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Re: [Haskell-cafe] Practical Haskell question.
by Jon Cast 25 Jun '07

25 Jun '07

On Monday 25 June 2007, Michael T. Richter wrote: > On Mon, 2007-25-06 at 01:05 -0500, Jon Cast wrote: > > What exactly are you trying to do? > > I'm trying to model a transactional system which could fail > mid-transaction without an easy rollback on conditions which could be > checked in advance. <snip> > Now if I could just check permissions on each operation > the monad emerging is clearer. Each call in the do block to C, then D > and finally E would be individually checked for permissions in the Monad > before the next step could progress. I can do this. But if I want to > change the model to pre-check the privileges (because of performance > issues or because of resource allocation issues or whatever) I see no > easy way to do the same thing. This intrigues me. Why? There's no easy or simple way to tell what operations you're going to execute in advance. Take a concrete example: do s <- readFile "/etc/ourProg" let Right xn = flip parse s $ many $ fileName flip mapM xn $ \ filename -> readFile xn Now, you can't possible pre-check this for permissions, because the argument to readFile in the fourth line is drawn from an external file; you would have to check for permission to read every file in the world. If your particular permissions issues are indeed independent of the arguments to performB, you can use an arrow: data MyArrow alpha beta = MyArrow (IO Bool) (alpha -> IO beta) instance Arrow MyArrow where arr f = MyArrow (return True) (return . f) MyArrow c0 a0 >>> MyArrow c1 a1 = MyArrow (liftM2 (&&) c0 c1) (\ x -> a0 x >>= a1) Then you can check the pre-conditions beforehand. (If you want to see why using an arrow helps, try implementing an instance of the class class Arrow a => ArrowApply a where apply :: a (a alpha beta, alpha) beta for MyArrow. Can't be done, but apply is built in to monads). Sincerely, Jonathan Cast http://sourceforge.net/projects/fid-core http://sourceforge.net/projects/fid-emacs

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Practical Haskell question.
by Michael T. Richter 25 Jun '07

25 Jun '07

Now I've got a situation I can't figure out how to resolve. I want to have a set of actions which are executed sequentially, but which, before I even start to execute the first one, have been inspected for legality and/or plausibility. Consider this kind of sequence: do x <- performActionA y <- performActionB z <- performActionC return $ calculateStuff x y z Now obviously this is going to be in a monad of some kind. Were this a regular, run-of-the-mill program I'd just use the IO monad. But what I want to do instead is, before executing any of the perform* functions, check that the actions desired are actually permitted (or possible) given a set of circumstances. For example let's say it's a permissions issue and performActionB can only be done if I'm root. If I'm not root I don't want performActionA done because I can't complete the transaction. (Maybe ActionA is non-reversible, say.) Or let's say this is code that's accessing databases on the network. If the network link to C can't be established, I don't want to screw around with A and B's links at all -- it's too expensive, too time-consuming or whatever. Were I programming this in C, C++, Python, Ruby, etc. I could do this in my sleep. Functions are addresses (C/C++) or objects with an ID (Python/Ruby) so it's possible to take them and do some kind of check based on identities before executing things (although the scaffolding around this would be nontrivial in any of these languages except, possibly, Ruby). Functions in Haskell don't have this property, however, so I can't figure out what I'd do to perform similar work. I'm sure there's a way to do it, but I just can't see it. -- Michael T. Richter <ttmrichter(a)gmail.com> (GoogleTalk: ttmrichter(a)gmail.com) I'm not schooled in the science of human factors, but I suspect surprise is not an element of a robust user interface. (Chip Rosenthal)

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FFI and Excel VBA
by Lewis-Sandy, Darrell 25 Jun '07

25 Jun '07

I am new to using the Haskell FFI, and have been trying to implement the example in section 11.6 of the GHC user's guide. I have finally gotten to the point where my dll compiles (there is a missing space in the mainDll.h code at line 4:12) using GHC 6.6.1, and used the declare statement to expose the "adder" function in Excel VBA. My VBA Code is given below: Private Declare Function adder Lib " adder.dll" Alias "adder@8" (ByVal x As Integer, ByVal y As Integer) As Integer Private Sub test() Debug.Print adder(1, 2) End Sub My problem is this: The function works fine (the immediate window displays 3), but when I terminate Excel, I get an application error ("The instruction at ... referenced memory at ... . The memory could not be read."). Does anyone else have any experience calling Haskell from VBA that might be relevant?

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SYB with class, simplified
by Benja Fallenstein 25 Jun '07

25 Jun '07

Hi all, The "scrap your boilerplate with class" sytstem [1] has two big advantages over the plain SYB system from Data.Generics, IMHO: One, it lets you declare an 'open' generic function as a type class, to which new cases can be added by adding new instances (emphasized in the paper); and two, it lets you write recursive functions that require other type class constraints in addition to Data (not emphasized in the paper, but something I've frequently found myself wanting with Data.Generics). [1] http://homepages.cwi.nl/~ralf/syb3/ However, when trying to convert the codebase I'm working on to SYB-with-class, I've found that the type proxies and explicit dictionaries used to simulate type class abstraction over type classes are... annoying. Today, I've hit on an alternative approach to implementing SYB-with-class (YAGS, yet another generics scheme...), with less boilerplate per generic function. The approach may or may not be new (I haven't studied *all* of the generics proposals out there yet); in any case, it shares the use of type-level functions with Smash Your Boilerplate, and it uses the same underlying gfoldl operator as SYB, but implements it in a quite different way. I believe that the equivalent of everywhere, mkT and friends can be implemented as type-level functions in this framework, but I haven't actually tried it yet. This mail is a literate script demonstrating the approach. I'm hoping to get some feedback on the idea. :) On to the code: > {-# OPTIONS_GHC -fglasgow-exts -fallow-overlapping-instances > -fallow-undecidable-instances #-} Yup, we need it all... I'll start with three example generic functions. * 'size' calculates the number of constructors in a term, except for lists, for which it returns one plus sum of the element sizes. * 'inc' increases all Ints in a term by one. * 'prints' prints out each subterm of a term on its own line, except for strings, for which it prints the string, but not its subterms. Thus, the following code: > test = ("Hello", 7::Int, [2,3::Int]) > main = do print (size test); print (inc test) > putStrLn ""; prints test; return () prints this: ---------------------------------------------------------------------- 11 ("Hello",8,[3,4]) ("Hello",7,[2,3]) "Hello" 7 [2,3] 2 [3] 3 [] ---------------------------------------------------------------------- Here is the 'size' function: > class Size a where size :: a -> Int > > data SizeF = SizeF > instance Size a => Apply SizeF a Int where apply _ = size > > instance Size a => Size [a] where size xs = 1 + sum (map size xs) > instance Apply (GMapQ SizeF) a [Int] => Size a where > size x = 1 + sum (gmapQ SizeF x) The constraint (Apply f x r) means that 'f' is a type-level function that, when applied to 'x,' returns 'r': > class Apply f x r | f x -> r where apply :: f -> x -> r Here is the 'inc' function: > class Inc a where inc :: a -> a > > data IncF = IncF > instance Inc a => Apply IncF a a where apply _ = inc > > instance Inc Int where inc = (+1) > instance Apply (GMapT IncF) a a => Inc a where inc = gmapT IncF And here is the 'prints' function; for illustration, the implementation is in a slightly different style, which does without the declaration of a new type class: > data PrintsF = PrintsF; prints x = apply PrintsF x > instance Apply PrintsF String (IO String) where > apply _ x = print x >> return x > instance (Show a, Apply (GMapM PrintsF) a (IO a)) => > Apply PrintsF a (IO a) where > apply f x = print x >> gmapM f x Note the 'Show' constraint: 'prints' can only be applied to values all of whose subterms implement 'Show.' This is the kind of constraint you can't have with the standard, not-with-class SYB code. So much for the demo code; now, onwards to the actual library. The core consists of the following three type classes: > class Constr x f where constr :: x -> a -> f a > class Param x p f where param :: x -> f (p -> a) -> p -> f a > > class GFoldl x a f where gfoldl :: x -> a -> f a Together, these classes form the equivalent of the standard SYB's 'gfoldl' method. (I'm ignoring the rest of the Data class at this time, but I believe that it could be implemented in a similar fashion.) * 'Constr' and 'Param' correspond to the first and second argument of the standard SYB's gfoldl. * The parameter 'x' specifies the type of fold to perform (GMapQ, GMapT and GMapM in the present module). * We give an instance 'Constr' and 'Param' for each type of fold. We give an instance of 'GFoldl' for each type we want to fold over. Here are the instances of GFoldl: > instance Constr x f => GFoldl x () f where gfoldl = constr > instance Constr x f => GFoldl x Char f where gfoldl = constr > instance Constr x f => GFoldl x Int f where gfoldl = constr > > instance (Constr x f, Param x a f, Param x [a] f) => GFoldl x [a] f where > gfoldl x [] = constr x [] > gfoldl x (y:ys) = constr x (:) `p` y `p` ys where > p a b = param x a b > > instance (Constr x f, Param x a f, Param x b f, Param x c f) => > GFoldl x (a,b,c) f where > gfoldl x (a,b,c) = constr x (,,) `p` a `p` b `p` c where > p a b = param x a b What remains is the code for GMapQ, GMapT and GMapM: > newtype GMapQ f = GMapQ f; gmapQ f = apply (GMapQ f) > > newtype K a b = K { fromK :: a } > > instance GFoldl (GMapQ f) a (K [r]) => Apply (GMapQ f) a [r] where > apply (GMapQ f) x = reverse $ fromK $ gfoldl (GMapQ f) x > > instance Constr (GMapQ f) (K [r]) where constr _ _ = K [] > instance Apply f a r => Param (GMapQ f) a (K [r]) where > param (GMapQ f) (K xs) x = K (apply f x : xs) > newtype GMapT f = GMapT f; gmapT f = apply (GMapT f) > > newtype I a = I { fromI :: a } > > instance GFoldl (GMapT f) a I => Apply (GMapT f) a a where > apply (GMapT f) x = fromI $ gfoldl (GMapT f) x > > instance Constr (GMapT f) I where constr _ = I > instance Apply f a a => Param (GMapT f) a I where > param (GMapT f) (I x) y = I (x (apply f y)) > newtype GMapM f = GMapM f; gmapM f = apply (GMapM f) > > instance (Monad m, GFoldl (GMapM f) a m) => Apply (GMapM f) a (m a) where > apply (GMapM f) x = gfoldl (GMapM f) x > > instance Monad m => Constr (GMapM f) m where constr _ = return > instance (Monad m, Apply f a (m a)) => Param (GMapM f) a m where > param (GMapM f) m x = do fn <- m; arg <- apply f x; return (fn arg) That ends the example. Comments would be appreciated! :-) Thanks, - Benja

