June 2013 - Beginners

TLS with RSA_PSK
by Friedrich Wiemer 22 Jun '13

22 Jun '13

Hey, is it possible to use RSA_PSK as a TLS-ciphersuit in any TLS library? I didnt found such a ciphersuit in Network.TLS. Thanks in advance, Friedrich

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haskell_doc.vim on Windows won't recognise g:haddock_docdir
by Tom Ayerst 21 Jun '13

21 Jun '13

Hi, Does anyone have any experience setting up haskell for vim on Windows? let g:haddock_browser = "C:\Users\Tom\AppData\Local\Google\Chrome\Application\chrome.exe" let g:haddock_docdir = "D:\tools\HaskellPlatform\2012.4.0.0\doc\html" in _vimrc the browser is getting recognised but the docdir isn't (or doesn't seem to be). Has anyone seen something like this (the problem or me making an obvious mistake?) Thanks Tom

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can this be made shorter?
by Emmanuel Touzery 21 Jun '13

21 Jun '13

hi, i want to know if a string contains only "." and "/" characters. i do something like that: null . dropWhile (\c -> c == '.' || c == '/') now it's a shame, because if I wanted only to check for "." then I would have: null . dropWhile (=='.') afaik in scala you can say: null . dropWhile (_ == '.' || _ == '/') which is a bit more compact than the haskell... I was thinking to use "and" but I'm not sure it would end up being readable... Any idea? Or I am trying too hard to make it compact? Emmanuel

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Re: [Haskell-beginners] Yesod error
by Lee Short 20 Jun '13

20 Jun '13

Thanks, Michael, persistLowerCase did the trick! (well, it showed another problem that looks to be related to diffs 1.0 -> 1.2, but I could figure that one out). > > I haven't finished updating the book for Yesod 1.2 yet; the function > persist has been removed in 1.2, and replaced with the two > alternatives > persistUpperCase and persistLowerCase. I recommend using the latter. > I'm > hoping to finish updating the Persistent chapter soon (it's about > halfway > done). > >

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Yesod error
by Lee Short 20 Jun '13

20 Jun '13

I am working my way through the Yesod book and I am having trouble writing any code that uses persist. I am consistently getting Not in scope: 'persist' at the point I am invoking the persist quasiquoter. This even happens when I am using the example code verbatim. Tried updating to the latest versions of yesod, persist, etc. Didn't help. The Hackage docs don't show any info on the quasiquoters in found in a package (presumably because they are a language extensio). Anybody seen this before? Or got any ideas how I could debut this? thanks, Lee

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Recursive XML schema and feeding into data model
by ashwin sathya 19 Jun '13

19 Jun '13

Hi, I have a recursive schema whose data model looks like the following {-data InformationField = InformationField { name, value :: String }-} {-deriving (Show, Eq) -} {-data Fields = Fields { fields :: [InformationField] }-} {-deriving (Show, Eq)-} {-data Children = Children { childNodes :: [BuildInformationNode] }-} {-data BuildInformationNode = BuildInformationNode { field :: Fields, children :: Children }-} BuildInformationNode has children which has multiple BuildInformationNode. I am getting started on HXT, using the samples that they had provided i was able to pull up individual nodes from xml. I am having difficult in starting to read the entire xml into the data model and have one single BuildInformationNode created which represents the entire xml. The idea is to implement a show on BuildInformationNode wherein i can implement for displaying the tree in a format that i require. Any pointers regarding this would be great. -- Thanks & Regards, R Ashwin Sathya

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iterateM
by Adrian May 18 Jun '13

