Re: [Haskell-cafe] COM and Haskell
Andrew Appleyard wrote: On 26/04/2007, at 12:12 am, Bulat Ziganshin wrote: Simon Marlow recently wrote paper about handling dynamic exceptions - for me it seems that he described general system to mimic OOP in Haskell I found the paper (titled 'An Extensible Dynamically-Typed Hierarchy of Exceptions'). The system described is not a complete OO mapping (it doesn't deal with method overloading, for example) however because it is a recent paper and the mapping is quite lightweight it could be useful to me. The best reference I know of for how to do OO in Haskell is the Kiselyov/Lämmel OOHaskell paper: http://homepages.cwi.nl/~ralf/OOHaskell/. Quite often you don't need the full range of OO functionality to implement a particular abstraction, and in that case you can pick from the menu of techniques described in that paper. Extensible exceptions is one particular example of an OO-like abstraction. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Poor first impression
5) The gigantic README with it's obscure note is here http://www.haskell.org/ghc/download_ghc_661.html a few lines away from the download link. You can probably read it in the time it takes you to find and click the download link. Much quicker than waiting for a configure script to detect the problem. Also, just in case one doesn't know where to look, doing a web search for 'ghc libreadline.so.4' gives enough hits to solve the problem. The OP insisisted on compiling from the source (which is OK), instead of using the distributions package (which would have been worked). In this case I expect enough know how to understand what the error message means and being able to install libreadline4. If one hasn't got the know how I expect that one asks nicely (what can I do / what does that mean) instead of just venting one's frustration in a rather destructive manner (you bloody program didn't work for me, so I won't use it ever again, there!). Regards -- Markus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANN: The Monad.Reader - Issue7
I am pleased to announce that the latest issue of The Monad.Reader is now available: http://www.haskell.org/haskellwiki/The_Monad.Reader Issue 7 consists of the following four articles: * Matthew Naylor A Recipe for controlling Lego using Lava * Yaron Minsky Caml Trading: Experiences in Functional Programming on Wall Street * Duncan Coutts Book Review: “Programming in Haskell” by Graham Hutton * Dimitry Golubovsky, Neil Mitchell, Matthew Naylor Yhc.Core – from Haskell to Core The Monad.Reader is a quarterly magazine about functional programming. It is less-formal than journal, but somehow more enduring than a wiki page or blog post. If you'd like to submit something to the next issue of The Monad.Reader, please get in touch. I haven't fixed the deadline for the next issue just yet; I hope to send out an official call for copy in the coming weeks. Wouter This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] Re: Newbie: what are the advantages of Haskell?
I'll study these ways to debugging Haskell, and in accordance with my final work plan I'll decide the better way to follow.. For the moment i'm studing Haskell things, but in the next times I'll might decide it. A lot of thanks Neil and Simon. On 4/30/07, Simon Marlow [EMAIL PROTECTED] wrote: Rafael wrote: Hi HTTP. We also need better availability of libraries, and a more standard and reliable way to install them and specify their dependencies. We could also do with a good debugger. These are being addressed by the Google Summer of Code project. Hi Neil, a good debugger ? What is in Google Summer of Code about it ? I just found it: http://hackage.haskell.org/trac/summer-of-code/ticket/6 that havent student related. I was studing about debuging techs for haskell to build a nice debugging enviorment in HIDE or some other IDE. It's my bacharelor final work. We're building some debugging features into GHCi. This started with Pepe Iborra's Summer of Code project last year, and more recently Bernie Pope reworked the breakpoint support during his internship here at MSR Cambridge. We are actively working on polishing what we have for inclusion in the next major GHC release. The GHCi debugger is based on imperative debugging techniques: breakpoints, single stepping, etc. It's a live debugger rather than post-mortem. We focussed on accessibilty, rather than functionality: so the debugger is always on, and it works with everything that you can compile in GHCi. It lacks some of the advanced debugging features you'll find in Hat, for example, but we hope it makes up for that by being more broadly accessible. More information on the debugger is here: http://hackage.haskell.org/trac/ghc/wiki/NewGhciDebugger although note that development is ongoing and more features have been added since the wiki was last updated. All the debugging functionality is exposed by the GHC API, so it's certainly possible to build debugging support into an IDE based on GHC. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Indexing of content at haskell.org by search engines
I have noticed that only parts haskell.org are indexed by Google. E.g. mail archives of Haskell-Cafe and HaskellWiki are not indexed, but cvs.haskell.org, darcs.haskell.org and hackage.haskell.org are. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Poor first impression
[EMAIL PROTECTED] wrote: 5) The gigantic README with it's obscure note is here http://www.haskell.org/ghc/download_ghc_661.html a few lines away from the download link. You can probably read it in the time it takes you to find and click the download link. Much quicker than waiting for a configure script to detect the problem. Also, just in case one doesn't know where to look, doing a web search for 'ghc libreadline.so.4' gives enough hits to solve the problem. The OP insisisted on compiling from the source (which is OK), Just wanted to clear up a misconception I've seen several times in this thread: the OP was *not* building from source, he was installing from a binary distribution tarball. The confusion no doubt arises because our binary tarball has a configure script and a Makefile. The real build system *does* check for readline, as it should. The tarball is built on an old RedHat 9 system with readline 4 on it. The main reasons we supply the binary tarball (as opposed to distro packages) are so that you can install GHC in your home directory, or install multiple versions of GHC, or try out new releases without installing them system-wide. I'm going to upgrade the machine that builds these distributions at some point, but it's in the data center and I find it a bit cold and windy in there :-) Typically the best way to get GHC on Linux is directly from your distro. The download page for 6.6.1 is currently not particularly helpful, we will be improving it in due course. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] RE:Cross-over from Haskell.org [Haskell] Re: Newbie: what are the advantages of Haskell?
