Mar '10

24 Mar '10

Hi all, Concerning INblobs, it's again online; a fire damaged the cable that links the university to the world!! I don't update the tool for a long time... but I'll take a look on that. Concerning visual functional programming, see this small chapter of my thesis about some of the existing languages. There are more subtleties on the visual side than those expected!! If you also consider debugging tools, take a look on Ghood http://hackage.haskell.org/package/GHood best regards Miguel Vilaça A 2010/03/23, às 05:31, Ronald Guida escreveu: > On Mon, Mar 22, 2010 at 7:02 PM, Dupont Corentin > <corentin.dupont(a)gmail.com> wrote: >> Hello, I’m relatively new to Haskell. >> I’m wondering if it exist a tool to graphically represent Haskell code. > ... >> Let’s try to do it on a simple example, as an exercise: >> f = Map (+1) > > Your graphic for "f = map (+1)" seems much more complex than the > corresponding code. I would agree with Ivan Miljenovic: >> I'm of the opinion that unless you just use it on small snippets, >> the generated images will be too large and unwieldy. > > The first question I would ask is /why/ would you like to visualize > some Haskell code? If you want to see the high-level structure of > a complex program, try SourceGraph. (I have never used it though.) > > On the other hand, if you are trying to visualize Haskell as part of > your efforts to learn the language, then I believe it would be best to > draw diagrams by hand, rather than relying on an automated tool. > The kinds of things that you'll want to depict are probably going to > vary considerably, depending on what you're trying to understand. > > Consider a few different implementations of the "map" function: > > -- version 1: recursion > map1 :: (a -> b) -> [a] -> [b] > map1 f [] = [] > map1 f (x:xs) = (f x) : map1 f xs > > -- version 2: fold > map2 :: (a -> b) -> [a] -> [b] > map2 f = foldr ((:) . f) [] > > -- version 3: continuation passing style > map3 :: (a -> b) -> [a] -> [b] > map3 f xs = map' (\x y -> f x : y) xs > where > map' k [] = [] > map' k (y:ys) = k y (map' k ys) > > -- version 4: list comprehension > map4 :: (a -> b) -> [a] -> [b] > map4 f xs = [f x | x <- xs] > > -- version 5: list monad > map5 :: (a -> b) -> [a] -> [b] > map5 f xs = xs >>= (return . f) > > These all do exactly the same thing, but each one uses different > techniques. If I'm trying to learn (or teach) Haskell, I would > probably need a slightly different visual "language" for each one > in order to capture the most relevant concepts in a useful way. > How would you visualize them? > > @Mihai Maruseac: > I think a visual debugger would be a wonderful idea. You may want > to consider how a visual debugger would work with each of these > versions of map. > > :-) You might also consider several versions of factorial :-) > http://www.willamette.edu/~fruehr/haskell/evolution.html > > -- Ron > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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State of the Haskell Web Application Stack
by Ozgun Ataman 24 Mar '10

24 Mar '10

Greetings all. I have been for quite some time trying to assess the feasibility of using Haskell in relatively large, high volume, high availability, long-running web application projects. I have enjoyed learning and using Haskell very much for the past year and I often find myself missing various language features when reasoning about alternatives like Ruby (on Rails). If I can identify the right set of tools for the job, I would really like to take the plunge and make Haskell my standard go-to language in web applications. Here are the couple of key questions that I wanted get your feedback on: Do you consider Haskell and its environment of libraries ready for prime time in web app development as defined above? What collection of libraries would you use in such an effort? What are the up and coming packages/technologies in Haskell-land you would watch out for? Also, here are some core requirements that I would define for such a project: Ease/speed of development in both back and front-ends, minimal boilerplate Extendability and flexibility in iterative development Robustness and reliability in production environment High performance Scalability Ability to interface with new technologies in the future: Cassandra, Redis, memcached, etc. Ease of implementing common/reusable features across web applications: user authentication, S3 file uploads, thumbnail/image handling, exception notifications, etc. In terms of libraries, I can think of a few key components (as pointed out by several others before) that one would need to arrange: Choice of server (happstack vs. alternatives) Templating (xhtml vs. file templates vs. newer efforts like BlazeHtml) Data/storage layer: HDBC vs. HaskellDB vs. others I know this is a common topic in Haskell-Cafe, but I have failed to identify conclusive opinions from experienced Haskellers out there in previous discussions. My apologies in advance if this is a blatantly redundant post. All the best, Ozgun

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Fwd: [Haskell-cafe] Bytestrings and [Char]
by Alberto G. Corona 24 Mar '10

24 Mar '10

2010/3/24 Stephen Tetley <stephen.tetley(a)gmail.com> > If you consider containers as the containers package, the data > structures are all (?) functorial - but they have different shapes, > so e.g. a cons operation makes sense on the linear ones > (Data.Sequence, Data.List) but not on Data.Map, Data.Tree. 'cons' is > analogous to 'insert' but 'insert' exactly describes the operation on > maps whereas 'cons' doesn't, similarly 'insert' doesn't describe the > 'cons' operation on lists exactly as it doesn't indicate the position > of where it adds to the list. > Some operations like insert in list could be inefficient, some operations like cons in maps could be an inefficient hack, but at this does not means that a list can do create, insert, map , lookup faster that a Map in the case of sort lists, for example. A generalized cons applied to a Map can make less sense, but a program made for lists with occasional cons's can happen to perform better with Maps and so on. So a library with generality in mind can perform optimally in two or more different scenarios with different instances/implementations. . A genetic algoritm can have the opportunity to detect this. I think also that a complex list of class hierarchies is neither necessary nor recommended. We are not talking about mathemathical concepts. This is a performance issue. Even if a definitive class hierarchy is a problem, hopefully it may be worth to define tentative and temporal classes for performance testing purposes, that warps the common primitives with the sole purpose of executing the testing algorithm for different instance implementations, compilation flags and fusion rules. > > Now, if you partition the type classes into small groups you get over > the fact that some operations make sense on certain 'shapes' of data > structure, but there are still subtle type differences that aren't > conducive to type classes - e.g. ByteString and Data.Text aren't > functorial so map is type restricted: > > map :: (Word8 -> Word8) -> ByteString -> ByteString > > For this purpose, specific testing classes for Word8 data -with no claim to be "official" can be defined by the programmer, leaving to the library user to run the testing process for the different instances in his particular program. I know that this add extra work and that not many would do this extra level of abstraction, but perhaps this is less work than spending even more time exchanging emails, discussing low level issues, performing tests, trying to figure out what will be the most common sceniarios for wich the library could be used, testing manually what is the container that has the most performance here and now, not tomorrow for these scenarios, to worry about the obsolescence of the library for reasons not related with the library algorithm etc Regards

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