18 Jun '13

Hi All, I just wrote a function to repeatedly transform something inside a monad using the same transforming function every time. I feel it might be dodgy though: iterateM :: (Monad m) => (a -> m a) -> m a -> m [a] iterateM f sm = sm >>= \s -> iterateM f (f s) >>= \ss -> return (s:ss) The context is that I was trying to write a state machine that responded to keyboard input: data Event = LoYes | LoNo | LoNum -- buttons on your phone | ReYes | ReNo | ReNum -- buttons on his phone data State = State { handler :: Event -> IO State } main = hSetBuffering stdin NoBuffering >> -- so you don't have to hit return iterateM (\st -> getEvent >>= handler st) (return idle) getEvent :: IO Event getEvent = getChar >>= \c -> case c of 'y' -> return LoYes 'n' -> return LoNo '0' -> return LoNum 'Y' -> return ReYes 'N' -> return ReNo '1' -> return ReNum _ -> getEvent idle, ringing, waiting, talking :: State idle = State $ \e -> case e of LoYes -> return idle LoNo -> return idle LoNum -> putStrLn "\tCalling somebody" >> return waiting ReYes -> return idle ReNo -> return idle ReNum -> putStrLn "\tIt's for you-hoo" >> return ringing -- other states similar The reason I'm worried is that this is the second time I've needed such a thing and it seems odd that it's not in the prelude already. Does it leak memory? Does it have a tail recursion problem? Is the functionality I want covered by something else? I guess I could consider [a] to be b in a regular monad but then the (\st -> getEvent >>= handler st) bit would have to juggle lists which seems meaningless. Am I missing something or does everybody else have this iterateM in their personal prelude? TIA, Adrian.

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Reform and blaze: trouble with example code
by Adrian May 18 Jun '13

18 Jun '13

Hi All, I'm trying to get this example working: http://patch-tag.com/r/stepcut/reform/snapshot/current/content/pretty/examp… It emitted what I took to be bitrot about ToHtml having apparently been generallised to ToMarkup and similar stuff, so I banged it into this form: {-# LANGUAGE FlexibleContexts, FlexibleInstances, TypeFamilies, OverloadedStrings #-} module Main where import Control.Applicative.Indexed import Control.Monad import qualified Data.ByteString.Char8 as C * import Text.Blaze * import qualified Text.Blaze.Html5 as H import qualified Text.Blaze.Html5.Attributes as A import Text.Blaze.Renderer.Utf8 (renderHtml) import Text.Reform * --import Text.Reform.Blaze.Common* * --import Text.Reform.Blaze.Text* * import Text.Reform.Blaze.String* import Text.Reform.Happstack import Happstack.Server import SharedForm instance *ToMarkup* (DemoFormError [Input]) where *toMarkup* InvalidEmail = "Email address must contain a @." *toMarkup* InvalidUsername = "Username must not be blank." *toMarkup* (CommonError (InputMissing fid)) = H.toHtml $ "Internal Error. Input missing: " ++ show fid *toMarkup* (CommonError (NoStringFound input)) = H.toHtml $ "Internal Error. Could not extract a String from: " ++ show input *toMarkup* (CommonError (MultiStringsFound input)) = H.toHtml $ "Internal Error. Found more than one String in: " ++ show input usernameForm :: (Monad m, FormInput input, *ToMarkup* (DemoFormError input)) => String -> Form m input (DemoFormError input) *Markup* NotNull Username usernameForm initialValue = ( *label "username: " ++>* (Username <<$>> inputText initialValue `prove` (notNullProof InvalidUsername))) {- usernameForm :: (Monad m, FormInput input, ToMarkup (DemoFormError input)) => String -> Form m input (DemoFormError input) Markup NotNull Username usernameForm initialValue = errorList ++> (*label "username: " ++> *(Username <<$>> inputText initialValue `prove` (notNullProof InvalidUsername))) -} blazeResponse :: *Markup* -> Response blazeResponse html = toResponseBS (C.pack "text/html;charset=UTF-8") $ renderHtml html blazeForm :: *Markup* -> *Markup* blazeForm html = H.form ! A.action "/" ! A.method "POST" ! A.enctype "multipart/form-data" $ do html H.input ! A.type_ "submit" formHandler :: (*ToMarkup* error, Show a) => Form (ServerPartT IO) [Input] error *Markup* proof a -> ServerPart Response formHandler form = msum [ do method GET html <- viewForm "user" form ok $ blazeResponse $ blazeForm html , do method POST r <- eitherForm environment "user" form case r of (Right a) -> ok $ toResponse $ show a (Left view) -> ok $ blazeResponse $ blazeForm view ] main :: IO () main = do let form = usernameForm "" simpleHTTP nullConf $ do decodeBody (defaultBodyPolicy "/tmp" 0 10000 10000) formHandler form where *italics* indicate the bits I changed, but now I'm stumped by the * bold* bit barfing with: Taser.hs:30:13: Ambiguous type variable `children0' in the constraints: (Data.String.IsString children0) arising from the literal `"username: "' at Taser.hs:30:13-24 (ToMarkup children0) arising from a use of `label' at Taser.hs:30:7-11 Probable fix: add a type signature that fixes these type variable(s) In the first argument of `label', namely `"username: "' In the first argument of `(++>)', namely `label "username: "' In the expression: (label "username: " ++> (Username <<$>> inputText initialValue `prove` (notNullProof InvalidUsername))) If I take out label "username: " ++>, then it all works fine, except I don't have a label. I also tried putting the label inside the Username constructor with the same result. I have the following versions installed: * blaze-markup (library) Versions installed: 0.5.1.5 * blaze-html (library) Versions installed: 0.6.1.1 * reform (library) Versions installed: 0.1.2 * reform-blaze (library) Versions installed: 0.1.2 The Glorious Glasgow Haskell Compilation System, version 7.4.2 Perhaps the markup thing is what broke it, but I can't see ToHtml in any of those modules. Thanks in advance, Adrian.

