December 2010 - Haskell-Cafe

haskell platform windows download url
by Albert Y. C. Lai 20 Dec '10

20 Dec '10

On http://hackage.haskell.org/platform/windows.html , the link for "download haskell for windows" is: http://lambda.galois.com/hp-tmp/2010.2.0.0/HaskellPlatform-2010.2.0.0-setup… Is this normal?

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REPL loop
by Aaron Gray 20 Dec '10

20 Dec '10

Is it possible to implement a REPL (Read-eval-print loop) in Haskell ? Many thanks in advance, Aaron

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Haskell on Debian - gcc compilation problem with cabal installed libs like hmatrix, etc.
by gutti 20 Dec '10

20 Dec '10

Hi, I'm very new to Haskell and this Forum, just doing my first steps ... -- Try to use the hmatrix package for vector and matrix calculations. The haskell compilation works (no problem in GHCi mode), the gcc however compilation fails with messages like: EFA.o: In function `r1bo_info': (.text+0x48): undefined reference to `hmatrixzm0zi10zi0zi0_NumericziVector_zdfNumVector1_closure' EFA.o: In function `r1bs_info': (.text+0x10a): undefined reference to `hmatrixzm0zi10zi0zi0_NumericziVector_zdfFractionalVector_closure' Obviously gcc doesn't know where the according libraries are installed. Same problem with other packages. I installed the haskell base system over debian (debian squeeze), gcc version 4.4.5. - Hmatrix and other packages are installed and build over cabel into local folder ~/.cabal/lib Do I need to set a system path, or change cabal options or .. ? Thanks in advance, Phil -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-on-Debian-gcc-compilation-prob… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Snap 0.3 Released!
by Gregory Collins 20 Dec '10

20 Dec '10

Hi all, After many months of hard work, the Snap team is proud to announce the release of the next major version of the Snap Framework: Snap 0.3. New Features ------------ - Snap now has SSL support! To enable SSL support, install the gnutls library and pass the "gnutls" in when you build snap-server: $ cabal install snap-server -fgnutls - The snap project starter executable has been pulled out of snap-core and put into its own package, called "snap". The snap package also contains some library code which the generated projects make use of, and the generated projects have a simplified and much cleaner layout. - Snap now comes with support for working in a "development mode" which runs your web handlers in an interpreter using the hint library. This makes working on Snap applications quicker and more convenient, since you no longer need to perform a kill/recompile/reload cycle to see your code changes reflected in your running application. - One of the new things in the snap package is an experimental "extensions" mechanism which contains some interfaces/helper code for writing reusable web components. - A new MonadSnap typeclass has been defined: class (Monad m, MonadIO m, MonadCatchIO m, MonadPlus m, Functor m, Applicative m, Alternative m) => MonadSnap m where liftSnap :: Snap a -> m a Most of the basic functions from Snap.Types have been redefined to accept any monad implementing MonadSnap, and MonadSnap instances have been defined for most of the standard monad transformers; this should make it much more convenient to use monad transformers with Snap without having to use "lift" all over the place. - Added getCookie :: MonadSnap m => ByteString -> m (Maybe Cookie), which allows you to fetch cookies from the request by name. - Snap now depends on the enumerator package for iteratee support instead of iteratee. - The snap-server now has a convenient configuration mechanism which will parse arguments from the command line for you. Bugfixes/Improvements --------------------- - Changed type signature of finishWith from Response -> Snap () to Response -> Snap a. - When using fileServe, file paths are now properly URL-decoded, meaning you can now download a file with spaces in its name. - Parameters grabbed by variable capture using route are also now URL-decoded. - Under the hood: snap server backend mechanism has been improved/factored out, making it possible to listen to two or more ports using different backends at once (e.g. SSL and non-SSL). Thanks, G -- Gregory Collins <greg(a)gregorycollins.net>

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handling multiple versions of a data structure
by Dmitry V'yal 20 Dec '10

20 Dec '10

Greetings, while developing my neural net simulator I stumbled upon a problem. I have a data type NeuralNet and use Show and Read instances for saving and loading configurations. As time passed, I changed the data type, so the program can no longer load files saved in previous versions. I want fix it. My current idea looks as follows. I'm going to create a bunch of types NN1, NN2, NN3..NNn for different versions and write converters c12 :: N1 -> N2, c23 :: N2 -> N3 and so on. But how to organize the whole process of parsing String into NNn so it's easy to change formats? Something based on using a list of parsers [read, c43 . read, c43 . c23 . read, c43, c23 . c12 . read, c43 . c32 . c21 . read] looks rather verbose and grows quadratically with N. I'm sure there must be a more elegant way. Any ideas? Dmitry

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cabal dependency on code repositories.
by Greg Weber 20 Dec '10

