- web-devel - Haskell.org

[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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[Yesod] widgets in default layout
by Dmitry Kurochkin 22 Feb '11

22 Feb '11

Hello. I am trying to make a menu widget for a site. It would render list of menu items and mark the active one. I started with a new Menu module that exports (mainMenu :: Widget ()) function. The function gets the current route, iterates over a list of (item title, item route) list and constructs the menu. Now I can import Menu in a handler module and use ^{mainMenu} in hamlet template. Next I tried to put the menu widget to default layout - menu should be on every page so default layout is where it belongs. Rest of the email describes problems I got. 1. Module import loop. I need to import the widget module in the module which defines defaultLayout, i.e. the main application module. But the widget module uses the main application module to have routes and probably other staff, hence import loop. I tried to separate foundation and route declaration from Yesod instance declaration, so that the widget module could import just routes declaration and Yesod instance could import the widget module. Turns out that does not work: Attempting to interpret your app... Compile failed: Menu.hs:20:23: Couldn't match expected type `Route m' against inferred type `TestAppRoute' NB: `Route' is a type function, and may not be injective In the first argument of `\ u[a7Kv] -> hamlet-0.7.0.2:Text.Hamlet.Quasi.urlToHamletMonad u[a7Kv] []', namely `AboutR' In a stmt of a 'do' expression: \ u[a7Kv] -> hamlet-0.7.0.2:Text.Hamlet.Quasi.urlToHamletMonad u[a7Kv] [] AboutR In the first argument of `hamlet-0.7.0.2:Text.Hamlet.Quasi.toHamletValue', namely `do { (hamlet-0.7.0.2:Text.Hamlet.Quasi.htmlToHamletMonad . preEscapedString) "<div id=\"menu\"><div class=\"left\"><div class=\"right\"></div><div class=\"container\"><a href=\""; \ u[a7Kv] -> hamlet-0.7.0.2:Text.Hamlet.Quasi.urlToHamletMonad u[a7Kv] [] AboutR; (hamlet-0.7.0.2:Text.Hamlet.Quasi.htmlToHamletMonad . preEscapedString) "\">itemTitle item</a></div></div></div>" }' I do not understand details, but it is clear that the widget needs the Yesod instance. So I have to put the widget in the main application module. This does not look good considering that application may have many widgets. I have also stumbled upon a bug in devel server: It tries to recompile the source in a loop, without changing source of course. Annoying but not a critical issue. 2. Widget does not work in default layout. I guess this is a known and expected behavior. My feeling is that hamletToRepHtml can not embed widgets because it may be too late to add cassius and julius. As a workaround I split default layout into outer and inner layout. Outer layout renders just HTML <head> and <body>. While outer layout is rendered as a widget that embeds the actual page contents. Since outer layout is rendered as a widget, it may embed other widgets like menu. I imagine that hamletToRepHtml could render all embedded widgets before the main body. Though, it may be difficult to implement, have performance or other issues. Anyway, I think it is not uncommon to include a widget in default layout. So Yesod should provide an easy way to do it. The last issue is that (mainMenu :: Widget ()) does not work, I had to change it to (GWidget sub TestApp ()). Again I do not know details, but this was unexpected to me. Perhaps the Widget type synonim should be changed? I would appreciate advices on how to solve the above problems. Perhaps I am just missing something and there is a proper way to do what I want. For now I will avoid using widgets in default layout and move part of layout to individual handler templates, primarily because I find it too ugly to define widgets in the main application module. Regards, Dmitry

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Sql join feature in persistent
by James 22 Feb '11

22 Feb '11

Hi all, I'm new to this mailing list. I did web development in python with Django framework before. But I really like the haskell language, so considering building my next site (a LBS-SN like service) in haskell. I learn snap for a while and think it's not so mature and powerful. After I learned the yesod docs in one day and read some yesod and haskellers.com codes in the following few days, I appreciate it and try to use it seriously. Now I need a database query across two table, so as to sql join feature. Currently, persistent doesn't provide this functionality. I think maybe adding a function to PersistBackend class, and implementing it in PersistBackend (SqlPersist m) may solve the problem. However, I am not sured. Will it be difficult and consistent to implement this feature? Thanks for any help. Sincerely, -- James Deng department of computer science, school of information science & technology, Sun-yat-sen University, Guangzhou, China mailTo: cnJamesDeng(a)gmail.com homepage: http://cnjdeng.appspot.com

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