- Haskell-Cafe - Haskell.org

Stupid newbie question
by Brian Hurt 06 Jan '07

06 Jan '07

My apologies for wasting bandwidth on what I'm sure is a stupid newbie question. Given: -- Reimplementing the wheel here, I know data Option a = Some a | Empty deriving (Eq,Ord,Show,Read) nth 0 (x:xs) = Some x nth i (x:xs) = if i < 0 then Empty else nth (i-1) xs nth i [] = Empty That: nth 1000000 [1..10000000] returns the right answer, while: nth 1000000 [1..] blows stack. Furthermore, given: makelist i = i : (makelist (i-1)) Why is it that: nth 1000000 (makelist 1) blows stack? What are the rules for being tail recursive in Haskell? I'm an Ocaml programmer, so I'm familiar with this problem, I just don't see the solution. Some help would be appreciated. Thanks in advance, Brian

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MVar style question
by Chad Scherrer 05 Jan '07

05 Jan '07

When using MVars, is there a reason to prefer using MVar (a,b) over (MVar a, MVar b), or vice versa? I'm not sure if this is really a question of style, or if there are practial implications I'm missing one way or another. Thanks! -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Composing functions with runST
by Yitzchak Gale 05 Jan '07

05 Jan '07

Can anyone explain the following behavior (GHCi 6.6): Prelude Control.Monad.ST> runST (return 42) 42 Prelude Control.Monad.ST> (runST . return) 42 <interactive>:1:9: Couldn't match expected type `forall s. ST s a' against inferred type `m a1' In the second argument of `(.)', namely `return' In the expression: (runST . return) 42 In the definition of `it': it = (runST . return) 42 Thanks, Yitz

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RE: [Haskell] Re: state of HaXml?
by Simon Marlow 05 Jan '07

05 Jan '07

[ moving to haskell-café... ] Norman Ramsey wrote: > > There seems to be a misunderstanding here: readFile in > itself is not the > > solution. readFile is defined thus: > > > > readFile name = openFile name ReadMode >>= hGetContents > > > and the original code was this: > > > > load fn = do handle <- IO.openFile fn IO.ReadMode > > contents <- IO.hGetContents handle > > IO.hClose handle > > return $ XP.xmlParse fn contents > > > Sure, you can replace the openFile/hGetContents pair by > readFile, but the > > real problem is the presence of the hClose. Removing that > will solve your > > problem (but note that you now have no control over when > the file is > > actually closed). > > Can I just leave it hanging and rely on the garbage collector to > close it in the fullness of time? Yes. The problem I was alluding to arises when you have many lazilly-closed files, and you run into the system's open file limit because the runtime doesn't close them eagerly enough. To be sure of closing the file at the right time, you need to force the entire file to be read (e.g. by forcing the result of the parse), then close the handle. > Because of laziness, I believe there's no point in my writing the > following: > > > load fn = do handle <- IO.openFile fn IO.ReadMode > > contents <- IO.hGetContents handle > > let xml = XP.xmlParse fn contents > > IO.hClose handle > > return xml > > Is that correct? Yes. Cheers, Simon

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Re: [Haskell] Re: state of HaXml?
by dons＠cse.unsw.edu.au 05 Jan '07

05 Jan '07

nr: > > Sure, you can replace the openFile/hGetContents pair by readFile, but the > > real problem is the presence of the hClose. Removing that will solve your > > problem (but note that you now have no control over when the file is > > actually closed). > > Can I just leave it hanging and rely on the garbage collector to close > it in the fullness of time? Yeah, once your program has demanded the entire file, it'll close the Handle. > Because of laziness, I believe there's no point in my writing the > following: > > > load fn = do handle <- IO.openFile fn IO.ReadMode > > contents <- IO.hGetContents handle > > let xml = XP.xmlParse fn contents > > IO.hClose handle > > return xml > > Is that correct? Yep. Its not neccessary in the usual programming cases to explicitly close the handle. IF you start really hammering the filesystem do you start to care about ensuring files are closed (so you don't hang on to too many FDs). Or if you start mutating files on disk. For these situations there are strict readFiles, or Data.ByteString.readFile -- Don

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Haskell Weekly News: January 02, 2007
by dons＠cse.unsw.edu.au 04 Jan '07

