- Haskell-Cafe - Haskell.org

Generalized list-comprehension
by Ron de Bruijn 01 Feb '04

01 Feb '04

Hi there, I have written this little function: f :: (Num a, Enum a) => a -> [[a]] f n = [[a]|a<-fu n] ++ [a:[b]|a<-fu n,b<-fu n] ++ [a:b:[c]|a<-fu n,b<-fu n,c<-fu n] fu n = [1..n] This is an example of the function in action: *Mod> f 4 [[1],[2],[3],[4],[1,1],[1,2],[1,3],[1,4],[2,1],[2,2],[2,3],[2,4],[3,1],[3,2],[3, 3],[3,4],[4,1],[4,2],[4,3],[4,4],[1,1,1],[1,1,2],[1,1,3],[1,1,4],[1,2,1],[1,2,2] ,[1,2,3],[1,2,4],[1,3,1],[1,3,2],[1,3,3],[1,3,4],[1,4,1],[1,4,2],[1,4,3],[1,4,4] ,[2,1,1],[2,1,2],[2,1,3],[2,1,4],[2,2,1],[2,2,2],[2,2,3],[2,2,4],[2,3,1],[2,3,2] ,[2,3,3],[2,3,4],[2,4,1],[2,4,2],[2,4,3],[2,4,4],[3,1,1],[3,1,2],[3,1,3],[3,1,4] ,[3,2,1],[3,2,2],[3,2,3],[3,2,4],[3,3,1],[3,3,2],[3,3,3],[3,3,4],[3,4,1],[3,4,2] ,[3,4,3],[3,4,4],[4,1,1],[4,1,2],[4,1,3],[4,1,4],[4,2,1],[4,2,2],[4,2,3],[4,2,4] ,[4,3,1],[4,3,2],[4,3,3],[4,3,4],[4,4,1],[4,4,2],[4,4,3],[4,4,4]] *Mod> This is ofcourse nice and all, but I need a function that does this same trick, but then doesn't only generate listelements of the maximum hard-coded length of three, but to length m, where m is an extra parameter. It's important that the elements are precisely in this order. I tried rewriting the listcomprehension to map, concat and filter, but that only complicated things. There is a possibility where I can write a function that gives the next of a list. So the next of [1,1,1] would be [1,1,2], but that would be somewhat inefficient when the list becomes large. Then I just call this function with (replicate n (head $ fu n)) and (last $ fu n). Then I just apply this function next to the rest, until I reach a state where all elements of the list equal the last value of the inputlist, so I have the required list of length n. Then to simply get the complete list as in function f. I need to map(\x->otherFunction x) [1..], but this "next"-function is almost identical to what the built-in listcomprehension does. I just don't like my solution. Then I can probably use Template Haskell(which I have never used), but that seems overkill. So does anyone has a better solution? Greets Ron __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free web site building tool. Try it! http://webhosting.yahoo.com/ps/sb/

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Producing fortran/C code with haskell?
by Vincenzo aka Nick Name 31 Jan '04

31 Jan '04

I seem to recall a discussion, don't know if it was here or on comp.lang.functional, where somebody said he uses haskell to generate fortran code. That fascinated me a lot, because that would mean being able to generate a program already specialized for a specific input, by first reading input in haskell and then producing code (fortran, but could be C either) - and because I guess it can add static safety exploiting haskell types. Since we already have that nice syntax for monads those programs should be readable, too. Where could I find information on such topics, or existing libraries to generate programs with haskell? Is somebody willing to share what (s)he already did? V.

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Favorite/recommended math books
by Greg Buchholz 29 Jan '04

29 Jan '04

I was wondering if the Haskellers out there have any favorite math texts they recommend for someone hoping to improve their programming skills by learning more math. I'm thinking about something at the undergraduate level that you enjoyed reading. What about Mr. Haskell Curry's _Foundations of Mathematical Logic_? Greg Buchholz

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Re: [Haskell-cafe] Interpreting fields in a buffer
by Tomasz Zielonka 28 Jan '04

28 Jan '04

On Mon, Jan 26, 2004 at 02:39:32PM -0800, John Meacham wrote: > On Mon, Jan 26, 2004 at 01:24:37PM +0100, Tomasz Zielonka wrote: > > 3) Roll your own (de)serialization framework > > > > That's what I did. It's a bit complicated, but I will try to describe it > > within a couple of days. Right now all I can say is that it uses TH and > > has a couple of implementations of low-lever decoders, one of which > > reads directly from UArray Int Word8. I managed to achieve throughput of > > 3 MB / s for quite complicated binary protocols, and I think I can > > improve that even further. > > Hi, I would be very interesting in looking at the design of this. Is it > available on the web somewhere? I've put the code in attachment. I compiled in GHC 6.0.1 (this may matter because of Template Haskell. I also used other GHC extensions). There is an Example.hs, which shows how to declare a record for IP header and how to use both supplied parsers. This is not a standalone program, so to try it, compile it with ghc -O2 --make Example and load it in GHCi (you have to compile it first, because the interpreter can't handle unboxed tuples). The library is really very simple, I created it quickly to solve some immediate problems. There is no support for bitfields - I think there would be if I had to deal with them ;) The original library used little endian encoding. I changed to big endian aka network order, but there really should be a possibility to choose. Right now I am not sure how this choice should be available to the user. The ability to easily switch from Parsec to an efficient UArray Parser and the benefit from specializing 'times' came as nice surprises. Well, not exactly unexpected surprises, because I was striving for it a bit, but it was easier than I thought. Template Haskell is used to automatically derive instances for record types. In my application I had to create some instances by hand, because some regions of binary files where not self-describing - they needed additional information from the outside. There is also an encoding part - if someone's interested, I can extract it from the application. The funny thing is that I was only supposed to produce files, not parse them. But I started by doing a parser and thanks to a declarative approach, when I finished the parser, the unparser was ready almost instantly. One way to introduce bitfields and varying endianness would be to design some description language like that in Erlang (some algebraic datatype should suffice). Then we could generate datatypes and class instances from it using TH. Something like that (just a sketch): ipHeaderLayout = Record "IPHeader" [ BitField "Word8" BE [ (4, Just ("iphHeaderLen", "!Word8")) , (4, Just ("iphVersion", "!Word8")) ] , Field "iphTOS" (Unsigned 1 BE) "!Word8" , Field "iphTotalLen" (Unsigned 2 BE) "!Word16" , Field "iphID" (Unsigned 2 BE) "!Word16" , Field "iphFragOff" (Unsigned 2 BE) "!Word16" , Field "iphTTL" (Unsigned 1 BE) "!Word8" , Field "iphProtocol" (Unsigned 1 BE) "!Word8" , Field "iphCheck" (Unsigned 2 BE) "!Word16" , Field "iphSAddr" (Unsigned 4 BE) "!Word32" , Field "iphDAddr" (Unsigned 4 BE) "!Word32" ] $(createDataType ipHeaderLayout) $(createDecodableClassInstance ipHeaderLayout) $(createEncodableClassInstance ipHeaderLayout) Best regards, Tom -- .signature: Too many levels of symbolic links

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Interpreting fields in a buffer
by Dominic Steinitz 27 Jan '04

27 Jan '04

I looked at the Erlang syntax when I wrote my helper functions and I agree it is very nifty. I didn't have the time to investigate how to do it in Haskell but it would be disappointing if it (or something like it) couldn't be done. Dominic. > > Mikael, > > Thanks, that's very helpful and seems to be just the sort of > thing I'm looking for. > > Greg > > > On Jan 26, 2004, at 6:05 PM, Mikael Brockman wrote: > > > > You'll probably want to take a look at Erlang's so called ``bit > > syntax'' > > at http://www.erlang.se/euc/00/bit_syntax.html. It's very nifty, and > > I'd love to see it (or something equally convenient) as a Haskell > > extension. > > > > Hugs and kisses, > > Mikael Brockman > >

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Simple "Maybe" question
by Jim Lewis 27 Jan '04

27 Jan '04

I'm new to Haskell and can't find an example to solve a trivial problem. I have code like this: findBlank :: [] -> Int findBlank str = findIndex (==' ') str But interpreter complains elsewhere of mismatch of Int with Maybe Int. I want to handle the maybe only here and nowhere else. This just makes matters worse: findBlank str = Just findIndex (==' ') str Thanks. Powered by the E-mail PIM - Info Select - www.miclog.com

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elemt from list
by Pajo Patak 27 Jan '04

27 Jan '04

I want to return a list, from and list of lists (all integers), where is the erroe in the code? nthList :: [[a]] -> Int -> [a] nthList ([x]:xs) 1 = [x] nthList ([x]:xs) (n+1) = nthListh xs n _________________________________________________________________ Schützen Sie Ihren Posteingang vor unerwünschten E-Mails. http://www.msn.de/antispam/prevention/junkmailfilter Jetzt Hotmail-Junk-Filter aktivieren!

