January 2009 - Haskell-Cafe

Fwd: [Haskell-cafe] Haskell Web Framework
by Artyom Shalkhakov 25 Jan '09

25 Jan '09

Sorry, forgot to forward to the ML. ---------- Forwarded message ---------- From: Artyom Shalkhakov <artyom.shalkhakov(a)gmail.com> Date: 2009/1/25 Subject: Re: [Haskell-cafe] Haskell Web Framework To: Michael Snoyman <michael(a)snoyman.com> Hello, 2009/1/25 Michael Snoyman <michael(a)snoyman.com>: > Just as a quick summary of the post, I would say the most salient points are > that the framework should work in a shared hosting environment and should > automatically abstract away most of the issues of writing an Ajax web > application. It should also leverage the strengths of Haskell, eg type > safety and speed. I'm doing some work on the same lines. I have a CGI test application running in a sandboxed environment that resembles shared hosting quite closely; nothing fancy ATM. I think more people will get interested in this over time. While HAppS is great, many people (I suppose :)) would like to use Haskell instead of PHP (well, at least I do). Regarding your blog post, let me comment it here. HStringTemplate can be used to handle templating in a principled fashion. There is however, some room for improvement (like, generating some structured data instead of plain strings, then enforcing constraints on that data, -- think (X)HTML validation at template "compile time"). User input validation is handled by formlets already. It is tempting to generate JS automagically, don't know if it will work out. WSGI-like abstraction would be great (I think of unifying FCGI/CGI apps, but AFAIK it's done already: there's a way to run Haskell FCGI applications as CGI applications, though it's not documented in tutorials). I prefer libraries to frameworks, because frameworks are just "preconfigured", "prewritten" bloated code bases (maybe, it's just me understanding it wrong...). Cheers, Artyom Shalkhakov.

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Re: list choices
by Yitzchak Gale 25 Jan '09

25 Jan '09

Both of the points raised by Malcolm are a matter of personal preference, in my opinion. In fact, my own preference is the opposite of Malcolm's in both cases. Both of Malcolm's suggestions would rob me of filtering capability. Malcolm Wallace wrote: > I would suggest that posting announcements *only* > to haskell@, but with followups set to haskell-cafe@, > is the ideal way to keep everyone informed... I am usually not interested in the posts to haskell - calls for papers and the like. So I receive it as a digest it and rarely look at it. Posts that interest me, such as announcements of new packages on hackage, tend to be cross-posted to haskell-cafe. That is ideal for me. > ...it would reduce _so_ much irritation > if all the haskell-related mailing lists could set the Reply-To header to > the list, instead of defaulting to the original poster. I filter haskell-cafe traffic into a place where I can browse it if I have time. But when I am participating in a thread, I want its messages to pop up in my inbox. Forcing active threads to the inbox would take work (given my setup), and the result would be far less accurate. I appreciate people's efforts to include me in a conversation by mentioning me as a recipient. Regards, Yitz

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ANNOUNCE: DecisionTree 0.0
by Adrian Neumann 25 Jan '09

25 Jan '09

Hello, it is my pleasure to announce the DecisionTree package. It provides an implementation of the ID3 algorithm [1] and can be used to classify data with discrete valued attributes. You can get it from * hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ DecisionTree * my own webspace: http://page.mi.fu-berlin.de/aneumann/decisiontree/ DecisionTree-0.0.tar.gz At this stage it is very likely buggy and slow, as I don't have a sufficiently large dataset to test with. You are encouraged to have a look at the source, it is quite short and well commented. Please feel free to make suggestions, send in patches or point me to some dataset I could use for testing. Regards, Adrian

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Fold that quits early?
by Andrew Wagner 24 Jan '09

24 Jan '09

This is almost a fold, but seemingly not quite? I know I've seem some talk of folds that potentially "quit" early. but not sure where I saw that, or if it fits. f xs [] = False f xs (y:ys) | any c ys' = True | otherwise = f (nub (y:xs)) (ys' ++ ys) where ys' = g y xs