04 Jan '07

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20070102 Issue 55 - January 02, 2007 --------------------------------------------------------------------------- Welcome to issue 55 of HWN, a weekly newsletter covering developments in the Haskell community. This week brings a new release of vty and HsColour, and some interesting discussion over the holiday break. Announcements hscolour-1.6. Malcolm Wallace [1]announced HsColour, a popular syntax-highlighter for Haskell code. It can generate ANSI terminal colour codes, HTML, and CSS, and can insert hyperlink anchors for function definitions (useful in conjunction with [2]Haddock). [3]HsColour-1.6 is now available. The major addition is a new LaTeX output mode. 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14677 2. http://haskell.org/haddock 3. http://www.cs.york.ac.uk/fp/darcs/hscolour Dimensional: Statically checked physical dimensions. Bj�rn Buckwalter [4]announced version 0.1 of [5]Dimensional, a module for statically checked physical dimensions. The module facilitates calculations with physical quantities while statically preventing e.g. addition of quantities with differing physical dimensions. 4. http://article.gmane.org/gmane.comp.lang.haskell.general/14691 5. http://code.google.com/p/dimensional/ vty 2.0. Stefan O'Rear [6]announced a new major version of [7]vty. Differences from 1.0 include: vty now uses a record type for attributes, instead of bitfields in an Int; vty now supports setting background colors; you can now explicitly specify 'default' colors; vty now supports Unicode characters on output, automatically setting and resetting UTF-8 mode. 6. http://article.gmane.org/gmane.comp.lang.haskell.general/14703 7. http://members.cox.net/stefanor/vty 'Lambda Revolution' tshirts. Paul Johnson [8]announced the creation of a new Haskell tshirt, on the theme of 'The Lambda Revolution'. Tshirts are available from [9]CafePress, and the designs are freely available. 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17916 9. http://www.cafepress.com/l_revolution Discussion Beautiful concurrency. Simon Peyton-Jones [10]mentioned that he's been writing a chapter on concurrency and transactional memory for a new book, 'Beautiful code'. [11]A first draft is available and Simon welcomes constructive suggestions for improvement. The book is aimed at a general audience of programmers, not Haskell geeks, so tries to explain everything necessary. If you are not a Haskell expert, your input would be particularly valuable. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14681 11. http://haskell.org/haskellwiki/Talk:SantaClausProblem Limits to implicit parallelism in functional applications. John DeTreville [12]announced a short paper about how much implicit parallelism there might be in ordinary functional applications. 12. http://article.gmane.org/gmane.comp.lang.haskell.general/14699 Inlining higher order functions. Norman Ramsey [13]asked about fine grained control for inlining in higher order functions. 13. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/11467 Red-black trees as a nested datatype. Jim Apple [14]described how to implement red-black trees as a nested datatype. 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17957 SYB for XML: deserialization and collections. Alexander Jacobson [15]asked about approaches to simplifying boilerplate in HAppS associated with XML serialization and state deserialization. 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17922 Flattening a lisp-style tree. pphetra [16]asked about flattening heterogeneous lists (or trees) in Haskell. 16. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17976/focus=17976 Jobs Functional programming at Jane Street Capital. Yaron Minsky [17]announced that [18]Jane Street Capital is again looking to hire some top-notch functional programmers. Of particular note is that Jane Street Europe Ltd. now has an office in London, and we are particularly interested in hiring someone for that office with strong systems administration skills in addition to experience with functional programming languages. The ideal candidate has: a commitment to the practical, experience with functional programming languages (such as Haskell). Applicants should also have experience with UNIX and a deep understanding of computers and technology and a strong mathematical background. 17. http://article.gmane.org/gmane.comp.lang.haskell.general/14683 18. http://www.janestcapital.com/tech.html Blog noise [19]Haskell news from the blogosphere. * [20]Secret Santas in Haskell III: Lather, Rinse, Repeat 1 * [21]More Haskell in Java 7 or 8? * [22]Generalized vs. dynamic interfaces * [23]Type classes in Java * [24]Static vs dynamic typing: do what thou wilt * [25]Hoogle 4 progress * [26]Hoogle Progress, HsMan features * [27]The York Haskell Compiler: 1000 patches! * [28]Evaluating cellular automata is co-monadic * [29]Tying Knots Generically * [30]Rails -> Ruby -> Haskell * [31]A Sermon on Programming Languages * [32]Thoughtful revision control * [33]readFile' * [34]Code unraveller * [35]The Haskell Meta Tutorial * [36]Monads * [37]My Haskell Experience * [38]Communities * [39]Darcs: The source code management system of the future? * [40]On strong type systems * [41]On Programming Languages and Productivity 19. http://planet.haskell.org/ 20. http://mult.ifario.us/articles/2006/12/22/secret-santas-in-haskell-iii-coll… 21. http://blogs.msdn.com/ralflammel/archive/2006/12/21/more-haskell-in-java-7-… 22. http://blogs.msdn.com/ralflammel/archive/2006/12/23/generalized-vs-dynamic-… 23. http://syntaxfree.wordpress.com/2006/12/23/et-tu-brute/ 24. http://syntaxfree.wordpress.com/2006/12/24/static-vs-dynamic-typing-do-what… 25. http://neilmitchell.blogspot.com/2006/12/hoogle-4-progress.html 26. http://neilmitchell.blogspot.com/2006/12/hoogle-progress-hsman-features.html 27. http://yhc06.blogspot.com/2006/12/1000-patches.html 28. http://sigfpe.blogspot.com/2006/12/evaluating-cellular-automata-is.html 29. http://sigfpe.blogspot.com/2006/12/tying-knots-generically.html 30. http://rubynations.wordpress.com/2006/12/28/evolutionrails-ruby-haskell/ 31. http://www.dailyspeculations.com/wordpress/?p=101 32. http://koweycode.blogspot.com/2006/12/distributed-chiming-in.html 33. http://koweynlg.blogspot.com/2006/12/meh.html 34. http://koweycode.blogspot.com/2006/12/unraveller.html 35. http://koweycode.blogspot.com/2006/12/haskell-metatutorial.html 36. http://hierodule.livejournal.com/69052.html 37. http://kawagner.blogspot.com/2006/12/my-haskell-experience.html 38. http://gravityboy.livejournal.com/31930.html 39. http://www.iovene.com/content/view/93/1/ 40. http://blog.tmorris.net/strong-type-systems/ 41. http://www.neilmix.com/2007/01/01/on-programming-languages-and-productivity/ Quotes of the Week * chessguy: [in regards to #haskell] man, it's amazing the difference between what happens when someone asks for help here, and what happens when they ask for help in another language channel * cjeris: It's amazing what some languages do to make thinking impossible, seemingly justified by the assumption that no one thinks anyway, so it's more important to make non-thinking programming as easy as possible. * edwinb: I've just walked past a poster advertising a gig by a band called 'The Awkward Squad'. I assume this means they provide output, play concurrently, and people take exception to them. * kfish: Apparently @pl also doubles as the command for producing an unintelligible flip-stream * Binkley: [Monads as clothes] using unsafePerformIO is kind of like going naked in public, might be safe in some contexts, but you really don't want to know what happens if you do it in a really bad one * quazimodo: I know why you guys are so ready to deal with me and put up with noob questions ... you program so fast compared to other language users that you have time to mess around? * iulus: IO, IO, it's off to bind we go ... * dons: all your imperative are belong to us * Logan Capaldo: I like constructing things with type errors. It lets me read the error and try and figure out what I'm really trying to do * glguy: In true Haskell form, after I realized what I was actually doing... all my functions melted down to about 2 lines each * jcreigh: I've found learning Haskell makes me feel vastly inferior to Haskell coders. ('Oh,', they say, 'That's just a fold over the hyper-monad fluxbox list. Here's the one-line replacement for your entire program.') * Saizan: [New Year's Fun] Yesterday I was so drunk I was trying to typecheck the people at the party... 'What's your monad!?' Code Watch Wed Dec 27 17:03:48 PST 2006. Manuel M T Chakravarty. [42]Parse and desugar equational constraints. With -findexed-types, equational constraints can appear in contexts wherever class predicates are allowed. The two argument types need to be boxed and rank 0. 42. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18551 About the Haskell Weekly News Each week, new editions are posted to [43]the Haskell mailing list as well as to [44]the Haskell Sequence and [45]Planet Haskell. [46]RSS is also available, and headlines appear on [47]haskell.org. Headlines are available as [48]PDF. The Haskell Weekly News is also [49]available in Spanish translation. To help create new editions of this newsletter, please see the [50]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get [51]http://www.cse.unsw.edu.au/~dons/code/hwn 43. http://www.haskell.org/mailman/listinfo/haskell 44. http://sequence.complete.org/ 45. http://planet.haskell.org/ 46. http://sequence.complete.org/node/feed 47. http://haskell.org/ 48. http://www.cse.unsw.edu.au/~dons/code/hwn/archives/20070102.pdf 49. http://haskell.org/haskellwiki/HWN/es 50. http://haskell.org/haskellwiki/HWN 51. http://www.cse.unsw.edu.au/~dons/code/hwn

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Arrays performance
by paolo.veronelli＠gmail.com 04 Jan '07

04 Jan '07

Good year everyone. I'm timing the following script.I'm not expert to evaluate th O'ness of this code, I hope someone can do it. The program clusters n integers in m buckets based on their distance. Anyway I thing should be linear.So I timed som executions changing the first arg. First argument n is the number of integers to be clustered.Their are choosen randomly between 0 and 100. Second argument m is the number of buckets.For them I choose a value choosen like before. In the timings second argument is always 10. Points Time (ms) 100 40 200 93 400 269 800 831 1600 3063 3200 12693 6400 54708 I'd like to know if the algorithm is the cause for this timings (it's not linear) or if I need to use other kinds of Arrays Thanks Paolino ---------------------------------------------------------------- import Data.Maybe import Data.List import Data.Array.Diff import System.Environment import Control.Monad.State import Control.Arrow import Random inc l i = l // [(i,l!i + 1)] switch l i = l // [(i,not (l!i))] constArray n v = listArray (0,n-1) (repeat v) data CState = CState {clusters :: [(Int,Int)],remi :: Int,colsHeap ::DiffArray Int Int ,rowsFlag :: DiffArray Int Bool} devil [] _ = return () devil (l@(row,col):ls) d = do s@(CState cs r hs fs) <- get let ns = s { clusters = (l:cs), rowsFlag = switch fs row, colsHeap = inc hs col } update | c < d = put ns | (c == d) && (r > 0) = put ns { remi = r - 1 } | otherwise = return () where c = hs ! col when (not (fs ! row)) update devil ls d comp f g x y = (f x) `g` (f y) swap = snd &&& fst collapse = (head &&& unzip) >>> (fst *** snd) mcluster :: [(Int,Int)] -> [(Int,[Int])] mcluster ls = let (lr,lc) = (f *** f) (unzip ls) where f = length.nub -- coo space width (k,r) = divMod lr lc CState cs _ _ _ = execState (devil ls k) CState{clusters = [],remi = r,colsHeap = constArray lc 0,rowsFlag = constArray lr False } in map collapse . groupBy (comp fst (==)) . sort . map swap $ cs coupage ls = zip [0..] ls delta fxy xs ys = [(abs(x-y),(n,m))|(n,x) <- coupage xs, (m,y) <- coupage ys] decoupage ls n = fromJust $ lookup n (coupage ls) test xs ys = let d = snd.unzip.sort $ delta (\x y -> abs (x -y)) xs ys in map (decoupage ys *** map (decoupage xs)) (mcluster d) -- call it with 2 args, the number ov values and the number of clusters -- <prog> 101 10 will cluster 101 values in 10 clusters main = do args <- getArgs gen <- getStdGen let [n,m] = map read args let (ps,bs) = splitAt n (take (m + n) (randomRs (0,100) gen)) print $ test ps bs

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GADT proofs of FunDeps?
by Roberto Zunino 04 Jan '07

04 Jan '07

I am investigating mixing FunDeps with the type equality GADT data Teq a b where Teq :: Teq a a Basically, I would like to write something like proof :: (C a b1, C a b2) => Teq b1 b2 proof = unsafeCoerce# Teq provided the FunDep class C a b | a -> b Is this safe? Any caveat? Regards, Roberto Zunino.