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RE: [Haskell-cafe] elemt from list
by Bayley, Alistair 27 Jan '04

27 Jan '04

There is a Prelude function "!!" which I think would do what you want: <http://haskell.org/ghc/docs/latest/html/libraries/base/Data.List.html#v%3A! !> You may find other useful list functions in this library... > -----Original Message----- > From: Pajo Patak [mailto:brutalbanal@hotmail.com] > Sent: 27 January 2004 09:32 > To: haskell-cafe(a)haskell.org > Subject: [Haskell-cafe] elemt from list > > > I want to return a list, from and list of lists (all > integers), where is > the erroe in the code? > > nthList :: [[a]] -> Int -> [a] > nthList ([x]:xs) 1 = [x] > nthList ([x]:xs) (n+1) = nthListh xs n ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. ***************************************************************** ________________________________________________________________________ This email has been scanned for all viruses by the MessageLabs Email Security System. For more information on a proactive email security service working around the clock, around the globe, visit http://www.messagelabs.com ________________________________________________________________________

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Interpreting fields in a buffer
by Dominic Steinitz 27 Jan '04

27 Jan '04

Gregory, I don't know if this helps but I ended creating functions like the ones below (I didn't need to use the C structures because the IP definition is language independent). I'm sure there are better ways and I didn't test the throughput but I was able to develop a Haskell version of ping and traceroute (both multi-threaded). It would be nice if there were a library so that we didn't end up re-inventing the wheel. Dominic. ipHeaderLength :: String -> Int ipHeaderLength s = (fromIntegral (ord (s !! 0)) .&. 0x0f) * 4 ipTTL :: String -> Int ipTTL s = fromIntegral (ord (s !! 8)) ipProtocol :: String -> IPProto ipProtocol s = toEnum (ord (s !! 9)) ipDestAddr :: String -> HostAddress ipDestAddr s = fromIntegral (ord (s !! 16)) `shiftL` 24 + fromIntegral (ord (s !! 17)) `shiftL` 16 + fromIntegral (ord (s !! 18)) `shiftL` 8 + fromIntegral (ord (s !! 19)) ----- Original Message ----- > > Date: Sun, 25 Jan 2004 22:44:58 -0500 > From: Gregory Wright <gwright(a)antiope.com> > To: haskell-cafe(a)haskell.org > Subject: [Haskell-cafe] Interpreting fields in a buffer > Message-ID: <0356DB09-4FB2-11D8-A3D2-00039398F084(a)antiope.com> > Content-Type: text/plain; charset=US-ASCII; format=flowed > MIME-Version: 1.0 (Apple Message framework v609) > Content-Transfer-Encoding: 7bit > Precedence: list > Message: 1 > > > Hi, > > I have a question related to a program I'm writing. I have to handle IP > packets, > which will be read into a buffer. What is the best haskell idiom for > getting access > to the fields in the buffer? > > The IP header has a number of fixed format fields. In C, I would define > a struct, > then cast the pointer to the beginning of the buffer to a pointer to > the struct. I would then be > able to access each field in the header as <packet_struct_ptr> -> > <field>. > > Is there a way in haskell to declare the format of the header, then > access the > components of the buffer as elements of those types? I'm only going to > do read > access on the buffer (an unboxed array). Most fields won't be examined > but I can't > tell in advance which ones will have to be looked at. > > I've not seen an example of this kind and was wondering if this was > especially awkward. > > Thanks. > > Best Wishes, > Greg > > Gregory Wright > Antiope Associates LLC > 18 Clay Street > Fair Haven, New Jersey 07704 > > gwright(a)antiope.com >

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Re: Simple Fudgets-question
by Thomas Hallgren 27 Jan '04

27 Jan '04

Henrik Berg wrote: >Hi! > >I can't find any function in the Fudgets-library to do this: > >(F a b) -> (F a (a, b)) > >... All I want to do is to resend the input out on the output. > > If that is all you want, this combinator is the right choice: throughF :: F a b -> F a (Either b a) -- Thomas H

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Matrix as list of list
by Pajo Patak 27 Jan '04

27 Jan '04

Hello i am new to Haskell and i could need some help :) I want to make a function getElement which returns me an Element from a martix, which ist represented as a list of lists, e.g. [[1,2,3] [4,5,6]] is a 2x3 Matrix. The Type of the function getElement is Int->Int->[[a]]->a Can someone help me please? _________________________________________________________________ 5 neue Buddies = 50 FreeSMS. http://messenger-mania.msn.de MSN Messenger empfehlen und kräftig abräumen!

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[jimlewis@miclog.com: Haskell web]
by John Peterson 26 Jan '04

26 Jan '04

Here are some comments regarding Haskell and the website. I think there are some things we can do to make it a little easier on people that enter haskell.org from outside the sheltered halls of academia. I think the comment about the organization of haskell.org is fairly accurate - we have been adding more and more content without a really systematic way of searching for things. I would really like to be able to sift through the information at haskell.org in a more useful way. Much of the best content on haskell.org is hidden in Mailman archives - I also have problems digging through old topics. I'm not sure we should change over to different discussion software but maybe we could do something better to access the archives. John ------- Start of forwarded message ------- Subject: Haskell web To: "peterson-john(a)cs.yale.edu" <peterson-john(a)cs.yale.edu> From: "Jim Lewis" <jimlewis(a)miclog.com> Hi John I would like to offer my input on how to make Haskell more successful. In running a software publishing company over the years I got a bit behind in language technology and only discovered FP and Haskell a few days ago. FP in general and Haskell specifically appear to be a great advance over OOPs (we use c++). So it's surprising that Haskell is not more mainstream. With years of experience in software publishing, I thought it might be helpful to offer some strategic feedback (aka constructive criticism), knowing well that I am probably not the first to make these points. Actually I am excited enough over the potential of Haskell that I would consider trying to help commercialize the language but unfortunately my plate is rather full at the moment. In summary, the problem is that the site and tools nearly stopped me from learning Haskell. I had a lot of trouble getting over the initial hump - not so much because of the concepts but because of the site, tools, and docs. (I still only know about 10% of the language but I can now get some trivial apps (a few pages of code) to run.) Here are some of the problems I encountered: 1) Site organization - I wasted hours finding SOEGraphics as I missed the link about older versions. It seems unlikely I am the first with this problem and must conclude that someone is not carefully listening to feedback. I almost gave up with Haskell due to this small glitch. 2) The documentation on the site and in two books I have starts out slow and then picks up speed too fast leaving the reader in the dust. 3) The archaic forum UI makes it impractical to find anything and the zip archive is a broken link. And the topics are mostly too advanced for a novice. For about $150 one can buy a modern forum management package. Downloading zip archives went out of style close to a decade ago. 4) The search options are usually unproductive. For example I could not find how to resolve how to handle the Maybe construct in my app. I only found advanced discussions that did not help. My use was very simple. 5) Someone is not listening to bugs. Hugs crashes on startup after changing any value in the options dialog. This also wasted hours. I can't be the first to experience this. I also almost gave up after this experience. 6) The language reference has numerous functions with no description or examples. I know you know this but find it hard to understand how it can remain so. The Prelude Tour on the other hand is a model of the proper approach - I found it very helpful in learning the language as each function showed the description, code, syntax, and examples. 7) There is so much complexity to the language that it should be made clear what minimal subset one needs to learn to start writing some code. It took a while to figure out what I needed. Once the user starts writing code successfully he will be much more hooked. I realize it must be difficult in a non-commercial environment to depend on volunteers to fix such problems but these types of glitches will stop all but the most dedicated newbies. I would like to see Haskell succeed and offer any advice on setting a better strategy for managing the spread of the language. No doubt you've heard most of the above many times before but perhaps I've added something new from the perspective of someone with almost two decades of experience in commercializing software. Regards, Jim Lewis President Micro Logic New Jersey 201-447-6991 x 113 ------- End of forwarded message -------

