May 2007 - Haskell-Cafe

Any Haskellers in Chicagoland?
by John Melesky 03 May '07

03 May '07

I'd love to post an ANN: Chicago Haskell user group, but i want to make sure there's more than one of me. -johnnnnnnn

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'Proper' use of the State monad
by Denis Volk 03 May '07

03 May '07

Hello all, I am trying to make a (turn-based) game in Haskell and need to pass around quite a bit of information, so using the State monad seems most appropriate. My question is, which is a better idea: 1) Using State GameState r and then call execState for each game event (i.e. user input) so I can do IO 2) Using StateT GameState IO () and have the entire game live in one big execStateT call. (I note XMonad does something similar.) There are difficulties with the first option, including keeping even more state about what we're doing (for instance, are we in a menu?), and adding stuff later would possibly require substantial rewrites. Other than the fact that I would have IO in places where it perhaps shouldn't be, the second option seems better, but I am new to all this and it may be that I'm missing something.

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Bloom Filter
by tom 03 May '07

03 May '07

Hi all, I'm pretty new to Haskell, I've been working on a Bloom filter[1] implementation as a learning exercise. I'd really appreciate it if someone more experienced would comment on the code. I'm sure there's plenty of places where I'm doing things in silly or overly complex ways. I've packaged it all with Cabal because I wanted to see how that all worked, I don't think it's ready for release yet! I would like to at some point get it polished up and release it if anyone thinks it might be useful. You can download it at: http://www.almostobsolete.net/bloom-0.0.tar.gz I've also put the Haddock docs for it online at: http://www.almostobsolete.net/doc/html/Data-yBloom.html All comments will be very much appreciated :p Thanks Tom [1] There's a nice description of what a Bloom filter is here: http://www.cs.wisc.edu/~cao/papers/summary-cache/node8.html

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FW: [Haskell-cafe] RE:Cross-over from Haskell.org [Haskell] Re: Newbie: what are the advantages of Haskell?
by Taillefer, Troy (EXP) 03 May '07

03 May '07

These Haskell lists seems to have a problem of having to many elitist pricks on it admittedly I am probably in this category as well so I will help you guys and by eliminating one prick and remove myself from the list good luck with the rest of them Troy -----Original Message----- From: u.stenzel(a)web.de [mailto:u.stenzel@web.de] Sent: Tuesday, May 01, 2007 5:35 PM To: Taillefer, Troy (EXP) Subject: Re: [Haskell-cafe] RE:Cross-over from Haskell.org [Haskell] Re: Newbie: what are the advantages of Haskell? Taillefer, Troy (EXP) wrote: > I really dislike Perl as a programming language but I have to strongly > disagree about your statements about CPAN and the quality of its > contents. Consider Sturgeon's Law: 90% of everything is crap. It's true of CPAN, too, which is only a stand in for any large collection of libraries. We don't gain anything from wrapping or reimplementing the crap, therefore, if you like some particular library, you should ask for *that* library, nor for more libraries in general. no need to get all touchy-feely about this. > Perl is popular so it must have some merit. So is Crack. I still won't smoke it, though. > I don't subscribe to the > flawed reasoning that Perl Hackers just don't know any better or that > they are dumb, or intellectual inferior in some way. Well, I didn't make that point in the mail you're replying to, but I subscribe to it. Perl *is* unadultered crap, it *is* a bad idea carried to perfection, a shoddy language that teaches bad habits, rewards stupidity and encourages you to attack every problem with the same blunt old tool, the regex, and most Perl Hackers, those who voluntarily use it to solve actual problem, *are* in fact dumb, don't want to know any better and are resistant to education. Perl is in every way the opposite of Haskell, and I happen to like Haskell. Imitating Perl is even worse than imitating C++. ...but I don't even wan't to discuss this, neither in private nor on a mailing list. If you want to argue, ask Google for Erik Naggum; nothing more needs to be said about it. > ... I am pro choice. And I *choose* to belittle Perl Hackers as much as I want. If that stops anyone from using Perl, it's their *choice*. And you can *choose* to hate me for belittling you, jackass. Political Correctness is even more misguided than Perl.

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Cabal, lib and exe in one
by Magnus Therning 02 May '07

02 May '07

I'm trying to create a single cabal file containing specs for both a library and an executable using that library. I'm not having much luck though :( This is what I have so far: name: foo version: 0.1 exposed-modules: Foo.Bar other-modules: Foo.Qux Foo.C2HS hs-source-dirs: src include-dirs: csrc c-sources: csrc/qux.c extensions: ForeignFunctionInterface build-depends: base, haskell98 executable: foo hs-source-dirs: test-src main-is: foo.hs other-modules: Foo.Bar When built this is the message I get: test-src/foo.hs:5:7: Could not find module `Foo.Bar': Use -v to see a list of the files searched for. How do I specify that Foo.Bar is found where cabal puts it (./dist/build/)? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. Finagle's Fifth Law: Always draw your curves, then plot your readings.