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darcs repo for yampa + gadt
by Joel Reymont 04 Jan '07

04 Jan '07

Folks, I have a version of Yampa with Henrik Nilsson's GADT optimizations that I cleaned up for ghc 6.6 and cabalized. Would it be possible to set it up at darcs.haskell.org and if so how should I go about it? Thanks, Joel -- http://wagerlabs.com/

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State separation/combination pattern question
by Reto Kramer 04 Jan '07

04 Jan '07

I'm looking for a pattern to use for "state separation" in my application. I need to write two "stateful libraries". One is a partitioned in- memory store, the other a disk based store with an in-memory cache fronting it. Both store modules need the IO and State monad. My aim is to write the libraries independently from one another in such a way that they are not aware of the context they will be used in. Both stores are called from a breadth first search loop. The example below extracts the essence of my current approach. foo is function in one of the store modules, bar is from the other type of store. Both manipulate their type of state (StateA, StateB). They extract their slice of the "global" application level state (AppStateRec). What I don't like is that the code in foo and bar know about the AppStateRec and that they need to get the "a", or "b" slot respectively. The store modules should not have such coupling with their client code. I've tried to thread the two states (StateA and StateB) using a chain of StateT ... StateT ..., but couldn't really make that work. It seems rather arbitrary in this case which state to make the inner/ outer one and depending on this ordering the "lifts" have to go with one or the other set of store calls. What I'm really looking for is not so much the chaining of StateT compositions, but rather the isolation of StateA from StateB while they both flow from the search loop into the respective library calls (foo, bar) transparently to the application programmer. I'm hoping there's a way to have the loop be in a State monad whose content is the sum of the two states that are needed for the foo and bar call made to the stores from inside the loop. The calls sites for foo and bar should then extract the right component of the global state and thread only that state through into the modules. Sounds like magic, but how close can I get? I've been unable to find a pattern on the WIKI or the web that refers to this type of "state composition" (or I may not have recognized the match). I trust many of you have run into this and there's an obvious and straight forward best way to address this type of state handling. Thanks, - Reto -------------------------- -- ghci -fglasgow-exts ... -- type StateA = [Integer] type StateB = [Integer] data AppStateRec = AppStateRec { a :: StateA, b :: StateB } deriving Show foo :: MonadState AppStateRec m => m () foo = do st <- get put $ st { a = 1:(a st) } bar :: MonadState AppStateRec m => m () bar = do st <- get put $ st { b = 2:(b st) } type Eval a = StateT AppStateRec Identity a exec :: Eval () exec = do foo bar foo foo bar go = runIdentity $ runStateT exec AppStateRec { a = [], b = [] } Prints: ((),AppStateRec {a = [1,1,1], b = [2,2]})

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Socket Programming
by Mark Goldman 04 Jan '07

04 Jan '07

I am trying to write a toy echo server that can handle multiple connections. I would like to be able to test and see if there are any connections waiting to be accepted on a socket. In C and related languages I would use something like select or poll to be nice to the OS, what would I use with the current haskell libs given that I can't seem to find a function to test if accept would block or not? -mdg -- Our problems are mostly behind us, now all we have to do is fight the solutions.

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Possible (GHC or HGL) bug or ??
by Calvin Smith 04 Jan '07

04 Jan '07

Dear haskell-cafe patrons, I've been working through an exercise in Hudak's _The Haskell School of Expression_ (ex. 3.2, creating a snowflake fractal image), and am seeing some strange drawing behavior that I'm hoping somebody can shed some light on. My initial solution is below (it requires HGL for Graphics.SOE): module Main where import Graphics.SOE main = runGraphics ( do w <- openWindow "Snowflake Fractal" (600, 600) fillStar w 300 125 256 (cycle $ enumFrom Blue) spaceClose w ) spaceClose w = do k <- getKey w if k == ' ' then closeWindow w else spaceClose w minSize = 2 :: Int fillStar :: Window -> Int -> Int -> Int -> [Color] -> IO () fillStar w x y h clrs | h <= minSize = return () fillStar w x y h clrs = do drawInWindow w (withColor (head clrs) (polygon [t1p1,t1p2,t1p3,t1p1])) drawInWindow w (withColor (head clrs) (polygon [t2p1,t2p2,t2p3,t2p1])) sequence_ $ map recur [t1p1,t1p2,t1p3,t2p1,t2p2,t2p3] where tanPiOverSix = tan(pi/6) :: Float halfSide = truncate $ tanPiOverSix * fromIntegral h hFrag = truncate $ tanPiOverSix * tanPiOverSix * fromIntegral h (t1p1,t1p2,t1p3) = ((x, y), (x-halfSide, y+h),(x+halfSide, y+h)) (t2p1,t2p2,t2p3) = ((x-halfSide, y+hFrag),(x, y+h+hFrag),(x+halfSide, y+hFrag)) reVert y = y - ((h - hFrag) `div` 3) recur pnt = fillStar w (fst pnt) (reVert (snd pnt)) (h`div`3) (tail clrs) This basically works, in that it does exactly what I want in Hugs, but GHC sometimes pauses partway through rendering, and does not continue rendering until I type any key (except space, which exits) or unfocus/refocus the window, or move the mouse pointer across the window. Sometimes, more often the first time in a GHCI session, it renders completely with no pauses, and it seems to pause more and more if I evaluate main, then close the window, evaluate again in the same GHCI session, repeatedly. The same pausing behavior is observed in a GHC-compiled executable. When the problem occurs, there is a message to the console that says: "thread blocked indefinitely". Versioning info: CPU: Pentium M OS: Gentoo GNU/Linux, kernel 2.6.18 GCC: 4.1.1 GHC: 6.6 HGL: 3.1 HUGS: March 2005 [all software compiled from source using gentoo ebuilds] Is anybody else familiar with this behavior? If not, any suggestions as to where I should file this as a potential bug? GHC? HGL? Both? Elsewhere? Thanks in advance for any information. Calvin p.s. Any stylistic or other comments about the code welcome too.

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Extensible static checking of dimensions?
by Mike Gunter 03 Jan '07

03 Jan '07

The very nice Buckwalter and Denney dimensional-numbers packages both work on a fixed set of base dimensions. This is a significant restriction for me--I want to avoid adding apples to oranges as well as avoiding adding meters to grams. Is it possible to have an extensible set of base dimensions? If so, how usable can such a system be made? Is it very much worse than a system with a fixed set of base dimensions? thanks, -m

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Generating Source Code with Haskell
by Martin Huschenbett 03 Jan '07

03 Jan '07

Hi, my aim is to transform an XML file into C++ source code with a Haskell program. The part that parses the XML is already finished (using HaXML) but the part that generates the C++ code is still remaining. Is there already any work on this topic? Maybe even on generating Java or any other object oriented or imperative language. My first approach was to simply generate the C++ code as text using a pretty printing library but this becomes ugly very fast. Next I thought about generating and rendering the AST of the resulting C++ code. But I don't want to reinvent the wheel. Regards, Martin.