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Recommended DB layer?
by harry 17 Jun '13

17 Jun '13

I've seen yesod-persistent described as "worst practices", e.g. http://www.reddit.com/r/haskell/comments/1dk73s/web_frameworks_benchmark_4_…. Is this just a one-off poster, or do others agree with it? What are considered best practices?

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Haskell Platform: Troubleshooting package configuration errors?
by egarrulo 15 Jun '13

15 Jun '13

Hello everyone, I'm trying to build the Haskell Platform 2013.2.0.0 (with required GHC 7.6.3) on Debian GNU/Linux 6 (AKA Debian Squeeze). So far, I've stumbled upon some a few errors, but I've managed to solve them and I have let the maintainers know. Now I'm stuck with an error while configuring the "network" package (see the end of this email). As such error has no accompanying diagnostics, I don't know how I could fix it. Any suggestions? Thanks for your attention. Have a nice day. -- ERROR MESSAGE STARTS HERE -- ************************************************** Building network-2.4.1.2 "/home/egarrulo/bin/ghc/ghc-7.6.3/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal-1.16.0" [1 of 1] Compiling Main ( Setup.hs, Setup.o ) Linking Setup ... "./Setup" "configure" "--package-db=../../packages/package.conf.inplace" "--prefix=/home/egarrulo/bin/haskell-platform/haskell-platform-2013.2.0.0" "--with-compiler=/home/egarrulo/bin/ghc/ghc-7.6.3/bin/ghc" "--with-hc-pkg=/home/egarrulo/bin/ghc/ghc-7.6.3/bin/ghc-pkg" "--with-hsc2hs=/home/egarrulo/bin/ghc/ghc-7.6.3/bin/hsc2hs" "--with-happy=../happy-1.18.10/dist/build/happy/happy" "--happy-options=--template=../happy-1.18.10" "--with-alex=../alex-3.0.5/dist/build/alex/alex" "--enable-library-profiling" "--ghc-pkg-option=--package-conf=../../packages/package.conf.inplace" "--user" Configuring network-2.4.1.2... Error: Configuring the network-2.4.1.2 package failed make: *** [build.stamp] Error 2 -- ERROR MESSAGE ENDS HERE --

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