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Proposal: Add Data.List.sortNub and sortNubBy
by Neil Mitchell 15 Mar '07

15 Mar '07

Hi, http://hackage.haskell.org/trac/ghc/ticket/1218 I propose the addition of sortNub and sortNubBy. Semantically sortNub = sort . nub Practically, if you are doing both a sort and a nub, this can be implemented as: sortNub = map head . group . sort This is O(n log n) [time to sort], rather than O(n^2) [time to nub]. I have seen this around several times, often called snub. People have also used snub to mean other things, snub itself has a meaning, and sortNub is a more accurate name (following the concatMap tradition). I personally have defined this function at least 25 times, I suspect others have to. I can find 5 identical versions of it on google code [1], and I know google code is lying because it's missing ones it found earlier. Thanks Neil [1] http://www.google.com/codesearch?hl=en&lr=&q=lang%3Ahaskell+%22map+head+.+g…

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Parsec documentation
by Paul Johnson 14 Mar '07

14 Mar '07

I'd like to map the Parsec reference manual into Haddock comments in the sources. Haddock didn't exist when Parsec was created. Which repository is considered the Master for Parsec? Paul.

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System.FilePath patch
by Neil Mitchell 14 Mar '07

14 Mar '07

Hi, A good 3 months after discussion came to a close, here is a patch to implement System.FilePath in the base library. THE IMPLEMENTATION: System.FilePath imports and exports one of System.FilePath.Windows or System.FilePath.Posix based that platform. System.FilePath.Internal.hs is not a real Haskell module, but has all the core code for both the Windows and Posix versions. System.FilePath.Windows/Posix just #include Internal, with the appropriate defines. The actual Internal.hs is all the reviewed code, identical to how it was when agreed upon. NOTES: I've tested the modules as best I can, and they do seem to work, but when it comes to base I'm not entirely sure what I'm doing so, someone might want to take a quick check that I haven't done anything silly. In order to check "am I running on Windows" I have done: #if defined(mingw32_HOST_OS) || defined(__MINGW32__) That is just a complete guess based on what was elsewhere in base. Introducing something semantically better like WINDOWS might be better. TESTING INFRASTRUCTURE: I have a load of tests in the code, along with a utility to generate a test script from Internal.hs. What wants doing with this is someone else's call, the options are: * Don't care about the tests (a bit of a shame) * Run the test generation utility, and shove the result in a testing repo somewhere * Add the test generation utility, and build code to get it all working This can be dealt with after the above patch goes in. Any last comments, or can this be committed? Thanks Neil

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'choice' in Control.Applicative?
by Greg Fitzgerald 14 Mar '07

14 Mar '07

Should there be a 'choice' function in Control.Applicative? choice :: Alternative<http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Applicat…>f => [f a] -> f a choice = foldr (<|>) empty or: choice = Data.Foldable.asum Thanks, Greg

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New ticket #1222 -- Add the "optionMaybe" combinator to Parsec
by Maxime Henrion 14 Mar '07

14 Mar '07

Hello all, I have just created a new ticket (#1222) to suggest the addition of a new parser combinator for Parsec, called "optionMaybe". It is just specialized version of the existing 'option' combinator: it wraps the result into a Maybe type with the expected semantics. As noted in the ticket, it has received positive feedback on the haskell-cafe mailing list and on the #haskell IRC channel. Cheers, Maxime

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Re: [Haskell-cafe] System.Random StdGen read fails on some strings?
by Kirsten Chevalier 14 Mar '07

