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beginner question re example in Hutton's "Programming in Haskell"
by David Cabana 24 Feb '07

24 Feb '07

I have been trying to work through Graham Hutton's "Programming in Haskell", but have hit something of a snag in chapter 8.4. Hutton presents some sample code which I am trying to run, with no luck so far. Here is the code as I constructed it by gathering snippets presented across three pages. <code> module Main where -- as per Hutton page 75 type Parser a = String -> [(a, String)] -- page 75 return :: a -> Parser a return v = \inp -> [(v,inp)] -- page 76 failure :: Parser a failure = \inp -> [ ] item :: Parser Char item = \inp -> case inp of [ ] -> [ ] (x:xs)-> [(x,xs)] parse :: Parser a -> String -> [(a,String)] parse p inp = p inp -- page 77 p :: Parser (Char, Char) p = do x <- item item y <- item return (x,y) </code> When I tried to load this code, I got this error message: Ambiguous occurrence `return' It could refer to either `return', defined at /Users/joe/haskell/ parse2.hs:8:0 or `return', imported from Prelude OK. My reasoning was that Hutton took the trouble to define return, so I decided to use the local definition instead of the one in the Prelude. I changed the last line from "return (x,y)" to "Main.return (x,y)". The new error message is worse: Couldn't match expected type `Char' against inferred type `[(Char, String)]' In the expression: x In the first argument of `return', namely `(x, y)' In the expression: return (x, y)</blockquote> Hutton provided explicit type signatures, so I did not expect type issues. I decided to take another approach. The book has a website that lists errata and provides code listings. The code for chapter eight is at http://www.cs.nott.ac.uk/~gmh/Parsing.lhs When I read Hutton's code, I noticed that he begins by importing Monad. The code I list above is from chapter 8, pages 75-77 of the book. Monads have not yet been mentioned. The book's index shows that monads aren't mentioned till page 113, in chapter 10. I also notice that in his code, Hutton makes repeated use of a symbol P whose meaning I do not know. What do I have to do to make this code work? I know I can use Hutton's code from the website, but I expected the code presented in the book to work, or the code on the website to restrict itself to what has been discussed in the book. Am I missing something here? Thanks, David Cabana

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Re: [Haskell-cafe] A "real" Haskell Cookbook
by P. R. Stanley 24 Feb '07

24 Feb '07

>>As a tip for anyone involved in writing and publishing scientific >>materials on the web, unless the maths is either written without any >>funny symbols or, better still, typeset in latex, it is not >>accessible to a screen-reader. > >I was under the impression that modern screen readers could pronounce >Unicode characters by looking up their name. I.e., your   would >get read as 'Non-breaking space' (perhaps a bad example, this one >wouldn't want to be read out due to its abuse as a layout tool, which >would make reading old pages very awkward). > >I don't see how images are going to be much better? I suppose <math> >images do, on MediaWiki, have an alt text which is their LaTeX, but >I'd hate to have to have that read to me. > I think latex is the perfect solution to the problem. It is > perhaps the only 100 percent accessible medium available right now. > It doesn't require any special software to read. All it needs is a > simple text editor. >Latex is a well-established tool/medium in the world-wide scientific >community and therefore its inclusion in the Haskell wikibook or any >other scientific document along with the unicode characters and >image files would be potentially beneficial to everyone. Paul >If you're interested in talking to the authors of the wikibook, >subscribe to the wikibook(a)haskell.org mailing list. > >-- >-David House, dmhouse(a)gmail.com

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Re: [Haskell-cafe] A "real" Haskell Cookbook
by P. R. Stanley 24 Feb '07