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Constructing a datatype given just its constructor as a string?
by Hugh Perkins 25 Jun '07

25 Jun '07

Trying to create a datatype/constructor given just its constructor as a string, something like: mkConstr :: String -> Constr parseData :: (Data a) => String -> a ***without knowing in advance anything about the datatype apart from the string contents*** So, not something like: parseData "Just 3" :: [Maybe] but simply: parseData "Just 3" (This is linked to the other post: the goal is to be able to deserialize xml without needing to know in advance the data type we are deserializing)

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Re: [Haskell-cafe] Installing HOpenGL
by Thomas Schilling 25 Jun '07

25 Jun '07

No, ffi is no extra package. It would be helpful if you posted you configure output though. BTW, please CC the list when replying. The easiest way should be to use the "reply all" feature of your mail client. Thanks, Thomas On 24 jun 2007, at 15.24, Jimmy Miller wrote: > That's all I know right now. Is there another package I need to > install for the C FFI? Also, is there any problem in Setup.hs saying > that sh was not found? > > On 6/24/07, Thomas Schilling <nominolo(a)googlemail.com> wrote: >> That looks like a missing C macro definition. It should probably >> expand to either stdcall or ccall. A bit more information would be >> helpful. >> >> >> On 24 jun 2007, at 13.11, Jimmy Miller wrote: >> >> > I'm trying to install HOpenGL 2.1, with ghc 6.6 on Windows XP. >> > runghc Setup.hs configure seems to work fine. It gives messages >> about >> > things missing, like sh, but there are no errors. >> > >> > When I run runghc Setup.hs build, I get the following error: >> > >> > [ 4 of 81] Compiling Graphics.Rendering.OpenGL.GL.Extensions >> > ( Graphics/Renderin >> > g/OpenGL/GL/Extensions.hs, dist\build/Graphics/Rendering/OpenGL/GL/ >> > Extensions.o >> > ) >> > >> > Graphics/Rendering/OpenGL/GL/Extensions.hs:46:15: >> > parse error on input `CALLCONV' >> > >> > Does anyone know what might be going wrong? >> > >> > -- >> > <a href="http://www.spreadfirefox.com/? >> > q=affiliates&id=153516&t=1"><textarea >> > rows="3" cols="40">Get Firefox!</a> >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > > > -- > <a href="http://www.spreadfirefox.com/? > q=affiliates&id=153516&t=1"><textarea > rows="3" cols="40">Get Firefox!</a>

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Re: copy-on-write monad?
by oleg＠pobox.com 25 Jun '07

25 Jun '07

Greg Meredith wrote: > First, has anyone worked out a monadic > approach to copy-on-write? (And, Is there any analysis of perf > characteristics of said monadic schemes?) If you use Zippers (Huet's or generic ones) with functional updates, copy-on-write comes out automatically and by default. This is explained in http://okmij.org/ftp/Computation/Continuations.html#zipper and, in a more readable form, in a recent paper http://okmij.org/ftp/papers/context-OS.pdf The web page also contains the complete code.

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Installing HOpenGL
by Jimmy Miller 24 Jun '07

24 Jun '07

I'm trying to install HOpenGL 2.1, with ghc 6.6 on Windows XP. runghc Setup.hs configure seems to work fine. It gives messages about things missing, like sh, but there are no errors. When I run runghc Setup.hs build, I get the following error: [ 4 of 81] Compiling Graphics.Rendering.OpenGL.GL.Extensions ( Graphics/Renderin g/OpenGL/GL/Extensions.hs, dist\build/Graphics/Rendering/OpenGL/GL/Extensions.o ) Graphics/Rendering/OpenGL/GL/Extensions.hs:46:15: parse error on input `CALLCONV' Does anyone know what might be going wrong? -- <a href="http://www.spreadfirefox.com/?q=affiliates&id=153516&t=1"><textarea rows="3" cols="40">Get Firefox!</a>

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stupid operator question
by Michael Vanier 24 Jun '07

24 Jun '07

I noticed that both the Data.Array library and the Data.Map library use the (!) operator for different purposes. How would it be possible to import both libraries usefully in a single module? I guess what I'm really asking is: how do I qualify operator names? Mike

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Re: [Haskell-cafe] avoiding command window with wxHaskell on Windows?
by Jens Fisseler 24 Jun '07

24 Jun '07

On Sat, 23 Jun 2007, Dean Herington wrote: > But if I double-click the .exe file, I get a command window that hangs around > (but doesn't appear to do anything, fortunately) until the program terminates. > How can I avoid this command window? With gtk2hs, using "-optl-mwindows" as a command line option for GHC lets me get rid of this window. Perhaps it will do the same for wxHaskell? Regards, Jens

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Existentials and type var escaping
by Roberto Zunino 24 Jun '07

24 Jun '07

In this function data C = C Int foo :: C -> C foo ~(C x) = C x foo is _not_ the identity: its result must be non bottom, i.e. the constructor C is "forced" to its argument. I wonder if a similar function is definable for existential types: data E = forall a . E a foo :: E -> E foo, as defined above does not work (lazy patterns not allowed), and in foo y = E (case y of E x -> x) a variable escapes. I also tried with CPS with no success. Is foo definable at all? I'm starting to think that it is not, and that there must be a very good reason for that... Thank you, Zun.

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Lambdabot
by Daniel Fischer 23 Jun '07

23 Jun '07

Greetings, now I have a working plugins package, I can refocus on my lambdabot problem. Let me recap: In Lib/Parser.hs, as_name &c are defined #if __GLASGOW_HASKELL__ > 606, however ghc-6.6.1 defines that as 606, so to go through the build, I had to change the '>' to '>='. Is that a bad idea? (otherwise that should be changed in Parser.ly/.hs) Next, in scripts/ShowQ.hs, there is defined an instance (Arbitrary a) => Arbitrary (Maybe a), but such an instance already is defined in Test.QuickCheck, so I commented the one in ShowQ out (after the build died there once). That probably should be fixed in the darcs repo. As anything of the above stupid/dangerous? Okay, so building succeeds without complaints, but then $ ./lambdabot Initialising plugins ................Speicherzugriffsfehler Changing BotPP so that it created a persisting copy of its output, I found out what modulesInfo does, then, letting ircInstallModule produce debugging output, I located the evildoers. lambdabot segfaulted when installing Fact Haddock Quote Source Todo Where What's special about them? I.e., why did they cause a segfault and the others not? And, how could I build a lambdabot _with_ them (though I'm not sure, I'll actually want them, but I might). Without these, I now have an apparently working lambdabot, well, not properly working. First time I started it, all seemed well, but from then on: $ ./lambdabot Initialising plugins ..................................sending message to bogus server: IrcMessage {msgServer = "freenode", msgLBName = "urk!<outputmessage>", msgPrefix = "", msgCommand = "NAMES", msgParams = [""]} ............... done. Main: caught (and ignoring) IRCRaised Data.ByteString.index: index too large: 0, length = 0 lambdabot> > 3+7 Main: caught (and ignoring) IRCRaised Data.ByteString.index: index too large: 0, length = 0 10 lambdabot> The bogus message thing also appeared the first time, I hope this is meant to be so. But what about the ByteString.index exception? Where might that come from? How to get rid of it? Finally, is there a tutorial/manual for using lambdabot? Thanks for any help, Daniel

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Odd lack of laziness
by Jefferson Heard 23 Jun '07

23 Jun '07

Alright, I've been hacking away at what I posted the other day, and I have something that works for files that will fit entirely into memory. And then I figured out why I've been restricted to files that fit entirely into memory... One of my functions is causing the entire thing to be read in, when, in the way I analyze the program, only a very small portion of the file should be read in. Here are the functions I've used to test this problem... import Data.Bits import qualified Data.ByteString.Lazy as BS import Foreign.C.Types ... {-# INLINE decodeLengthBits #-} decodeLengthBits :: BS.ByteString -> CInt decodeLengthBits doc = (shift (pieces !! 3) 24) .|. (shift (pieces !! 2) 16) .|. (shift (pieces !! 1) 8) .|. (pieces !! 0) where pieces::[CInt] = map fromIntegral . BS.unpack . BS.take 4 $ doc breakIntoDocuments :: RawDocument -> [RawDocument] breakIntoDocuments f | BS.length f > 0 = if len > 0 then (BS.take bytes f) : (breakIntoDocuments (BS.drop bytes f)) else (breakIntoDocuments (BS.drop bytes f)) | otherwise = [] where len = decodeLengthBits f bytes = fromIntegral (len * 2 + len * 4 + 4) and a main function of: main = do f <- B.readFile "Documents.bin" print (take 1 (breakIntoDocuments f)) Shouldn't the program only read in enough of the lazy byte-string to create the first return value of breakIntoDocuments? The return value of decodeLengthBits is only 277. I watched it, and it's reading in my whole 2gb file... -- Jeff

