March 2012 - web-devel

"Wiki" section of the Yesod doc site
by Michael Snoyman 13 May '12

13 May '12

Hi everyone, I've added a new section to the Yesod doc site: the wiki[1]. This section is intended to be user-writable, so if you have any content you'd like to submit, please go ahead. In particular, this section contains the "Powered by Yesod" page: please add your sites. Unfortunately, the list from the old wiki was lost when my previous VPS died. (All my databases were backed up, I just forgot to backup the Wiki content as well...) Michael [1] http://www.yesodweb.com/page/wiki

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ANN: Happstack 7
by Jeremy Shaw 30 Mar '12

30 Mar '12

We are pleased to announce the release of Happstack 7! Happstack is a fast, modern, web application framework written in Haskell. Please check out the brand new happstack.com website to read about what is new in Happstack 7, and what we are planning for Happstack 8, and what makes Happstack great! http://www.happstack.com/ - jeremy

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Re: [web-devel] [Haskell-cafe] Fail-back monad
by Alberto G. Corona 28 Mar '12

28 Mar '12

Hi Stephen: It could be: It performs a rollback indeed. I guess that this monad can be used in something similar to nonblocking (multilevel) transactions. if "GoBack" is changed to "Goback String", and a trace string is added to each NoBack step, then each NoBack step could sum the traces of all the previous steps. The when GoBack condition is reached, it can transport back the entire trace of the failed execution. to the BackPoint 2012/3/28 Stephen Tetley <stephen.tetley(a)gmail.com>: > Maybe this is a version of William Harrison's DebugT monad with > rollback, listed in his periodic table of effects? > > http://www.cs.missouri.edu/~harrisonwl/Presentations/UIUCFM05.ppt > > I've never seen a definition of the monad itself... > http://www.haskell.org/pipermail/beginners/2010-January/003371.html

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GSoC project idea: haskell-type-exts
by Niklas Broberg 28 Mar '12

28 Mar '12

See ticket here: http://hackage.haskell.org/trac/summer-of-code/ticket/1620 == What? == The haskell-src-exts library (HSE) is very popular for manipulating Haskell source code in a wide range of scenarios, such as (syntactic) preprocessors (e.g. hsx), syntax highlighting, refactoring and code inspection (e.g. hlint), etc. A commonly sought feature, that would enable a multitude of new and improved scenarios, is the ability to type-check the code, and annotate the AST with type information. Type-driven refactoring, type-aware code inspection, type-based preprocessing, ... Lots of pure awesomeness lies in that direction. Sadly, HSE has no type-checking abilities. For those who seek such, we are forced to point in the direction of the dark cave that is the GHC API. While GHC is an utterly awesome compiler, there are several reasons why this is not an ideal situation. First, GHC was designed and built for optimization, which means that even the early stages of the compilation pipeline contain features that cater to later optimization, but that are (at best) unnecessary for a front-end tool. Second, the API into GHC internals is defined as an afterthought. Some good work has been done to make it somewhat palatable, but it remains a fact that GHC's primary purpose is not to be a library for integration in tools, and this shows. Overall, depending on GHC when all you want is front-end capabilities like parsing and typechecking, is really like using the proverbial sledge hammer. Also, at the meta level, GHC deserves "competition". Right now GHC is de facto the only implementation of Haskell, and as such it more or less defines the progress of Haskell. Too often this leads to overly pragmatic choices, which lead to inconsistencies and headaches for tool programmers. Having a separate, stand-alone front end library would help set higher standards. So - what we need is a type checker that works on the HSE AST: haskell-type-exts! == How? == I now hear you think: "This is too much, an enormous project, surely it can't be done as a GSoC project?" Sure it can! By walking in the footsteps of giants, and following GHC's approach. This can be achieved by closely observing the documentation provided by [1] and [2], with a bit of [3] thrown into the mix. I already have a basic library that provides most of the core framework necessary to cover extensions, but at this time it's mostly just scaffolding. It needs a lot of love, to provide cover for all the various available extensions, but I'm confident it can be done satisfactorily as a GSoC project. Still, it would be reasonable to modularize the task. Certain features are more crucial than others to support, simply because they have a more profound impact on the overall type-checking. Higher-rank types, GADTs and type families (and fundeps) are likely the most important ones to get right. There are also some design issues to get right (e.g. using pure substitution vs efficient mutable variables). I (Niklas Broberg) am more than willing to mentor this project. == Further reading: == [1] S. Peyton Jones, D. Vytiniotis, S. Weirich, and M. Shields. Practical type inference for arbitrary-rank types. [2] D. Vytiniotis, S. Peyton Jones, T. Schrijvers, M. Sulzmann. OutsideIn(X) – Modular type inference with local assumptions [3] M. P. Jones. Typing Haskell in Haskell