14 Mar '07

[redirecting from haskell-cafe to libraries] On 3/13/07, Fritz Ruehr <fruehr(a)willamette.edu> wrote: > > According to the documentation for System.Random (see <http://haskell.org/ghc/docs/latest/html/libraries/base/System-Random.html>): > > > > > In addition, read may be used to map an arbitrary string (not necessarily one produced by show) onto a value of type StdGen. In general, the read instance of StdGen has the following properties: > > > * It guarantees to succeed on any string. > * ... > > On the other hand, I get the following on the (admittedly stupid, spur-of-the-moment) String argument "whateva": > > > > > Hugs> :l System.Random > System.Random> read "whateva" :: StdGen > > > Program error: Prelude.read: no parse > > > System.Random> map read $ words "this is a test of the System dot Random StdGen read, which seems to fail only on ... whateva" :: [StdGen] > [4580 1,440 1,101 1,4584 1,445 1,1485 1,35261 1,1377 1,32825 1,34047 1,13422 1,14037 1,13637 1,469 1,4132 1,4514 1,453 1,626 1, > Program error: Prelude.read: no parse > > > Am I missing something here? Or am I just being punished for the stupidity of my particular choice of String? :) > You're being punished for using a string with more than six characters :-) I took a look at the code for System.Random; in particular, the Read instance for StdGen. If you look at the implementation of readsPrec in that instance, it clearly fails on any string longer than six characters that doesn't match [0-9]*[:space:][0-9]* (in that case, stdFromString is invoked, which consumes only the first six characters of the string, hence read says "no parse" if the input string has length greater than six). I'm not sure whether the code is wrong or whether the comment that says "It guarantees to succeed on any string." is wrong. (Or both.) Does anyone know? Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "The blues isn't about feeling better, it's about making other people feel worse, and making a few bucks while you're at it." -- Bleeding Gums Murphy

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Ann: regex-base-0.91
by Chris Kuklewicz 13 Mar '07

13 Mar '07

I have updated the darcs repository and the hackage repository to have regex-base-0.91 Changes: GHC uses late instance overlap detection, whereas Hugs demands no potential overlap. Thus Hugs rejected some of the polymorphic results for matching given by RegexContext instances. The solution in version 0.91 of regex-base is to use newtypes. To this end there are 4 newtypes which are used to disambiguate some of the more complicated return types of match/matchM/=~/=~~. The new haddock documentation for Text.Regex.Base.Context has more details and lists all the instances. The 4 newtypes are: > newtype AllSubmatches f b = AllSubmatches {getAllSubmatches :: (f b)} > newtype AllTextSubmatches f b = AllTextSubmatches {getAllTextSubmatches :: (f b)} > newtype AllMatches f b = AllMatches {getAllMatches :: (f b)} > newtype AllTextMatches f b = AllTextMatches {getAllTextMatches :: (f b) } The 'f' parameters are containers types, such as [] or (Array Int). The two *Submatches types report all the captured (i.e. parenthesized) groups of the first match found. The two *Matches types report on the all the non-overlapping matches found. This clarifies the desires of the programmer and allows choice of either result to be made. The two *Text* version have the type of the searched text included in the type of the result 'b', unlike the other two types. This avoids overlapping. These newtypes are not used for all the instances, but only where they are needed to either avoid overlapping or remove possible confusion. Using this newtype schema has allowed a few new instances to be added. In particular all the instances should have a [] or (Array Int) option for the returned container. -- Chris Kuklewicz

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New I/O library
by Bulat Ziganshin 13 Mar '07

13 Mar '07

Hello libraries, i've further developed my idea of making new I/O library. look at http://haskell.org/haskellwiki/Library/IO Please edit it if you know how networking/async capabilities may be added to this design i've also added a proposal of developing it as SoC project: http://hackage.haskell.org/trac/summer-of-code/ticket/1113 -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: [Haskell] Composing applicative functors?
by Ross Paterson 13 Mar '07

13 Mar '07

[redirecting to libraries] On Wed, Feb 21, 2007 at 04:23:50PM -0800, Conal Elliott wrote: > Is the composition instance of Control.Applicative defined somewhere? I see > it in the McBride & Paterson paper [1], but not in the library docs [2]. > I'd rather pull it in from a library than (re)define it myself. > > [1] http://www.soi.city.ac.uk/~ross/papers/Applicative.pdf > [2] > http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Applicat… It's not defined in the libraries. The type (and the identity functor) should probably go in the module that defines Functor (currently Control.Monad) with a Functor instance, and an Applicative instance in Control.Applicative.

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Re: [Haskell-cafe] Program Command Line Arguments
by Ross Paterson 11 Mar '07

11 Mar '07

On Sun, Mar 11, 2007 at 12:51:16PM +1100, Donald Bruce Stewart wrote: > Dave: > > How are command line arguments to a ghc-compiled program > > accessed in Haskell? Or is that even possible? > > Simplest: System.Environment:getArgs > Complex: System.Console.GetOpt Which suggests that these belong in the same place in the module hierarchy. Certainly GetOpt doesn't belong with Readline and SimpleLineEditor. I'd suggest System.Console.GetOpt -> System.Environment.GetOpt, or to create a new node in the hierarchy.

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