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Hugs/nhc getting progressively slower
by Neil Mitchell 02 May '07

02 May '07

Hi, I like to develop on Hugs, because its a nice platform to work with, and provides WinHugs, auto-reloading, sub-second compilation etc. Unfortunately some of the newer libraries (ByteString/Binary in particular) have been optimised to within an inch of their lives on GHC, at the cost of being really really slow on Hugs. Taking the example of Yhc Core files, which are stored in binary. Using a very basic hPutChar sequence is miles faster (10x at least) than all the fancy ByteString/Binary trickery. Taking the example of nobench, Malcolm told me he reimplemented ByteString in terms of [Char] and gained a massive performance increase (6000x springs to mind, but that seems way too high to be true) using nhc. Could we have a collective thought, and decide whether we wish to either kill off all compilers that don't start with a G, or could people at least do minimal benchmarking on Hugs? I'm not quite sure what the solution is, but it probably needs some discussion. Thanks Neil

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Is Hs-Plugins dead?
by Philipp Volgger 02 May '07

02 May '07

Is Hs-Plugins still under develeopment; is there still somebody who is updating it?

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Type-level programming problem
by oleg＠pobox.com 01 May '07

01 May '07

Thomas Schilling wrote: > data T > class Foo ns a b c | ns -> a, ns -> b, ns -> c where > mkFoo :: ns > defaultA :: a > defaultB :: c -> IO b > defaultC :: [T] -> c > f :: c -> b -> a -> (b, Int) > data DefaultA > instance Foo ns a b c => Apply DefaultA ns a where > apply _ _ = defaultA > > but I get: > > Could not deduce (Foo ns1 a b1 c1) > from the context (Apply MakeAs ns a, Foo ns a b c) > arising from use of `defaultA' Indeed. Let us examine the inferred type of defaultA: *Main> :t defaultA defaultA :: (Foo ns a b c) => a We see it is a value polymorphic over four type variables: ns, a, b, and c. The type variable 'a' is also the type of the value, so we have a way to instantiate it. There is no direct way to instantiate the remaining three. If there were a functional dependency a -> ns, a->b, a->c, we could have instantiated the remaining variables. But there are no such dependencies. So, there is really no way we can ever instantiate the type variables ns, b and c -- and so the typechecker will complain. So, we need either a functional dependency a -> ns in the definition of Foo, or defaultA should have a signature defaultA :: ns -> a (and ditto for other defaults). As I understand, the function 'defaultA' can be present in different components, identified by ns. When we write 'defaultA' however, how can we say that we mean defaultA of component X rather than of component Y? There isn't any way to name the desired component... Incidentally, if we represent components by records data XRec = XRec { defaultA :: XA } then the type of defaultA is Xref -> XA. It is the function from the type of the `namespace'. This seems to suggest the signature of defaultA should be ns -> a ... BTW, there are other ways to add the name of the namespace to the signature of defaultA. For example: newtype TaggedT ns a = TaggedT a class Foo ns a b c | ... defaultA :: TaggedT ns a or class Foo ns a b c | ... defaultA :: ns a etc.

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Getting the number of seconds since epoch from System.Time.ClockTime
by Martin Percossi 01 May '07

01 May '07

Hello haskell-cafe, In System.Time, data ClockTime = TOD Integer Integer , where the first integer represents the number of seconds since epoch, and the other represents the number of picoseconds. Is there a way of retrieving the first part? (In Haskell 98, the ClockTime type is abstract). TIA Martin My music: http://www.youtube.com/profile?user=thetonegrove

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what exactly does "deriving (Functor, Monad, MonadIO)" do?
by Thomas Hartman 01 May '07

01 May '07

I was trying to follow the reasoning in Don's article on using haskell for shell scripting http://cgi.cse.unsw.edu.au/~dons/blog/2007/03/10 In the source listing at the end we is newtype Shell a = Shell { runShell :: ErrorT String IO a } deriving (Functor, Monad, MonadIO) and I don't understand it what "deriving" is doing here, nor have I been able to find documentation on it. http://en.wikibooks.org/wiki/Haskell/Class_declarations claims: "You can only use deriving with a limited set of built-in classes. They are: Eq Ord Enum Bounded Show Read " But, here we are deriving classes not in that list. So, is this a recently added feature? Or something that came in from {-# OPTIONS -fglasgow-exts #-} ? I would just like to understand this, and I can't figure out how to begin. Thanks for any help! thomas.

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