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"Graphical Haskell"
by peterv 23 Jun '07

23 Jun '07

In the book "Haskell School of Expression", streams are nicely explained using a graphical flow graph. This is also done more or less in http://research.microsoft.com/~simonpj/papers/marktoberdorf/Marktoberdorf.pp t to explain monads and other concepts. I would like to create a program that allows you to create such flow graphs, and then let GHC generate the code and do type inference. I found a paper where Haskell is used to create a GUI application with undo/redo etc for creating graphical Basian networks (http://www.cs.uu.nl/dazzle/f08-schrage.pdf) so this gave me confidence that I could it do all in Haskell. Now, instead of generating Haskell code (which I could do first, would be easier to debug), I would like to directly create an AST, and use an Haskell API to communicate with GHC. I already found out that GHC indeed has such an API, but how possible is this idea? Has this been done before? I only found a very old attempt at this, confusingly also called "Visual Haskell", see http://ptolemy.eecs.berkeley.edu/%7Ejohnr/papers/visual.html, but I can't find any source code for that project. I did a similar project in C# that generated C++ code, so I've done it before, just not in Haskell. Thanks a lot, Peter

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Shouldn't this be lazy???
by Olivier Boudry 23 Jun '07

23 Jun '07

Hi all, I'm playing with the TagSoup library trying to extract links to original pictures from my Flickr Sets page. This programs first loads the Sets page, open links to each set, get links to pictures and then search for original picture link (see steps in main function). It does the job, but for the tests I just wanted to take 10 links to reduce the time the program runs. Just hoping that haskell laziness would magically take the minimum amount of data required to get the first 10 links out of this set of pages. I did this replacing: (putStrLn . unlines . concat) origLinks with (putStrLn . unlines . take 10 . concat) origLinks in the main function. With the last version of that line, I effectively only get 10 links but the runtime is exactly the same for both main functions. As I'm a newbie haskell programmer I certainly missing something. By the way I know Flickr has an api I could use, but the purpose was playing with TagSoup. Thanks for any advice. Olivier. Here's the code: module Main where import Data.Html.TagSoup import Control.Monad (liftM) import Data.List (isPrefixOf, groupBy) import Data.Maybe (mapMaybe) import System (getArgs) import System.Time import IO (hPutStrLn, stderr) base = "http://www.flickr.com" setsUrl name = "/photos/" ++ name ++ "/sets/" main :: IO () main = do args <- getArgs tStart <- getClockTime setLinks <- getLinksByAttr ("class", "Seta") (base ++ setsUrl (args !! 0)) picLinks <- mapM (getLinksByAttr ("class", "image_link")) setLinks origLinks <- mapM (getLinksAfterImgByAttr ("src", "http://l.yimg.com/www.flickr.com/images/icon_download.gif")) $ (mapMaybe linkToOrigSize . concat) picLinks (putStrLn . unlines . concat) origLinks tEnd <- getClockTime hPutStrLn stderr ( timeDiffToString $ diffClockTimes tEnd tStart ) -- | extract all links from "a" tag types having given attribute getLinksByAttr :: (String, String) -> String -> IO [String] getLinksByAttr attr url = do sects <- getSectionsByTypeAndAttr "a" attr url return $ hrefs sects -- | get "a" tags following a "img" having a specific attribute getLinksAfterImgByAttr :: (String, String) -> String -> IO [String] getLinksAfterImgByAttr attr url = do sects <- getSectionsByTypeAndAttr "img" attr url return $ hrefs $ map (dropWhile (not . isTagOpen) . drop 1) sects -- | create sections from tag type and attribute getSectionsByTypeAndAttr :: String -> (String, String) -> String -> IO [[Tag]] getSectionsByTypeAndAttr tagType attr url = do tags <- liftM parseTags $ openURL $ url (return . filterByTypeAndAttr tagType attr) tags where filterByTypeAndAttr :: String -> (String, String) -> [Tag] -> [[Tag]] filterByTypeAndAttr t a = sections (~== TagOpen t [a]) -- | extract href values from sections of "a" tags hrefs :: [[Tag]] -> [String] hrefs = map (addBase . fromAttrib "href" . head) where addBase :: String -> String addBase s | "http://" `isPrefixOf` s = s addBase s | otherwise = base ++ s -- | transform a link to a picture into a link to the original size picture linkToOrigSize :: String -> Maybe String linkToOrigSize link = if parts !! 3 == "photos" then Just $ newUrl parts else Nothing where parts = map tail $ groupBy (const(/='/')) link newUrl p = "http://www.flickr.com/photo_zoom.gne?id=" ++ p !! 5 ++ "&size=o&context=" ++ p !! 7

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A probably-stupid question about a Prelude implementation.
by Michael T. Richter 23 Jun '07

23 Jun '07

So, I'm now going over the code in the 'Report with a fine-toothed comb because a) I'm actually able to read it now pretty fluently and b) I want to know what's there in detail for a project I'm starting. I stumbled across this code: any :: (a -> Bool) -> [a] -> Bool any p = or . map p or :: [Bool] -> Bool or = foldr (||) False Now I see how this works and it's all elegant and clear and all that. But I have two nagging problems with it (that are likely related): 1. Using foldr means I'll be traversing the whole list no matter what. This implies (perhaps for a good reason) that it can only work on a finite list. 2. I don't see any early bale-out semantics. The way I read this it's going to expand a whole list of n and perform n comparisons (including the one with the provided False). Considering that I only need a single True result to make the whole expression true, I'd have expected there to be some clever semantics to allow exactly this. But what I'm seeing, unless I'm really misreading the code, is that if I give it a list of a million boolean expressions, it will cheerfully evaluate these million boolean expressions and perform a million calls to (||) before giving me a result. Please tell me I'm wrong and that I'm missing something? -- Michael T. Richter <ttmrichter(a)gmail.com> (GoogleTalk: ttmrichter(a)gmail.com) There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies. (Charles Hoare)

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copy-on-write monad?
by Greg Meredith 23 Jun '07

23 Jun '07

Fellow Functionally-Caffeinated, i have a few questions regarding copy-on-write semantics. i am working for a client that is stuck with a legacy in-house language that chose copy-on-write as a way to provide aliasing-issue-free semantics to a user population they perceived as not sophisticated in programming. Despite the fact that they are now realizing there are a lot of very sophisticated and performant ways of providing ways to avoid aliasing problems, they are stuck having to support copy-on-write for legacy codes. So, i have two basic questions. First, has anyone worked out a monadic approach to copy-on-write? (And, Is there any analysis of perf characteristics of said monadic schemes?) Second, i have worked out a scheme which is like a version control system. The mutable collection is treated like a source tree. A reference to a mutable collection is like a tag for a branch of the tree. Updates attach deltas to a given branch of the tree. Accesses have to be matched to see if they are impacted by any update-deltas. There is an optimization/memo-ization strategy in which certain events (access or update) trigger a copy to be made, finally, and the updates applied to the copy. This shifts perf characteristics so that access becomes slower and update considerably faster in the common case. Does anyone know if such a scheme has been studied? i'd like to compare implementations, too, if possible. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Telling the time
by Andrew Coppin 23 Jun '07

23 Jun '07

Greetings. Is there a standard library function anywhere which will parse a string into some kind of date/time representation? And, further, is there some function that will tell me how many seconds elapsed between two such times? (I see there's a giant pile of modules to do with dates and times, but I can't make much sense out of them - and in at least one place, the documentation on the Haskell website doesn't actually match what's installed on my computer!) I want to write a program which reads a file of the form 2007-05-26 11:23:16 PM|Begin processing... 2007-05-26 11:23:16 PM|Done stage #1 2007-05-26 11:23:21 PM|Done stage #2 2007-05-26 11:24:05 PM|Done stage #3 and produce a new file of the form 2007-05-26 11:23:16 PM|Begin processing... +0|Done stage #1 +5|Done stage #2 +44|Done stage #3 Ideally I'd like to have it able to parse several slightly differing date/time formats. (It turns out my programs all log using a very subtly different format!) Before I go on an 8-month expedition to reimplement this myself, is there something I can use in the library?