20 Dec '10

Michael Snoyman and I were discussing the need for beta releases of Yesod and he encourage me to post this to the cafe. Beta releases could be built into the hackage system. However, this can be viewed as a more general problem of distributing multiple versions of code (stable vs. experimental, forks, etc). This is a problem that has been largely solved by version control, but in this instance just needs some integration with an installer system. In the ruby world there is a tool called bundler that is now the standard installer- basically bundler combined with rubygems it is like cabal combined with hackage. In addition to referencing a published library version, with bundler one can reference a git repository, and even reference a particular branch or revision. Essentially with beta releases you are trying to release multiple branches of your repository. This is also very useful for dealing with multiple forks of a package. If I have problems with a package I can look at the github repo and look for forks with bug fixes, and then point the Gemfile (cabal file) to that repo. The repo must have a gemspec (cabal file). Essentially this removes the step of the code writer of needing to edit the cabal file or publish to hackage. This is good because many code writers do not want to publish their minor variations of packages on hackage or do not want to publish new packages that they feel are of low quality. This is also good from the perspective of the code user- I normally don't want to see minor variations on hackage. If I am interested in minor variations I will go to the github repo and look at the different branches and forks where it will be clear to me what the difference is- it would be difficult to accurately portray this information on hackage. Productivity in modern programming is largely about code re-use, and I only see this as becoming more important. Type safety and Cabal do a great job facilitating code re-use. I think we can make cabal even more useful here. Has this ability been discussed before for cabal? t would be great if it could at least be worked on for the next Google Summer of Code. Sorry if I offended anyone by exclusively mentioning git, but that is the only version control system used by Rubyists :)

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Is there a tutorial interpreter to teach haskell?
by Larry Evans 20 Dec '10

20 Dec '10

By tutorial interpreter, I means something like an expert system having a list of rules and than a problem which is solved by using those list of rules. The tutorial means the trace of the "problem state" before and after each rule is applied along with which parts of the rule are matched with which part of the "problem state". W.R.T. haskell, the "problem state" would be a haskell expression and each "rule application" would be simply reducing the haskell expression (using some rule, which would be cited during the reduction) to a simpler form until the final answer was achieved. Obviously the rule applications to be trace should be user selectable to avoid way too much output, but that seems similar to setting breakpoints in selected functions; hence, I guess it wouldn't be hard to do. The attached file illustrates what I'm after. I laboriously composed that attached to enable me to understand what the haskell code was doing. It would help other novices if such a trace could be automated. I'm currently trying to understand: sequence (c:cs) = return (:) `ap` c `ap` sequence cs from p. 2 of: http://www.soi.city.ac.uk/~ross/papers/Applicative.pdf and again I'm having a lot of difficulty what the code is doing. So far I've got: --{--eshell-- Prelude Monad> :t ap ap :: (Monad m) => m (a -> b) -> m a -> m b Prelude Monad> :t return (:) return (:) :: (Monad m) => m (a -> [a] -> [a]) Prelude Monad> (:) 'a' "bc" "abc" Prelude Monad> :t return (:) `ap` "a" return (:) `ap` "a" :: [[Char] -> [Char]] --}--eshell-- so now I must "manually" figure out what the a and b in the ap declaration correspond to in the return(:) type: m ( a -> b ) __ _ _ 1: [] Char -> [Char]->[Char] 2: [] Char->[Char] -> [Char] IOW, is it choice 1: in the above table for choice 2:? I'd guess, since application associates to the left, that it's choice 1:. But you see, I'm not sure, and I have to work too hard to figure it all out. I *may* get it eventually, but it's a *lot* of work. I tutorial interpreter would make this so much easier! -regards, Larry

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enumsets in ArchLinux but not in Hackage
by Henning Thielemann 20 Dec '10

20 Dec '10

I found the package 'enumsets' in ArchLinux: http://aur.archlinux.org/packages.php?ID=18116 This points to Hackage, but there is no such package. It sounds like something I could need: A simple Haskell 98 wrapper around Word32, Word16, Word8, that are interpreted as bitfields, providing set operations with elements of an Enum type. I frequently like to have this when interfacing to C. I know of Edison.EnumSet, but my emphasis is on simplicity (i.e. Haskell 98).

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IO, sequence, lazyness, takeWhile
by Jacek Generowicz 19 Dec '10

19 Dec '10

-- Is it possible to rewrite code written in this style untilQuit = do text <- getLine report text if text == "quit" then return () else untilQuit -- in a style using higher order functions for abstract iteration? For -- example, something along these lines: untilQuit' = (fmap (takeWhile (/= "quit"))) (sequence $ map (>>= report) (repeat getLine)) -- The latter version shows the report, but it doesn't stop at the -- appropriate place, so I'm guessing that I'm being bitten by my -- ignorance about the interaction of actions and lazyness. -- For completeness, here's a definition of report report text = do putStrLn $ "You wrote " ++ text return text

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Re: [Haskell-cafe] Bifold: a simultaneous foldr and foldl
by Henning Thielemann 19 Dec '10

19 Dec '10

Noah Easterly wrote: > Somebody suggested I post this here if I wanted feedback. > > So I was thinking about the ReverseState monad I saw mentioned on r/haskell a > couple days ago, and playing around with the concept of information flowing > two directions when I came up with this function: > > bifold :: (l -> a -> r -> (r,l)) -> (l,r) -> [a] -> (r,l) > bifold _ (l,r) [] = (r,l) > bifold f (l,r) (a:as) = (ra,las) > where (ras,las) = bifold f (la,r) as > (ra,la) = f l a ras > > (I'm sure someone else has come up with this before, so I'll just say I > discovered it, not invented it). > > Basically, it's a simultaneous left and right fold, passing one value from > the start of the list toward the end, and one from the end toward the start. I also needed a bidirectional fold in the past. See foldl'r in http://code.haskell.org/~thielema/utility/src/Data/List/HT/Private.hs You can express it using foldr alone, since 'foldl' can be written as 'foldr': http://www.haskell.org/haskellwiki/Foldl_as_foldr You may add your example to the Wiki.

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