- Haskell-Cafe - Haskell.org

Problem with haskell types
by Anupam Jain 31 Jul '10

31 Jul '10

Hi, I am having trouble getting a small program to compile. The helpful folks at #haskell created a version of the program that does compile - http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28406#a28408 but it is not very clear to them (and to me) why the original program wouldn't type compile in the first place. Here's the program that refuses to compile - module Delme () wheredata DecisionState = A | B | C | Dd_test :: Eq b => b -> b -> DecisionState -> DecisionState -> ()d_test test testVal trueState falseState = if (test == testVal) then d trueState else d falseStated :: DecisionState -> ()d A = d_test True True B Cd B = d_test 1 2 C Dd C = d_test True False A Bd D = () I get an error like - Delme.hs:13:0: Contexts differ in length (Use -XRelaxedPolyRec to allow this) When matching the contexts of the signatures for d_test :: forall b. (Eq b) => b -> b -> DecisionState -> DecisionState -> () d :: DecisionState -> () The signature contexts in a mutually recursive group should all be identical When generalising the type(s) for d_test, d Putting in the extension does get the program to type check but the original program should have type compiled in the first place. The ironic thing we discovered is that if we remove the type declaration for 'd', the program type checks, and GHC then derives the exact same type which we removed! Can some of the smarter people in the room please shed more light on this? -- Anupam

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couchDB
by Andrew U. Frank 30 Jul '10

30 Jul '10

a few weeks ago a question was posed on this list for examples how to use couchDB. i post the 'hello world' for couchDB using the haskell interface - perhaps it helps other to start! a more extensive and complicate example is found at www.maztravel.com/haskell/mySqlToCouchDB.html I created with Futon a db names "first" (i had no luck with names containing a - or a _ , but have not investigated further). the runcouchDB' affects the couchDB at localhost. andrew ps: if there is a better place to document examples? a wiki on haskell.org would be nice and should be available for any project in hackage. ---- {-# LANGUAGE DeriveDataTypeable #-} module Main ( ) where import Database.CouchDB import Data.Data (Typeable) import Text.JSON s1 = JSString $ toJSString "Peter" m1 = makeObj [("FirstName", s1), ("FamilytName", JSString . toJSString $ "Miller")] mydb1 = db "first" -- convert db name to checked couchdb -- problem with "-" or "_" in db names in haskell?? main = do putStrLn "start couchdb tests" (doc, rev) <- runCouchDB' $ newDoc mydb1 m1 -- works return ()

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[Yi Editor]Cabal Problem
by Alessandro Stamatto 30 Jul '10

30 Jul '10

Installing Yi Editor i get the following error: ------------------------------ ----------------------------------------------- Missing dependencies on foreign libraries: * Missing C libraries: gobject-2.0, glib-2.0, intl, iconv ----------------------------------------------------------------------------- Im on windows, using Cabal in Cygwin. How should i install those missing libs?

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Fail to install SDL with Cabal
by Eitan Goldshtrom 30 Jul '10

30 Jul '10

I'm trying to install SDL through Cabal -- I don't know another way to install it. However, I'm getting this: setup.exe: The package has a './configure' script. This requires a Unix compatibility toolchain such as MinGW+MSYS or Cygwin. cabal: Error: some packages failed to install: SDL-0.5.10 failed during the configure step. The exception was: exit: ExitFailure 1 I have MinGW and MSYS, so I don't understand why I'm having this problem. Do I need to set something special up so that Cabal can access their tools? -Eitan

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ANN: weighted-regexp-0.1.0.0
by Sebastian Fischer 30 Jul '10

30 Jul '10

Hello, this year's ICFP features A Play on Regular Expressions where two Haskell programmers and an automata theory guru develop an efficient purely functional algorithm for matching regular expressions. A Haskell library based on their ideas is now available from Hackage. For more information (and a link to the play) visit: http://sebfisch.github.com/haskell-regexp/ Cheers, Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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data.binary get reading beyond end of input bytestring?
by Conrad Parker 30 Jul '10

30 Jul '10

Hi, I am reading data from a file as strict bytestrings and processing them in an iteratee. As the parsing code uses Data.Binary, the strict bytestrings are then converted to lazy bytestrings (using fromWrap which Gregory Collins posted here in January: -- | wrapped bytestring -> lazy bytestring fromWrap :: I.WrappedByteString Word8 -> L.ByteString fromWrap = L.fromChunks . (:[]) . I.unWrap ). The parsing is then done with the library function Data.Binary.Get.runGetState: -- | Run the Get monad applies a 'get'-based parser on the input -- ByteString. Additional to the result of get it returns the number of -- consumed bytes and the rest of the input. runGetState :: Get a -> L.ByteString -> Int64 -> (a, L.ByteString, Int64) The issue I am seeing is that runGetState consumes more bytes than the length of the input bytestring, while reporting an apparently successful get (ie. it does not call error/fail). I was able to work around this by checking if the bytes consumed > input length, and if so to ignore the result of get and simply prepend the input bytestring to the next chunk in the continuation. However I am curious as to why this apparent lack of bounds checking happens. My guess is that Get does not check the length of the input bytestring, perhaps to avoid forcing lazy bytestring inputs; does that make sense? Would a better long-term solution be to use a strict-bytestring binary parser (like cereal)? So far I've avoided that as there is not yet a corresponding ieee754 parser. Conrad.

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Announce snm-0.0.2: The Simple Nice-Looking Manual Generator
by Johnny Morrice 30 Jul '10

30 Jul '10

Hi list, Recently, I've been working on a compiler/programming language project.[2] I became rather unstuck when I started spending more time documenting the semantics instead of actually getting to work on the implementation. Hence this program! snm allows you to write clean, web-friendly reports, user guides and manuals without having to edit fickle html. snm allows you to structure your document in a modular fashion. snm document sections are written in yaml and are easy to write and understand. snm is a generator of small, valid xhtml files. Read the snm user guide here: http://www.killersmurf.com/static/snm_help.html snm uses Yaml and generates XHTML, so it's a little like John MacFarlane's yst[1], but yst is for making websites, while snm is exclusively for creating structured reports. It still has a lot of problems, not to mention no support for images, or multiple pages, but I'm finding it usable (enough to document itself!) so I thought it best to share :) Sorry if I have left typos, it's very late Have fun, Johnny [1] John MacFarlane's yst: http://hackage.haskell.org/package/yst [2] Yon aforementioned programming language project, in case anyone fancies a peep. Demoted to reference #2, on account of it not yet compiling code :) http://github.com/elginer/Obelisk