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Generic typeclass for converting between types
by John Goerzen 24 Jan '09

24 Jan '09

Hi folks, I've been thinking today that I frequently need to convert data beween types: * Between various numeric types * Between various calendar types (both within the new calendar system, and between the old and new) * Marshalling data back and forth to a database in HDBC It's hard to remember all the functions to use to do these. I often resort to a chart I made for numeric conversions. It occurs to me that it would be nice to be able to (convert (5.8::Double))::Int or (convert calendarTime)::ZonedTime So, the first question is: does something like this exist already? I'm not aware of it, but I'm not sure how to search either. I'm thinking of something like the below. With a little magic, it's quite possible to make errors easy to generate in the safe fashion (for instance, when converting from String to Integer using reads). {-# LANGUAGE MultiParamTypeClasses #-} import Control.Monad.Error type ConvertResult a = Either ConvertError a class Convertible a b where safeConvert :: a -> ConvertResult b instance Convertible Int Double where safeConvert = return . fromIntegral instance Convertible Double Int where safeConvert = return . truncate -- could do bounds checking here instance Convertible Integer Double where safeConvert = return . fromIntegral instance Convertible Double Integer where safeConvert = return . truncate convert :: Convertible a b => a -> b convert inp = case safeConvert inp of Left e -> error (show e) Right x -> x -- rudimentary error type for this example data ConvertError = ConvertError { sourceValue :: String, errorMessage :: String } deriving (Eq, Read, Show) instance Error ConvertError where strMsg x = ConvertError "(unknown)" x The other option is to use an approach more like I have in HDBC. In HDBC, there is a direct need to encapsulate data for transport, so I've got this: class (Show a) => SqlType a where toSql :: a -> SqlValue safeFromSql :: SqlValue -> FromSqlResult a data SqlValue = SqlString String | SqlByteString B.ByteString | SqlWord32 Word32 | SqlWord64 Word64 ... many more .... ... instance SqlType Int32 where sqlTypeName _ = "Int32" toSql = SqlInt32 safeFromSql (SqlString x) = read' x safeFromSql (SqlByteString x) = (read' . byteString2String) x safeFromSql (SqlInt32 x) = return x safeFromSql (SqlInt64 x) = return . fromIntegral $ x The advantage of this is that if you've got a whole slew of types and you're going to be converting between all of them (for instance, numeric types), if you turn on -Wall the compiler will help you know when your safeFromSql instance doesn't convert everything. The disadvantage is that the type system doesn't enforce whether or not it is even possible to convert certain things (for instance, a TimeOfDay to a Char), and so we have to return a Left for those. Any thoughts? -- John

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ANN: orchid-0.0.7
by Sebastiaan Visser 24 Jan '09

24 Jan '09

I am pleased to announce a new release of Orchid, just another Haskell Wiki. Some new features/fixes: * Attached wiki to FileStore package which means we now also have Git backend. * Orchid-demo now has a flag which allows user to choose Git or Darcs. * Searching document is now possible. * Deleting and renaming documents is now possible. * Better keyboard support in user interface. * Fixed the in-page HTML anchors. * Password are now saved in md5, not plain text. * Use extensible exceptions to be more compatible with older GHC. * Fixed bugs in directory listings. * Users now also have a email. Watch the demo at: http://funct.org/wiki/ Or get the demo from cabal: $ cabal install orchid-demo

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ANN: gitit 0.5
by John MacFarlane 24 Jan '09