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Fail-back monad
by Alberto G. Corona 28 Mar '12

28 Mar '12

Hi Haskellers. In my package MFlow [1] I program an entire web navigation in a single procedure. That happened in the good-old WASH web application framework. The problem is the back button in the Browser. To go back in the code to the previous interactions when the data input does not match the expected because the user pressed the back button one or more times, i came across this Monad specimen,: that solves the problem. data FailBack a = BackPoint a -- will go back to this point | NoBack a . -- Normal outcome | GoBack -- "exception":: must go to the last backPoint newtype BackT m a = BackT { runBackT :: m (FailBack a ) } -- this monad nas a loop instance Monad m => Monad (BackT m) where fail _ = BackT $ return GoBack return x = BackT . return $ NoBack x x >>= f = BackT $ loop where loop = do v <- runBackT x case v of NoBack y -> runBackT (f y) -- business as usual BackPoint y -> do z <- runBackT (f y) case z of GoBack -> loop -- if x was a backpoint, then redirects the flow to this backpoint other -> return other GoBack -> return GoBack --propagate the signal back This monad does not perform exploration of alternatives as is the case of MonadPlus instances. It does not perform the kind of backtracking of "nondeterministic" three navigations in the Prolog style. It just go back to the last point where the computation can restart again in a sequence of actions. In this example: liftBackPoint f= BackT $ f >>= \x -> return $ BackPoint x test= runBackT $ do lift $ print "will not return back here" liftBackPoint $ print "will return here" n2 <- lift $ getLine lift $ print "second input" n3 <- lift $ getLine if n3 == "back" then fail "" else lift $ print $ n2++n3 Whenever the second input is "back" The procedure will go back to where liftBackPoint is. Otherwise, it will return the concatenation of the two inputs. If the underlying monad is an instance of MonadState, it can transport the state that caused the failure to the backpoint. Are there something similar? May it be functionally equivalent to something simpler or with more grounds? I looked at some exception monads out there, but they did not seems to share the same idea [1] http://haskell-web.blogspot.com.es/2012/02/web-application-server-with-stat…

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GSoC project ideas for web technology
by Jeremy Shaw 25 Mar '12

25 Mar '12

Hello, If you are looking for some ideas for a GSoC project, I have written down some web technology related projects I would like to see. So far I mostly have ideas for improvements to HSX (a templating solution) and acid-state (a pure, haskell persistent datastore). Both these technologies can be used with any Haskell web framework (though they are embraced most fully by Happstack). http://code.google.com/p/happstack/wiki/GoogleSummerOfCode I am an officially registered Haskell GSoC mentor. I am happy to help expand any of these ideas into a genuine GSoC proposal and to act as a mentor on the project. If you have other ideas you wish to pitch, I am happy to hear those as well. - jeremy

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ANN: sendfile 0.7.6
by Jeremy Shaw 21 Mar '12

21 Mar '12

Hello, I just uploaded sendfile 0.7.6 to hackage. This should fix issues with the build failing on 32-bit Linux with GHC 7.4.1. (Such as the hackage build bot). Let me know if you still experience any build failures. - jeremy