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Search for documentation as data fails with Haskell Platform on Windows 7
by John D. Ramsdell 29 Jul '10

29 Jul '10

I contributed the CPSA package to Hackage, and tried using it on a PC running Windows 7. The package installs six executables and documentation as PDF and XHTML into the package's data directory. Haskell Platform with Cabal installs the package without a problem. It was actually quite smooth and simple. So far so good. The trouble came when I tried to locate the documentation. The XHTML document is called cpsauser.html, but the search command on Windows 7 cannot find the document. Is there some way to tell it to try harder? How are users supposed to know how to locate program documentation when using Windows 7? I'm not much of a Windows user, so let me apologize in advance if this is a silly question, and search is, in fact, easy. John

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Memory and Threads - MVars or TVars
by Eitan Goldshtrom 29 Jul '10

29 Jul '10

Hi everyone. I was wondering if someone could just guide me toward some good information, but if anyone wants to help with a personal explanation I welcome it. I'm trying to write a threaded program and I'm not sure how to manage my memory. I read up on MVars and they make a lot of sense. My real question is what is "atomic" and how does it apply to TVars? I don't understand what atomic transactions are and I can't seem to find a concise explanation. I also saw some stuff about TMVars? But I can't find much on them either. Any help would be appreciated. -Eitan

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Re: Fun with uu-parsinglib
by S. Doaitse Swierstra 29 Jul '10

29 Jul '10

On 29 jul 2010, at 05:04, David Place wrote: > Hi, Doaitse. > > I am making good progress transcribing my parser to use your library. I think some ways that I have grown accustomed to working with Parsec will not work, though. Now, I am getting the run time error: > >> Result: *** Exception: cannot compute minimal length of right hand side of monadic parser > > Is there an explanation of this error in the documentation? I am trying to combine with alternation some parsers which return the semantic data structures of my program. Perhaps there are restrictions on the kind of parsers that can be combined with alternation. > > Cheers, > David > ____________________ > David Place > Owner, Panpipes Ho! LLC > http://panpipesho.com > d(a)vidplace.com > > > Dear David, I am cc-ing this answer to Haskell-café since the answer may be of a wider interest. Let me first explain a a sequence of design choices I made: 1) my parser combinators perform error correction by trying to insert missing tokens into and to delete superfluous tokens from the input 2) of course there are many possibilities to do so 3) hence I implemented a search process which currently prunes the tree of possible corrective choices up to depth three 4) this process may return several solutions with the same overall cost and I have to pick one of those 5) in case of a draw I give preference to those alternatives which delete something, so progress is guaranteed 6) at the end of the input this does not work, since there is nothing to delete 7) if I pick an arbitrary alternative this may lead to a choice where we get an infinite sequence of insertions; suppose e.g. we want to insert an expression, and the system picks the if_alternative. If it does so it inserts an if-token and then the process starts all over again by trying to insert a conditional expression, which in general will be an expression, which will lead to the insertion of another if-token, etc. 8) in order to prevent this behaviour internally an abstract interpretation is made which computes the minimal number of tokens a parser will recognise (and thus can insert). The recursive alternatives will get an infinite length, and will thus never be chosen unless you grammar is well-formed and all choices only can lead to an infinite sequence of insertions 9) if a choice has to be made for the correction process always a finite alternative is chosen, so the correction process is guaranteed to complete Now the monads come in; they raise the question how to compute the minimal length of a parser WHICH DEPENDS ON A PREVIOUSLY RECOGNISED part of the input. In general this is impossible. So I made the decision to generate an error message in such cases, since I did not expect many people to run into this. But now you, and probably many people used to writing parsers in the monadic (i.e. Parsec) style turn up, and you continue to use the monadic style bacuse you have become accustomed to a different way of writing parsers. Let me give an example, based on the following grammar for well-formed nested parentheses: S -> (S)S ; epsilon. We want to return e.g. the maximal nesting depth. This is one of the functions in the Examples.hs file in the uu-parsinglib. The prototypical way of writing this in applicative style, without using monads is: wfp :: Parser Int wfp = max . (+1) <$> pParens wfp <*> wfp `opt` 0 Now let us take a look at the monadic formulation: wfp = do lt <- pParens wfp rt <- wfp -- < second call to wfp return ((lt + 1) `max` rt) <|> return 0 Now we see that the second call to wfp is within the scope of the binding of lt, and hence may depend on it. In this case it does not, but unfortunately the abstract interpretation cannot see this, and thus cannot do very much. This shows in my view the bad thing about monadic parser combinators: they prohibit the self-analysis of parsers, they inhibit optimisations, and they inhibit feedback based on the analysis. More on this below. Since I assume that the monadic formulation is only used as a last resort, i.e. when a parser REALLY depends on previous input, and that this will in general not be part of an alternative construct, I have decided to generate the above error message. Another solution I could have chosen is to make the choice in the correction process based on the order in which the alternatives occur in the program, but this would create the need to explain to users that the order of their alternatives matters, and it is one of the nice things that thus far it does not. In the old uulib the order even gets completely lost because we internally build tables which assist in speeding up the parsing process. There are several solutions now: a) I just remove the error message and make an educated guess, leading to situations where users might get stuck in an infinite correction process (only at the end of the file; normally in case of a draw preference is given to delete something in order to guarantee progress), and then start to ask questions about what is going on. Notice that they will not get feedback except probably a stack overflow. b) We leave it like this assuming that when people use a monadic style this will not be in the branch of an alternative construct c) We add an extra combinator which removes the error message, and we require the user to indicate that a specific alternative should never be chosen in the insertion process d) Internally I check to see whether more than specific number of insertions occur in a row, and then stop. This will however make parsing a bit more expensive, which I always try to avoid. Let me now come back to explain g further why I think the monadic style should be avoided where-ever possible because they prohibit abstract interpretation. If in the uu-parsinlib you write a parser like: pList (pList (pSym 'a')) you will get an an error message telling that this parser does not make sense, since the outer pList gets as an argument a parser which can accept the empty string. The good news is that this is something we can statically recognise, EVEN IN THE CASE we use monadic parsers. When we have p >>= q then either p is not empty, and if it is we know the value it returns and we can use this to see whether the q part may return empty. Notice that this is an analysis we have to do only once for each parser, and hence costs can be almost neglected. My conclusion is: avoid monadic parsers as much as possible, and only use them where you really want to continue parsing with a parser which really depends on the input, like in: n_as = do n <- pInt pEaxct (pSym 'a') pExact 0 p = pReturn [] pExact n p = (:) <$> p <*> pExact (n-1) p I have explained this observation before in my lecture notes for the Advanced Functional Programming summer school in 1996, section 5.2: @inproceedings{SwieDupo96, Author = {Swierstra, S. D. and Duponcheel, L.}, Booktitle = {Advanced Functional Programming}, Editor = {Launchbury, John and Meijer, Erik and Sheard, Tim}, Pages = {184-207}, Publisher = {Springer-Verlag}, Series = {LNCS-Tutorial}, Title = {Deterministic, Error-Correcting Combinator Parsers}, Urlpdf = {http://www.cs.uu.nl/people/doaitse/Papers/1996/DetErrCorrComPars.pdf}, Volume = {1129}, Year = {1996}} Good luck with your further attempts, and let me know which of the a)-d) you prefer, and do not hesitate to inform me about the next problem you may run into ;-}} Doaitse PS: if you want to do any further semantic processing then you might consider using the uuagc compiler to build te functions which typically arise in from of the <$>'s: http://www.cs.uu.nl/wiki/bin/view/HUT/AttributeGrammarSystem