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RE: [Haskell] Gallopping Tab characters
by Simon Marlow 26 Jan '04

26 Jan '04

> Simon Marlow wrote: > > As for the width of the tab character: tab stops are every > 8 columns. > > Period. The Haskell report says so > > Yes, true. I think it was Leslie Lamport who wrote in TeXHaX > that anyone > defining an input format which includes tabs should be > sentenced to ten years > programming COBOL in Nijny-Novgorod. Off you go, Simon ... :) /me grabs COBOL book and bottle of Vodka sniff Simon

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Interpreting fields in a buffer
by Gregory Wright 26 Jan '04

26 Jan '04

Hi, I have a question related to a program I'm writing. I have to handle IP packets, which will be read into a buffer. What is the best haskell idiom for getting access to the fields in the buffer? The IP header has a number of fixed format fields. In C, I would define a struct, then cast the pointer to the beginning of the buffer to a pointer to the struct. I would then be able to access each field in the header as <packet_struct_ptr> -> <field>. Is there a way in haskell to declare the format of the header, then access the components of the buffer as elements of those types? I'm only going to do read access on the buffer (an unboxed array). Most fields won't be examined but I can't tell in advance which ones will have to be looked at. I've not seen an example of this kind and was wondering if this was especially awkward. Thanks. Best Wishes, Greg Gregory Wright Antiope Associates LLC 18 Clay Street Fair Haven, New Jersey 07704 gwright(a)antiope.com

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Re: [Haskell] Re: Use of tab characters in indentation-sensitive code
by Graham Klyne 26 Jan '04

26 Jan '04

[Switching to Haskell-cafe] At 11:38 26/01/04 +0100, Andreas Rossberg wrote: >The most flexible but safe solution is to simply define the indentation as >the sequence of indentation characters used. Two consecutive lines are >indented consistently whenever one indentation is a prefix of the other. >Hence you may freely mix different indentation characters, but you must be >consistent across lines. Any decent editor should be able to ensure that. > >With this solution, tab width is irrelevant and indentation may include >whatever Unicode has. Neat. But unfortunately I don't think this, or any of the alternatives suggested, actually addresses the problem I have encountered, namely when an editing environment set up to handle local editing preferences is presented with code imported from another environment using different conventions. Conversion of all tabs to spaces (which I do to avoid this kind of problem when exporting my code to other environments) can cause correct code to become incorrect. There is, of course, a reasonable argument that my editing environment is at fault. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Converting String
by Alex Gontcharov 26 Jan '04

26 Jan '04

Hi, I'm fairly new to Haskell, I'd like to know how to convert a list of Strings of type IO [String] to Int. I used map read p, where p is a list of IO Strings ["1", "2"] Thanks

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RE: Library Infrastructure code & help wanted
by Simon Marlow 22 Jan '04

22 Jan '04

> > Having a single package database for all Haskell > implementations seems a > > hard thing to implement: when GHC is installed, it will > have to check > > whether the database and haskell-config already exist, and > similarly for > > the other Haskell implementations. When a Haskell system is > > deinstalled, it has to decide whether to de-isntall > haskell-config and > > the package database. > > I don't think that's so bad. Figuring out if a package database > already exists should be a matter of having an agreed upon > System.Directory.systemConfigDir. Then maybe the package database > itself can tell you where to find haskell-config. What about multiple versions of GHC installed on a system, each with different sets of packages installed? Can I still install GHC in my own home directory, without needing to modify files in /etc? > I think that the compilers / interpreters installed on the system will > also be stored in the "package config" database. If you're > deinstalling yourself, the package database can tell you whether > you're the only compiler left and thus whether to get rid of > haskell-config. > > That said, I think there are still issues with having a single package > database, for instance, what if a package configuration for one > Haskell Implementation has a field that conflicts with the package > configuration for another implementation. Yuck! But maybe that won't > come up. For instance, right now there are separate flags for ghc, > nhc, and hugs opts, and probably there should be flags for > shared opts. Also, there doesn't seem to be provision in the package configuration for naming libraries specific to a particular implementation (eg. to use this package on GHC you must link in libHSwibble.a from /foo/bar, but on nhc98 you link in libHSnhc_wibble.a from ...). It seems to me that every field needs to be duplicated for each compiler installation; either that or you could put the compiler name in the package identifier, so you'd have packages like "ghc-6.2-base-1.0", "nhc98-1.3-base-1.0". Incedentally, GHC never uses the extra_ghc_opts in the package configuration. In fact, I don't think it should be there: it doesn't make sense in conjunction with auto packages (if I don't know whether I'm using a package or not, I don't know whether to add the flags). The flags fields should be disabled if auto is True. > > I'd rather just admit that the Haskell systems are > separate, and each > > provides its own implementation of Distribution.Package. Yes, this > > means that if you run Setup.lhs with Hugs it is different > than running > > it with GHCi: if I want to install a package for GHC I > have to run the > > Setup.lhs script using GHC. > > I think this would dash my hopes of being able to install a package > with a single command. The idea so far has been to have some kind of > haskell-interactive / runhugs-type executable /usr/bin/haskell which > will execute the setup script to install and configure the package > based on the already-installed compilers. It doesn't prevent you doing this. Here's how it might work: - Each compiler has their own package database, and their own implementation of Distribution.Package with an implementation-defined location for the package database(s). - There is a suite of tools, including haskell-config and /usr/bin/haskell, which are installed only once. These tools know about all the installed compilers on the system. We could install these tools with a compiler, or separately; installs/deinstalls of compilers need to update some state so that the tools are aware. - /usr/bin/haskell could run the script it is given using each compiler on the system, if that's what you want. eg. $ /usr/bin/haskell script.hs -- runs script.hs using the default compiler $ /usr/bin/haskell --all script.hs -- runs the script using each compiler on the system so to do an installation for all compilers, you say './Setup.lhs --all --install', assuming that Setup.lhs begins with '!#/usr/bin/haskell'. > Likewise, when you install a new compiler, it knows about what > packages are there already for the other ones and might take a crack > at building them for itself. > > This isn't so far fetched: elisp for emacs and xemacs already do > something similar on Debian at least. We can aim for this in the long term if you like - I still believe this is something that the package management system should do for us, though. The end result is the same, and it's a lot less work. Cheers, Simon

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RE: Library Infrastructure code & help wanted
by Simon Marlow 22 Jan '04