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happstack code reloading
by Asafe Ribeiro 19 Mar '12

19 Mar '12

Hi, Since one extra parameters was added to the plugin lambda my hmt code does no update when I refresh the page. As my code is in hierarchical modules , I tried the hack of changing System.Plugin.Make.build but to no sucess. Maybe am I missing something? {-# LANGUAGE CPP, TemplateHaskell,NoOverloadedStrings #-} module Main where import Control.Monad(msum) import Happstack.Server import Control.Monad.IO.Class(liftIO) import qualified Page as P import qualified Handler as H #define DESENVOLVIMENTO --plugins #ifdef DESENVOLVIMENTO import Happstack.Server.Plugins.Dynamic #else import Happstack.Server.Plugins.Static #endif main = do dev <- initPlugins simpleHTTP nullConf $ do decodeBody (defaultBodyPolicy "/tmp/" 106496 106496 106496) msum [ $(withServerPart 'H.user) dev devHandler , dir "book" $ $(withServerPart 'H.book) dev devHandler } where devHandler = (\ _ handler -> handler)

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WAI routing?
by Stephen Paul Weber 18 Mar '12

18 Mar '12

I looked around, but could not find a lot of routing packages for WAI. There is wai-routes, but QuasiQuote makes me nervous. So, I hacked together a very simple router (attached). It works, though it is not efficient for large routing tables (it just does a linear scan over all possible routes in the worst case). I'm curious: (a) what are other people using? (b) are there glaringly stupid things I've done in my code? Thanks :) -- Stephen Paul Weber, @singpolyma See <http://singpolyma.net> for how I prefer to be contacted edition right joseph

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WAI and ^C
by Stephen Paul Weber 17 Mar '12

17 Mar '12

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 My simple WAI apps will not die when I hit ^C, but when I use Scotty, ^C seems to work fine. I've read through the Scotty code and do not see anything that would cause this. Any tips? - -- Stephen Paul Weber, @singpolyma See <http://singpolyma.net> for how I prefer to be contacted edition right joseph -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) iQIcBAEBCAAGBQJPZTv0AAoJENEcKRHOUZze7CkQAMz6Hm9szR8td15Nc67uq1HD lYVsAkPnrwk3vPkLbzYmnVxIjkaXejVXS7GDT/snSHFA/JjxWsQxL4EoDhHjqVOB zSJFutGXPO8yPUSSy2B6NVFLeXG8f+l+8sf+SEq0N4xdq3gpdMfb6J+BoHJ/MkW9 E0CEAWHlOFkePuBL3L18KWcZpaFXVpcrWr8SNv1/ZfIDgo3NYX9Yz2sfjGzuBnDq da6uAPwkEtNHJTCDNS47OZf8jsqi9yVH5xSuQ388WzD3oK1uXl79nRhOsptXGm/b 2xehxCnkFeW/KALvFl6zzlL4pzMSt2u/FnhFOIfhFHs+A2ZJfzJwl0nN/9Af5Vqe Rfx6jQyKOMgvjNp2f3ng9BCfgcj9J7UjpLmjBBqtaTWeGVY+Fx9LNYIeiPK4oFLb 8OMaPbVCU93Fz+3yUSily3rw4nH+/7e8fOksUnZAd1UTThIvL7n9rjZlu2Pg+My8 8G410tMSabGPagkTkm682E1gaW8aHLhsWvCcNFaknElYso13aUxBFN5TcivnnxSh qf0Hpic3G9/AqGhUmoEgC/HPq7LTeZk4o3cfplUedsyvLrTW8Fv3AwIcvh16Y5UQ CW8WVzpoZEUIwUnbeZKLmPsJVACw+e2Sv8PUPpMXOR2bH5YNThWelhDeW+Pyo1M4 7q3E422kOUIHXjlDDL2b =Hd77 -----END PGP SIGNATURE-----

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