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[fun] HaskellDB Talk trailer
by Ahn, Ki Yung 17 Oct '08

17 Oct '08

There is an impressive HaskellDB Talk trailer on the web. http://www.vimeo.com/1983774 Cheers to the HaskellDB developers :-)

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Arch Haskell News: Oct 4 2008
by Don Stewart 17 Oct '08

17 Oct '08

A weekly update about Haskell on Arch Linux. Highlights, * Arch now has 609 Haskell packages in AUR. That’s a record increase of 19 new packages in the last 7 days. Noteworthy, * mp3decoder-0.0.1: “MP3 decoder for teaching.” * yi-0.4.6.2: “The Haskell-Scriptable Editor” * haskell-haskore-0.0.5.1: “The Haskore Computer Music System” Full update list, http://archhaskell.wordpress.com/2008/10/04/arch-haskell-news-oct-4-2008/ -- Don

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What I wish someone had told me...
by John Lato 17 Oct '08

17 Oct '08

I was just thinking about what I wish someone had told me when I started working with Haskell (not that long ago). It would have saved me a great deal of trouble. The Difference Between Interfaces and Type Classes. Many "Introduction to Haskell for the OOper" style tutorials claim that type classes are like Interfaces (for languages with that feature). I now think that, although this is technically true, it's fundamentally misleading. Here's a simple example demonstrating my line of thought: In C# (and presumably Java), this sort of function is common: public IList GetListOfData() In Haskell, a similar function may be GetListOfData :: (Foldable a, Indexable a) => IO a In C#, it doesn't matter what the actual type returned by GetListOfData is, as long is it supports the IList interface. It's not uncommon for GetListOfData to make a choice between several different implementations, depending on the nature of the data to be returned. The following code is perfectly reasonable in C# : // List and MyList are different classes if (something) { return new List(); } else { return new MyList(); } The equivalent won't compile in Haskell, because the actual return type does matter, and *is determined by the calling code*. Our fictional GetListOfData can't return a List or a Mylist depending on some conditional, in fact it can't explicitly return either one at all, because the actual type of the result, as determined by the caller, could be either one, or something else entirely (ignoring the IO bit for the time being). So I've come to the conclusion that stating type classes are like interfaces is misleading to newcomers to Haskell, because it leads people to think they should use type classes for the same sorts of problems OO-languages solve with interfaces. In turn this means new programmers are encouraged to use OO-style architecture in programs, reassured that they're in a "functional" idiom because they're using the "functionally-approved" feature of type classes. I think that if I had understood this earlier, I would have embraced functional idioms more quickly. Incidentally, I'm still horrible at designing functional APIs and modules, but at least now I know it, and I'm no longer trying to force OO interfaces into Haskell. So I've made progress.

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Haskell newbie indentation query.
by Ramaswamy, Vivek 17 Oct '08

17 Oct '08

Hello All~ I have just started with Haskell, and I must confess; I am in love with it. However one area that I am really confused about is indentation. Lets take a look at if-else if- else block. The way I understand it: {------} if something then do something 1 something2 else if nothing then do something3 something4 else do different {-------} The code above gives out an error. I have been programming in python and the above appears fine. But it does not work. What works is: if something then do something1 something2 else if then do something3 something4 else do different I find the above scheme extremely confusing. I tried going to: http://en.wikibooks.org/wiki/Programming:Haskell_indentation after reading I am even more confused. Can somebody please explain how the Haskell indentation works? The else-if and else seem to be aligning up with "then". They should be aligning with "If" in my opinion. Thanks in advance. Regards -Vivek Ramaswamy-

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Re: [Haskell-cafe] What is this function? (RESOLVED)
by John Ky 17 Oct '08