22 Jan '04

> 2) I also would like someone to help out with the package > configuration file IO. Right now, it uses HaXmL, with no generated > code. Someone who has seen HaXmL before could give me advice about > how my code could be better, submit patches, or rewrite it. As > discussed on libraries(a)haskell.org, I'm open to not using XML if > someone wants to redo what I've got there (not a huge task). This > parsing code is in Distribution.Package. I do want something more > flexible than straight-up read/show. I rather think that this should be considered a "sample implementation" of the Distribution.Package API. A consumer of the API should neither know nor care about the on-disk representation of the package database. I think we should specify the form of the library meta-data, at least for the default build system (by meta-data I mean the form of toolInfo in the example in sec. 3.4 of the doc). We can take the approach that we specify a very simple form, which works for most straightforward Haskell-only libraries, or we could define a rich form which will provide enough for all but the most complex library packages. I think we should start with the simple form - complex libraries will probably continue to use their own build systems for a while yet. The idea, as I understand it, is that we're going to provide two main APIs: (b) The high-level API (Distribution.Core in the example). Used by simple Setup.lhs scripts to provide generic build, install, and package registration services. Consumes Distribution.Package internally. (a) The low-level API (Distribution.Package). This is for custom build-systems which provide their own build & install, so that they can register a package. Also used by the command-line tool (haskell-config?). There may also be a mid-level API (Distribution.Build, Distribution.Install), but I'm not sure I can see a clear use for that at the moment. If not, then we don't need to specify these APIs, and they can be internal to the default build system. Also, I really think we should clarify the situation as regards multiple Haskell implementations, which seems to be glossed over at the moment. Having a single package database for all Haskell implementations seems a hard thing to implement: when GHC is installed, it will have to check whether the database and haskell-config already exist, and similarly for the other Haskell implementations. When a Haskell system is deinstalled, it has to decide whether to de-isntall haskell-config and the package database. I'd rather just admit that the Haskell systems are separate, and each provides its own implementation of Distribution.Package. Yes, this means that if you run Setup.lhs with Hugs it is different than running it with GHCi: if I want to install a package for GHC I have to run the Setup.lhs script using GHC. I believe that's the right way to go, and it's much easier from our point of view: all we have to do is agree on the Distribution.* APIs, we don't have to have any clever install/deinstall magic to deal with shared components. Cheers, Simon

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RE: MVar swap
by Simon Marlow 20 Jan '04

20 Jan '04

> I took a look at the implementation of the MVar function swap > and I am > just woundering where the atomicy comes from. > > -- |Swap the contents of an 'MVar' for a new value. > swapMVar :: MVar a -> a -> IO a > swapMVar mvar new = > block $ do > old <- takeMVar mvar > putMVar mvar new > return old > > Is it really just the Exception block, that makes it not > interruptable? swapMVar is only atomic with respect to other well-behaved threads. By well-behaved I mean that they only do takeMVar followed by putMVar on the MVar, never a putMVar on its own. Does that answer your question? Cheers, Simon

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MVar swap
by Georg Martius 20 Jan '04

20 Jan '04

Hi, I took a look at the implementation of the MVar function swap and I am just woundering where the atomicy comes from. -- |Swap the contents of an 'MVar' for a new value. swapMVar :: MVar a -> a -> IO a swapMVar mvar new = block $ do old <- takeMVar mvar putMVar mvar new return old Is it really just the Exception block, that makes it not interruptable? Thanks! Georg

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GHC kit for Windows
by Graham Klyne 20 Jan '04

20 Jan '04

I note that the GHC kit for Windows includes some of the (I presume) MinGW binutil components, but does not include 'ar'. I find that 'ar' is needed for building HaXml. May I suggest that 'ar' also be included in this kit? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Library Infrastructure code & help wanted
by Isaac Jones 20 Jan '04

20 Jan '04

Greetings, I'm looking for some help on the library infrastructure project. The project and its goals are explained here: http://www.haskell.org/libraryInfrastructure/ And the code is here: http://www.haskell.org/libraryInfrastructure/code.html In brief, the Library Infrastructure Project is an effort to provide a framework for developers to more effectively contribute their software to the Haskell community. I'm hoping to not have to implement this entire proposal myself, and I'm very flexible about implementation details. I've made some concrete progress, and I just mirrored my code repository on my web site, so now is the time to get involved! Specifically, at the moment, I want a couple of things: 1) A version parser. I implemented a parser which parses versions that are made of dates, or simple numeric versions. I'd like it to do more kinds of versions, though. Distribution.Version is attached. What is this version stuff used for? Well for one thing, you can use it to say something like: ./haskell-config --user list "MyPackage>1.2" where "MyPackage" is the name of a package. This will list all of the user-installed packages whose versions are greater than 1.2. If you look at the HUnit tests at the bottom of the attached file, you'll get a good idea of what it does right now. It uses Parsec. BTW, I understand that the Debian project has some standard version number parser which might give you some ideas. 2) I also would like someone to help out with the package configuration file IO. Right now, it uses HaXmL, with no generated code. Someone who has seen HaXmL before could give me advice about how my code could be better, submit patches, or rewrite it. As discussed on libraries(a)haskell.org, I'm open to not using XML if someone wants to redo what I've got there (not a huge task). This parsing code is in Distribution.Package. I do want something more flexible than straight-up read/show. 3) Someone to make it work with in nhc & hugs. The HaXmL page says that it doesn't work w/ Hugs yet (can anyone tell me why?), so that's one step. Also, at least for the version of NHC I have on my system, Parsec is different. So maybe we have to move away from Parsec, or maybe a different version thereof? It is absolutely vital for this project to be completely cross-platform and cross-compiler, so if we have to avoid depending on such things, I'm very willing to change. 4) Someone to make it work with Windows. Avoid using a filename that's called "Aux" ;) ABOUT THE CODE: It only works with GHC at the moment (see above) :( Right now what I've got is a first stab at Distribution.Package and haskell-config as explained in the proposal [1], though some things may have changed. It's hosted with Arch[2], a newish version control system. You can branch my code on your own machine and submit patches. Arch has some tools to help with patches. I've also provided a tarball. Arch is cool, though. You should try it. The HUnit tests in some of the modules should be pretty illuminating. The interactive help for haskell-config is pretty OK. Get the code here: http://www.haskell.org/libraryInfrastructure/code.html peace, isaac [1] http://www.haskell.org/libraryInfrastructure/proposal/index.html [2] http://regexps.srparish.net/www/

2 1

LIP and the AAP project
by Isaac Jones 20 Jan '04

20 Jan '04

Greetings, Now that I have at least gotten something started on the Library Infrastructure Project, I'm going to bend some of my energy toward the build/install issues for Haskell modules. The plan at this point, which is summarized on the project web site[1] is to attempt to leverage HMake's usefulness as a non-compiler-specific build system. Since I wrote the proposal, I've been made aware of the AAP project[2]. Does anyone have any experience with this? Does anyone have a sense for how it might fit in with LIP? I know that Peter Simons seems to think it might. I haven't read through all of the documentation, but I suspect it might help us out with the CPAN-style distribution stuff, and perhaps the building stuff. One big objection I have with using it would be that Haskell modules would depend on Python (!), but I thought I'd let everyone know about it anyway. Seems like an interesting project, and we could quite possibly learn something from it. peace, isaac [1] http://www.haskell.org/libraryInfrastructure/ [2] http://www.a-a-p.org/

1 0

Haskell binaries for AIX?
by Dean Herington 16 Jan '04

16 Jan '04

Does anyone out there have a prebuilt Haskell system for AIX? I'm trying to find the easiest route to running a Haskell interpreter on AIX (5.1 and 5.2). Dean

1 0

FFI preprocessor for GHC
by Gour 15 Jan '04

15 Jan '04

Hi! I'd like to create Haskell bindings for swisseph C library for calculating ephemeris. Which preprocessor would be a suitable for ghc compiler with the ability to run the code both on Linux & Win32? Sincerely, Gour -- Gour gour(a)mail.inet.hr Registered Linux User #278493