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Fwd: [Haskell-cafe] fine control of bytestring streaming
by Alberto G. Corona 29 Jul '10

29 Jul '10

Thanks Henning , I finally found the problem and an elegant solution (I guess) The problem is not, as I feared, the coalescence of bytestring chunks in more bigger chunks, but the buffering mode of the stream. The stream simply waits until the buffer is full to stream its content. So the solution is either to change the blocking to NoBlocking (with hSetBuffering, and very inefficient) or else to modify Data.BytreString.Lazy.hPut to insert an hFlush after each chunk delivered to the stream: import qualified Data.ByteString as S(hPut) import Data.ByteString.Lazy.Internal(foldrChunks) myhPut :: Handle → ByteString → IO () myhPut h cs = foldrChunks (λc rest → S.hPut h c >> System.IO.hFlush h >> rest) (return ()) cs the original Data.BytreString.Lazy.hPut has not the hFlush step. I had to modify simpleServer to do that and now I control exactly the size of the fragment and the moment when it is delivered, because the mappend instance of ByteString.Lazy respect the chunks created with "pack (String)". I give it to the consideration of the maintainers of Data.ByteString.Lazy (Dons),, because lazy bytestrings, since are a concatenation of user made blocks, have such possibility of controlling precise streaming. Something that neither ordinary strings nor strict bytestring, and I guess nothing else has. Don´t miss out this!. 2010/7/27, Henning Thielemann <lemming(a)henning-thielemann.de>: > > On Tue, 27 Jul 2010, Alberto G. Corona wrote: > >> The question is: are there some way to control bytestring streaming?. >> Can It be done without the stream handler? > > I think there is a function that converts a lazy ByteString to a list of > strict ByteStrings, that should work without copying the chunk data > around. On this chunk list you can do all kind of manipulations, like > merging adjacent small chunks. >

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ANN: syz-0.2.0.0
by Michael D. Adams 28 Jul '10

28 Jul '10

Hi all, This years Workshop on Generic Programming includes a generic zipper for Haskell that defines a Zipper type permitting zipper traversals over arbitrary instances of Data. A Haskell library based on those ideas is now available from Hackage at: http://hackage.haskell.org/package/syz Right now the documentation is very minimal so the best guide would be to have a look at the paper which includes a small tutorial. A pre-print is available at: http://www.cs.indiana.edu/~adamsmd/papers/scrap_your_zippers/ Bug reports as well as suggestions, feedback or contributions to the library or the documentation are most welcome and should be sent to me at adamsmd-AT-cs-DOT-indiana-DOT-edu. Michael D. Adams Ph.D. Student Indiana University http://www.cs.indiana.edu/~adamsmd/

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Lists and monads
by Kevin Jardine 28 Jul '10

28 Jul '10

As a Haskell neophyte, one of the things I find confusing is the way that the usual list functions (map, fold, ++, etc.) often cannot be used directly with monadic lists (m [a] or [m a]) but seem to require special purpose functions like ap, mapM etc. I get the idea of separating pure and impure code but am constantly frustrated by the fact that stuff that would be easy outside a monad seems to get more difficult inside, especially list manipulation. As a result I tend to write ugly recursive list functions that would be more appropriate with Python, say. I suspect that things are not quite as difficult as they appear, however, but cannot find any tutorials on monadic list manipulation. Any suggestions for resources in that area? Kevin

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Parsec "Best Practices" examples
by David Place 28 Jul '10

28 Jul '10

Hi, All. I have a compiler written in Parsec and Haskell for a DSL. It's quite rudimentary since I made it for my own use only. I would like to make it useable by a slightly wider circle. For this, I need better error reporting. Currently, it terminates after finding one syntax error. I would like to detect as many syntax errors as possible and report them before terminating. Are there any "best practices" examples of how to do this available? It seems like something many people would have solved already. Cheers, David ____________________ David F. Place Owner, Panpipes Ho! LLC http://panpipesho.com d(a)vidplace.com

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Template Haskell sees into abstract data types
by Serguey Zefirov 28 Jul '10

28 Jul '10

Data.Map.Map and Data.Set.Set are exported abstractly, without exposing knowledge about their internal structure. I cannot directly create my own class instances for them because of that. But I found that I can write Template Haskell code that could do that - those data types could be reified just fine. This is somewhat strange situation. Was it a design decision?

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Class instances on class constraints
by Dušan Kolář 28 Jul '10

28 Jul '10

Hello all, For school purposes, I would like to define something like this: module SetOverList( SetFromList , SetOL(..) ) where type SetFromList a = [a] addEq :: Eq a => a -> SetFromList a -> SetFromList a addEq v [] = [v] addEq v l@(x:xs) = if x==v then l else x : addEq v xs addOrd :: Ord a => a -> SetFromList a -> SetFromList a addOrd v [] = [v] addOrd v l@(x:xs) = case compare x v of GT -> v : l EQ -> l LT -> x : addOrd v xs class SetOL a where addElem :: a -> SetFromList a -> SetFromList a instance (Eq a) => SetOL a where addElem = addEq instance (Ord a) => SetOL a where addElem = addOrd which does not work, of course (Flexible or Undecidable instances won't help). The aim is to have addElem function that works differently according to situation whether a type, which is base of the list/set, is a member of class Eq or Ord. Could you point me or hint me how to get as close as possible to the required solution? Maybe I'm not able to see an obvious way... Thanks, Dušan

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Haskell Forum
by Daniel Díaz 28 Jul '10

28 Jul '10

Hi all, I want to open a Haskell forum based on phpBB, but I need some collaborators for organize its content, and moderate its use. When we have finished, I will open this forum for the entire community of Haskell! If you are interested, mail me: danieldiaz(a)asofilak.es Thanks in advance.