17 Oct '08

Hi Luke, Thankyou so much. I need the sequence function. What I was after was this: class Scramblable a where scramble :: a -> [a] instance Scramblable [MyType] where scramble values = sequence (map scramble values) instance Scramblable MyType where scramble myType = {- blah blah -} I was basically after exhaustively generating lots ASTs given a template AST with lots of leaf values changed. Thanks, -John On Thu, Oct 16, 2008 at 8:39 PM, Luke Palmer <lrpalmer(a)gmail.com> wrote: > 2008/10/16 John Ky <newhoggy(a)gmail.com>: > > Hi, > > > > I've written this function here: > > > > scramble [] = [] > > scramble [x] = [[z] | z <- scramble x] > > scramble (x:xs) = > > [(y:z)|y <- scramble x, z <- scramble xs] > > > > and (I think) it roughly does what I want it to: > > > > *Main> scramble ([]::[Int]) > > [] > > *Main> scramble ([1]::[Int]) > > [[1],[2]] > > So, um, this is nonsense. You've given it only 1, and yet it outputs > a 2, yet there is no mention of addition or the literal 2 anywhere in > your function. > > This function looks a lot like the more sensible: > > scramble' n xs = sequence (replicate n xs) > > >>> scramble' 0 [1,2,3] > [[]] > >>> scramble' 1 [1,2,3] > [[1],[2],[3]] > >>> scramble' 2 [1,2,3] > [[1,1],[1,2],[1,3],[2,1],[2,2],[2,3],[3,1],[3,2],[3,3]] > > Where your examples all had [1,2] as the argument. > > Luke >

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About do notation.
by Magicloud 16 Oct '08

16 Oct '08

Hi, As some articles say, do notation is expand to (>>) and (>>=) when being compiled. So I want to know the details. Like: main = do a <- getArgs b <- getLine myFunc1 (head a) b myFunc2 b (head a) I cannot figure out what is the (>>) and (>>=) way of this. Thanks.

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Re: Where did House/hOp go?
by Kenny Graunke 16 Oct '08

16 Oct '08

Hello! On Thu, Oct 16, 2008, Donnie Jones <donnie at darthik.com> wrote: [snip] > The site is working for me even now. > Unfortunately, the site hasn't really been updated much over the past > couple of years and when I e-mailed some people on the Programatica > project about the status of House & Osker I never got anything back. > The biggest obstacle you may find when trying to set up House & hOp is > that hOp is based upon a modified version of a very old GHC and from > the last time I looked at it the GHC runtime has changed enough that > making a new hOp will be non-trivial. Indeed! House 0.8, as listed on the official programatica page, is based on GHC 6.2...not so fun to work with these days. I've spent the last year or so working on House as part of my research, and have updated it to GHC 6.8.2 (with a lot of help). You can get a copy of my unofficial "House 0.8.91" sources here: http://web.cecs.pdx.edu/~kennyg/house/ It's still a bit of a challenge to build. Notably, you'll need a 32-bit Linux system - it doesn't build on 64-bit linux nor Mac OS X (yet). If you just want a floppy image, you may as well stick with House 0.8 from http://programatica.cs.pdx.edu/House/ ... it's basically the same. --Kenny

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Re: Haskell-Cafe Digest, Vol 46, Issue 118
by Alberto G. Corona 16 Oct '08

16 Oct '08

>Personally, I'm loving the whole concept of this puppy: > GHC.Prim.*reallyUnsafePtrEquality# >I have absolutely no idea what it does, but it must be something really >unsafe! ;-)i I really need reallyUnsafePtrEquality# !!!!! . I need it in inside GHC.Exts if GHC.Prim is being removed. Please!.. I´m involved in a project for storage and retrieval of heavily autoreferenced data structures. This functionality is absolutely needed for that!!.

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2008-10-12 Hackage status with GHC 6.10 release candidate
by Don Stewart 16 Oct '08