3 2

RE: ghc optimization of incremental update
by Simon Marlow 13 Jan '04

13 Jan '04

> Does ghc make any attempt to analyze dataflow and eliminate > unnecessary > copying for incremental updates? I compile the code below > with -O, and > based on the time it takes to run I'd guess it's copying the > array with > every update, even though there's no need in this particular > case to do > that. Perhaps there's a compiler flag or pragma to turn on > the analysis? > > module Main where > import Array > main = print (take 1 (reverse (elems (array_neg (setup > 100000))))) > setup i = listArray (1,i) [1..i] > -- the next three lines are the important part. > array_neg array = array_neg2 (length (elems array)) array > array_neg2 0 array = array > array_neg2 i array = array_neg2 (i-1) (array // [(i,-i)]) > > I realize I can force it to stop copying if I use a Monad, and there > are of course more efficient ways to code this contrived testcase. > I want to know if ghc can make it efficient as it stands. GHC doesn't do this kind of optimisation, no. There is the DiffArray type which provides constant-time array update, although it has been reported to have a high constant factor overhead. Your best bet for algorithms using mutable arrays is to encapsulate it in a state thread with the ST monad. Cheers, Simon

1 0

EDI X12
by Christopher Milton 13 Jan '04

13 Jan '04

Has anyone in the Cafe written EDI X12 generator/parser in Haskell? ===== Christopher Milton cmiltonperl(a)yahoo.com

1 0

pet project - 7 Millennium Prize problemss
by Christopher Milton 13 Jan '04

13 Jan '04

Folks, I think Haskell can be used to solve several, if not all, of the seven problems. Now I have to decide which problem to tackle first. Chris ===== Christopher Milton cmiltonperl(a)yahoo.com

4 6

ghc optimization of incremental update
by Doug Brown 13 Jan '04

13 Jan '04

Does ghc make any attempt to analyze dataflow and eliminate unnecessary copying for incremental updates? I compile the code below with -O, and based on the time it takes to run I'd guess it's copying the array with every update, even though there's no need in this particular case to do that. Perhaps there's a compiler flag or pragma to turn on the analysis? module Main where import Array main = print (take 1 (reverse (elems (array_neg (setup 100000))))) setup i = listArray (1,i) [1..i] -- the next three lines are the important part. array_neg array = array_neg2 (length (elems array)) array array_neg2 0 array = array array_neg2 i array = array_neg2 (i-1) (array // [(i,-i)]) I realize I can force it to stop copying if I use a Monad, and there are of course more efficient ways to code this contrived testcase. I want to know if ghc can make it efficient as it stands.

1 0

RE: no continuations
by Kevin S. Millikin 12 Jan '04

12 Jan '04

On Tuesday, December 30, 2003 5:04 PM, Kevin S. Millikin [SMTP:kmillikin@atcorp.com] wrote: > Oh, sure. I didn't mean to quibble with the idea that continuations > are computational effects. Just wanted to point out that (I think) > you can't macro express mutation with call/cc, unless you've already > got mutation anyway. [snip] > Yup. If you do that, you can use d as your setter and c as your > getter: > > > (define c (make-cell)) > > (define d c) > > ((d 'set) 9) > > (c 'get) > 9 > > ((d 'set) 17) > > (c 'get) > 17 It sure looks like the example contradicts the assertion, but I happen to know that there is a set! (or some other assignment) in the macro expansion of define. I'm just using call/cc to get at that, rather than getting at the one in the expansion of letrec. Moved to Haskell Cafe.

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Hi, I have a question!
by NamO 12 Jan '04

12 Jan '04

Hi. I am a student, who get interesting in Haskell. So, nowdays, I study Hakell with "Introduction to Functional Programming using Haskell - second edition" And today, I face an error from bellow code. ---------------------------- module OperationTest where data Nat = Zero | Succ Nat deriving (Show) (+) :: Nat -> Nat -> Nat m + Zero = m m + Succ n = Succ (m + n) ---------------------------- But above source make an error like bellow Reading file "g:\Haskell\OperationTest.hs": Dependency analysis ERROR "g:\Haskell\OperationTest.hs":8 - Ambiguous variable occurrence "+" *** Could refer to: OperationTest.+ Prelude.+ above source is from the text book - "Introduction to Functional Programming using Haskell - second edition" and I use Hugs to interpret it. I understand what interpreter say, but I don't know How I can solve this problem. Please, Help me!!! - 5 - P.S. Tank you very much to read my broken english letter.

4 5

JVM bridge
by Mark T.B. Carroll 11 Jan '04

11 Jan '04

With haskell-jvm-bridge-0.2.1 I get: ghc -package javavm HelloWorld.o HelloWorld_JVM.o Class_java_io_PrintStream.o Class_java_lang_System.o -o HelloWorld gcc: unrecognized option `-rpath' gcc: unrecognized option `-rpath' gcc: unrecognized option `-rpath' /usr/bin/ld: cannot open /usr/lib/jvm-bridge/lib/: File format not recognized $ ld --version GNU ld version 2.12.90.0.1 20020307 Debian/GNU Linux Copyright 2002 Free Software Foundation, Inc. This program is free software; you may redistribute it under the terms of the GNU General Public License. This program has absolutely no warranty. $ gcc --version 2.95.4 $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.2 I don't suppose anyone can hand-hold me through tracking this problem down? -- Mark

4 5

RE: no continuations
by Kevin S. Millikin 09 Jan '04

09 Jan '04

On Friday, January 09, 2004 2:48 AM, Ashley Yakeley [SMTP:ashley@semantic.org] wrote: > > Do you have an example of use of Y for letrec where a program would > violate R5RS? Sure, take a look at my implementation of Ben Rudiak-Gould's implementation of Alan Bawden's implementation of boxes. In 4.2.2 of R5RS, it says, re letrec: Semantics: The <variable>s are bound to fresh locations holding undefined values, the <init>s are evaluated in the resulting environment (in some unspecified order), each <variable> is assigned to the result of the corresponding <init>, the <body> is evaluated in the resulting environment, and the value(s) of the last expression in <body> is(are) returned. Each binding of a <variable> has the entire `letrec' expression as its region, making it possible to define mutually recursive procedures. The result of the corresponding <init> is *assigned to* each <variable> (anyone know why the wording is backward above?), and that is after the <init>s are evaluated, which is after the <variable>s are bound. There was a discussion on comp.lang.scheme a couple of years ago about this. http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&th=fdcf3 554852a3cad&seekm=3AC66F16%40MailAndNews.com#link1 http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&th=a47e0 3e456b2dc2a&seekm=200102220358.TAA77339%40adric.cs.nps.navy.mil#link1 http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&th=58a68 6525be78d16&rnum=1