24 Jan '09

We are pleased to announce the latest release of Gitit, the multitalented distributed wiki written in Haskell. What's new in this release? * 'Gitit' is now something of a misnomer, as Gitit now makes use of the filestore library. The upshot is that Gitit can now work in Darcs repositories; if before you thought Gitit was cool but didn't like git, you now have no excuse! * Gitit has been optimized (thanks to the efforts of Gwern Branwen): - Pages are now gzipped whenever possible. - Expire headers are set so that CSS, images, and javascripts will be cached by the browser. - Those previously mentioned elements have been optimized for space, and rearranged in the generated pages for faster rendering. * A new configuration option allows reStructuredText to be used instead of markdown (thanks to a patch from Simon Michael). * Gitit now sports a 'Go' box. You can now either search through Search, or go to a specific page title via Go. * Search is smarter, and can also return page titles. * Revisions are better formatted: the author field now include the user's email as well. * Diffs are of much higher quality, thanks to filestore. * UTF8-related fixes. You can check out a running example at http://gitit.johnmacfarlane.net/. Or try it locally: cabal update cabal install -fhighlighting pandoc gitit gitit # note: this will create two subdirectories in the working directory # then browse to http://localhost:5001. The git repository for Gitit is at http://github.com/jgm/gitit/tree/master

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Arch Haskell News: Jan 24 2009
by Don Stewart 24 Jan '09

24 Jan '09

A regular update of Haskell in Arch Linux http://archhaskell.wordpress.com/2009/01/24/arch-haskell-news-jan-24-2009/ * Arch now has 864 Haskell packages in AUR. That’s an increase of 37 new Haskell packages in the last 13 days, or 2.8 new Haskell apps and libraries a day packaged so far in January. Cheers, Don

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Factory methods in Haskell
by Jeremy Shaw 24 Jan '09

24 Jan '09

Hello, I was reading about the "Factory Method Pattern" on wikipedia, and noticed that the very first example was written in Haskell. Sweet! http://en.wikipedia.org/wiki/Factory_method_pattern#Haskell Unfortunately, it looks to me like it is missing the 'factory' part. I have attempted to implement something more factory like (see attached). I am wonder what other people think. Is the code on wikipedia really demoing a factory method? Is the code attached any better? Is there an even better what to write this in Haskell? Thanks! - j {-# LANGUAGE ExistentialQuantification #-} import Numeric (showFFloat) -- * A type which can hold different types of pizzas data Pizza = forall a. (PizzaMethods a) => Pizza a -- * A type class with functions common to different types of pizza class (Show a) => PizzaMethods a where price' :: a -> Double -- * Getter functions for the pizza type price :: Pizza -> Double price (Pizza p) = price' p pizzaType :: Pizza -> String pizzaType (Pizza p) = show p -- * Some types of pizza data HamAndMushroom = HamAndMushroom deriving (Read, Show) data Deluxe = Deluxe deriving (Read, Show) data Hawaiin = Hawaiin deriving (Read, Show) -- * Prices of various pizzas instance PizzaMethods HamAndMushroom where price' _ = 8.50 instance PizzaMethods Deluxe where price' _ = 10.50 instance PizzaMethods Hawaiin where price' _ = 11.50 -- * A pizza factory pizzaFactory :: String -> Pizza pizzaFactory pizzaType | pizzaType == "HamAndMushroom" = Pizza HamAndMushroom | pizzaType == "Deluxe" = Pizza Deluxe | pizzaType == "Hawaiin" = Pizza Hawaiin | otherwise = error "We don't serve your kind here." -- * An order at the pizza factory main = let pizza = pizzaFactory "HamAndMushroom" in putStrLn $ "You can get a " ++ pizzaType pizza ++ " for $" ++ showFFloat (Just 2) (price pizza) "."

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ANNOUNCE: Scurry :: A cross platform (if you put your mind to it) P2P VPN
by John Van Enk 24 Jan '09

24 Jan '09

Hello All! I've finally brought Scurry, my P2P VPN application written in Haskell (and a little C), to a point where I'm comfortable letting the world more formally know that it's out there. Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Scurry SVN: http://code.google.com/p/scurry/ Reddit: http://www.reddit.com/r/haskell/comments/7s653/a_cross_platform_p2p_vpn_app… Patches, questions, and criticism are more than welcome! /jve

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