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Type-level Programming
by Vincenz Syntactically 23 Jun '07

23 Jun '07

Dear all, Recently I was playing around with encoding matrices in the type-level system. Thereby one can enable the multiplication of matrices. The general idea (which can be read about at ( http://notvincenz.blogspot.com/2007/06/generalized-matrix-multiplication.ht…) is that there is more than one way to multiplly a matrix. Given two matrices A and B, with M and N dimensions: a_1*...*a_m and b_1*...*b_n then whenever the last L dimensions of A match the first L dimensions of B, they can be multiplied to have a matrix of dimension: a_1*..*a_(m-l)*b_(l+1)*...*b_n What one does is a dot-product on those middle L dimensions. This is what I tried to do in the code in the blogpost. However, I was unable to formulate the constraints for the final multiplication class that does the actual proper cross-multiplication. Is this at all possible, or was I chasing ghosts? Best regards, Christophe

1 0

haskell crypto is reaaaaaaaaaally slow
by Anatoly Yakovenko 22 Jun '07

22 Jun '07

$ time ./md5sum ./md5sum [105,252,52,138,187,192,216,17,225,123,185,3,124,101,86,132] real 0m4.790s user 0m3.688s sys 0m0.492s $ time md5sum ./md5sum 69fc348abbc0d811e17bb9037c655684 ./md5sum real 0m0.023s user 0m0.000s sys 0m0.008s this is my implementation using crypto (http://www.haskell.org/crypto/) Am I doing something wrong? module Main where import System import qualified Data.Digest.MD5 as MD5 import qualified Data.ByteString as BS main = do args <- getArgs dt <- BS.readFile $ head args putStrLn $ show $ MD5.hash . BS.unpack $ dt

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Re: Re[2]: [Haskell-cafe] IDE?
by Peter Verswyvelen 22 Jun '07

22 Jun '07

That's just my point. Although I have no practical experience with Haskell (besides writing a simple L-System using HOpenGL), from what I've read Haskell is indeed much better than typical OO languages... So it *deserves* an easy entry level IDE that will get many many more people started with it. Like Concurrent Clean has, although their IDE is also far from perfect (e.g. they don't have multi-level undo, sigh) Anyway, it seems many people use Emacs for their Haskell edit/compile/run cycle. I've used Emacs on IBM OS/2 a long time ago so I guess I can get back into it. But man, was I happy back then when I could switch over to Visual Studio... The productivity I nowadays have with Visual Studio 2005 and Resharper for doing compilation, code-documentation-tips, code-completion, refactoring, navigation, debugging, boiler plate code generation, is amazing. Some of my colleagues still use Emacs, and maybe they are not using it correctly, but at first sight their development is much much slower. >From this cafe talk I now know such an IDE for Haskell does not exist. So I won't search any further for a great IDE before starting to do some real Haskell programming, because my L-Systems experiment was a lot of fun! So I just installed XEmacs with the latest Haskell mode. I'll go from here... If that doesn't work, Notepad++ and GHCI/GHC in a command prompt also works, although it does make me feel I'm back in the eighties. Thanks for all the help folks! >----- Oorspronkelijk bericht ----- >Van: Bulat Ziganshin [mailto:bulat.ziganshin@gmail.com] >Verzonden: zaterdag, juni 16, 2007 08:50 PM >Aan: bf3(a)telenet.be >CC: haskell-cafe(a)haskell.org >Onderwerp: Re[2]: [Haskell-cafe] IDE? > >Hello bf3, > >Saturday, June 16, 2007, 3:23:40 PM, you wrote: > >> The point I wanted to make is, that I can't find an >> easy-to-install-ready-to-use-and-rock-n-roll IDE for Windows that comes with >> all or most of those features. I mean something like Borland TurboPascal > >it's well-known trap. haskell is an order of magnitude better than >widespread OOP languages. why it's not used by everyone? just due to >shortage on libs, training and - yes - IDEs. "programming" in Delphi >in many cases need just clicking here and there > >so, you got something, you lost something > >ps: i use editor which supports only syntax highlighting. it's very >like working in tp 3.0 or quickc 1.0 - are you had such experience? :) > > > >-- >Best regards, > Bulat mailto:Bulat.Ziganshin@gmail.com > > >

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Re: Type classes vs C++ overloading Re: [Haskell-cafe] Messing around with types [newbie]
by Cristiano Paris 22 Jun '07

22 Jun '07

I sent this message yesterday to Bulat but it was intended for the haskel cafe, so I'm resending it here today. Thank to everyone who answered me privately. Today I'll keep on experimenting and read the reference you gave me. Cristiano ---------- Forwarded message ---------- From: Cristiano Paris <cristiano.paris.ml(a)gmail.com> Date: Jun 21, 2007 6:20 PM Subject: Re: Type classes vs C++ overloading Re: [Haskell-cafe] Messing around with types [newbie] To: Bulat Ziganshin <Bulat.Ziganshin(a)gmail.com> On 6/21/07, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > > Hello Cristiano, > > Thursday, June 21, 2007, 4:46:27 PM, you wrote: > > > class FooOp a b where > > foo :: a -> b -> IO () > > > > instance FooOp Int Double where > > foo x y = putStrLn $ (show x) ++ " Double " ++ (show y) > > this is rather typical question :) I knew it was... :D unlike C++ which resolves any > overloading at COMPILE TIME, selecting among CURRENTLY available > overloaded definitions and complaining only when when this overloading > is ambiguous, type classes are the RUN-TIME overloading mechanism > > your definition of partialFoo compiled into code which may be used > with any instance of foo, not only defined in this module. so, it > cannot rely on that first argument of foo is always Int because you may > define other instance of FooOp in other module. "10" is really > constant function of type: > > 10 :: (Num t) => t > > i.e. this function should receive dictionary of class Num in order to > return value of type t (this dictionary contains fromInteger::Integer->t > method which used to convert Integer representation of 10 into type > actually required at this place) > > this means that partialFoo should have a method to deduce type of 10 > in order to pass it into foo call. Let's consider its type: > > partialFoo :: (FooOp t y) => y -> IO () > > when partialFoo is called with *any* argument, there is no way to > deduce type of t from type of y which means that GHC has no way to > determine which type 10 in your example should have. for example, if > you will define > > instance FooOp Int32 Double where > > anywhere, then call partialFoo (5.0::Double) will become ambiguous > > shortly speaking, overloading resolved based on global class > properties, not on the few instances present in current module. OTOH, > you build POLYMORPHIC functions this way while C++ just selects > best-suited variant of overloaded function and hard-code its call > > further reading: > http://homepages.inf.ed.ac.uk/wadler/papers/class/class.ps.gz > http://haskell.org/haskellwiki/OOP_vs_type_classes > chapter 7 of GHC user's guide, "functional dependencies" Mmmmmhhhh... your point is hard to understand for me. In his message, I can understand Bryan Burgers' point better (thanks Bryan) and I think it's somewhat right even if I don't fully understand the type machinery occuring during ghc compilation (yet). Quoting Bryan: "*From this you can see that 10 is not necessarily an Int, and 5.0 is *not necessarily a Double. So the typechecker does not know, given just 10 and 5.0, which instance of 'foo' to use. But when you explicitly told the typechecker that 10 is an Int and 5.0 is a Double, then the type checker was able to choose which instance of 'foo' it should use." So, let's see if I've understood how ghc works: 1 - It sees 5.0, which belongs to the Fractional class, and so for 10 belonging to the Num class. 2 - It only does have a (FooOp x y) instance of foo where x = Int and y = Double but it can't tell whether 5.0 and 10.0 would fit in the Int and Double types (there's some some of uncertainty here). 3 - Thus, ghci complains. So far so good. Now consider the following snippet: module Main where foo :: Double -> Double foo = (+2.0) bar = foo 5.0 I specified intentionally the type signature of foo. Using the same argument as above, ghci should get stuck in evaluating foo 5.0 as it may not be a Double, but only a Fractional. Surprisingly (at least to me) it works! So, it seems as if the type of 5.0 was induced by the type system to be Double as foo accepts only Double's. If I understand well, there's some sort of asymmetry when typechecking a function application (the case of foo 5.0), where the type signature of a function is dominant, and where typechecking an overloaded function application (the original case) since there type inference can't take place as someone could add a new overloading later as Bulat says. So, I tried to fix my code and I came up with this (partial) solution: module Main where class FooOp a b where foo :: a -> b -> IO () instance (Num t) => FooOp t Double where foo x y = putStrLn $ (show x) ++ " Double " ++ (show y) partialFoo :: Double -> IO () partialFoo = foo 10 bar = partialFoo 5.0 As you can see, I specified that partialFoo does accept Double so the type of 5.0 if induced to be Double by that type signature and the ambiguity disappear (along with relaxing the type of a to be simply a member of the Num class so 10 can fit in anyway). Problems arise if I add another instance of FooOp where b is Int (i.e. FooOp Int Int): module Main where class FooOp a b where foo :: a -> b -> IO () instance (Num t) => FooOp t Double where foo x y = putStrLn $ (show x) ++ " Double " ++ (show y) instance (Num t) => FooOp t Int where foo x y = putStrLn $ (show x) ++ " Int " ++ (show y) partialFoo = foo 10 bar = partialFoo (5.0::Double) while I thought could be solved likewise: module Main where class FooOp a b where foo :: a -> b -> IO () instance (Num t1, Fractional t2) => FooOp t1 t2 where foo x y = putStrLn $ (show x) ++ " Double " ++ (show y) instance (Num t1, Num t2) => FooOp t1 t2 where foo x y = putStrLn $ (show x) ++ " Int " ++ (show y) partialFoo = foo 10 bar = partialFoo 5.0 but it didn't work. Here's ghci's complaint: example.hs:7:0: Duplicate instance declarations: instance (Num t1, Fractional t2) => FooOp t1 t2 -- Defined at example.hs:7:0 instance (Num t1, Num t2) => FooOp t1 t2 -- Defined at example.hs:10:0 Failed, modules loaded: none. It seems that Num and Fractional are somewhat related. Any hint? Were my reasonings correct or was it only crap? Thanks, Cristiano

2 2

Filesystem access
by Andrew Coppin 22 Jun '07

22 Jun '07

OK, a few questions... 1. Is there *any* way to determine how large a file is *without* opening it? The only library function I can find to do with file sizes is hFileSize; obviously this only works for files that you have permission to open! 2. Is there any way to discover Windoze-style "attributes" for files? (Read-only, archive, system, hidden, et al.)