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Glasgow Distributed Haskell
by Joel Reymont 02 Jan '07

02 Jan '07

Is anyone using GdH? Can someone tell me why it's not part of the GHC distribution? It seems that GdH is not being developed anymore and I think it's a real pity! Thanks, Joel -- http://wagerlabs.com/

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Re: [Haskell-cafe] Composing functions with runST
by Yitzchak Gale 02 Jan '07

02 Jan '07

Hi Thomas, You wrote: > How do I "import" Control.Monad.ST so I can experiment with it from > ghci and just do > runST > like you had. Instead of qualifying it in some way. In GHCi, use the :module command (:m) for built-in modules, and :load and :add for source files. In Hugs, use :load and :also for both. You need to run Hugs with the -98 flag to use Control.Monad.ST. > how do I do runST as fully qualified? Like this: Prelude> Control.Monad.ST.runST (do {r <- Data.STRef.newSTRef 2007; Data.STRef.readSTRef r}) 2007 Have fun with monads! Regards, Yitz

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porting ghc
by Brian McQueen 02 Jan '07

02 Jan '07

I was trying to get a ghc going in my shell account the other day and found that the data at http://haskell.org/ghc/docs/6.6/html/building/sec-porting-ghc.html didn't apply at all. The system is a netbsd alpha which turns up as alpha-unknown-netbsd through configure. I didn't find any configure.in to modify, but there is a config.guess. Is that what I'm supposed to modify? I was using the source distribution of ghc 6.6. So how do I get off to s start on the netbsd system at my ISP? Here is the uname data for my system with the hostname x'd out: NetBSD xxx 2.1.0_STABLE NetBSD 2.1.0_STABLE (sdf) #0: Mon Nov 13 08:11:09 UTC 2006 root@ol:/var/src/netbsd/src/sys/arch/alpha/compile/sdf alpha Brian McQueen

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Strange type behavior in GHCi 6.4.2
by Grady Lemoine 01 Jan '07

01 Jan '07

Hello, I've been playing around with Dan Piponi's work on automatic differentiation (from http://sigfpe.blogspot.com/2005/07/automatic-differentiation.html and http://sigfpe.blogspot.com/2006/09/practical-synthetic-differential.html) and I'm getting some odd behavior with the inferred types of functions in GHCi 6.4.2. My test case is as follows: > data Dual a = D a a deriving (Show,Eq) > instance Num a => Num (Dual a) where > fromInteger i = D (fromInteger i) 0 > (D a a')+(D b b') = D (a+b) (a'+b') > (D a a')-(D b b') = D (a-b) (a'-b') > (D a a')*(D b b') = D (a*b) (a*b'+a'*b) > evalDeriv f x = (v, v') > where D v v' = f (D x 1) > f x = x^3 > f' = snd . (evalDeriv f) When I load this in GHCi, I get: *Main> :t f f :: (Num a) => a -> a *Main> :t snd . (evalDeriv f) snd . (evalDeriv f) :: (Num a, Num (Dual a)) => a -> a *Main> :t f' f' :: Integer -> Integer Why is the type of f' Integer -> Integer, especially when the type of the expression it's defined as is more general? Is this something I'm not understanding about Haskell, or is it more to do with GHC 6.4.2 specifically? Any help appreciated, --Grady Lemoine

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Second Call for Papers: TFP 2007, New York, USA
by TFP 2007 01 Jan '07

01 Jan '07

CALL FOR PAPERS Trends in Functional Programming 2007 New York, USA April 2-4, 2007 http://cs.shu.edu/tfp2007/ The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming languages, focusing on providing a broad view of current and future trends in Functional Programming. It aspires to be a lively environment for presenting the latest research results through acceptance by extended abstracts. A formal post-symposium refereeing process then selects the best articles presented at the symposium for publication in a high-profile volume. TFP 2007 is co-hosted by Seton Hall University and The City College of New York (CCNY) and will be held in New York, USA, April 2-4, 2007 at the CCNY campus. SCOPE OF THE SYMPOSIUM The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles leading-edge, previously unpublished research work Position Articles on what new trends should or should not be Project Articles descriptions of recently started new projects Evaluation Articles what lessons can be drawn from a finished project Overview Articles summarizing work with respect to a trendy subject Articles must be original and not submitted for simultaneous publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or more experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Articles on the following subject areas are particularly welcomed: o Dependently Typed Functional Programming o Validation and Verification of Functional Programs o Debugging for Functional Languages o Functional Programming and Security o Functional Programming and Mobility o Functional Programming to Animate/Prototype/Implement Systems from Formal or Semi-Formal Specifications o Functional Languages for Telecommunications Applications o Functional Languages for Embedded Systems o Functional Programming Applied to Global Computing o Functional GRIDs o Functional Programming Ideas in Imperative or Object-Oriented Settings (and the converse) o Interoperability with Imperative Programming Languages o Novel Memory Management Techniques o Parallel/Concurrent Functional Languages o Program Transformation Techniques o Empirical Performance Studies o Abstract/Virtual Machines and Compilers for Functional Languages o New Implementation Strategies o any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2007 program chair, Marco T. Morazan, at tfp2007(a)shu.edu. SUBMISSION AND DRAFT PROCEEDINGS Acceptance of articles for presentation at the symposium is based on the review of extended abstracts (6 to 10 pages in length) by the program committee. Accepted abstracts are to be completed to full papers before the symposium for publication in the draft proceedings and on-line. Further details can be found at the TFP 2007 website. POST-SYMPOSIUM REFEREEING AND PUBLICATION In addition to the draft symposium proceedings, we intend to continue the TFP tradition of publishing a high-quality subset of contributions in the Intellect series on Trends in Functional Programming. IMPORTANT DATES Abstract Submission: February 1, 2007 Notification of Acceptance: February 20, 2007 Registration Deadline: March 2, 2007 Camera Ready Full Paper Due: March 9, 2007 TFP Symposium: April 2-4, 2007 PROGRAMME COMMITTEE John Clements California Polytechnic State University, USA Marko van Eekelen Radboud Universiteit Nijmegen, The Netherlands Benjamin Goldberg New York University, USA Kevin Hammond University of St. Andrews, UK Patricia Johann Rutgers University, USA Hans-Wolfgang Loidl Ludwig-Maximilians Universität München, Germany Rita Loogen Philipps-Universität Marburg, Germany Greg Michaelson Heriot-Watt University, UK Marco T. Morazán (Chair) Seton Hall University, USA Henrik Nilsson University of Nottingham, UK Chris Okasaki United States Military Academy at West Point, USA Rex Page University of Oklahoma, USA Ricardo Pena Universidad Complutense de Madrid, Spain Benjamin C. Pierce University of Pennsylvania, USA John Reppy University of Chicago, USA Ulrik P. Schultz University of Southern Denmark, Denmark Clara Segura Universidad Complutense de Madrid, Spain Jocelyn Sérot Université Blaise Pascal, France Zhong Shao Yale University, USA Olin Shivers Georgia Institute of Technology, USA Phil Trinder Heriot-Watt University, UK David Walker Princeton University, USA ORGANIZATION Symposium Chair: Henrik Nilsson, University of Nottingham, UK Programme Chair: Marco T. Morazan, Seton Hall University, USA Treasurer: Greg Michaelson, Heriot-Watt University, UK Local Arrangements: Marco T. Morazan, Seton Hall University, USA ************************************************************************************ Dr. Marco T. Morazan TFP 2007 Program Committee Chair http://cs.shu.edu/tfp2007/

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Re: [Haskell-cafe] Literate Haskell source files. How do I turn them into something I can read?
by Kirsten Chevalier 01 Jan '07

01 Jan '07

[redirecting to the mailing list] On 12/31/06, Michael T. Richter <ttmrichter(a)gmail.com> wrote: > On Sat, 2006-30-12 at 23:48 -0800, Kirsten Chevalier wrote: > > I'm probably not the right person to explain why David Roundy chose to > > write his code the way he did, since I've never even met him. However, > > since as of 12/8/2006 there were 115 contributors to darcs, perhaps > > reading the source code isn't as difficult as you seem to think it is. > > What's the secret then? pdflatex vomits. unlit generates a few dozen > pages of whitespace per five-line block of code. I'm at a loss to take > this "literate" format and turn it into something which I can read. > The secret lies in adjusting your perspective. The literate format is readable by humans as is. If there are specific things about it that you find difficult, you can post about them here with *specific* examples you find perplexing. But one reason why you may feel like people aren't being helpful is that your question isn't specific enough. > > > I fail to see how making code which can't be read makes it more... > > > readable. > > > "Can't" is an awfully strong word, isn't it? > > Nope. The source files as they are are gibberish. Again, that's an awfully strong statement. Clearly, there exist any number of people who can read them without finding them gibberish. Perhaps it might be better to listen to what they are telling you than to make contradictory declarations. > There's some kind of > markup (of a type I can't identify) mixed up with Haskell source. Given > that I can't identify the markup and I'm a little hazy on Haskell at > this point (part of the point of this exercise was to learn to read > Haskell as it's used in real projects), the net effect is executable > line noise. You know, like a perl program written circa 3.0. > Right, if you're trying to learn to read Haskell as it's used in real projects, reading the .lhs files as they are is the way to go. Again, if you find specific things hard to comprehend, ask here, or on the IRC channel. A general "all Haskell looks like line noise!" complaint is hard for us to answer. > > I can only explain how I did it myself, which was through practice. > > OK. How do you read the literate source code? Which tools do you use > to separate the code from the commentary? Which tools actually work to > turn .lhs files into something human-readable? (pdflatex failed on the > GHC code as well.) > My eyes. And brain. I'm sorry that I can't be more helpful, but that's what most other people I've watched reading Haskell code use, too. Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "Memo to myself: Do the dumb things I gotta do. Touch the puppet head." -- They Might Be Giants