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Int and ByteStrings
by wren ng thornton 28 Jul '10

28 Jul '10

Hey all, Is there a library function (f :: [Int] -> ByteString) such that it's prefix-preserving, and platform independent? GHC-only is fine for now. There are a bunch of functions of that type, but I need those guarantees and I'm hoping someone else has already done it. If there isn't one for Int, has someone already made a package that offers IntMap on Word32, Word64, etc? -- Live well, ~wren

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Need Control.Monad.State
by michael rice 28 Jul '10

28 Jul '10

How do I import Control.Monad.State? I see this note in http://en.wikibooks.org/wiki/Haskell/Understanding_monads/State Note: in some package systems used for GHC, the Control.Monad.State module is in a separate package, usually indicated by MTL (Monad Transformer Library). Michael ===============rand.hs ============== import Control.Monad.State type GeneratorState = State StdGen rollDie :: GeneratorState Int rollDie = do generator <- get let( value, newGenerator ) = randomR (1,6) generator put newGenerator return value ======================= [michael@localhost ~]$ ghci rand GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. rand.hs:1:7: Could not find module `Control.Monad.State': Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude>

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fine control of bytestring streaming
by Alberto G. Corona 28 Jul '10

28 Jul '10

Hi, I´m streaming content using lazy bytestrings in a web application. The problem is that the output comes in huge blobs (presumably of 32k) one at a time. This is good for some purposes, but not for console-like interfaces or runtime log visualization. (one of my purposes is to web-alize ghci. I know that this can be done without streaming the standard output, but I want it that way). I can not control when the IO bytestring primitives flush the data to the stream, because this is not part of my application (nor does it should do it). I suppose here that it is in a chunk by chunk basis. But maybe I misinterpreted something or everything. I use the package hack with the server hack-handler-simpleserver. this serves does Data.ByteString.hPut the bytestring content... I use mappend to add content to the stream. mappend uses "append" which uses. "foldrChunks" which seems not to compact two bytestrings in max size chunks . So I do not know what happens. The question is: are there some way to control bytestring streaming?. Can It be done without the stream handler? Alberto.

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Haskell at CERN
by Alexey Khudyakov 28 Jul '10

28 Jul '10

Hello I'm wondering is there any haskellers in CERN? Given quality and usability of software there must be people looking for better alternatives.

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transform function for a GADT
by Ozgur Akgun 27 Jul '10

27 Jul '10

Café, I've tried several things already, but I am not including any of them for now. My question is, how would you define the 'transform' function for a GADT, say the one in the linked gist: http://gist.github.com/492364 (also attached to this e-mail) To be concise, I want 'transform' to apply the transformation function (its first parameter) to every immidiate child of its second parameter as long as the types match. Similar to what the 'tranform' function of Uniplate does for normal ADTs. (But just one level, so I guess it is more similar to the 'descend' of Uniplate. See http://hackage.haskell.org/packages/archive/uniplate/1.2.0.1/doc/html/Data-… ) I think I got closest to a sensible solution using multi-param type classes, and defining many instances for different combinations of ExprType's but still there were problems. Waiting for suggestions and/or insights. Best, Ozgur Akgun

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Actors and message-passing a la Erlang
by Yves Parès 27 Jul '10

27 Jul '10

Hello ! I've been studying Erlang and Scala, and I was wondering if someone has already implemented an actors and message passing framework for concurrent and distributed programs in Haskell.

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An experimental Zipper using Thrists and first-class Labels. Help or thoughts?
by Brandon Simmons 27 Jul '10

27 Jul '10

I had the idea for a simple generic Zipper data structure that I thought would be possible to implement using type-threaded lists provided by Gabor Greif's thrist package: http://hackage.haskell.org/package/thrist ...and the fclabels package by Sebastiaan Visser, Erik Hesselink, Chris Eidhof and Sjoerd Visscher: http://hackage.haskell.org/package/fclabels It would (ideally) work as follows: - the zipper would consist simply of a tuple: (type threaded list of constructor sections , current "context") - in the type threaded list we store functions (constructor with hole -> complete constructor), so the "one hole context" is represented as a lambda expression where the free variable will be filled by the current "context" (the snd of the tuple) - we "go down" through our structure by passing to our `moveTo` function a first-class label corresponding to the constructor we want to descend into. `moveTo` uses this both as a "getter" to extract the next level down from the current level, and as a "setter" to form the lambda expression which acts as the "constructor with a piece missing" - "going up" means popping the head off the thrist and applying it to the current context, making that the new context, exiting the zipper would be a fold in the same manner After throwing together a quick attempt I realized that I'm not sure if it would be possible to make the `moveUp` function type-check and be usable. I'm still new to GADTs, existential types, template haskell etc. and am stuck. Here is the code I wrote up, which doesn't currently compile: ---------------------------------- START CODE ------------------------------- {-# LANGUAGE TypeOperators, GADTs #-} module ZipperGenerator ( viewC --lets user pattern match against context , moveTo , moveUp , genZippers , zipper , unzipper , (:->) , ZipperGenerator , Zipper ) where -- these provide the secret sauce import Data.Record.Label import Data.Thrist import Language.Haskell.TH type ZipperGenerator = [Name] -> Q [Dec] -- the Template Haskell function that does the work of generating -- first-class labels used to move about the zipper: genZippers :: ZipperGenerator genZippers = mkLabels -- hide the innards: newtype Zipper t c = Z (Thrist (->) c t, c) -- returns the current "context" (our location in the zipper) for pattern -- matching and inspection: viewC :: Zipper t c -> c viewC (Z(_,c)) = c -- takes a first-class label corresponding to the record in the current context -- that we would like to move to: moveTo :: (c :-> c') -> Zipper t c -> Zipper t c' moveTo lb (Z(thr,c)) = Z (Cons (\a-> set lb a c) thr , get lb c) -- backs up a level in the zipper, returning `Nothing` if we are already at the -- top level: moveUp :: Zipper t c -> Maybe (Zipper t b) moveUp (Z (Nil,_)) = Nothing moveUp (Z (Cons f thr,c)) = Just $ Z (thr, f c) -- create zipper with focus on topmost constructor level: zipper :: t -> Zipper t t zipper t = Z (Nil,t) -- close zipper unzipper :: Zipper t c -> t unzipper (Z(thr,c)) = undefined --foldThrist ($) id thr c ---------------------------------- END CODE ------------------------------- Thanks, Brandon Simmons http://coder.bsimmons.name/blog/