5 6

Collections
by Andrew Coppin 22 Jun '07

22 Jun '07

When I was at university, we learned a programming language known as Smalltalk. I was rather good at it. [Ironically, making "small talk" is one of the things I do worst IRL! But anyway, back to the topic...] In Smalltalk, there is a wide selection of collection types, all with different facilities and efficiency trade offs. There is bag, set, list, array, ordered list, dictionary, hash table, weak array, etc. A whole menagerie of collection types. However, Haskell only has 1 type of collection: linked lists. (And only single-linked at that.) While other "normal" programming languages spend huge amounts of effort trying to select exactly the right collection type for the task in hand, Haskell programs only ever use linked lists. Why is that, exactly? Does writing software in Haskell magically change the properties of these data structures such that lists become more efficient than all the other types? Or is it that other data structures are only efficient when used with in-place updates? (The latter statement appears to be isomorphic to stating that Haskell programs must necessarily be less efficient than impure ones.) Thoughts?

25 55

WMI via Haskell?
by Dean Herington 22 Jun '07

22 Jun '07

Has anyone built a library providing an interface to WMI (Windows Management Instrumentation) in Haskell?

1 0

Parsec question
by Levi Stephen 22 Jun '07

22 Jun '07

Hi, Fairly new to Haskell and trying some parsec. (Also, new to parsers/interpreters) I had come up with this, which works, but I can't help thinking there's a better way :) |> newtype Identifier = Identifier String > newtype Literal = StringLiteral String -- to be extended later > data Primary = PrimaryLiteral Literal | PrimaryIdentifier Identifier > ||> primary = do { > i <- identifier; > return $ PrimaryIdentifier i; } > <|> do { > l <- stringLiteral; > return $ PrimaryLiteral l; } > ||> identifier = do > i <- many1 letter > return $ Identifier i > > stringLiteral = do > (char '\'') > s <- manyTill anyChar (char '\'') > return $ StringLiteral s Is there a way through combining types/parsers that the double do block in primary could be avoided? I understand it's necessary right now because the parsers identifier and stringLiteral return different types, so I can't just write: > i <- identifier <|> stringLiteral So, I'm not sure whether my types need work, the parsers, or if this is the simplest way. Thanks, Levi lstephen.wordpress.com PS, have lurked on the list for a while, enjoy the discussions and content. |

6 8

Template Haskell, information about data constructor types
by Neil Mitchell 22 Jun '07

22 Jun '07

Hi I'm trying to write a function: reaches :: Type -> Q [Type] The intention is that reaches (Either Bool [Int]) would return [Either Bool [Int], Bool, [Int], Int] - i.e. all types which are contained by the initial type at any level. I took a shot at this: getTypes :: Type -> Q [Type] getTypes t = do let (ConT c, cs) = typeApp t TyConI dat <- reify c return $ concatMap ctorTypes $ dataCtors dat reaches :: Type -> Q [Type] reaches t = f [] [t] where f done [] = return done f done (t:odo) | t `elem` done = f done odo | otherwise = do ts <- getTypes t f (t:done) (odo ++ ts) Where typeApp splits a type to find its constructor, ctorTypes gets the types of the fields, and dataCtors gets the constructors in a data type. Unfortunately reify doesn't seem to work on types. Is it possible to do what I am after? Thanks Neil

3 4

memory usage in repeated reading of an external program's output
by Andrea Rossato 22 Jun '07

22 Jun '07

Hello, I have this very simple program that executes an external program, reads its output and prints it (the program is "date"). The readings is done with pipes. The problem is that memory usage constantly increases over time. Profiling does not show garbage collection of any sort. File descriptors and handles seem to be properly closed. Still I cannot find out where the problem lays. Can it be related to the fact that runProcess closes the handles so that the write file descriptor of the pipe is left open? using a "closeFd w" after runProcess gives a Bad fd error. Moreover, fdToHandle "converts" the fd into a handle, so I presume that closing the second should be enough. And indeed removing or inserting rc <- handleToFd rh and closeFd rc doesn't change anything. Thanks for your help. Andrea The code: ---------------- module Main where import System.Process import System.Posix.IO import System.IO import Control.Concurrent runComLoop :: String -> IO () runComLoop command = do (r,w) <- createPipe wh <- fdToHandle w hSetBuffering wh LineBuffering p <- runProcess command [] Nothing Nothing Nothing (Just wh) (Just wh) rh <- fdToHandle r str <- hGetLine rh rc <- handleToFd rh hClose rh closeFd rc -- get and print the status of handles swh <- hShow wh srh <- hShow rh putStrLn $ show swh putStrLn $ show srh putStrLn str threadDelay $ 100000 * 1 runComLoop command main = runComLoop "date"

4 7

lazy patterns versus where-clauses
by Peter Padawitz 22 Jun '07

22 Jun '07

Is f(~p(x))=e(x) semantically equivalent to: f(z)=e(x) where p(x)=z?

3 3

Messing around with types [newbie]
by Cristiano Paris 22 Jun '07

22 Jun '07

Hi, I'm making my way through Haskell which seems to me one of the languages with steepest learning curve around. Now, please consider this snippet: {-# OPTIONS_GHC -fglasgow-exts #-} module Main where class FooOp a b where foo :: a -> b -> IO () instance FooOp Int Double where foo x y = putStrLn $ (show x) ++ " Double " ++ (show y) partialFoo = foo (10::Int) bar = partialFoo (5.0::Double) I hope the indentation looks ok in your email client. I'm experimenting with currying and typeclasses at the moment. If I try to import this in ghci, it works flawlessy. Now, if I remove the type signature from 10 and 5.0, ghci complaints saying: example.hs:12:6: Ambiguous type variable `t' in the constraint: `Num t' arising from use of `partialFoo' at example.hs:12:6-19 Probable fix: add a type signature that fixes these type variable(s) example.hs:12:6: Ambiguous type variables `t', `t1' in the constraint: `FooOp t t1' arising from use of `partialFoo' at example.hs:12:6-19 Probable fix: add a type signature that fixes these type variable(s) example.hs:12:17: Ambiguous type variable `t1' in the constraint: `Fractional t1' arising from the literal `5.0' at example.hs:12:17-19 Probable fix: add a type signature that fixes these type variable(s) I switched off the monomorphism restriction (btw, is this bad? No flame war please :D) otherwise it'd have complained louder. Can you explain how to fix the code (if possible) and give some explanation? Thanks, Cristiano

5 5

RE: [Haskell-cafe] FFI and Excel VBA
by Lewis-Sandy, Darrell 22 Jun '07

22 Jun '07

Whoops - I posted the wrong version of the exports list. Compilation with that adder.def will fail (since it the Haskell code doesn't export either suber or hello). Below is the revised example: KEYWORDS: Foreign, Export, Win32, DLL, VBA, Excel, GCH, Example The contents of adder.hs are as follows: -------------------------------------------- module Adder(adder) where import Foreign adder :: Int -> Int -> IO Int adder x y = return (id$!(x+y)) foreign export stdcall adder :: Int -> Int -> IO Int -------------------------------------------- The contents of the dllMain.c file are below: -------------------------------------------- #include <windows.h> #include <Rts.h> extern void __stginit_Adder(void); static char* args[] = { "ghcDll", NULL }; BOOL STDCALL DllMain ( HANDLE hModule, DWORD reason, void* reserved ) { if (reason == DLL_PROCESS_ATTACH) { startupHaskell(1, args, __stginit_Adder); return TRUE; } if (reason == DLL_PROCESS_DETACH) { shutdownHaskell(); return TRUE; } return TRUE; } -------------------------------------------- Note that the GHC manual omits the space between the "void" and the __stgint_Adder(void) which causes a compile error. The second case (if (reason == DLL_PROCESS_DETACH) is essential for garbage collection, and shuts down the Haskell runtime when the DLL is unloaded. To execute the Haskell code, I created a VBA module in an Excel workbook with the following code snippet: -------------------------------------------- Private Declare Function adder Lib "adder.dll" _ (ByVal x As Integer, ByVal y As Integer) As Integer Private Sub test() Debug.Print adder(1, 2) End Sub -------------------------------------------- NOTE: If the dll is not ultimately placed in your environments' PATH, then you may have to modify the library name to include the actual path information (or else you will get an error indicating that the library can't be found). Example: ... adder Lib "c:\myDlls\adder\adder.dll" ... Finally, you need to explicitly define an exports file to keep the c compiler from mangling the names too badly. The contents of adder.def are as follows: -------------------------------------------- EXPORTS adder -------------------------------------------- With these preliminaries out of the way, you can compile this to a dll using the following sequence of commands: -------------------------------------------- ghc -c adder.hs -fglasgow-exts ghc -c dllMain.c ghc --mk-dll -optdll--def=adder.def -o adder.dll adder.o adder_stub.o dllMain.o -------------------------------------------- When this is all done, open the XLProject and press F5 to run, or simply type "=adder(1,2)" in any cell of the workbook. My apologies for any confusion caused by my earlier posting. -Darrell Lewis-Sandy _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