24 Feb '07

Andrew and co, I have a big fat apology to make. It was actually the wikipedia page on Lambda Calculus where there are many occurrences of special symbols. The Wikibook, as far as I've explored, is fully accessible. Nonetheless, I'd still be more than happy to make my contributions in any way I can. I am a beginner to functional programming and so I could play a useful role in ensuring the text takes nothing for granted with regard to the fundamental principles and beyond. As a tip for anyone involved in writing and publishing scientific materials on the web, unless the maths is either written without any funny symbols or, better still, typeset in latex, it is not accessible to a screen-reader. One small point about the Haskell source code is that the column indentations do not always appear in the screen reader browse buffer. Of course, there are ways of getting around the issue and if the author of the Wikibook is interested we could discuss the details off list. Cheers, Paul At 16:22 22/02/2007, you wrote: >Can you guys point out an example of this in the wiki book? I've never >heard of this accessibility issue. I may even be willing to go back >through and modify the wikibook to make it more accessible of you can >explain what you're referring to. Accessibility is important to me. > >On 2/21/07, P. R. Stanley <prstanley(a)ntlworld.com> wrote: >>and can I please ask anyone thinking of using special symbols to >>resist the temptation. >>Symbols such as the &160 used liberally in the Haskell wikibook are >>totally invisible to screen readers. >>I would be happy to proof read any document before it goes to the >>wikibook to ensure it's fully accessible to screen readers. >> >>Regards, >>Paul >> >>At 03:17 22/02/2007, you wrote: >> >I made a preliminary page, and fleshed out some of the >> >headers/sub-headers on the wiki page for a good Haskell Cookbook >> >(aka NOT a PLEAC clone). Please contribute and/or fix the examples >> >and explanations so we can make a really nice Cookbook for newbies. :) >> > >> > http://haskell.org/haskellwiki/Cookbook >> > >> >_______________________________________________ >> >Haskell-Cafe mailing list >> >Haskell-Cafe(a)haskell.org >> >http://www.haskell.org/mailman/listinfo/haskell-cafe >> >>_______________________________________________ >>Haskell-Cafe mailing list >>Haskell-Cafe(a)haskell.org >>http://www.haskell.org/mailman/listinfo/haskell-cafe

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process
by h. 24 Feb '07

24 Feb '07

Hello, I need to interact with some other program, and wrote the following code: module Main where import System.Process import System.IO main :: IO () main = do putStrLn "Running proc9..." (inp,out,err,pid) <- runInteractiveProcess "prog1" [] Nothing Nothing hSetBuffering inp LineBuffering hSetBuffering out LineBuffering hSetBuffering err LineBuffering hPutStrLn inp "1" a <- hGetLine out hPutStrLn inp a a <- hGetLine out waitForProcess pid putStrLn a But it does not work as I expected. As long as there is no need to put some input after having received some output it is no problem, but real interaction seems not possible. Is there a solution to work with the Handles as with stdin,... ? using ghc 6.6 on Windows Thanks in advance for your help.

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exists . a psuedo-standard non-empty list module
by oleg＠pobox.com 24 Feb '07

24 Feb '07

Nicolas Frisby wrote: > Some of the code from the previous wiki link, type-level decimal > numbers I would rather advice against type-level decimal numbers, if we are looking at a lightweight solution. The [complete] code at http://www.haskell.org/haskellwiki/Non-empty_list is Haskell98! It also provides for a head and tail functions -- which are total and so raise no errors. Incidentally, inserting NList into the existing Safe.List does not seem like a good match as NList critically relies on being in a separate module with a limited export. NList constitutes a security kernel, whose invariants are to be described explicitly (and hopefully, automatically proven at some point). For that reason, it is beneficial to keep module NList separate and compact. It could of course be placed within Safe.* hierarchy, etc -- as a separate module. Neil Mitchell wrote: > Fortunately the problem of pattern match errors is being tackled by at > least two projects: > > * Catch: http://www-users.cs.york.ac.uk/~ndm/projects/catch.php > * ESC-Haskell: http://www.cl.cam.ac.uk/~nx200/research/escH-hw.ps I would submit the approach of the lightweight static capabilities (cf above Wiki link) to be counted as the third project in that area. The latter has an advantage that it is available in Haskell right now and requires no extra tools.

