February 2011 - web-devel

[Yesod] Teach me how to write to access second database.
by Katsutoshi Itoh 28 Feb '11

28 Feb '11

Hi I write an application, which must to access 2 databases. 1st is main database for this application. 2nd database is used as main database by another application. Could you teach me how to write to access 2nd database? Since there is no access to the 2nd database only partially, I'm a little problem in an ad hoc way.

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ANN: Hakyll 3
by Jasper Van der Jeugt 28 Feb '11

28 Feb '11

Hello all, I've just uploaded the 3.0.0.0 version of Hakyll [1] to Hackage [2]. This is a complete rewrite, and completely backward-incompatible with previous versions. Sorry for that. On the other hand, I believe almost all aspects of the library have improved tremendously. Hakyll now uses a declarative EDSL in Haskell to specify compilation rules, and an arrow-based EDSL to describe the compilation process. If you want to have a quick look, I recommend have a glance at the example in the tutorial [3]. More documentation needs to be written, but I wanted to make sure I can incorporate your feedback into that process. [1]: http://jaspervdj.be/hakyll/ [2]: http://hackage.haskell.org/package/hakyll-3.0.0.2 [3]: http://jaspervdj.be/hakyll/tutorial.html Cheers, Jasper

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[WAI] Switching to WAI/Warp...
by Bardur Arantsson 27 Feb '11

27 Feb '11

Hi all, I've recently switched my "Hums" UPnP server over to using WAI/Warp, but I'm seeing quite high CPU usage compared to monadic I/O. (1-7% with WAI as opposed to 0-1% for monadic). (Just to be clear: The server is using a constant amount of memory, so it's definitely not leaking or anything like that.) I think the source of the efficiency problem is that I'm enumerating strict ByteStrings from a file, but the WAI response enumerator requires Builder chunks, so I'm forced to use "fromByteString" to convert between the strict ByteString representation and the Builder representation -- I'm assuming this performs a memory copy. Is there any way to avoid this extra "fromByteString"? Other than this little issue, WAI seems to be working very well and it seems like a great fit if you just want low-level access to the HTTP protocol. On a slightly related note: Is there an elegant way to map enumerator chunk elements from type a -> b using a mapping function? Right now, I've embedded the ByteString -> Builder conversion deep in my enumerator, but it should really be happening at a higher level. Cheers, Bardur

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Problem with multipart request on yesod/wai
by Nubis 27 Feb '11

27 Feb '11

Hi guys, I'm developing a webservice in Yesod that needs to accept multipart POST's with image data in them. This webservice will be used mainly by mobile devices. My handler that takes care of receiving the image works correctly when receiveng a POST crafted by google chrome (from a multipart/form-data encoded form), but when receiving a multipart post from a mobile device it fails to parse the multipart request arguments. I've setup a ruby on rails application with a similar handler, and both request from my mobile device and from a browser work, so I'm leaning towards thinking this is a yesod/wai issue. This is an excerpt from my handler, that I use to debug this: myHandler = do (params, files) <- runRequestBody liftIO $ putStrLn $ "params " ++ show params When requesting from the mobile device it just shows an empty list. I have dumps of both multipart requests, they seem pretty similar with the main differences being: * the multipart boundaries are longer in the browser request. * the request from the mobile device adds a trailing semicolon to the content-type of each part (except for the image) I attached the request that's not parsing correctly Any help will be greatly appreciated, thanks in advance! --nubis :)

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Enumerations?
by Vesa Kaihlavirta 25 Feb '11

25 Feb '11

Hey, I was just reading the new Persistent section and a thought came to me at the Custom Fields section. Have you considered implementing them as database enums? They would be vastly more efficient than storing as strings. --vk #yesod on freenode!

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Get Request Page
by Eduardo Ribeiro 25 Feb '11

25 Feb '11

Hi, I want to use the api of this site http://www.vagalume.com.br/ to get lyrics. They say to send a get request with params indicating what I want. How do I send a GET request to a page ( and retrieve the json output) using Yesod?

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Re: [web-devel] Sql join feature in persistent
by Greg Weber 24 Feb '11

24 Feb '11

The basic Persistent backend is designed without joins ("web-scale"), and with the idea that it should be SQL agnostic. App-level join functionality is definitely needed. If someone wanted to design a layer on top of it with other SQL features that just the SQL backends can use that is fine. However, this might start re-inventing the wheel. For those interested in SQL joins and otherwise advanced SQL queries, it would be great if they started trying out a better tool for the job, like the recently released DSH [1] or (bitrotting?) HaskellDB. Relational Algebra sql query libraries can be very productive to use for 95% of use cases, even if they aren't "web-scale". [1] http://hackage.haskell.org/package/DSH [2] http://hackage.haskell.org/package/TableAlgebra

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[Yesod] rendering different templates for different languages
by Dmitry Kurochkin 24 Feb '11

24 Feb '11

Hi all. I want a handler to render different templates for different languages. I have getCurrentLanguage function and now I try to do something like: getRootR = do currentLanguage <- getCurrentLanguage defaultLayout $ do addWidget $(widgetFile $ currentLanguage ++ "/homepage") This results in: GHC stage restriction: `currentLanguage' is used in a top-level splice or annotation, and must be imported, not defined locally This makes sense to me, because TH is calculated at compile time. I would like to hear ideas how to work around this restriction. Perhaps there is an existing solution in Yesod? At the moment, the best I could think of is smth like this: getRootR = do currentLanguage <- getCurrentLanguage defaultLayout $ do case currentLanguage of "en" -> addWidget $(widgetFile "en/homepage") ... and so on for each language ... Obviously, this is not a solution taking in account that there are many languages and many handlers. I was considering creating a global (template file name -> rendered template) map. But I am not sure this is really feasible. Regards, Dmitry

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[Yesod] Why required textarea fields are not required?
by Dmitry Kurochkin 22 Feb '11

22 Feb '11

Hi Michael. textareaFieldProfile defines fpWidget as: , fpWidget = \theId name val _isReq -> addHamlet [HAMLET|\ <textarea id="#{theId}" name="#{name}">#{val} |] Can you tell why the required flag is ignored? It feels like a bug, but perhaps it's done on purpose? Regards, Dmitry

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Fwd: Sql join feature in persistent
by Paolo Losi 22 Feb '11

22 Feb '11

2011/2/22 Антон Чешков <acheshkov(a)gmail.com> > I want join its by some rule and take only first "N" results. In this case > i do not want serve all datasets entirely. I would like to make as little > as possible. > I think this algorithms are exist. > Those algorithms exist indeed and have been implemented in last 25 years inside RDBMS as query optimizers. The question is, how much is that reasonable to re-implement in Haskell/Persistent? My 2 cent guess is that there are two viable options: 1) For Relational Backends, when scalability concerns raised by Max are not important, the approach of a monadic DSL a la HaskellDB is the best. Basically the library job is to build the SQL query and let the RDBMS optmizer do its job. 2) For NOSQL Backends, the solution is to code your hand rolled queries as it can be done with persistent now (the "do nothing" solution). That is the approach suggested for redis, for example... Paolo -- Paolo Losi e-mail: paolo(a)enuan.com mob: +39 348 7705261 ENUAN Srl Via XX Settembre, 12 - 29100 Piacenza

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