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Mo' comprehensions
by Diego Navarro 31 Dec '06

31 Dec '06

I was solving some programming puzzles today[1], and found myself pining for Map comprehensions. Maybe there should be a Comprehensible class that's automatically mapped to comprehension syntax. It's rather odd to have them only for lists. That would be both more general and more elegant than just bringing back monad comprehensions. Is there any obvious reason why this wouldn't work? [1] www.projecteuler.net <-- fun stuff! -- -- Diego Navarro

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HaXML - creating a simple document
by Terrence Brannon 31 Dec '06

31 Dec '06

Hello, I don't know why my simple example will not render a simple HTML document. For the moment, I simply want to render this: <html> </html> or whatever, the simplest document is. When I run my source code, it simply hangs (as the transcript below shows) {- SOURCE CODE -} import Text.XML.HaXml.Html.Generate import Text.XML.HaXml.Combinators import Text.XML.HaXml.Wrappers main = processXmlWith go go = html [] {- EXECUTION TRANSCRIPT  ghci mk.hs ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.6, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. [1 of 1] Compiling Main ( mk.hs, interpreted ) Ok, modules loaded: Main. *Main> main Loading package haskell98 ... linking ... done. Loading package HaXml-1.17 ... linking ... done. -}

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Re: [Haskell-cafe] Literate Haskell source files. How do I
by Eric Kow 31 Dec '06

31 Dec '06

Hi, > On 30/12/06, Michael T. Richter <ttmrichter(a)gmail.com> wrote: > > Are there any tools which I can use to render .lhs files readable? I'm fine with having them converted into documented source (i.e. source code embedded in documentation) or as pure Haskell source (but without the huge whitespace gaps) -- but I can't figure out how to get either. The darcs code is a bit different in that the LaTeX "comments" are actually the user's manual (for the most part). The code is normally hidden when you compile the pdf. To see the code, you can apply the attached darcs patch. (You could also do it by hand; it's just a couple of changes). When you run make manual/darcs.ps, you will have a ps file that is simultaneously the darcs manual, and various bits of darcs code interspersed and highlighted in blue. (This is > 300 pages of literate darcs, mind you. Maybe just browsing it in a text editor is simpler) -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français.

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Seeking advice on a style question
by Steve Schafer 31 Dec '06

31 Dec '06

In my text/graphics formatting work, I find myself doing a lot of "pipeline" processing, where a data structure will undergo a number of step-by-step transformations from input to output. For example, I have a function that looks like this (the names have been changed to protect the innocent--and to focus on the structure): process :: a -> b -> c -> d -> e process x1 x2 x3 x4 = let y01 = f01 x1 x2 x3; y02 = f02 x1; y03 = f03 y02; y04 = f04 y03; y05 = f05 x1 y01 y04; y06 = f06 x2 y05; (y07,y08) = f07 y01 y06; y09 = f08 y07; (y10,y11) = f09 x2 x4 y09 y08; y12 = f10 y10; y13 = f11 y12; y14 = f12 x1 x2 x3 y01 y13; y15 = f13 y14; y16 = f14 y15 y11 in y16 As you can see, the process is somewhat imperative in overall appearance, with each intermediate function f01..f14 accepting some combination of the original input values and/or intermediate values and returning a new value (or, in some cases, a tuple of values). Obviously, not all of the steps need to be broken out this way. We can, for example, skip the second and third steps and directly write: y04 = f04 $ f03 $ f02 x1; Laying it out this way has a couple of advantages. It makes the steps in the process transparently clear, and if I discover at some point that I need to make a change (e.g., a new requirement causes f13 to need access to x2), it's perfectly obvious where to make the modifications. Nevertheless, it also looks like something that would be amenable to processing with a State monad, except for one thing: The "shape" of the state isn't constant throughout the process. At any given step, new information may be added to the state, and old information may be thrown away, if there is no further need for it. In principle, it could be managed with a bunch of nested StateT monads, but my attempts to do so seem to get so caught up in the bookkeeping that I lose the advantages mentioned above. Alternatively, I can wrap all of the state up into a single universal structure that holds everything I will ever need at every step, but doing so seems to me to fly in the face of strong typing; at the early stages of processing, the structure will have "holes" in it that don't contain useful values and shouldn't be accessed. So here's the question: Is there a reasonable way to express this kind of process (where I suppose that "reasonable" means "in keeping with Haskell-nature") that preserves the advantages mentioned above, but avoids having to explicitly pass all of the various bits of state around? The (unattainable?) ideal would be something that looks like this: process = f14 . f13 . ... . f01 or process = f01 >>= f02 >>= ... >>= f14 Steve Schafer Fenestra Technologies Corp. http://www.fenestra.com/

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Idiomatic Haskell equivalent of "keyword arguments" to functions
by Paul Moore 30 Dec '06

30 Dec '06

I'm thinking around the design of a couple of things, and am hitting an issue which I know how I would solve in Python, but I'm not sure what a good idiomatic Haskell approach would be. The problem is that I am trying to write a function which takes a rather large number of arguments, many of which are optional (ie, have sensible defaults). The canonical example of this is creating a GUI window, which involves a lot of style options, most of which would typically be left to default. In Python, this type of interface is often handled either as a function with many keyword arguments, or as a mutable object which has attributes set, and then a method called to handle the function call. Neither of these approaches seems plausible in Haskell. I looked at wxHaskell for inspiration - its approach (button f [text := "Quit", on command := close f]) looks quite readable, but slightly unusual (to me) for Haskell. It also seems fairly complex to implement (ie, my head hurt when I tried to follow the types involved, but maybe that's just because it's getting late :-)) To make things concrete, the example I'm really thinking of is a "send an email" function, which would take a subject, a body, a list of recipients, optional lists of cc and bcc recipients, an optional mailserver (default localhost), an optional port (default 25), and possibly optional authentication details. I found a couple of Haskell modules implementing a SMTP client, but they both just used a list of positional parameters, which I'm not really happy with. At the very least, I'd like to wrap them in a nicer interface for my code. I'd appreciate any ideas about how to think of this sort of problem - I'm pretty sure that what I need to do is think differently about the issue, rather than just mechanically translating the code I'd write in Python. But I don't really know how. Any pointers would be very helpful! Thanks, Paul.

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how unsafe is this?
by Ben Clifford 30 Dec '06

30 Dec '06

I've defined a helper function to let me do regexps in functional style: > sed exp str = unsafePerformIO $ do > regexp <- regcomp exp 0 > regexec regexp str Is this always safe? or where is it not? (I'm using any one regexp more than once so it doesn't bother me that it compiles each time) --

2 1

Efficient way to break up a lazy bytestring
by Ranjan Bagchi 30 Dec '06

30 Dec '06

Hi -- I'm loading a big file (~2G), again, with 72-byte C structs. I've done pretty well [thanks to everyone] interpreting the fields, but I'm finding the traversal of the file to be pretty inefficient. I do the following: breakUp s | L.null s = [] | otherwise = h:(breakUp r) where (h,r) = L.splitAt 72 s Running this on the 2G file blows up the stack pretty quickly, taking the first 1 million records (there are 20M of them) with a big stack parameter gives about 25% productivity, with GC taking the other 75%. My understanding is that while this looks tail recursive, it isn't really because of laziness. I've tried throwing seq operators around, but they don't seem to do much to help the efficiency. Any help's appreciated. Ranjan

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Newbie question
by Pieter Laeremans 30 Dec '06