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monoids and monads
by John Lato 27 Jul '10

27 Jul '10

Hello, I was wondering today, is this generally true? instance (Monad m, Monoid a) => Monoid (m a) where mempty = return mempty mappend = liftM2 mappend I know it isn't a good idea to use this instance, but assuming that the instance head does what I mean, is it valid? Or more generally is it true for applicative functors as well? I think it works for a few tricky monads, but that's not any sort of proof. I don't even know how to express what would need to be proven here. Any resources for how I could develop a means to reason about this sort of property? Thanks, John

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Is 'flip' really necessary?
by aditya siram 27 Jul '10

27 Jul '10

Eta-reducing is nice, and sometimes it makes code more readable. But 'flip' is one of those functions that always seems to hinder rather than help readability, conversely factoring out flip always makes code easier to comprehend. I don't see a need for its existence - maybe I'm missing something and more experienced Haskellers can comment. -deech

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1st attempt at parallelizing
by Günther Schmidt 27 Jul '10

27 Jul '10

Hi all, I'm spidering web pages, the implementation currently is synchronous. I'd like to parallelize this for speed-up, ie. get up to 6 pages in parallel and recycle those threads. Now I have come across good examples for this on the web before, but I doubt I'd find it again right away. I'd appreciate some good pointers. Günther

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Re: Instances for Set of Functor, Traversable?
by oleg＠okmij.org 27 Jul '10

27 Jul '10

Lennart Augustsson wrote: > Try to make Set an instance of Functor and you'll see why it isn't. > It's very annoying. And yet the very simple, and old solution works. http://okmij.org/ftp/Haskell/types.html#restricted-datatypes We just properly generalize Functor, so that all old functors are new functors. In addition, many more functors become possible, including Set. In general, we can have functors fmap' :: (C1 a, C2 b) => (a -> b) -> f a -> f b Incidentally, even an Integer may be considered a functor: we can define the fmap' operation fitting the above signature, where the constraint C1 a is a ~ Integer. Although the use of OverlappingInstances is not required, the extension leads to the nicest code; all old functors just work. {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-} module FunctorEx where import Control.Monad import Data.Set as S class Functor' f a b where fmap' :: (a -> b) -> f a -> f b -- The default instance: -- All ordinary Functors are also extended functors instance Functor f => Functor' f a b where fmap' = fmap -- Now define a functor for a set instance (Ord a, Ord b) => Functor' S.Set a b where fmap' = S.map -- Define a degenerate functor, for an integer newtype I a = I Integer deriving Show instance Functor' I Integer Integer where fmap' f (I x) = I $ f x -- tests -- Lists as functors test_l = fmap' (+10) [1,2,3,4] -- [11,12,13,14] -- Sets as functors test_s = fmap' (\x -> x `mod` 3) $ S.fromList [1,2,3,4] -- fromList [0,1,2] -- Integer as functor test_i = fmap' (* (6::Integer)) $ I 7 -- I 42

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Re: Announce snm-0.0.2: Oops
by Johnny Morrice 27 Jul '10

27 Jul '10

> Sorry if I have left typos, it's very late I knew it was a bit late to be uploading things, I forgot to say where anyone interested might download this: it's on Hackage so just cabal install snm Cheers Johnny

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Instances for Set of Functor, Traversable?
by Gregory Crosswhite 27 Jul '10

27 Jul '10

Is there a specific reason why Set doesn't have instances for Functor and Traversable? Or have they just not been written yet? :-) Cheers, Greg

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Re: [Haskell-cafe] data type declaration
by John Lato 27 Jul '10

27 Jul '10

> From: Patrick Browne <patrick.browne(a)dit.ie> > > Andrew, > Thanks for your detailed feedback, it is a great help. > I appreciate that the code does not do anything useful, nor is it an > appropriate way to write Haskell, but it does help me > understand language constructs. I have seen statements like > >>>> data C3 c3 a => Address c3 a = Address c3 a > Incidentally, there seems to be a consensus that this a Bad Idea [1]. Even when you specify a type class context on a data declaration, Haskell still requires you to specify the context on functions that use that data (Address c a). What's worse is that you need the class restriction for *all* functions that use an Address, even if they don't operate on the component parts and don't make use of the type class at all. Basically, it ends up making all your type signatures longer with no benefit. If you really want to ensure that every "Address c a" has this context, then use a smart constructor: address :: C3 c3 a => c3 -> a -> Address c3 a address = Address and don't export the Address data constructor. Also, Andrew said that type classes are closer to Java interfaces than C++ objects. This is true, but you may find the difference useful too. A small example of a common OOP-er mistake should demonstrate: -- return a Float if True, else an Int. At least it would if type classes were interfaces. toFloat :: Num a => Int -> Bool -> a toFloat x True = (fromIntegral x) :: Float toFloat x False = x OOP-style interfaces have two features: they specify methods that are available for all types which implement the interface, and they are an existentially-quantified data constructor. Equivalent code in Java would return an existentially-quantified type - we know the interface methods are supported but not the data type, so the interface methods are all we can do. The exact type (either Float or Int) is wrapped in this existentially quantified box. Haskell type classes don't do existential quantification, which is why the above doesn't compile. The return value of this function must be any a with a Num instance, and the caller (not toFloat) gets to choose the exact type to instantiate. Translating an interface to Haskell requires that you write both parts: class Num a where ... data INum = forall a. Num a => INum a --requires the ExistentialQuantification extension -- now toFloat works toFloat :: Int -> Bool -> INum toFloat x True = INum ((fromIntegral x) :: Float) toFloat x False = INum x Now the type variable is stuck in the INum type instead of being universally quantified (top-level forall), which is what an interface does. I think most Haskellers would prefer to choose a design that doesn't require existential quantification, but it really depends on the problem. John [1] GHC sources refer to this context as "stupid theta", see further discussion at http://hackage.haskell.org/trac/haskell-prime/wiki/NoDatatypeContexts

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Typeclasses question in "Real World Haskell" book
by Angel de Vicente 27 Jul '10