Functional Data Structures
by Michael T. Richter 22 Jun '07

22 Jun '07

Is there a good book or web site outlining decent pure-lazy-functional data structures, with or without code samples? -- Michael T. Richter <ttmrichter(a)gmail.com> (GoogleTalk: ttmrichter(a)gmail.com) I'm not schooled in the science of human factors, but I suspect surprise is not an element of a robust user interface. (Chip Rosenthal)

3 2

RE: [Haskell-cafe] FFI and Excel VBA
by Lewis-Sandy, Darrell 21 Jun '07

21 Jun '07

I was able to figure out how to get the example in chapter 11 of the GHC manual to work. Since it was not intuitive (to a non C programmer), I thought that I should post this so that others might take advantage of it. KEYWORDS: Foreign, Export, Win32, DLL, VBA, Excel, GCH, Example The contents of adder.hs are as follows: -------------------------------------------- module Adder(adder) where import Foreign adder :: Int -> Int -> IO Int adder x y = return (id$!(x+y)) foreign export stdcall adder :: Int -> Int -> IO Int -------------------------------------------- The contents of the dllMain.c file are below: -------------------------------------------- #include <windows.h> #include <Rts.h> extern void __stginit_Adder(void); static char* args[] = { "ghcDll", NULL }; BOOL STDCALL DllMain ( HANDLE hModule, DWORD reason, void* reserved ) { if (reason == DLL_PROCESS_ATTACH) { startupHaskell(1, args, __stginit_Adder); return TRUE; } if (reason == DLL_PROCESS_DETACH) { shutdownHaskell(); return TRUE; } return TRUE; } -------------------------------------------- Note that the GHC manual omits the space between the "void" and the __stgint_Adder(void) which causes a compile error. The second case (if (reason == DLL_PROCESS_DETACH) is essential for garbage collection, and shuts down the Haskell runtime when the DLL is unloaded. To execute the Haskell code, I created a VBA module in an Excel workbook with the following code snippet: -------------------------------------------- Private Declare Function adder Lib "adder.dll" _ (ByVal x As Integer, ByVal y As Integer) As Integer Private Sub test() Debug.Print adder(1, 2) End Sub -------------------------------------------- NOTE: If the dll is not ultimately placed in your environments' PATH, then you may have to modify the library name to include the actual path information (or else you will get an error indicating that the library can't be found). Example: ... adder Lib "c:\myDlls\adder\adder.dll" ... Finally, you need to explicitly define an exports file to keep the c compiler from mangling the names too badly. The contents of adder.def are as follows: -------------------------------------------- EXPORTS adder suber hello -------------------------------------------- With these preliminaries out of the way, you can compile this to a dll using the following sequence of commands: -------------------------------------------- ghc -c adder.hs -fglasgow-exts ghc -c dllMain.c ghc --mk-dll -optdll--def=adder.def -o adder.dll adder.o adder_stub.o dllMain.o -------------------------------------------- When this is all done, open the XLProject and press F5 to run, or simply type "=adder(1,2)" in any cell of the workbook. I hope that this was helpful. -Darrell Lewis-Sandy

1 0

advice on GADT type witnesses needed
by David Roundy 21 Jun '07

21 Jun '07

Hi all, Jason and I have been working on getting GADT type witnesses into darcs, and have run into a puzzling error. For the life of me, I can't see how this is an actual error, to the point that I'm wondering if it's actually a bug in ghc. You can check out the code and reproduce this with darcs get http://physics.oregonstate.edu/~roundyd/darcs-gadts cd darcs-gadts autoconf ./configure --with-type-witnesses make darcs At this point ghc will compile for a bit. It shouldn't get past Viewing, since we haven't converted over much of the code yet, but it fails inexplicably on Show. The error message we get is: ghc -DPACKAGE_NAME=\"darcs\" -DPACKAGE_TARNAME=\"darcs\" -DPACKAGE_VERSION=\"1.0.9\" -DPACKAGE_STRING=\"darcs\ 1.0.9\" -DPACKAGE_BUGREPORT=\"bugs@darcs.net\" -DSTDC_HEADERS=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_MEMORY_H=1 -DHAVE_STRINGS_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_UNISTD_H=1 -DGADT_WITNESSES=1 -DHAVE_TERMIO_H=1 -cpp -package regex-compat -package QuickCheck -package mtl -package parsec -package html -package unix -O -funbox-strict-fields -Wall -Werror -I. -I./src -i./src -DHAVE_CURSES -DHAVE_CURL -c src/Darcs/Patch/Show.lhs src/Darcs/Patch/Show.lhs:50:0: Quantified type variable `y' is unified with another quantified type variable `x' When trying to generalise the type inferred for `showPatch' Signature type: forall x y. Patch x y -> Doc Type to generalise: Patch y y -> Doc In the type signature for `showPatch' When generalising the type(s) for showPatch, showComP, showSplit, showConflicted, showNamed make: *** [src/Darcs/Patch/Show.o] Error 1 The relevant code is showPatch :: Patch C(x,y) -> Doc showPatch (FP f AddFile) = showAddFile f ... showPatch (Conflicted p ps) = showConflicted p ps and the trouble comes about because of (in Core.lhs) data Patch C(x,y) where NamedP :: !PatchInfo -> ![PatchInfo] -> !(Patch C(x,y)) -> Patch C(x,y) ... Conflicted :: Patch C(a,b) -> FL Patch C(b,c) -> Patch C(c,c) Still, this makes no sense to me. If any type gurus could help clarify, that would be great. Thanks! -- David Roundy Department of Physics Oregon State University

4 8

Using Template Haskell to automate QuickCheck testing?
by Brent Yorgey 21 Jun '07

21 Jun '07

(I originally tried to send this on Jun 8, but it seems that due to various issues it did not actually get sent over the list. Apologies if anyone gets multiple copies.) Hi all, Following some recent discussions in #haskell, I've decided to try my hand at a module to allow automation of QuickCheck testing. I'm aware that there are already various "homegrown" solutions out there, (e.g. [1],[2]) as well as idiomatic boilerplate to do this (e.g. [3]), but it feels like there ought to be a more elegant/DRY-style solution. In my ideal vision, you'd be able to do something like this: import ModuleToBeTested import Test.AutoQuickCheck -- the library module I want to write prop_foo = blah -- some QuickCheck tests prop_bar = blah blah main = $(runChecks) ...where runChecks would use TH to reflect on the current module, find any top-level function declarations beginning with "prop_", and generate code to run them all through QuickCheck, printing the results. I think the attractions of such a system (if it could actually work) should be obvious. However, after reading all about TH it doesn't seem like there's a way to do this (reflecting on the current module to pull out the names of certain top-level declarations). Since I don't know much about how TH is implemented, I'm not sure whether (1) there's a good reason this is pretty much impossible with TH; (2) it would be possible but it's currently unimplemented; or (3) there actually is a way to do what I want, but I'm not clever enough in the ways of TH to figure it out. (Or (4) none of the above?) Any thoughts or advice would be greatly appreciated! thanks, -Brent [1] http://www.cs.chalmers.se/~rjmh/QuickCheck/quickcheck<http://www.cs.chalmers.se/%7Erjmh/QuickCheck/quickcheck> [2] http://blog.codersbase.com/2006/09/01/simple-unit-testing-in-haskell/ [3] http://www.haskell.org/haskell wiki/How_to_write_a_Haskell_program#Add_some_automated_testing:_QuickCheck

3 2

Re: [Haskell-cafe] Useful IDE features -
by Peter Verswyvelen 21 Jun '07

21 Jun '07

That looks nice, just unfortunate you need to cast to ::Float in homer2?Age::Float. I don't see why this is needed, but I must say I don't understand your code completely yet, working on that :) Also, wouldn't this approach be less performant? Or is GHC that good that ist compiles away all the overhead? >----- Oorspronkelijk bericht ----- >Van: Claus Reinke [mailto:claus.reinke@talk21.com] >Verzonden: dinsdag, juni 19, 2007 01:37 PM >Aan: haskell-cafe(a)haskell.org >Onderwerp: Re: [Haskell-cafe] Useful IDE features - "implement instance" > >> Just another wild idea which I might find useful, but is more like >> refactoring, is to convert the fields of a record to get/set type-classes, >> and refactor all usages of those fields. > >you could use a preprocessor (DrIFT, Data.Derive) to derive the >instances, but you need to share the class declarations between all >client modules. > >> ------------------- >> data Person = Person { name :: String, age :: Float } >> >> main = print $ name p ++ " is " ++ show (age p) ++ " years old" >> where p = Person { name = "Homer", age = 41 } >> ------------------- >>.. > >alternatively, you could generalise this a bit, so that there is only one >class for all combinations of records, fields, and field value types, and >then generalise it further so that you only need one pair of instances >to define selection and update for all records. that kind of operates at >the borders of the language, so you lose portability (the nicest version >is ghc-only; nearly all language extensions used are also supported >by hugs, but with a slightly different interpretation). you'd still need >to share the label types between all client modules. > >claus > >---------------------------------------------------------- >{-# OPTIONS_GHC -fallow-undecidable-instances #-} >{-# OPTIONS_GHC -fallow-overlapping-instances #-} >{-# OPTIONS_GHC -fglasgow-exts #-} > >infixl ? >infixr <:,:< > >------------------- poor man's records > >-- record extension >-- (ghc only: infix constructor; for hugs, use (,) instead) >data fieldValue :< record = fieldValue :< record > >-- field selection (?) and update (<:) >-- needs overlapping instances to recurse down record extensions >-- for hugs: drop the functional dependency, use more type annotations >class Has field value record | field record -> value where > (?) :: record -> field -> value > (<:) :: (field,value) -> record -> record > >-- if the first field matches, we're done >instance Has field value ((field,value) :< record) where > ((_f,v) :< _) ? f = v > (f,v) <: ((_f,_) :< r) = ((f,v) :< r) > >-- otherwise, try again, with the remaining record >instance Has field value record => Has field value ((f,v) :< record) where > ((f',v') :< r) ? f = r ? f > (f,v) <: ((f',v') :< r) = ((f',v') :< ( (f,v)<:r ) ) > >-- some field labels >data Name = Name >data Age = Age > >------------------- a generic version, no separate Person type or instances > >type Person1 = (Name,String) :< (Age,Float) :< () > >homer :: Person1 >homer = (Name,"Homer") :< (Age,41) :< () > >test1 = print $ homer?Name ++ " is " ++ show(homer?Age) ++ " years old" > >------------------- a more down-to-earth version, closer to the original > >data Person = Person String Float > >instance Has Name String Person where > (Person name age) ? Name = name > (Name,newName) <: (Person name age) = Person newName age > >instance Has Age Float Person where > (Person name age) ? Age = age > (Age,newAge) <: (Person name age) = Person name newAge > >defaultPerson = Person "" 0 > >homer2 = (Name,"Homer2") <: (Age,42::Float) <: defaultPerson > >test2 = print $ homer2?Name ++ " is " ++ show(homer2?Age::Float) ++ " years old" > >------------------- > >main = test1 >> test2 > > > >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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1d-rubik
by Arie Groeneveld 21 Jun '07