16 Oct '08

Hey all. The GHC 6.10 RCs are out, and we're preparing the release. To help manage the transistion to GHC 6.10 it is now possible to actually build all the 3rd party Haskell packages, and publish their results wrt. the release candidate. For the first time ever, we're able to have all the 3rd party code tested and ready to go *prior* to the release of the new compiler and base libraries. Using GHC 6.10 RC, Cabal 1.6 and cabal-install 1.16, of 682 libraries and apps tried in total, 1 UnpackFailed 2 DownloadFailed 2 InstallFailed 16 ConfigureFailed 73 DependencyFailed 132 BuildFailed 456 InstallOk Note that these builds are with "soft deps", provided on hackage, base < 4 parsec < 3 HaXml == 1.13.* QuickCheck < 2 which train cabal-install to build a larger set of packages. The important result: *46 packages produce different results to ghc 6.8.2* These packages and their logs are listed below. If you maintain one of the following packages, and are able to fix it before GHC 6.10 is released, your users will be happy. The most common issues for these differences are, * Changes to Arrow class definition * Changes to types of Map and Set functions * Cabal changes * Changes to ghc-api * Changes to when 'forall' is parsed (add Rank2Types) * GHC.Prim was moved, * Changes to -fvia-C and headers * GADT changes, * pragma warnings tightened * Integer constructors have moved * New warnings and used -Werror How to address these, as library maintainers, is addressed here, http://haskell.org/haskellwiki/Upgrading_packages Build reports for everything, produced today, are here, http://galois.com/~dons/tmp/build-logs-2008-10-12/ The following packages are producing different results than with ghc 6.8.2. Package maintainers are invited to look at them. ArrayRef-0.1.2 CLASE-2008.9.23.2 EdisonCore-1.2.1.2 HPDF-1.4 HaLeX-1.1 Hashell-0.15 Hipmunk-0.2 MemoTrie-0.0 NewBinary-0.1.1 PArrows-0.1 TypeCompose-0.5 WebBits-0.9.2 YamlReference-0.9.2 Yampa-0.9.2.2 arrows-0.4 bytestring-show-0.2 cabal-setup-1.2.1 chp-1.1.0 cmath-0.3 fixpoint-0.1 hasim-0.1 hask-home-2007.12.6 hetris-0.2 hexpat-0.2 hinstaller-2008.2.16 hint-0.2.4.1 hslackbuilder-0.0.1 hxt-8.1.0 iException-0.0.1 libGenI-0.16.1 list-extras-0.2.2 logfloat-0.9.1 mage-1.1.0 numeric-prelude-0.0.4 plugins-1.3 quantum-arrow-0.0.4 regex-tdfa-0.94 streamproc-1.1 stringtable-atom-0.0.4 typalyze-0.1.1 xmonad-utils-0.1 yhccore-0.9 If you'd like to try your own build of all of hackage, grab a package list (such as this one), http://www.galois.com/~dons/tmp/pkgs-6.10 Install a GHC 6.10 release candidate, upgrade to Cabal 1.6 (on hackage), and cabal-install 0.6 (on hackage), and then simply, cabal install -v -O0 $(cat pkgs-6.10) --build-reports This will construct a clever plan to install all the packages in the right order, and write logs to ~/.cabal/logs and a full structured build report into ~/.cabal/packages/hackage.*/build-report.log -- Don

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Re: [Haskell-cafe] Very silly
by Andrew Coppin 16 Oct '08

16 Oct '08

Juan Carlos Arevalo Baeza wrote: > Andrew Coppin wrote: >> Some guy told me that templates are "the best feature in the >> language", and proceeded to show me a huge chunk of highly >> complex-looking code which is approximately equivilent to >> >> join :: Array x -> Array x -> Array x >> >> I was unimpressed. > > Now I'm curious to know what chunk of code he showed you. FWIW, that > declaration you put there is done in C++ like so: > > template < typename X > > Array<X> join(Array<X> const&, Array<X> const&); > > Nothing more. I like both languages for very different reasons. C++ > for its insane amount of flexibility, and Haskell for its > expressiveness. And I've encountered equally harsh roadblocks in both > whenever I try to get "too crazy" while using them. Well no, he included the implementation as well as just the signature. >> Actually, that's a lie. I was impressed that such a low-level >> language could manage even that much abstraction. But I still prefer >> the Haskell way... > > The dynamic range of the C++ language is probably unparalleled. In > particular, templates enable implementing high level abstractions in a > way such that we're still discovering how far it can go. But of > course, it's not just templates: it's templates when united to all > other language features like overloading. C++ has some interesting ideas. I haven't learned how to use templates yet, but what I do find interesting is that there is no automatic memory management, and yet you can still do fairly dynamic programming. I've never seen any other language that allows this. (I had assumed it's impossible...) This makes me wonder just now necessary GC really is, and whether there is some way to avoid it...

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