27 Jul '10

Hi, I'm stuck at page 151 of Real World Haskell and hoping that perhaps some of you can give me a hand here... The code that is giving me trouble is below. data JValue = JString String | JNumber Double | JBool Bool | JNull | JObject [(String, JValue)] | JArray [JValue] deriving (Eq, Ord, Show) type JSONError = String class JSON a where toJValue :: a -> JValue fromJValue :: JValue -> Either JSONError a instance JSON JValue where toJValue = id fromJValue = Right instance JSON Bool where toJValue = JBool fromJValue (JBool b) = Right b fromJValue _ = Left "not a JSON boolean" I don't understand how the JSON typeclass is defined, in particular the fromJValue definition. For instance, when defining the instance for Bool types, then I understand that both functions (toJValue and fromJValue) will be called upon when we supply a Bool type, but then the (JBool b) type in function fromJValue doesn't match.... toJValue is no problem, but I cannot understand how fromJValue is supposed to work, and the comments in the online book (http://book.realworldhaskell.org/read/using-typeclasses.html) don't help with this either. *Main> :load ch6 [1 of 1] Compiling Main ( ch6.hs, interpreted ) Ok, modules loaded: Main. *Main> toJValue False JBool False *Main> :type it it :: JValue *Main> fromJValue False <interactive>:1:11: Couldn't match expected type `JValue' against inferred type `Bool' In the first argument of `fromJValue', namely `False' In the expression: fromJValue False In the definition of `it': it = fromJValue False *Main> fromJValue (JBool False) <interactive>:1:0: Ambiguous type variable `a' in the constraint: `JSON a' arising from a use of `fromJValue' at <interactive>:1:0-23 Probable fix: add a type signature that fixes these type variable(s) *Main> Any pointers? Thanks a lot, Ángel de Vicente -- http://www.iac.es/galeria/angelv/ High Performance Computing Support PostDoc Instituto de Astrofísica de Canarias --------------------------------------------------------------------------------------------- ADVERTENCIA: Sobre la privacidad y cumplimiento de la Ley de Protección de Datos, acceda a http://www.iac.es/disclaimer.php WARNING: For more information on privacy and fulfilment of the Law concerning the Protection of Data, consult http://www.iac.es/disclaimer.php?lang=en

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Please report any bug of gtk2hs-0.11.0!
by Andy Stewart 27 Jul '10

27 Jul '10

Hi all, We plan to release bug fix version : gtk2hs-0.11.1 Please report any bug of gtk2hs-0.11.0, we will fix it before release gtk2hs-0.11.1 We plan to add many new APIs in gtk2hs-0.12.0, so gtk2hs-0.11.1 will be the last stable version with current APIs. Thanks for your help! -- Andy

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ANNOUNCE: darcs 2.5 beta 2
by Reinier Lamers 26 Jul '10

26 Jul '10

Hi all, The darcs team would like to announce the immediate availability of darcs 2.5 beta 2 (also known as darcs 2.4.98.2 due to Cabal restrictions). Important changes since darcs 2.4.4 are: * trackdown can now do binary search with the --bisect option * darcs always stores patch metadata encoded with UTF-8 * obliterate has a -o flag to save obliterated patch * amend-record now supports --ask-deps * apply now supports --match * amend-record has a new --keep-date option * inventory-changing commands (like record and pull) now operate in constant time with respect to the number of patches in the repository * the push, pull, send and fetch commands no longer set the default repository by default * the --edit-description option is now on by default for the send command If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.4.98.2.tar.gz and build it by hand as explained in the README file. Kind Regards, the darcs release manager, Reinier Lamers

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datatype contexts
by Gregory Crosswhite 26 Jul '10

26 Jul '10

I agree with prior discussion on this list that adding contexts to datatype declarations seems to be more trouble than its worth, since these contexts just have to be added again to every function using the datatype. However, I have often wondered: why do function *have* to have these contexts? What would it affect in the language if we made the contexts be implicit, so that if we have data Datatype a = Context a => Datatype a then for function declarations f :: D a -> ... the context "Context a" is automatically asserted by the compiler? Cheers, Greg

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ANN: Moio, a library for compositional event-driven programming
by Patai Gergely 26 Jul '10

26 Jul '10

Hello all, I created a little library that provides first-class event sources and event stream transformers, both allowing side effects. For the time being, the code is only available on GitHub [1]. The library is called Moio, short for 'multiple-occurrence I/O', since event sources are effectively a generalisation of the IO monad that outputs a stream of values over time instead of just a single result at the end of its execution. To summarise in a short phrase, Moio could be considered as 'Rx meets Fudgets', but that's only a superficial characterisation. Check out the example application, a simple bulletin board server, which shows how to treat a callback-based GTK GUI and blocking I/O based networking uniformly using the abstractions provided. Gergely [1] http://github.com/cobbpg/moio -- http://www.fastmail.fm - Or how I learned to stop worrying and love email again

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Techniques for ensuring parser correctness?
by Jason Dagit 26 Jul '10

26 Jul '10

Hello, I find that parser correctness is often hard to verify. Therefore, I'm interested in techniques that others have used successfully, especially with Haskell. Techniques I'm aware of: * Round trip checks: Generate a datastructure, render as a string, parse back, and compare. Quickcheck can be used to automate this. * Fuzz testing: What tools exist to help me? * Formal verification: Has anyone been using this with Haskell parsers? Other than general theorem provers, say Isabelle, what tools exist? My specific need: The immediate challenge I have is that I'm modifying the parser that Darcs uses and we would like to improve the parser's test suite as well. The parser abstraction used in this case follows parsec's API. Left to my own devices I would use round trip checks, written with quickcheck, for this exercise. Because we're using a parsec style parser, I don't have a nice neat grammar handy. Thanks in advance for any advice you have! Thanks, Jason

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How to do this with associated types?
by Alexey Karakulov 26 Jul '10

26 Jul '10

Suppose I have one piece of code like this: > class Result r e | r -> e where > failure :: e -> r a > success :: a -> r a > at :: Result r String => [a] -> Int -> r a > at xs i = if i >= 0 && i < length xs > then success (xs !! i) > else failure "Wrong index" Either instance of Result is quite straightforward: > instance Result (Either e) e where > failure e = Left e > success x = Right x Maybe instance is discarding failure information: > instance Result Maybe e where > failure _ = Nothing > success x = Just x Some tests it in ghci: ghci> let xs = [0,1,2] ghci> at xs 2 :: Either String Integer Right 2 ghci> at xs 3 :: Either String Integer Left "Wrong index" ghci> at xs 2 :: Maybe Integer Just 2 ghci> at xs 3 :: Maybe Integer Nothing I'd like to replace functional dependencies with type families (associated types in my case): > class Result r where > type Failure > failure :: Failure r -> r a > success :: a -> r a Either instance is ok: > instance Result (Either e) where > type Failure (Either e) = e > failure e = Left e > success x = Right x But what to do with Maybe? > instance Result Maybe where > type Failure Maybe = forall e. e -- can't do this > failure _ = Nothing > success x = Just x