21 Jun '07

Hi all, I read this on the J -programming forum http://www.jsoftware.com/pipermail/programming/2007-June/007004.html Maybe of interest, especially the part of generating the subgroup or composing a more intelligent solver. quote I found an interesting game, as found on Andrew Nikitin's MSX-BASIC page http://nsg.upor.net/msx/basic/basic.htm , and I am not sure if its solver has been given as a puzzle. Here it goes. 1D Rubik's Cube is a line of 6 numbers with original position: 1 2 3 4 5 6 which can be rotated in 3 different ways in groups of four: _______ _______ (1 2 3 4)5 6 --(0)-> (4 3 2 1)5 6 _______ _______ 1(2 3 4 5)6 --(1)-> 1(5 4 3 2)6 _______ _______ 1 2(3 4 5 6) --(2)-> 1 2(6 5 4 3) Given a scrambled line, return the shortest sequence of rotations to restore the original position. Examples: solve 1 3 2 6 5 4 1 2 1 solve 5 6 2 1 4 3 0 2 solve 6 5 4 1 2 3 0 1 2 end quote What follows is a kind of emulation (in the sense of the nature of the J-program) of the solution of Roger Hui. see http://www.jsoftware.com/pipermail/programming/2007-June/007006.html Remarks: Rewards for me are learning and understanding J programming and programming a Haskell solution for the same problem, plus a bit of group theory. Roger Hui says that for his solution it's not guaranteed that the rotation-sequence is the shortest one --------------------- PROGRAM -------------------------------------------- -- subgroup generators -- all rotations are permutations of order 2 because they leave 2 elements in place -- f.e. [3,2,1,0,4,5] has cycle product (1 4)(2 3) rotaties :: [[Int]] rotaties = [[3,2,1,0,4,5],[0,4,3,2,1,5],[0,1,5,4,3,2]] ident :: [Int] ident = [1..6] rotix :: [[Int]] rotix = [ [e] | e <- [0..2]] -- flip consecutive part of the 1d-rubik ds `roteer` d = map (ds !!) d -- number of misplacements -- equivalent to the parity-function of permutations or the order of a permutation -- it seemed that only even order permutations are solvable, indeed the subgroup generated -- contains only even permutations mispl = sum . map (\[a,b] -> if b-a > 0 then 0 else 1) . combinationsOf 2 -- equivalent to J's : rotaties , ,/{"1/~ rotaties -- to keep in order with the J program I had to change 'map (roteer x)' to 'map (flip roteer x)' rotzelf xs = concat $ xs : [ map (flip roteer x) xs | x <- xs ] -- rotseqs equivalent to J-program line: q , , ,&.>/~ q rotseqs xs = xs ++ (map concat $ [ [x,y] | x <- xs, y <- xs ]) -- mark duplicates function equivalent to J's ~: p -- boolean array where a '0' marks a duplicate markdups = domark [] domark _ [] = [] domark ys (x:xs) | x `elem` ys = 0 : domark (x:ys) xs | otherwise = 1 : domark (x:ys) xs -- b <select> a -- selects elements from array 'a' according to bool array 'b' -- equivalent to J's m#n [] `boolsel` _ = [] (b:bs) `boolsel` (x:xs) | b==1 = x: bs `boolsel` xs | otherwise = bs `boolsel` xs -- after 5 iterations no further change occurs -- the number of elements then reaches the order of the subgroup = 360 (subg, rseqs) = head . drop 4 . iterate tab $ (rotaties, rotix) -- or : head . dropWhile ((/=360) . length . fst) . iterate tab $ (rotaties, rotix) tab (ps , qs) = (bs `boolsel` rs, bs `boolsel` ts) where rs = rotzelf ps ts = rotseqs qs bs = markdups rs solve :: [Int] -> [Int] solve rs | odd . mispl $ rs = error " no solution possible...." | rs == ident = [] -- identity of the subgroup | otherwise = as where rs' = map (flip (-) 1) rs is = fromJust $ elemIndex rs' subg as = rseqs !! is test = map solve [[1,3,2,6,5,4],[5,6,2,1,4,3],[6,5,4,1,2,3],[6,4,2,5,3,1]] greetings @@i = Aai

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Re: [Haskell-cafe] Collections
by Chaddaï Fouché 21 Jun '07

21 Jun '07

Andrew Coppin writes: > Maybe it's just a culture thing then... In your typical OOP language, > you spend five minutes thinking "now, what collection type shall I use > here?" before going on to actually write the code. In Haskell, you just > go "OK, so I'll put a list here..." > Well lists are really useful, but I don't think all Haskell programmers are like you, in fact I think only the enthusiast newbies (like.. you maybe ?) only use lists without asking themselves if there is not a data structure better suited to their problem. A lot of work in the functional world goes to improve existing data structures, traversal and upgrade means and inventing new data structures to resolve real-world problems. Real Haskell programs don't limit themselves to lists, eg. if you look at xmonad you'll see that zippers play a really important role in the powerfulness of the code and it's elegance. -- Jedaï

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Perl-style numeric type
by Brent Yorgey 21 Jun '07

21 Jun '07

Hi all, I've started developing a library to support a "Perl-style" numeric type that "does the right thing" without having to worry too much about types. Explicit static typing of numeric types is really great most of the time, and certainly a good idea for larger projects, but probably everyone's had one of those experiences where you just want to write some simple, "one-off" numeric code that ends up getting cluttered with all sorts of fromIntegers and whatnot just to make the type checker happy. The idea would be to internally use either an Integer, Rational, or Double, and transparently convert between them as necessary. I know I would enjoy having such a numeric type for use in, e.g. programs to solve Project Euler problems. But before I get too far (it looks like it will be straightforward yet tedious to implement), I thought I would throw the idea out there and see if anyone knows of anything similar that has already been done before (a cursory search of the wiki didn't turn up anything). I don't want to reinvent the wheel here. thanks! -Brent PS Also, did anyone get my e-mail to this list of June 8 about Template Haskell and QuickCheck? If you did and it's just that no one knows the answer to my questions, no problem. But I was subscribed in a strange way (through the fa.haskell Google group) and I'm beginning to suspect that perhaps my message never actually got sent over the list. If so I could resend it now that I'm subscribed by more conventional means.

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Avoiding "Non-exhaustive patterns in function f"
by peterv 21 Jun '07

21 Jun '07

Haskell is known for its very strong static type checking, which eliminates a lot of runtime errors. But the following simple program crashes at runtime: data D = A | B f A = True main = print (f B) I understand this has nothing to do with type checking, but why can't the compiler give a warning about this? Or is this by design or because it is impossible to check with more complex recursive data types? Thanks, Peter

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Reading/writing packed bytes from file
by Jefferson Heard 20 Jun '07

20 Jun '07

I've read the documentation for some of the marshalling packages out there for Haskell, and I'm left confused as to which one I should be using and how to actually do what I want to do. I have a file, a little over 2gb, of packed data in the format (recordcount) records of: 4-byte int (count), (count) 2-byte unsigned shorts, (count) 4-byte floats all in little-endian order. What I want to do is read each record (lazily), and unpack it into Data.IntMap.IntMap Float where the unsigned shorts become the keys and the 4-byte floats become the values. Then I want to do a lot of interesting processing which we'll skip here, and write back out packed data to a file in the format of 4-byte float, 4-byte float, 4-byte float for each record. I need these output records to be four-byte C floats. I've gotten as far as datatypes and a couple of signatures, but I can't figure out the functions themselves that go with the signatures, and then again, maybe I have the signatures wrong. -- import qualified Data.IntMap as M import qualified Data.ByteString.Lazy.Char8 as B data InputRecord = M.IntMap Float data OutputRecord = (Float, Float, Float) -- open a file as a lazy ByteString and break up the individual records -- by reading the count variable, reading that many bytes times -- sizeof short + sizeof float into a lazy ByteString. readRawRecordsFromFile :: String -> IO [B.ByteString] -- take a bytestring as returned by readRawRecordsFromFile and turn it -- into a map. decodeRawRecord :: B.ByteString -> M.IntMap Float -- Can anyone help with how to construct these functions? I'm going to have to make a few passes over this file, so I'd like the IO to be as fast as Haskelly possible. -- Jeff