Udo, I am cross posted/migrated this to Haskell-cafe per Simons request sorry for the late response I don't check this email on weekends 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. I have worked professional in Perl (Feel free to feel sorry for me) for over a year and I have always been impressed with what I could find in CPAN. Perl obviously has a niche and is well supported by its community. Perl is popular so it must have some merit. 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. Programmers have very good and valid reason for choosing their tools and I will have words for those who irrational state otherwise especially in a belittling manner I am pro choice. I am going to borrow some Ebonics Don't language Hate Appreciate Troy -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Udo Stenzel Sent: Saturday, April 28, 2007 7:06 AM To: [EMAIL PROTECTED] Subject: Re: [Haskell] Re: Newbie: what are the advantages of Haskell? Michael T. Richter wrote: I wish I knew the language better so I could start working on those libraries. Which ones? those libraries cannot come into existence until someone says what's actually missing. (The bulk of CPAN is crap and is certainly not worth being reimplemented.) -Udo -- Object-oriented programming is an exceptionally bad idea which could only have originated in California. -- E. W. Dijkstra ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Getting the number of seconds since epoch from System.Time.ClockTime
Martin Percossi wrote: 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). Better to use the new time package, in particular Data.Time.Clock.POSIX: http://www.haskell.org/ghc/docs/latest/html/libraries/time/Data-Time-Clock-POSIX.html getPOSIXTime will do what you want. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Bloom Filter
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Poor first impression
4) The fix to the bug is simply download and install the libreadline4 shared object. No recompilation or reinstallation necessary. i'm not sure if this has been addressed - but is there a specific reason an older version of the readline library is in use? v5 appears to be stable and has been in use for some time now. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Type-level programming problem
Hi, I have a type class similar to this one. 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) The idea is, that I define classes of components where the data types of the methods are component-specific. I therefore use a namespace to define the component types (associated types would be nicer, but are not widely supported). Given a few sample instances, I can define a configuration: data X; data XA = XA; data XB = XB; data XC = XC instance Foo X XA XB XC where mkFoo = undefined defaultA= XA defaultB XC = return XB defaultC _ = XC f _ b _ = (b,0) mkX = mkFoo :: X data Y; data YA = YA; data YB = YB; data YC = YC instance Foo Y YA YB YC where mkFoo = undefined defaultA= YA defaultB YC = return YB defaultC _ = YC f _ b _ = (b,1) mkY = mkFoo :: Y config = mkX .*. mkY .*. HNil Using this configuration, I now want to define various functions that work on As, Bs or Cs, respectively. For example, I'd like to have a :: XA :*: XB :*: HNil a = hMap DefaultA config but I fail to figure out how to define DefaultA. I tried with this one: 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' at /Users/nominolo/Devel/Haskell/testcase1.hs:126:16-23 Possible fix: add (Foo ns1 a b1 c1) to the class or instance method `apply' In the expression: defaultA In the definition of `apply': apply _ _ = defaultA In the definition for method `apply' I guess I need some type class that also binds b and c. Or maybe I need to do some things completely different at all. Ideally, I'd hope to keep the work for adding new Foo instances as low as possible, though. I attached the relevant parts of the HList implementation and the code. Any suggestions? /Thomas testcase1.hs Description: Binary data ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] FIT for Haskell
Who wrote FIT for Haskell on http://darcs.haskell.org/FIT/? Does anybody know if the version is stable? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Poor first impression
brad clawsie [EMAIL PROTECTED] writes: 4) The fix to the bug is simply download and install the libreadline4 shared object. No recompilation or reinstallation necessary. i'm not sure if this has been addressed - but is there a specific reason an older version of the readline library is in use? v5 appears to be stable and has been in use for some time now. Actually some sources recommend to just symlink libreadline.so.4 to libreadline.so.5. I haven't tried, but since version 5 is supposed to be upwards compatible to version 4 it's reasonable to expect that it works (to a certain extend). To me (as an outsider) the whole thing certainly looks like a quirk/buglet in the ghc build process (why not just link to the available readline?), but IMO that is no reason for a ghc is not ripe for prime time outcry. Ghc is for programmers and there is a certain extend of ability to help yourself I'd be expecting from programmers. If one doesn't have it, he/she should just use the packages of his/her distro or another distro. Regards -- Markus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Poor first impression