1 0

Re: Combining distinct-thread state monads?
by Graham Klyne 09 Jan '04

09 Jan '04

[Reply thread moved to Haskell-cafe] At 18:19 06/01/04 +1030, Dr Mark H Phillips wrote: >I am still learning about monads. I have a problem >in mind and am wondering whether state monads are >able to solve it. The difficulty is that it would >necessitate the interaction of two state threads >and I'm not sure whether Haskell state monads >allow this. Let me explain what I'm getting at. I'm not an expert in this, but I think what you are proposing is possible, to a point, possibly assuming that your monads have associated functions to combine and separate the monadic parts. Hmmm, let's try something... Given: combine :: ma -> mb -> mab separate :: mab -> (ma,mb) (where ma, mb, mab are the separate and combined state monads) f :: ma () -> mb () -> mc () f a b = do { ma1 <- fa1 a -- process state in a, returning ma1 -- fa1 :: ma -> mc ma ; mb1 <- fb1 b -- process state in b, returning mb1 ; let mab1 = combine ma1 mb1 ; mab2 = fab mab1 ; let (ma2,mb2) = separate mab2 ; ma3 <- fa3 ma2 -- process state in ma2 ; mb3 <- fb3 mb2 -- process state in mb2 ; return (fc ma3 mb3) } (This code is speculative, not tested in any way.) In this case, a third monad is used to schedule the operations on the separate monads, so in that respect the entire sequence is performed in a composite monad, within which methods defined for the separate monads can be invoked. To get the results, Monad 'mc' would need to provide a way to pick them out. It looks as if the combined monad 'mab' is probably superfluous. I think the composite monad 'mc' might be avoided, but some of the efficiency advantage of monads would be lost as the single-threading of each monad is potentially broken. I think that this may be all be achieved more cleanly using the monad transformer libraries and 'lift' methods -- can a state transformer be applied to a state monad? What I have noticed in my work with monads is that in most respects they can be treated just like any other value. Although they look different, a do sequence is just a monad-returning function, and any monad-returning function may be a do sequence. <aside> In my own work, I was pleasantly surprised how easy it was to use a Parsec parser monad (effectively a state monad, I think) to parse some data and return a combined state+IO monad, effectively precompiling a script, which which could then be executed by applying the resulting monad to an initial state, all within an IO monad. The code which does this can be seen at: http://www.ninebynine.org/Software/Swish-0.2.0/SwishScript.hs The main parser declaration is: script :: N3Parser [SwishStateIO ()] where 'script' is a Parsec parser monad which parses a script and returns a list of 'SwishStateIO ()' values, each of which is a combined state+IO monad. </aside> #g -- >Consider two state threads. The first has each state >being a non-negative int, thought of as a string of >binary digits. The second thread has each state >being a bool. > >Now I want to have a state monad which modifies >both threads as follows. Consider input states i (the >int thought of as binary string) and b (the bool), >and output states i' and b'. > > b' = not (b && (i `mod` 2)) > i' = i `div` 2 > >As you can see, both of these should be able to do >update-in-place provided the above order is adhered to. >We could achieve this using state monads where state >is an (Int, Bool) pair. We would have one monad >which did the first line, leaving i unchanged and >a second monad which did the second line, leaving >b' unchanged. > >But... what if before this interaction, the int >thread and the bool thread were separate monads >doing their own thing, and we just wanted to >combine these threads briefly (using the above >interaction) before letting the threads do their >own thing again? Is this possible? > >Also, suppose we have previously defined an int thread >monad which takes i, returns a value of i `mod` 2, >and changes the state to i' = i `div` 2. Suppose >also we have previously defined a bool thread >monad which takes b, returns a nothing value, and >changes the state to b' = not b. Can we use >these two monads (each acting on different >threads), to form a combined-interaction monad >that does (same as before): > > b' = not (b && (i `mod` 2)) > i' = i `div` 2 > >I hope this is possible. It would facilitate >both code reuse and readability. However I >fear that it is not, requiring one to instead >explicitly rewrite the two separate thread monads >into (Int, Bool) pair acting ones. > >Cheers, > >Mark. > > >_______________________________________________ >Haskell mailing list >Haskell(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

3 2

Type checking
by Lee Dixon 09 Jan '04

09 Jan '04

Hi, Can anyone explain to me how hugs manages to derive that f x y z = y (y z) x is of type f :: a -> ((a -> b) -> a -> b) -> (a -> b) -> b Many thanks and a happy new year to all! Lee _________________________________________________________________ Stay in touch with absent friends - get MSN Messenger http://www.msn.co.uk/messenger

3 2

RE: Help!!!!
by Garner, Robin 09 Jan '04

09 Jan '04

It looks like the only data structure you use here is a list, which has an access time of O(n). Perhaps if you used a log(n) data structure (like a FiniteMap) or a constant structure (like an array) the runtime might improve. It's difficult to see from your program exactly what the algorithm you're using is. Or even how to run it. Have you done an analysis of its time complexity ? cheers -----Original Message----- From: Bhrein Brannick [mailto:bhrein_brannick@hotmail.com] Sent: Friday, 09 January, 2004 3:25 AM To: haskell-cafe(a)haskell.org Subject: Help!!!! Hi Guys, I'm a fourth year computer science in Ireland and I'm doing my final year project on Haskell programming. What I'm doing is writing a program in Haskell for the maths department for my college. What it involves is this: There is a class of 50 students and there are about 25 staff members. Now some staff member's take on three students, two students or even just one student depending on the staff member for supervising students. The students rank the projects the staff members have on offer in order of preference from 1 to 4 (see Haskell code in mail attachment). The program finds the best assignment of projects for students based on their ranking. The program works fine and that but my problem is that it takes way too much time to execute the program. When I reduced class number to 10 students and only included 10 staff for allocation projects it took thirty seconds to execute. That was fine. But when I increased class number to 20 students it took 10 minutes to execute. I tried it once for a class of 50 students and I wouldn't let it finish since it was taking far too long. I'm just wondering is there any way that the program can be improved so that execution of program can be faster. Even if there were software available that makes Haskell programs runs faster that would be a great help. I have a presentation in this, which I have only ten minutes to show the department what I'm talking about, and I want to include a demonstration of the program in it. Can you please help me with this since I tried a number of things with this and it never once seemed to improve performance. Thank you, Bhrein Brannick, Maths department, NUI Galway, Ireland. <http://graphics.hotmail.com/emsmile.gif> beano #7 <http://www.music.com/> _____ The new MSN 8: <http://g.msn.com/8HMAEN/2737??PS=> smart spam protection and 2 months FREE*

1 0

Re: Help!!!!
by Thomas Davie 08 Jan '04

08 Jan '04

> <html><div style='background-color:'><P>Hi Guys,</P> > <P>Im a fourth year computer science in Ireland and Im doing my > final year project on Haskell programming. What Im doing is writing a > program in Haskell for the maths department for my college. What it > involves is this: There is a class of 50 students and there are about > 25 staff members. Now some staff members take on three students, two > students or even just one student depending on the staff member for > supervising students. The students rank the projects the staff members > have on offer in order of preference from 1 to 4 (see Haskell code in > mail attachment). The program finds the best assignment of projects > for students based on their ranking. The program works fine and that > but my problem is that it takes way too much time to execute the > program. When I reduced class number to 10 students and only included > 10 staff for allocation projects it took thirty seconds to execute. > That was fine. But when I increased class number to 20 students it > took 10 minutes to execute. I > tried it once for a class of 50 students and I wouldnt let it > finish since it was taking far too long. Im just wondering is there > any way that the program can be improved so that execution of program > can be faster. Even if there were software available that makes > Haskell programs runs faster that would be a great help. I have a > presentation in this, which I have only ten minutes to show the > department what Im talking about, and I want to include a > demonstration of the program in it. Can you please help me with this > since I tried a number of things with this and it never once seemed to > improve performance. </P> One thing you might consider is whether or not it needs to run that fast - what happens if a student runs out of project choices? Do they need to pick new choices? We have a similar system here at York, and it ends up taking several days to run because students have to modify their choices to allow it to work. I've snipped your code and ignored it, because I'm guessing that your uni's rules on collusion are as strict as they are here, and they wouldn't be too chuffed if I were to give you a hand with it. Hope this helps Bob