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Re: [Haskell] ANN: Uniplate 1.0
by Isaac Dupree 20 Jun '07

20 Jun '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Neil Mitchell wrote: > Hi, > > I am pleased to announce Uniplate (formerly known as Play), a library > for boilerplate removal requiring only Haskell 98 (for normal use) and > optionally multi-parameter type classes (for more advanced features). > This library has been tested with Yhc, Hugs and GHC. > > The Uniplate library can do some of the same things as Scrap Your > Boilerplate (SYB), and has functions that can be used in a similar > manner to everywhere and everything. > > LINKS > > Project home page: http://www-users.cs.york.ac.uk/~ndm/uniplate/ > Hackage release: > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/uniplate-1.0 > Haddock docs: http://www.cs.york.ac.uk/fp/haddock/uniplate/ > Short Manual: http://www.cs.york.ac.uk/fp/darcs/uniplate/uniplate.htm > Draft Paper: > http://www-users.cs.york.ac.uk/~ndm/downloads/draft-uniplate-15_jun_2007.pdf > > > The manual contains a basic overview of some bits of the library, the > paper goes into more detail, but is intended to still be a readable > introduction to the library. The draft paper feels more readable and up-to-date than the manual. However I have one comment about it. In the section on DERIVE for Uniplate instances, the type appearing directly in one of its own constructors is demonstrated being recognized, as is the type appearing in a list in one of its own constructors. But no mention is made of just what powers the deriving has. I presume lists are a special case (how else would it be possible), but a naive reading gives no particular reason to suspect that the type in a Data.Map or Data.Set or any other collection, in one of its own constructors, would not be allowed. Also is there any restriction on the placement of lists in the constructor? I can't see how data Foo = CFoo Foo [Foo] Foo [Foo] Foo could be translated. Does putting the list somewhere other than last work? Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGdyHsHgcxvIWYTTURAiWuAKCUKf7aGf1rO58gcgxYkqbO7oKNCwCfTRpo 0UsQHx9riP+cZ/S/gloAy3g= =cAxY -----END PGP SIGNATURE-----

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Can't build Lambdabot
by Daniel Fischer 20 Jun '07

20 Jun '07

Hi all, so I got myself a brand new lambdabot-4.0 today, but failed miserably building it. First, build-depends lists fps>=0.7. Now Data.ByteString(.*) is in base(since 6.6.1, I believe). Easy to fix, just delete the entry in lambdabot.cabal. After apparently successful configuration, ./build fails, because 'breakChar' is no longer present (in fps-0.8 or the 6.6.1 base). Okay, change breakChar '\n' to break (== '\n') in BotPP.hs and hope for the best. Trying ./build again, fails with Preprocessing executables for lambdabot-4.0... Building lambdabot-4.0... Plugin.hs:46:7: Could not find module `Text.Regex': it is a member of package regex-compat-0.71, which is hidden which would be easy to fix if regex-compat-0.71 WERE hidden, bu it's NOT, it's definitely exposed. So what's going on here and how can I get myself a working lambdabot? I'm on an i386-linux, if that matters. Thanks, Daniel

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csv library
by Jaap Weel 20 Jun '07

20 Jun '07

This is an announcement of something so tiny (about 100 lines of code including comments), I'd feel bad posting it to the haskell list, so it'll go on haskell-cafe. This was pretty much just a project for me to learn cabal and hackage, but it may very well actually come in useful some day. csv 0.1.0 CSV loader and dumper This library parses and dumps documents that are formatted according to RFC 4180, "The common Format and MIME Type for Comma-Separated Values (CSV) Files". This format is used, among many other things, as a lingua franca for spreadsheets and for certain web services. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/csv-0.1.0 -- /jaap

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hSetBuffering woes
by Eric 20 Jun '07

20 Jun '07

Hi all, I tried to turn off buffering with the command hSetBuffering (from System.IO) but my app still blocks on hGetContents (from Data.ByteString). Does anyone know what's happening? E.

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Recursive instance dictionaries in GHC
by Benja Fallenstein 20 Jun '07

20 Jun '07

Hi, I can't figure out why the following program compiles with this instance declaration > instance Size (Maybe [a]) => Size [a] where size x = size (foo x) but has GHC loop forever with this one: > instance (Foo a b, Size b) => Size a where size x = size (foo x) Anybody here know? Thanks, - Benja > {-# OPTIONS_GHC -fglasgow-exts -fallow-overlapping-instances > -fallow-undecidable-instances #-} > > class Foo a b | a -> b where foo :: a -> b > instance Foo [a] (Maybe [a]) where foo (x:xs) = Just xs; foo [] = Nothing > > class Size a where size :: a -> Int > > instance Size a => Size (Maybe a) where > size (Just x) = 1 + size x; size Nothing = 0 > > --instance (Foo a b, Size b) => Size a where size x = size (foo x) > -- OR > --instance Size (Maybe [a]) => Size [a] where size x = size (foo x) > > main = print $ size "foo"

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OS design & FP aesthetics
by Creighton Hogg 20 Jun '07

20 Jun '07

On 6/18/07, Andrew Coppin <andrewcoppin(a)btinternet.com> wrote: > > Creighton Hogg wrote: > > Well, since we're on the subject and it's only the Cafe list, what is > > it that you find messy about Linux that you would want to be solved by > > some hypothetical Haskell OS? > > This is drifting off-topic again, but here goes... Yeah, so I'll just split this off into a different thread. There are lots of things to like about Linux. It doesn't cost money. > It's fast. It's reliable. It's flexible. It's secure. Okay, I'm not sure if I'd agree with the reliable & secure points. I mean, relative to what could be done. I'm a rank amateur when it comes to OS work but when I've looked at recent papers Linux really isn't that cutting edge. I mean, it may be reliable in comparison to Windows 98 & has less known exploits than any Windows system, but in terms of how good it *could* be I think there's an awful lot of room for growth. However, > unfortunately it's still Unix. In other words, it's a vast incoherant > mess of largely incompatible ad-hoc solutions to individual problems > implemented independently by unrelated hackers over the 40+ years of > history that this software has been around. New software has to emulate > quirks in old software, and client programs work around the emulated > quirks in the new software to get the functionallity it actually wants. > One vast tangled mess of complexity and disorder. Exhibit A: Package > managers exist. Exhibit B: Autoconf exists. I rest my case. Okay, but these don't seem to really be design flaws so much as the inevitable results of age and the need for backwards compatibility. I'm looking more for technical problems that you would want to see fixed in our magical UberOS. An operating system should have a simple, clear, consistent design. Not > unlike a certain programming language named after a dead mathematition, > come to think of it... > > (Have you ever programmed in C? You can certainly see where Unix gets > its features from - terse, cryptic and messy.) This is another thing we're just going to disagree on. I think C++ is a pretty messy language, but feel that straight up C is rather simple and elegant. I had only used C++ before, but a friend rather easily convinced me that C is in fact a very sexy language when used in its intended design space. Still, I don't have the skill to write a functioning operating system - > much less one that's "ready for the desktop" - so that's that I suppose... > > (I did seriously investigate the task once. Indeed, I got as far as > writing a bootloader. It worked too!) Would you mind sharing the code? I'd be interested.

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Useful IDE features - What abaut automatically adding import statements?
by Marc Weber 19 Jun '07

19 Jun '07

Eclipse does have this which saves you a lot of time: Fix imports. Did I miss anyone mentioning such a feature request? Or is there already a solution around ? Marc Weber

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latex table and backgound color
by Adrian Dusa 19 Jun '07

19 Jun '07

Hello all, I'm a newb on this list, hoping to get (what I suppose is) a straightforward answer to a problem. Is there any possibility to set a background color for different rows in a tabular environment? What I hope to get is easily done in OpenOffice for example: http://www.roda.ro/table.png Many, many thanks in advance, Adrian -- Adrian Dusa Romanian Social Data Archive 1, Schitu Magureanu Bd 050025 Bucharest sector 5 Romania Tel./Fax: +40 21 3126618 \ +40 21 3120210 / int.101

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Building Yi (and wider Cabal stuff)
by Michael T. Richter 19 Jun '07

19 Jun '07

I'm trying to build Yi (from the darcs repository) to take a look at it. The README that comes with it says "it's a standard Cabal project so do what you normally do" (paraphrased slightly). The problem is that I'm not a cabal user just yet and have no idea where to go from here. Just throwing caution to the wind and typing "make" built a setup program which was then executed and failed complaining about not having yi-lib0.3 (which I kind of thought the project should be building for me) and such. Editing the config.mk (which has some options left almost entirely undocumented) got me a step farther (specifically giving it a prefix that actually existed), but then had it choking out that it couldn't find the vty library -- even when I specifically told it to build the gtk version. Is there some painfully obvious thing I'm missing in trying to build this? And is it something that's applicable in the broader Cabal world or is it just this project? Compiling GHC (which is a lot bigger and more complex, I'd have guessed) was painless in comparison, so I'm kind of in the dark here. -- Michael T. Richter <ttmrichter(a)gmail.com> (GoogleTalk: ttmrichter(a)gmail.com) If there's one thing that computers do well, it's to make the same mistake uncountable times at inhuman speed. (Peter Coffee)

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