On Mon, Apr 30, 2007 at 08:26:35PM +0200, [EMAIL PROTECTED] wrote: Actually some sources recommend to just symlink libreadline.so.4 to libreadline.so.5. I haven't tried, but since version 5 is supposed to be upwards compatible to version 4 it's reasonable to expect that it works (to a certain extend). there is an even simpler solution, upgrade the linux version on the build box. from a previous email by simon marlow: The tarball is built on an old RedHat 9 system with readline 4 on it rh9 is over four years old, and those four years there have been countless upgrades in many key free software packages used by linux distros. installing a modern linux on this box is a thirty minute exercise. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Poor first impression
brad clawsie wrote: installing a modern linux on this box is a thirty minute exercise. Ah - a volunteer! :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] 'Proper' use of the State monad
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. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 'Proper' use of the State monad
On 4/30/07, Denis Volk [EMAIL PROTECTED] wrote: 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. I like the second option better too. Also, you can make this type opaque (stick it in a newtype, use newtype deriving to derive the appropriate classes) and only export whatever IO actions you want to support (e.g. you may want to be able to spawn off a thread in a game for concurrent AI, so you could wrap that in a function that can only fork off Game actions, and not general IO actions, also you may want to be able to store certain game properties to disk for e.g. save games, but not allow general disk writes, so you only suply a savegame abstraction). You still get a nice encapsulation of what kind of IO actions people can do, because you don't have to allow them the use of lift or liftIO. -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 'Proper' use of the State monad
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Denis Volk wrote: 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. Are you going to be making lots of separate little sequenced changes to the state? If not, passing it around explicitly may be clearer. It may be helpful to distinguish between logical game state and the user-interface parts (such as in a menu), possibly putting them in different 'data', different parts of the code. Hoping that was any help, Isaac -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGNl9WHgcxvIWYTTURAr2LAJ9uGh09lTvfJ9wIRHmGszN5VQH2BwCfb2kQ 6GMUVZrggM3y1oZyk+UU49w= =kJhf -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 'Proper' use of the State monad
On Mon, Apr 30, 2007 at 11:16:47PM +0200, Denis Volk wrote: 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. You can also give some of your actions the type Monad m = StateT GameState m (), so that parametricity guarantees no actual IO will occur. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Poor first impression
On Mon, Apr 30, 2007 at 09:53:06PM +0100, Andrew Coppin wrote: brad clawsie wrote: installing a modern linux on this box is a thirty minute exercise. Ah - a volunteer! :-) absolutely! for the low cost of one round-trip business-class seat from san jose to wherever this box is, and i will happily insert a recent ubuntu cd and click the install icon. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Creating pseudo terminals
On 4/29/07, Georg Sauthoff [EMAIL PROTECTED] wrote: On 2007-04-29, Tom Hawkins [EMAIL PROTECTED] wrote: Hi, [..] I haven't done this before in any language, so any tips would be appreciated. From what I gather, a call to posix_openpt or openpty returns a master and a slave, or alternatively opening /dev/ptmx followed by calls to grantpt and unlockpt. well, then I suggest 'Stevens, Advanced programming in the unix environment' for basic pseudo terminal programming. Thanks for the reference. Does Haskell's standard library support pseudo terminals, or will I have to write foreign interfaces for grantpt and unlockpt? -Tom ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Creating pseudo terminals
tomahawkins: On 4/29/07, Georg Sauthoff [EMAIL PROTECTED] wrote: On 2007-04-29, Tom Hawkins [EMAIL PROTECTED] wrote: Hi, [..] I haven't done this before in any language, so any tips would be appreciated. From what I gather, a call to posix_openpt or openpty returns a master and a slave, or alternatively opening /dev/ptmx followed by calls to grantpt and unlockpt. well, then I suggest 'Stevens, Advanced programming in the unix environment' for basic pseudo terminal programming. Thanks for the reference. Does Haskell's standard library support pseudo terminals, or will I have to write foreign interfaces for grantpt and unlockpt? I'm not aware of an existing binding, so I'd suggesting writing your own. Should be pretty simple. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Bay Area Haskell users' group, anyone?