30 Dec '06

Hi, I'm reading the Haskell school of expression by Paul Hudok. Great book. However I would like some feedback about a solution to an exercise The problem is quite simple : define f1 and f2 (using higher order functions ) such that f1 (f2 (*) [1..4]) 5 => [5,10,15,20] I have come up with the following solution : f2 :: (a->b)->[a] -> [b] f2 f xs = map f xs f1 fs a = map (applyOp a) fs applyOp b f = f b But I guess there must be some more elegant way. f2 isn't at all original. And the applyOp sounds silly. Any comments or suggestions are welcome. thanks, Pieter

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constant functions
by michael rice 30 Dec '06

30 Dec '06

I'm trying to learn Haskell and translating some Lisp functions as exercises. How would I write a Haskell function named ALWAYS that behaves like this: one = always 1 bozo = always "clown" > map one [2,3,4,5,6] [1,1,1,1,1] > one 62 1 > map bozo [2,3,4,5,6] ["clown","clown" ,"clown", "clown"," clown"] > bozo 62 "clown" i.e. ALWAYS returns a function with a single parameter that is ignored, returning instead the value given to ALWAYS when the function was created. This is what I've been trying: always :: (a -> a) -> a -> a always x = (\y -> x) one = always 1 Michael __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

8 13

Combine list of sorted lists
by Quan Ta 30 Dec '06

30 Dec '06

Hi all, I have this function which combines (zip) list of sorted lists into a sorted list (sorted by sum). The function works with infinite list and seems to give correct result. But after I see the code for the Hamming sequence from the Wiki, I wonder if it can be written better, or more clearly, or succint? import Data.List comb [] = [] comb (a:as) = foldl f2 (f1 a) as where f1 :: [Int] -> [[Int]] f1 [] = [] f1 (a:as) = [a] : f1 as f2 :: [[Int]] -> [Int] -> [[Int]] f2 la [] = [] f2 [] lb = [] f2 la@(a:as) lb@(b:bs) = (a ++ [b]) : (f3 (f2 [a] bs) (f2 as lb)) f3 :: [[Int]] -> [[Int]] -> [[Int]] f3 [] lb = lb f3 la [] = la f3 la lb = let a = head la b = head lb in if sum a <= sum b then a : f3 (tail la) lb else b : f3 la (tail lb) t1 = take 500 (comb [[1,2..],[1,23..],[1,5..],[1,9..]]) t2 = take 500 (sortBy (\x y -> compare (sum x) (sum y)) [[a,b,c,d] | a<-[1,2..80],b<-[1,23..80], c<-[1,5..80],d<-[1,9..80]]) --t3 = take 500 (sortBy (\x y -> compare (sum x) (sum y)) -- [[a,b,c,d] | a<-[1,2..],b<-[1,23..], -- c<-[1,5..],d<-[1,9..]]) main = print (show ((map sum t1) == (map sum t2))) -- thanks for looking, -- Quan

4 6

flatten a nested list
by pphetra 29 Dec '06

29 Dec '06

I would like to write a program that can do something like this. ;; lisp syntax * (my-flatten '(1 (2 (3 4) 5))) (1 2 3 4 5) I end up like this. data Store a = E a | S [Store a] deriving (Show) flat :: [Store a] -> [a] flat [] = [] flat ((E x):xs) = [x] ++ flat xs flat ((S x):xs) = flat x ++ flat xs so *Main> flat [E 1, S[E 2, S[E 3, E 4], E 5]] [1,2,3,4,5] Compare to a Lisp solution, It 's not looking good. Any suggestion. Thanks, PPhetra -- View this message in context: http://www.nabble.com/flatten-a-nested-list-tf2893713.html#a8084726 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Coverage Condition?
by lists＠qseep.net 29 Dec '06

29 Dec '06

Hi folks, I'm working on a program that I've been dabbling with for years. For the first time, I tried to compile it with GHC 6.6, and got an error, explaining that I was violating the Coverage Condition in my instance declaration. The instance declaration looks like this: instance MonadReader r m => MonadReader r (CPST o m) where ... The MonadReader class definition, which doesn't appear to have changed since 6.4.2, looks like this: class Monad m => MonadReader r m | m -> r where ... Apparently, the Coverage Condition disallows my instance declaration, because the variable 'r' is not mentioned in the '(CPST o m)' term. Now this would make sense to me if I didn't have the assertion 'MonadReader r m'. Because of that assertion, m -> r, so '(CPST o m)' shouldn't need to explicitly mention 'r'. I will try using -fallow-undecidable-instances, and see if the message goes away. But can someone explain to me why this is wrong, and what would be the preferred way to write it? I've attached the two relevant source files. (Try compiling both files.) Thanks, Lyle

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Is there a printable copy of the (GHC) library references anywhere?
by Paul Moore 29 Dec '06

29 Dec '06

I'd like to print out a copy of the GHC manuals, for reference. I've got the Haskell 98 report, and the GHC user guide, but the only documentation I've found for the hierarchical libraries is in HTML format (generated from Haddock). Is there a printable (preferably PDF) form of the library documentation? Either all of it, or alternatively reasonably large subsets? Thanks, Paul.

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SYB and/or HList for XML, deserialization and collections
by S. Alexander Jacobson 29 Dec '06

29 Dec '06

Having just done a major refactor of the HAppS HTTP API to make it much much easier to use, I am now thinking about simplifying the current boilerplate associated with XML serialization and state deserialization. In the case of XML, Currently the developer must manually write ToElement for every type he/she wants to output. HAppS provides functions that makes doing this a lot easier if the types are Haskell records with field labels of the form: data Animal = Animal {key::Int, name:: String, breed::Breed, price::Float} Looking at the XML example from the SYB site, a shift to SYB means shifting from Record types with field labels to identifying fields by their types so e.g. data Animal = Animal Key Name Breed Price type Key = Int type Name = String type Species = Cow | Sheep type Price = Float This model seems ok, but adds some verbosity. It also entails a learning curve because either you also have field labels or you end up with non-standard extra code to access fields. In this context, HList provides good record functionality and a little template haskell makes it easy to code so we end up with e.g. $(hList_labels "Animal" "key name breed price") type Breed = Cow | Sheep This seems superior from a programming perspective because we have good record functionality (and implicitly support for XML namespaces!), but we lose something on the XML side. In particular, we end up with two new problems: 1. We no longer have type names, so we can't automatically generate an XML element name from an default HList record. The solution may be to define something like this: typeName name item = typeLabel .=. name .*. item animal = typeName "animal" myAnimal = animal .*. breed .=. Cow And since update and extension are different functions, if we hide "typeLabel" from the apps then this gets very safe (or we expose it and get a form of typesafe coerce). 2. We don't know how to deal with non-atomic field values e.g. <animal><breed>Cow></breed></animal> makes sense, but we probably don't want read/show for product types. Perhaps it is possible to use overlapping instances to do something like: instance ToElement HCons a b where ... instance ToElement a where toElement = show But perhaps we can use SYB to allow us to distinguish between atomic and non-atomic types and handle appropriately. I have to say I don't think I undersand the SYB papers well enough to give an example. Note: I can't find an actual example of generating XML from HLists in any of the HList docs so it may be that it is not actually as easy as it looks. All of this may be an open issue in theory as well as practice. == Deserialization == HAppS periodically checkpoints application state to disk. Developers may want to add or remove fields from their state types for from data types used by their state types. The current solution is to have the developer assign a version number to state. If state changes then the developer provides dispatch to a deserialization function based on that version number. It is not at all clear from either the SYB or the HList papers how to deserialize generally. That being said, the HList library provides a way to call functions with optional keyword arguments that looks like it would also generalize to schema transitions. Anyone who has some experience with this issue in the context of HList or SYB? == Haskell Collections == Currently HAppS developers use e.g. Data.Set or Data.Map as collection types for their application. If we push devlopers to use HList for eveything then they are going to need a way of handling collections of Hlist items. My sense is that HList style data structures can't be stored by Data.Map or Data.Set because they are not heterogenous collection types (and will break simply from varying the order of field names). If we use HList as the assumed record type, then I think we need to recode Data.Map and Data.Set for them. Has anyone implemented more interesting collection types for HList objects? -Alex- PS Obviously people can continue to use regular haskell types and implement stuff manually. My goal here is to support people who want more automation and are willing to learn HList or SYB in order to get it.