1 0

Help!!!!
by Bhrein Brannick 08 Jan '04

08 Jan '04

2 1

HSQL Linking Problem with ghc6
by Charles Perkins 06 Jan '04

06 Jan '04

Hello Haskellians, I have been looking for a way to get ghc-compiled haskell programs to talk to a PostgreSQL database, and HSQL seems to be exactly what I need... And what's more, it appears to have downloaded and installed with minimal hassle... Alas (and I am sure this is a problem with my own lack of knowledge or misconfiguration of my system) I can't get ghc to link my program to the HSQL libHSSql.a library. At least I think that is the problem. On my debian-stable system, with some packages and libraries upgraded to -testing so I could install ghc6, I downloaded HSQL 1.0 from sourceforge and then ./configured with --enable-postgress and told it to make and then make -install the HSQL package. When I put an import HSQL statement in my program ghc compiles it without errors but the Linking phase reports the following (my program name is EnvPassed:) Linking ... EnvPassed.o: In function `__stginit_main_': EnvPassed.o(.text+0x25): undefined reference to `__stginit_HSQL_' Collect2: ld returned 1 exit status I get the same error when I do a standard compile and when I specify the library to link to as follows: ghc --make EnvPassed.hs -o EnvPassed -lHSsql -L/software/HSQL/ (I had expanded and built HSQL in the /software directory.) Am I doing something stupid? What should I check for this kind of error? Any help would be appreciated. Chuck

3 3

Re: Perspectives on learning and using Haskell
by Derek Elkins 05 Jan '04

05 Jan '04

On Tue, 23 Dec 2003 17:26:20 +0000 Graham Klyne <gk(a)ninebynine.org> wrote: (moved to Haskell-Cafe as this reply might generate several more) > I've spent part of the past few months learning Haskell and developing > a moderately sized application. I came to this from a long background > (20 years or so) of "conventional" programming in a variety of > languages (from Fortran and Algol W to Java and Python). For me, > learning Haskell has been one of the steepest learning curves of any > new language that I have ever learned. Before this project, I was > aware of some aspects of functional programming, but had never > previously done any "in anger" (i.e. for real). Well, the obvious question is: after climbing partway up that curve, what do you think of the view? Was it worth learning? Is it worth continuing to use? Does the code seem 'better' than what you might produce in other languages you've used? > Throughout this period, I've been accumulating some notes about some > things that I found challenging along the way. The notes are not > organized in any way, and they're certainly not complete. I've > published them on my web site [1] in case the perspective might be > useful to any "old hands" here. > > [1] http://www.ninebynine.org/Software/Learning-Haskell-Notes.html When I saw this page earlier, I was thinking about suggesting to you to add it and any new thoughts to the wiki so that it's more readily accessible to other Haskell newbies and can be annotated with pointers to resources and comments; kind of like an annotated unidirectional version of "A Newbie's on-going tutorial" on the Squeak wiki (http://minnow.cc.gatech.edu/squeak/1928) > Also on the topic of perspectives: > > In recent conversation with a colleague, he mentioned to me that the > term "functional programming" has an image problem. He suggested that > the term conveys an impression of an approach that is staid, > non-progressive or lacking novelty, and is prone to elicit a response > of "been there, done that" from programmers who don't realize the full > significance of the term "functional". I've also noticed that when I > talk about "functional programming", some people tend to think I'm > talking about using techniques like functions in C or Pascal (which is > course is very desirable, but old hat and not worthy of great > excitement). Yes. People often use the term 'procedural' to describe the C style break down of a problem. Of course, some people use procedural to refer to functional and others functional to refer to procedural. Then, of course, there are also dynamic typers who equate static typing with C/Pascal. On the OO side, there are the people who equate OO with C++/Java much to the chagrin of Smalltalkers and CLOS users. At any rate, functional programming is a pretty well established term so there isn't much Haskell can do about it. Perhaps throwing in 'higher-order' will help, and certainly laziness/purity can't be said to be lacking novelty.

8 13

Limiting resources on a per-function basis?
by Jeff Newbern 05 Jan '04

05 Jan '04

Hi all, I would really like to be able to set resource limits on a per-function basis. I am envisioning something like: limit :: l -> (a -> b) -> (a -> Maybe b) limit lim fn = ... which would convert a function so that it returns Nothing if a limit (on stack, heap, time, etc.) is exceeded during evaluation of that function. Is there any hope of being able to do this within the existing GHC libraries? If not, is it even possible to determine after-the-fact how much stack or heap space a function evaluation required? Thanks, Jeff Newbern -- Jeff Newbern <jnewbern(a)nomaware.com> Nomaware, Inc.

5 5

Stack usage with a state monad
by Joe Thornber 02 Jan '04

02 Jan '04

Hi, I was wondering if anyone could give me some help with this problem ? I'm trying to hold some state in a StateMonad whilst I iterate over a large tree, and finding that I'm running out of stack space very quickly. The simplified program below exhibits the same problem. This is the first time I've hit space problems in Haskell, I hope judicial use of 'seq' or '$!' would be enough to fix it, but I don't know where to start. Any ideas as to what I'm doing wrong would be much appreciated. Thanks, - Joe module Main (main) where -- Program to count the leaf nodes in a rose tree. Written to try and -- reproduce a stack space leak present in a larger program. -- How can I use a state monad to count the leaves without eating all -- the stack ? import Control.Monad.State data Tree = Tree [Tree] | Leaf buildTree :: Int -> Int -> Tree buildTree order = buildTree' where buildTree' 0 = Leaf buildTree' depth = Tree $ map (buildTree') $ take order $ repeat (depth - 1) countLeaves1 :: Tree -> Int countLeaves1 (Tree xs) = sum $ map (countLeaves1) xs countLeaves1 (Leaf) = 1 incCount :: State Int () incCount = do {c <- get; put (c + 1); return (); } countLeaves2 :: Tree -> Int countLeaves2 t = execState (aux t) 0 where aux :: Tree -> State Int () aux (Tree xs) = foldr1 (>>) $ map (aux) xs aux (Leaf) = incCount main :: IO ()B main = print $ countLeaves2 $ buildTree 15 6

5 10

foldr
by James Ealing 02 Jan '04

02 Jan '04

Hi all, If I have a function: it f [a0, a1, a2, ...] = [a0, f a1, f (f a2), ...] Is there any way of expressing it f as an instance of foldr? Many thanks, Jim ________________________________________________________________________ Yahoo! Messenger - Communicate instantly..."Ping" your friends today! Download Messenger Now http://uk.messenger.yahoo.com/download/index.html