Hello all, With all the user groups that seem to have been forming lately, I figure it's high time to start a Bay Area group for people interested in Haskell (on any level). I interpret Bay Area broadly, since I'm in Monterey -- perhaps encompassing all of Northern California. If you're potentially interested, reply to me off-list, and I'll see what kind of response I get. I imagine that the first meeting, at least, would consist of people gathering informally over food, drink, or such. Cheers, Tim -- Tim Chevalier * [EMAIL PROTECTED] * Often in error, never in doubt Confused? See http://catamorphism.org/transition.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bloom Filter
G'day. Quoting tom [EMAIL PROTECTED]: I'm pretty new to Haskell, I've been working on a Bloom filter[1] implementation as a learning exercise. Excellent! Sounds like a fun test. 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. Sure. All in all, very well done. It works, and it looks pretty efficient. My quibbles are mostly stylistic or syntactic in nature. Please understand that the relative triviality of my quibbles is a sign that there are really no major problems. This is not a criticism, but more an advertisement: What are you using for source control here? Darcs is nice, and as a bonus, it's trivially browsable from a web browser, which saves downloading and unpacking. General comments: You overuse parentheses. A lot. Definitions like this: ary = (listArray (0, wordc-1) (repeat 0)) don't need parentheses around them, and just add to the general noise level. And (.. ((size b)-1)) is much more cleanly expressed as (.. (size b - 1)). Rather than carrying around a hash function, it might be better to use a type class: class BloomHash k where bloomHash :: k - [Word8] In wordsize: You don't need to hard-code this. You can use: wordsize = bitSize (undefined::Word32) -- Or Int, of course! bitSize is defined in Data.Bits. In splitup: I got a bit confused by the local binding names. It's usual, especially in generic code, to use xs, ys etc for a list of x and y. Something like this might be more idiomatic: splitup n xs = let (xs1, xs2) = splitAt n xs in xs1 : splitup n xs2 In indexes: (fromIntegral $ x `div` wordsize, fromIntegral $ x .. (wordsize-1)) Seems intuitively wasteful. Either use divMod or bit operations. Similarly, (hashfunc b) key is the same as hashfunc b key. But even better is: split bytecount . hashfunc b $ key That makes it obvious that it's a pipeline of functions applied to the key. This looks cool: bytes2int = foldr ((. (256 *)) . (+)) 0 . (map toInteger) but I'm not smart enough to parse it. This is both more readable and shorter: bytes2int = foldr (\x r - r*256 + fromInteger x) 0 Integer log2's are probably better done using integers only, or at least abstracted out into a separate function. In bloom: Function guards are your friends! This: bloom hf sz hc = if condition then b else error Badness is almost always better expressed as: bloom hf sz hc | condition = b | otherwise = error Badness You can now inline b. (I can see why you put it in a where clause; now you don't have to.) wordc, again, only needs integral arithmetic: wordc = ceiling ((fromIntegral a) / (fromIntegral b :: Double)) is more or less: wordc = (a+b-1) `div` b And drop the parentheses around the definition of ary. In add: Try to use function names that are close to names in existing libraries, like Data.Set. insert sounds better here. Also, rather than this: add :: Bloom a - a - Bloom a a better argument order is this: insert :: a - Bloom a - Bloom a That way, you can use it with foldr. In test: Again, probably misnamed. Data.Set calls this member. And again, arguably the wrong argument ordering. Once again, well done. Cheers, Andrew Bromage ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bloom Filter
ajb: Quoting tom [EMAIL PROTECTED]: This looks cool: bytes2int = foldr ((. (256 *)) . (+)) 0 . (map toInteger) but I'm not smart enough to parse it. This is both more readable and shorter: bytes2int = foldr (\x r - r*256 + fromInteger x) 0 Integer log2's are probably better done using integers only, or at least abstracted out into a separate function. Reminds me of this code from Data.Binary: unroll :: Integer - [Word8] unroll = unfoldr step where step 0 = Nothing step i = Just (fromIntegral i, i `shiftR` 8) roll :: [Word8] - Integer roll = foldr unstep 0 where unstep b a = a `shiftL` 8 .|. fromIntegral b Which is a bit stream-fusion inspired, I must admit. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Poor first impression
On Mon, 2007-30-04 at 18:35 -0700, brad clawsie wrote: On Mon, Apr 30, 2007 at 09:53:06PM +0100, Andrew Coppin wrote: brad clawsie wrote: installing a modern linux on this box is a thirty minute exercise. Ah - a volunteer! :-) absolutely! for the low cost of one round-trip business-class seat from san jose to wherever this box is, and i will happily insert a recent ubuntu cd and click the install icon. You beat me to it. -- Michael T. Richter [EMAIL PROTECTED] (GoogleTalk: [EMAIL PROTECTED]) The most exciting phrase to hear in science - the one that heralds new discoveries - is not Eureka! but That's funny... (Isaac Asimov) signature.asc Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Type-level programming problem
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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