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Red-black trees as a nested datatype
by Jim Apple 28 Dec '06

28 Dec '06

-- Inspired by Chris Okasaki's reference to Ross Patterson's AVL -- trees as a nested datatype, here are (I think) red-black trees -- as a nested datatype. -- ref: http://www.haskell.org/pipermail/haskell/2003-April/011693.html module RedBlackTree where {- Red-black trees satisfy the following conditions, according to Wikipedia: 1. A node is either red or black. 2. The root is black. 3. All leaves are black. 4. Both children of every red node are black. 5. Every simple path from a node to a descendant leaf contains the same number of black nodes. -} data Node a n = Node n a n {- a is the carrier type: the type of the values contained in the nodes. r0 and b0 are red and black trees with one more level of black nodes than r1 and b1. -} data Tree a r0 b0 r1 b1 = Zero b1 -- The top node of a tree is black -- We recurse by adding one to the number of levels of black nodes | Succ (Tree a {- Red trees have black children and reduce the count of black nodes to the descendent leaves by 0 -} (Node a b0) {- Black trees have children of either color and reduce the count of black nodes to the descendent leaves by 1 -} (Node a (Either r1 b1)) r0 b0) -- The type for black-rooted trees with two levels of black nodes. type Black2 a = Node a (Maybe a) type RedBlackTree a = Tree a -- A red tree with two levels of black nodes is just a red node on -- top of two black nodes. (Node (Black2 a) a) (Black2 a) a () -- Jim Apple

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Re[2]: [Haskell-cafe] Dealing with incoherent instances
by Bulat Ziganshin 28 Dec '06

28 Dec '06

Hello Vyacheslav, (returning to cafe) Wednesday, December 27, 2006, 6:10:30 PM, you wrote: > Ah, so the moment something is passed through a polymorphic function > its type information is lost... This seems like a bug in the > specification/implementation, no? This is most certainly not the > desired behavior. It seems like the compiler has all the information > it needs but still can't select the right instance. sorry, but i think that it is how type class system should work in this case. look at the page for details > Are there ways to get around this problem? existential types, for example. we need exact specification of your problem. "show me the code" >> are you seen http://haskell.org/haskellwiki/OOP_vs_type_classes ? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: [Haskell] state of HaXml?
by dons＠cse.unsw.edu.au 28 Dec '06

28 Dec '06

nr: > I have some XML things to take care of, and I had hoped to use > a nice functional language with combinators. But I'm having > trouble getting HaXml to do anything useful. Here is a program > that I wrote just to read in the XML and prettyprint it. > But it fails with an error message. The 'expat' tool 'xmlwf' > claims the XML is well formed, and the XML was generated from > gpsbabel, a tool I trust to behave properly. And yet: > > : nr@curlycoat 10429 ; xmlwf 2006-12-27-backup.gpx > : nr@curlycoat 10430 ; $AWD/Main 2006-12-27-backup.gpx > Main: Parse error: unexpected EOF > > Here is Main.hs: > > module Main where > import qualified Text.XML.HaXml as X > import qualified Text.XML.HaXml.Parse as XP > import qualified Text.XML.HaXml.Pretty as XPP > import qualified IO > import qualified System > > load :: String -> IO X.Document > load fn = do handle <- IO.openFile fn IO.ReadMode > contents <- IO.hGetContents handle > IO.hClose handle > return $ XP.xmlParse fn contents > > main = do [xml] <- System.getArgs > d <- load xml > IO.putStrLn $ show $ XPP.document $ d > > > I'm using HaXml version 1.13.2-5 as distributed by Debian, > with GHC 6.6. > > Does anyone have thoughts or suggestions? Is there software I should > prefer to HaXml? (I notice that HaXml does not understand XML Schema, > which I seem to be stuck with...) You might also want to have a peek at HXT, the XML toolbox. That's been used to good effect recently, for a little RSS aggregator: http://cale.yi.org/index.php/HRSS -- Don P.S. Redirected to the -cafe

1 0

Dealing with incoherent instances
by Vyacheslav Akhmechet 27 Dec '06

27 Dec '06

I'm looking at GHC's overlapping instances docs here: http://web.mit.edu/ghc/www/users_guide/type-extensions.html#instance-decls and I've ran into the incoherent instances problem. Basically, I have a "catch all" instance that handles all types in a generic manner using SYB introspection, and then I have type specific instances that specialize behavior for certain types. This works with overlapping instances extension but whenever I take advantage of this functionality from polymorphic functions I run into the incoherent instances. If I enable incoherent instances GHC always picks the general case which seems like the wrong thing to do. What I want it to do is delay comitting to an instance until it's processing a specific invocation of a polymorphic function. All the information is available at compile time but I found no way to do this. Is there a way to get around this problem? Thanks, - Slava.

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hiii
by omit singh rathore 27 Dec '06

27 Dec '06

Send free SMS to your Friends on Mobile from your Yahoo! Messenger. Download Now! http://messenger.yahoo.com/download.php

1 0

Lambda Revolution tee shirts
by Paul Johnson 25 Dec '06

25 Dec '06

I was inspired by Don Stewart's Blog posting "The Lambda Revolution" to create a tee shirt design on this theme. Samples are available from CafePress at http://www.cafepress.com/l_revolution , but the designs are freely available. If anyone wants to send them to a more local tee-shirt printer than CafePress then let me know and I will email the SVG files. Paul.

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Getting my feet wet - small browser game
by Joachim Durchholz 25 Dec '06

25 Dec '06

OK, after years of looking and discussing and doing HSoE exercises, I have finally decided that Haskell is far enough into practical usefulness that I should give it a try in a "real" project. The basic idea is a browser game - this touches all the potentially hard issues without miring me too deeply in target platform details. I'd like to lay out my plans here and ask how they are going to work out, and take advice. THE PLAN I'll start with http://haskell.org/haskellwiki/How_to_write_a_Haskell_program and get a toolchain together. I haven't decides which compiler (interpreter?) to choose; I'll probably go for the one that give the least amount of trouble. Next would be library selection. I'm willing to undergo some modest amount of hassle here, since I don't expect all libraries that I need to be mature yet. My preliminary plan is to split the application into a central world simulation process, and satellite processes that accept HTTP requests, feed them into the simulation, read back the results, and generate the response HTML. The interface between simulation and satellite is: * Satellites can read a snapshot of the game data. * Satellites cannot directly write game data. What they can do is to post commands to a blackboard, which are marked as "no more updatable" as soon as the simulation starts executing them. I expect the simulation and the satellites to be separate OS processes, so I'll need a way to marshall command and game data between processes. The simulation will have to store its entire state to disk, too - in theory, it could run forever and never write to disk (and I don't need a database, too), but in practice, I have to plan for the occasional reboot. Since the server will be running Apache for other purposes anyway, and I don't want to force the players to use a URL with a port number, I think I'll set up Apache so that it proxies game-related URLs to the Haskell software. I just hope that Apache doesn't incur much overhead in that mode. I have no idea how to organize software upgrades. The satellites are easy, but how do I make sure that revision N+1 of the simulation can read the marshalled data from revision N? The final software should be efficient. Ideally, the satellites are able to saturate the network card of today's typical cheap rootserver if the simulation is a no-op. I have two data points for a "typical cheap rootserver": * 100 MBit/s Ethernet, 256 MB RAM, 1.2 GHz Celeron (~3 years old) * 1 GBit/s Ethernet, 1 GB RAM, 2.2 GHz Athlon (current) Of course, not needing an RDBMS will give the system a head start efficiency-wise. Comments and suggestions welcome :-) Regards, Jo

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Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Kirsten Chevalier 24 Dec '06