2 1

Parsing library
by James Ealing 02 Jan '04

02 Jan '04

I'm trying to make use of the combinatorial parsing library to process strings. However, I can't figure out the correct syntax for the (|||) (^^^) (>>>) (<^^) and (^^>) functions. Can anyone see how to do it? If so it'd be really useful if you could put down a couple of examples of how each is used. Thanks Jim The library: Generic parsing functions >module Parse ( > Parse, succeed, token, spot, > (|||), (^^^), (<^^), (^^>), (>>>), > many, listOf, topLevel, > white, ws, parseVar, word, parseNum) >where --------------------------------------------------- -- Combinatory parsing library using Maybe types -- -- suitable for non-ambiguous grammars. -- -- sja4(a)mcs.le.ac.uk (21/10/96) -- --------------------------------------------------- >infixr 5 `into`, ^^^, <^^, ^^> >infixl 4 >>> >infixr 3 ||| Maybe defined in prelude: data Maybe a = Just a | Nothing --------------------- -- Type of parsers -- --------------------- >type Parse a = String -> Maybe (a, String) ------------------- -- Basic parsers -- ------------------- Succeed with the value given. >succeed :: a -> Parse a >succeed val inp = Just (val, inp) Recognize a specified token at the head of the input. >token :: Char -> Parse Char >token t (u:x) = if t == u then Just (t,x) else Nothing >token t [] = Nothing Recognize a token with a certain property. >spot :: (Char -> Bool) -> Parse Char >spot p (t:x) = if p t then Just (t,x) else Nothing >spot p [] = Nothing ------------------------- -- Parsing combinators -- ------------------------- A choice between two parsers. The function p1 ||| p2 returns the result of p1 whenever it succeeds and the result of p2 otherwise. >(|||) :: Parse a -> Parse a -> Parse a >(p1 ||| p2) inp = case p1 inp of > Nothing -> p2 inp > Just (v,x) -> Just (v,x) Sequencing of parsers. The function p1 ^^^ p2 returns the result, if any, of applying p1 to the input and then p2 to the remainder. >(^^^) :: Parse b -> Parse c -> Parse (b,c) >(p1 ^^^ p2) inp = > case p1 inp of > Nothing -> Nothing > Just (v,x) -> case p2 x of > Nothing -> Nothing > Just (u,y) -> Just ((v,u),y) Semantic action. The results from a parser p are transformed by applying a function f. >(>>>) :: Parse b -> (b -> c) -> Parse c >(p >>> f) inp = case p inp of > Nothing -> Nothing > Just (v,x) -> Just (f v, x) Sequencing of parsers, choosing one component or the other >(<^^) :: Parse b -> Parse c -> Parse b >p <^^ q = (p ^^^ q) >>> fst >(^^>) :: Parse b -> Parse c -> Parse c >p ^^> q = (p ^^^ q) >>> snd Repetition. The parser p is used as many times as possible and the results are returned as a list. >many :: Parse b -> Parse [b] >many p = ((p ^^^ many p) >>> cons) ||| (succeed []) >cons (x,xs) = x:xs ListOf p c applies parser p as many times as possible, with the instances separated by instances of c, and returns the result as a list. >listOf :: Parse b -> Char -> Parse [b] >listOf p sep = p ^^^ many (token sep ^^> p) >>> cons The top level parser is a function which maps a list of tokens to a value. A value p :: Parse a b can be converted to such a function by applying topLevel: >topLevel :: Parse b -> String -> b >topLevel p inp > = case p inp of > Just (result,[]) -> result > Just (result,rest) -> > error ("parse unsuccessful; input unconsumed:"++show rest) > other -> error "parse unsuccessful" Note there is an error if the input is not fully consumed. It is sometimes useful to test whether a given parser will accept the input without actually returning a result. >acceptedBy :: Parse b -> String -> Bool >acceptedBy parser inp > = case parser inp of > Just (result,[]) -> True > other -> False A more sophisticated form of sequencing. The into combinator allows the second parser to be chosen according the result of the first. >into :: Parse b -> (b -> Parse c) -> Parse c >into p f inp = case p inp of > Nothing -> Nothing > Just (v,x) -> f v x ========================================================================== Absorb white space >white = many (token ' ' ||| token '\t') >ws p = white ^^> p <^^ white ========================================================================== Consume and return variable: must start with small letter, and continue with alphanumberic characters >parseVar :: Parse String >parseVar = spot isLower ^^^ many (spot isAlphaNum) >>> cons Consume and return particular word >word :: String -> Parse String >word [c] = token c >>> (\ c -> [c]) >word (c:cs) = (token c ^^^ word cs) >>> cons Consume and return number >parseNum :: Parse Int >parseNum = spot isDigit ^^^ many (spot isDigit) >>> mknum > where mknum (d, ds) = foldl f (digitToInt d) ds > f n d = 10*n + digitToInt d ________________________________________________________________________ Yahoo! Messenger - Communicate instantly..."Ping" your friends today! Download Messenger Now http://uk.messenger.yahoo.com/download/index.html

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RE: foldr
by Garner, Robin 02 Jan '04

02 Jan '04

It's not too difficult to do: think 'fold map' and put it in the form that foldr needs. cheers -----Original Message----- From: James Ealing [mailto:jamesealing2000@yahoo.co.uk] Sent: Friday, 02 January, 2004 1:54 PM To: haskell-cafe(a)haskell.org Subject: foldr Hi all, If I have a function: it f [a0, a1, a2, ...] = [a0, f a1, f (f a2), ...] Is there any way of expressing it f as an instance of foldr? Many thanks, Jim ________________________________________________________________________ Yahoo! Messenger - Communicate instantly..."Ping" your friends today! Download Messenger Now http://uk.messenger.yahoo.com/download/index.html _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Module creation
by J & M van Berchem 02 Jan '04

02 Jan '04

New to Haskell, I got a version, through Fink, for my Mac OS X. I like the interpreter very much, but I do not see how I may get out of "Prelude" in order to write a program. Could you advise me ? Thanks in advance Jacques van Berchem

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Parsec question
by Mark Carroll 01 Jan '04

01 Jan '04

I tried posting this before but, from my point of view, it vanished. My apologies if it's a duplicate. In http://www.cs.uu.nl/~daan/download/parsec/parsec.html we read, > testOr2 = try (string "(a)") > <|> string "(b)" > > or an even better version: > > testOr3 = do{ try (string "(a"); char ')'; return "(a)" } > <|> string "(b)" Why is the latter better? (BTW, I like Parsec. Thanks, Daan. (-:) -- Mark

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Monads
by Mark Carroll 01 Jan '04

01 Jan '04

Omitting the typeclass bit, I'm trying to write something like (s1 -> s2) -> StateT s1 m () -> StateT s2 m a -> StateT s1 m a That is, it sequences two StateT computations, providing a way to translate from the first's state to the second to keep the chain going. I can easily write something for when s1 and s2 are the same, and my understanding of much of Control.Monad.* remains tenuous at best, but if it's easy for anyone to provide me with some tips, then I thought I should mention that it'd certainly be helpful. And Happy New Year, everyone! -- Mark

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Type checking/inference
by Lee Dixon 28 Dec '03

28 Dec '03

Hi, I've run into a small problem whilst doing some manual type checking, to see if I could match the results given by hugs prelude> :t foldr filter foldr filter :: [a] -> [a -> Bool] -> [a] -- This was fine and was the same as my answer, so I tested it with prelude> foldr filter [1,2,3,4] [even,odd] -- and the answer was indeed an empty list -- However I got stuck when attempting to derive the result of map (foldr filter) -- Hugs says that: prelude> :t map (foldr filter) map (foldr filter) :: [[a]] -> [[a -> Bool] -> [a]] Two main questions: 1/ How does hugs derive this answer? 2/ What input can I give so that it yields a correct result? I've tried giving it a list of lists but it fails... Any pointers would be greatly appreciated! Many Thanks, Lee _________________________________________________________________ Stay in touch with absent friends - get MSN Messenger http://www.msn.co.uk/messenger

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Re: Perspectives on learning and using Haskell
by Graham Klyne 24 Dec '03

24 Dec '03

[switching to Haskell-cafe] At 19:37 23/12/03 +0100, Tomasz Zielonka wrote: >On Tue, Dec 23, 2003 at 05:26:20PM +0000, Graham Klyne wrote: > > > [1] http://www.ninebynine.org/Software/Learning-Haskell-Notes.html > >Thanks, that was a nice reading :) Thanks! (If by any chance there's anything here that might be useful for the Wiki, anyone may feel free to plunder it.) >I have some comments: > >8. Your explanation of Functor excludes many useful Functors which are > rather not collections. For example, every monad (like IO) can > be a Functor if you take fmap = Monad.liftM. > > For [] and Maybe this would give the same operation as in their > normal instances. That's an interesting perspective that I wasn't aware of... I need to think about that. Meanwhile, I've added your observation to my notes. [later] It now seems to me that (some?) Monads are kinds of Functors, generalized to handle the "no value" case, and also composition. This also had me thinking about sequence: is there a generalization to arbitrary monads that rearranges the monadic structure? >11 and 18. > If you define an instance of Monad for ((->) e) then > > return (putStrLn "Hello!") 'x' > > is a proper IO () value. Probably still not sensible ;) Ah, I think I see your point. It would apply where monads are "nested", right? > Special treatment of 'return' could be helpful, but I am afraid that > it could also make it look special, like a return keyword in C. I certainly wouldn't argue for special treatment _in the language_, but OTOH, I think it might be helpful if compiler diagnostics hinted at the possibility when a type error is detected in a form like return x y. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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