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Re: [Haskell-cafe] Lists and monads
by John Lato 26 Jul '10

26 Jul '10

> From: Kevin Jardine <kevinjardine(a)gmail.com> > > As a Haskell neophyte, one of the things I find confusing is the way > that the usual list functions (map, fold, ++, etc.) often cannot be > used directly with monadic lists (m [a] or [m a]) but seem to require > special purpose functions like ap, mapM etc. > > I suspect that things are not quite as difficult as they appear, > however, but cannot find any tutorials on monadic list manipulation. > It's not really a tutorial, but have you read the Typeclassopedia, http://haskell.org/sitewiki/images/8/85/TMR-Issue13.pdf ? I found it very helpful to get a good understanding of functors, applicatives, and monads, after which it became much simpler to get good usage from Control.Monad and Control.Applicative. John

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Iteratee package: combining enumerators
by Max Cantor 26 Jul '10

26 Jul '10

I have a series of files with binary encoded data in them, and want to create an enumerator iterates on the first element at the front of all the files. Something like the pseudocode: return . minimum =<< mapM (fmap (heads . lines) readFile) listOfFileNames I can use convStream to create an enumerator which runs iteratees on each tuple in a single file: (convStream decodeStrings) :: Monad m => IterateeG [] String m a -> IterateeG WrappedByteString Word8 m (IterateeG [] MyDataType m a) or, with the EnumerateeGMM tysyn: (convStream decodeStrings) :: Monad m => EnumerateeGMM WrappedByteString Word8 [] String m a My question is if there is a simple way to combine the Enumeratees to enumerate on a set of files or if I have to write an enumerator from scratch. Thank you in advance, Max P.S. John, apologies for the duped email, sent from the wrong address by mistake.

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Random this! ;-)
by michael rice 26 Jul '10

26 Jul '10

Hi All, From: http://en.wikibooks.org/wiki/Haskell/Understanding_monads/State Exercises 1. Implement a function rollNDiceIO :: Int -> IO [Int] that, given an integer, returns a list with that number of pseudo- random integers between 1 and 6. After a lot of learning what not to do, this is the best I could come up with. rollNDiceIO :: Int -> IO [Int] rollNDiceIO n = mapM (\x -> randomRIO(1,6)) (replicate n 1) I know, ugly, but at least I got it to work. What's a better way to generate this list? Michael

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Re[Haskell-cafe] ad large file and match lines to a pattern
by grzyb 26 Jul '10

26 Jul '10

Hi, I'm a beginner in haskell, I was trying to write the following code, but I still encourage some problems, can you help me with that? I need to read a large file and try to match each line to a pattern which is int,int value="string" for example: 0,1 value="string1" 1,5 value="string2" when the line matches the pattern then I need to get those 3 values (int, int, string) and pass them to my function. -- View this message in context: http://old.nabble.com/Read-large-file-and-match-lines-to-a-pattern-tp292603… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Tiger compiler in Haskell: annotating abstract syntax tree
by José Romildo Malaquias 26 Jul '10

26 Jul '10

Hello. In his book "Modern Compilder Implementation in ML", Appel presents a compiler project for the Tiger programming language where type checking and intermediate code generation are intrinsically coupled. There is a function transExp :: Absyn.Exp -> (Tree.Exp,Types.Type) that do semantic analysis, translating an expression to the Tree intermediate representation language and also do type checking, calculating the type of the expression. Maybe the compiler can be made more didatic if these phases are separate phases of compilation. The type checker would annotate the abstract syntax tree (ast) with type annotations, that could be used later by the translater to intermediate representation. In an imperative language probably each relevant ast node would have a field for the type annotation, and the type checker would assign the type of the node to this field after computing it. I am writing here to ask suggestions on how to annotate an ast with types (or any other information that would be relevant in a compiler phase) in Haskell. As an example, consider the simplified ast types: data Exp = IntExp Integer | VarExp Symbol | AssignExp Symbol Exp | IfExp Exp Exp (Maybe Exp) | CallExp Symbol [Exp] | LetExp [Dec] Exp data Dec = TypeDec Symbol Ty | FunctionDec Symbol [(Symbol,Symbol)] (Mybe Symbol) Exp | VarDec Symbol (Maybe Symbol) Exp Expressions can have type annotations, but declarations can not. Comments? Regards, Romildo -- Computer Science Department Universidade Federal de Ouro Preto, Brasil

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Yet another monad transformer or silly usage of Either?
by Eugeny N Dzhurinsky 25 Jul '10

25 Jul '10

Hello, everybody! I am trying to develop some sort of library, which supposed to sign into a WEB service, then perform some requests with it. Initially I designed methods in the following way data DServError = InvalidCredentials | InvalidRequest | ... newtype Result a = Result { getOpResult :: Either DServError a } data DSession = Session { ... } data DLoginResponse = LoginResponse { currentSession :: DSession, ... } login :: String -> String -> IO ( Result LoginResponse ) servRequest1 :: DSession -> ParamType1 -> ParamType2 -> ... -> IO ( Result DServResponse ) Now I want to be able of doing something like authenticatedReq = do loginResponse <- login "username" "password" let session = currentSession loginResponse servRequest1 session ... ... ... servRequest2 session ... ... ... ... so if login succeeds - I will be able to extract Right data from the Either response ( with explicit or implicit usage of getOpResult), if any of operations within "do" block will fail with DServError - then an error should be reported. I think the solution for this may be using Control.Exception and it's try/catch? Or may be there's some trick available for Either? I looked at EitherT, and it seems that I have to wrap every invocation into EitherT and then chain them with >>/>>= -- Eugene Dzhurinsky

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data type declaration
by Patrick Browne 25 Jul '10