24 Dec '06

It's not as if this is the first time that this has been suggested, but some people have suggested that a practical book about Haskell would be a good idea. I agree. Some people have also suggested that the right moment for this hasn't arrived yet, and I see that as a challenge. I'm willing to take the lead in at least thinking about what such a book might look like. I'm potentially about to have some free time for such things, and am still young and foolish enough to think that writing a book would be a good idea. Of course, there are many good Haskell books out there already, but many of them are intended as class textbooks or are aimed at more theoretical-minded people. There's nothing wrong with that, but I think that it would be nice if a friendly, conversational, informal book about Haskell existed, since after all this is such a friendly and informal community. (If there already is a book like this, point it out, but I get the impression there's not.) There's also excellent Haskell documentation available on the web already, but people like to buy books and they like to have an artifact that they can hold in their hands without getting laser printer toner all over themselves. But if I were going to do this, I'd need all the help I could get, so if you're interested in working with me on this, email me off-list and we'll talk. Don't feel like you need to be named "Simon" for this; I don't think you need to be a Haskell guru to contribute to a book like this (I know I'm not one), though it wouldn't hurt. Being interested in good writing and explaining things to a wider audience is more important. And, the more people who are interested in working on this, the more we can all pool our various talents to create something awesome. Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "Everyone's too stupid." -- _Ghost World_

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Followup on writing "Haskell for Dummies..."
by Kirsten Chevalier 23 Dec '06

23 Dec '06

I think I've decided that this isn't the right time for me to take the lead on this book-writing project, and in addition, I've come to think that maybe the things *I'd* like to say about Haskell hacking can't be transmitted so easily in book form. However, if any others would like to organize the writing of one (or several) books based on any of the ideas brought up in the mailing list discussion, they should feel free to step forward and do so, and know that they won't be duplicating work (or at least not my work). Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "You can't learn everything you need to know legally."--John Irving

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"Secret Santas" in Haskell / better data structure?
by Paul Brown 23 Dec '06

23 Dec '06

Cafe Crew -- I posted my simple-but-slow solution to Ruby Quiz problem #2: http://mult.ifario.us/articles/2006/12/22/secret-santas-in-haskell-iii-coll… (Or http://tinyurl.com/y3l2re if your mail client messes up the URL...) It's more than practically fast enough unless you have, say 10^8 friends to do secret Santa with, but the problem just cries out for a well-chosen data structure. (One obvious improvement over the approach I posted is to sort the list first, but maybe there's something even better...) As I'm learning, critiques of syntax and usage welcome. I'll collect the source and post to the Haskell Quiz wiki as well. -- Paul

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partial functions / failure, Maybe and MonadError and good style
by Steve Downey 23 Dec '06

23 Dec '06

Although terse, the subject really says it all. If i've a partial function, like a parser, what is considered good style for a library. The tradeoffs that I can see are that Maybe is a binary operation, while Error can communicate more information in the type of the error case. Is there some way to defer the error handling Monad to the calling context?

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Cabal licence files
by Dougal Stanton 23 Dec '06

23 Dec '06

The cabal setup recognises a small set of licences which I don't think are well explained. I'm trying to put together a canonical list for setting up new projects. GPL: <http://www.gnu.org/licenses/gpl.txt> LGPL: <http://www.gnu.org/licenses/lgpl.txt> Thankfully the FSF are particularly anal^W well-organised about this kind of thing and they make it very easy to determine what each licence is and how to apply it BSD3/BSD4: There appears to be one explicitly-named BSD licence on the OSI site. There is also a closely related MIT licence. I don't know which would be which, if these are the two referred to in the licence data type. <http://www.opensource.org/licenses/bsd-license.php> <http://www.opensource.org/licenses/mit-license.html> PublicDomain: The Creative Commons people have a public domain licence, though I am uncertain whether it is intended for code. Maybe this doesn't matter when what you're saying is "I give up all rights"? <http://creativecommons.org/licenses/publicdomain/> AllRightsReserved: This is the very opposite of a PD licence and, I believe, the default licence when there is no other explicitly stated. However it would be nice to say as much rather than relying on people's legal knowledge. ;) OtherLicense: This one's easy enough... If anyone can fill in gaps and correct the errors in the above I think it would be useful. Obviously if this has been discussed elsewhere just point me off in the right direction! :) Cheers, D.

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Showing the 1 element tuple
by Neil Mitchell 23 Dec '06

23 Dec '06

Hi, A weird question, what does the 1 element tuple look like? () -- 0 element tuple (,) a b -- 2 element tuple (,,) a b c -- 3 element tuple () a - meaningless (a) - a in brackets GHC has the 1 element unboxed tuple, (# a #), and all the other sizes unboxed as well, but how would you visually represent the 1 element boxed tuple? As it happens, Yhc _does_ have the 1 element tuple, you just can't use it from normal programs, its only created by desugarings of class instances. I'd quite like to change this, and I also need to render the 1 element tuple in some way, so wondered if anyone had any good ideas (or if there is even some sort of convention) I currently use ()1, but don't like that as it doesn't follow Haskell rules - the () and 1 would be separate lexemes. My other thought is (?), where ? is something appropriate - but all the appropriate things I can think of would either not be a lexeme (i.e. 1), having an existing meaning (i.e. . | etc) or seem wrong. Thoughts? Neil

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ST and Transformers
by J. Garrett Morris 23 Dec '06

23 Dec '06

Hello everyone, I recently found myself attempting to use ST at the base of a stack of transformers, ala: type Test t r = ErrorT String (ST t) r I imagined, given the type of ST, that I would need a run function along the lines of: runTest :: (forall t. Test t r) -> Either String r (which was, in fact, exactly what I wanted). Unfortunately, even with that type signature, I can't find a way to convince the compiler that the state thread is properly universal in the call to runST. Is it possible to write this function, or is ST (and other monads that use its style of threading) not transformable? Thanks, /g -- It is myself I have never met, whose face is pasted on the underside of my mind.

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"automonadization" of code?
by Adam Megacz 23 Dec '06

23 Dec '06

Is there any work on automatic translation of code in some tiny imperative language into Haskell code that uses the ST and/or IO monads (or perhaps even pure functional code)? For example, the user writes something vaguely like array = newArray (1,100) 1 for x=2 to 99 array[x] := array[x-1]+array[x+1] And it is transformed into something like foldl (>>=) (newArray (1,100) 1) $ map (\n arr -> do a <- readArray arr (n-1) b <- readArray arr (n+1) writeArray arr n (a+b) return arr) [2..99] Obviously the "small imperative language" would have to be highly restricted and carefully chosen in order for the translation to always work and be predictable. I'm interested in any existing work on choosing such a sublanguage. Thanks! - a -- PGP/GPG: 5C9F F366 C9CF 2145 E770 B1B8 EFB1 462D A146 C380

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Re: Shrinking the Prelude: The "categorical" approach
by Imam Tashdid ul Alam 22 Dec '06

22 Dec '06

so the main issues are these: Bulat Ziganshin raised: Num is intrinsically bound to the compiler. sad. so let's leave Num out. this basically means we will avoid "abstract algebra" in general. that is, forget "groups", "rings", for now, the principle focus will be "functor", "monad"... Henning Thielemann provided links to CAL: hi Henning, would you mind if we "steal" some of the good looking names in your Prelude? Brian Hulley raised one serious issue: even if we were to define a "newbie" friendly Prelude, which should be preferred, alphanumeric names or operators? I have a thing for operators, they look good! but Brian's issue is solid, the precedence isn't documented by Haddock. any Haddock developer reading this? so mplus stays. I would still suggest plus/zero over mplus/zero but Haskell's scoping is rather liberal. I suggest the convention that Control.Monad is always imported as M. this would mean we use M.plus, M.zero, M.lift and so on. Diego Navarro said: this will confuse beginners. I disagree. I think type classes and monads are the basics of Haskell (before that, it's just commonsense ;)... ) so if the error message successfully conveys the message that there's a problem, that should be enough to inspire the beginner to start on the basics. besides, people will ultimately use the standard Prelude. what I am trying to do is to prove that with a good choice of names programs written in Haskell can look more "comprehensible" to beginners. like Gosling said about Java, the language syntax has one very good property: it's easy to read it. Garrett Morris seems to agree with me on this. Greg Fitzgerald raised the further issue of monads being overlooked by the beginners. I think "do notation considered harmful" blog was at least partially right: it's better to expose the concept rather than hiding it. so with enough people interested (I assume) it's time to move this discussion to the wiki. the wiki code is not to be taken seriously, it's a "proof-of-concept", a "guideline". cheers! Imam __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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