25 Jul '10

Hi, I am trying to understand the data type declaration below. What is the relation between class C3 and the data type Address below? Where is such a technique used? Thanks, Pat module A where data Person = Person String Integer deriving Show data Employee = Employee String Integer deriving Show class C1 c1 where age :: c1 -> Integer instance C1 Person where age(Person "P1" 1) = 1 instance C1 Employee where age(Employee "E1" 1) = 1 class C1 c2 => C2 c2 where name :: c2 -> String instance C2 Person where name(Person "P2" 1) = "P2" instance C2 Employee where name(Employee "E2" 1) = "E2" class C2 c3 => C3 c3 a where address :: c3 -> a instance C3 Person String where -- ** What are the semantics or purpose of this construct. -- ** Is the type declared in the context of a class. data C3 c3 a => Address c3 a = Address c3 a instance C3 Person (Address String String) where This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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default function definitions
by Patrick Browne 25 Jul '10

25 Jul '10

Hi, I am studying the Haskell type class system as part of a language comparison thesis. At this point I am looking at how default function definitions are handled in classes. Functions are usually defined in instances, not classes. I appreciate that the code below may not be an appropriate way to write Haskell programs, but it does help me understand how defaults work. I am not trying to construct a running program. Any feedback on the questions below and defaults work in general would be appreciated. Pat module A where data Person = Person String Integer deriving Show data Employee = Employee String Integer deriving Show class C1 c1 where age :: c1 -> Integer -- add default impl, can this be defined only once at class level? -- Can this function be redefined in a *class* lower down the heirarchy? age(c1) = 1 -- Is it true that instances must exists before we can run function or make subclasses? instance C1 Person where instance C1 Employee where -- Is it true that C2 can inherit age, provided an instance of C1 exists class C1 c2 => C2 c2 where name :: c2 -> String name(c2) = "C2" instance C2 Person where instance C2 Employee where -- Is it true that C3 cannot override C1 or C2 existing defaults? -- Is it true that this must be done at instance level? -- class Cx c3 => C3 c3 where -- age(c3) = 3 -- This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Type problems
by michael rice 25 Jul '10

25 Jul '10

This works: Prelude System.Random> do { randomRIO (1,6) >>= (\x -> putStrLn $ "Value = " ++ show x) } Value = 5 So does this: Prelude System.Random> do { x <- randomRIO (1,6); putStrLn $ "Value = " ++ show x } Value = 2 But not this: 1 import Control.Monad 2 import System.Random 3 4 foo :: IO () 5 foo = do 6 x <- randomRIO (1,6) 7 putStrLn $ "Value = " ++ show x foo.hs:6:18: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `1' at foo.hs:6:18 `Random t' arising from a use of `randomRIO' at foo.hs:6:7-21 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. Prelude System.Random> Or this: 1 import Control.Monad 2 import System.Random 3 4 foo :: IO () 5 foo = randomRIO (1,6) >>= (\x -> putStrLn $ "Value = " ++ show x) foo.hs:5:17: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `1' at foo.hs:5:17 `Random t' arising from a use of `randomRIO' at foo.hs:5:6-20 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. How to fix? Michael

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Porting ELF statifier to Haskell!
by C K Kashyap 25 Jul '10

25 Jul '10

Hi, At my work we ran into a situation where we started wishing there was a way to take a dynamically linked executable and create a statically linked bundle out of it. Little bit of googling got me to statifier - http://statifier.sourceforge.net/statifier/main.html. The project seems a little old and when I tried it out on my 32bit RHEL5 box, the statically linked file did not run. So I thought it would be a good exercise to try and use Haskell's Elf module (Data.Elf) and attempt to build a statifier. Just wanted to understand if anyone's tried this before or have any advice for me. -- Regards, Kashyap

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recommendation for (best) sqlite3 bindings
by Gour 25 Jul '10

25 Jul '10

Hello! We are looking for recommendation which Haskell bindings for sqlite3 to use for destkop GUI app where we want, among other things to store *.png and/or *.jpg images. (Yeah, I know about the hint to store iamges in the filesystem and just store filepaths in the db, but for portability reasons so that user can easily carry/backup database, we want everything stored in one file.) By looking at Hackage, it seems there are 3 candidates: 1) sqlite - bindings by Galois (http://hackage.haskell.org/package/sqlite) - looks quite complete & low-level interface 2) direct-sqlite - it says "It is not as complete as bindings-sqlite3 (1), but is slightly higher-level...it supports strings encoded as UTF8, and BLOBs represented as ByteStrings." and 3) HDBC-sqlite3 - higher level but without support for BLOBs. Now, based on the above it looks that 2) is the best one - not high-level as 2), but adding BLOBs support which, iirc, is missing in HDBC. Otoh, having highr-level abstraction ala HDBC is nice, although at the moment we believe that we won't have need to go to PostgreSQL since it means that setup would be greatly complicated for the end-user, so we're staying focused on Sqlite3. I know there are also Takusen & HSQL, but based on my past experiences when watching those two projects, it seems they aren not supported as well as the above itemized ones. Any recommendation? May I add that, according to the recent "Merge hsql and HDBC -- there can only be one!" thread, I can only "+1" for having slight less & more complete database packages. Sincerely, Gour -- Gour | Hlapicina, Croatia | GPG key: F96FF5F6 ----------------------------------------------------------------

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build and use ghc's rts without a full unregistered ghc port?
by Korcan Hussein 25 Jul '10

25 Jul '10

Hi, I was just wondering if this is possible, I would like to use a gcc port which cross compiles to the PPC architecture (Wii DevkitPPC to be specifically: http://wiibrew.org/wiki/DevkitPPC) for a platform that is not POSIX compatible I believe (at least not fully or maybe I'm wrong entirely). Ideally I would make an unregistered port of GHC but the problem is because GHC doesn't support true cross compiling yet I need to bootstrap on to the target machine, which isn't running a POSIX environment. Still I did try to attempt to build with both the host and target set to 'powerpc-unknown-linux' and build it with devkitPPC, I did use a custom build.mk which should pull in all the required dependencies, defines, etc for devkitPPC. In the end I didn't get very far with it, it looked like missing posix functions. I didn't want to spend to much time trying to make it work this way (maybe i'm doing it wrong completely). It's quite possible that I could make this work more easily if I used a linux port for the wii but I would like to avoid the need of using linux on the wii because I guess most home-brew users don't have such a setup. So I thought maybe it's possible with less effort to build GHC's RTS and then compile Haskell in C with the RTS as a library dependency. Is this possible? maybe I'm missing something or doing something stupid? Thanks in advance. _________________________________________________________________ http://clk.atdmt.com/UKM/go/197222280/direct/01/ We want to hear all your funny, exciting and crazy Hotmail stories. Tell us now

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