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exists . a psuedo-standard non-empty list module
by Nicolas Frisby 24 Feb '07

24 Feb '07

I don't particularly like using fromJust or head, and there's been plenty of discussion on those issues. For the cases where it makes good sense to do so, I'm about to write a module for non-empty lists that looks to strike a balance between usability and static checks. But before I re-invent the wheel, I was wondering if there's already a popular module along those lines. I would also appreciate references to your favorite discussion about uses of head and other unsafe functions or various Oleg posts showing how they're all unnecessary. I'll find these on my own, but I would also like to know which ones strike a chord with the community. Thanks for your time. -Nick

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Map list of functions over a single argument
by Paul Moore 23 Feb '07

23 Feb '07

I'm after a function, sort of equivalent to map, but rather than mapping a function over a list of arguments, I want to map a list of functions over the same argument. The signature would be [a -> b] -> a -> [b], but hoogle didn't come up with anything. It seems like an obvious analogue of map, so I'm pretty sure I'm missing something (otherwise, I'd just write it myself, and not bother :-)) Can anyone point me in the right direction? Thanks, Paul.

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Re: [Haskell-cafe] TFP 2007: Registration and Program
by TFP 2007 23 Feb '07

23 Feb '07

>> >> Dear Colleagues, >> >> You may now resgister for TFP 2007! TFP 2007 will be held April 2-4, >> 2007 in New York City, USA. > April 2 is the first night of Passover. This is not one of those Your point is well taken. It is very unfortunate that the overlap you have pointed out exists. The organizing committee had to consider _many_ variables including, among others, the availability of a venue and of hotels for participants in NYC. Sometimes the logistics of a situation force your hand. ************************************************************************************ Dr. Marco T. Morazan TFP 2007 Program Committee Chair http://cs.shu.edu/tfp2007/

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Prime finding
by Ruben Zilibowitz 23 Feb '07

23 Feb '07

I see that there has been some discussion on the list about prime finding algorithms recently. I just wanted to contribute my own humble algorithm: primes :: [Integer] primes = primesFilter 1 [2..] primesFilter :: Integer -> [Integer] -> [Integer] primesFilter primorial (n:ns) | (gcd primorial n == 1) = n : (primesFilter (primorial*n) ns) | otherwise = primesFilter primorial ns Comparing it to some of the algorithms in: http://www.haskell.org/pipermail/haskell-cafe/2007-February/022765.html It seems to perform reasonably well. It also has the advantage of being quite short. Ruben

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HList, hOccurs and MonadReader
by Marc Weber 23 Feb '07

23 Feb '07

Having the module given below I can't see why using printAndRerun l1 printAndRerun2 l2 but not printAndRerun l1 printAndRerun l2 ? They only differ in their name. Can you point me in the right direction? ------------------------------------------------ {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-incoherent-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} module HListTest.MonadReader where import HList hiding (liftIO) import Monad import Control.Monad.Reader printAndRerun = do v <- liftM hOccursFst ask liftIO $ print (v :: Int ) v2 <- liftM hOccursFst ask return (v2 :: String) printAndRerun2 = do v <- liftM hOccursFst ask liftIO $ print (v :: Int ) v2 <- liftM hOccursFst ask return (v2 :: String) a = (2 :: Int) b = "str" l a b = HCons a (HCons b HNil) --- l1 :: ( HCons Int (HCons String HNil)) l1 = l a b l2 = l b a printBoth f l = runReaderT f l >>= print printBoth2 l = do print (hOccurs l :: Int) print (hOccurs l :: String) hlistMonadReaderTest = do print "hlistMonadReaderTest" printBoth printAndRerun l1 -- printBoth printAndRerun l2 -- < this doesn't work but the next line ? Where is the difference between printAndRerun printAndRerun2 ? printBoth printAndRerun2 l2 -- ^ printBoth2 l1 -- here is no trouble.. printBoth2 l2 main = hlistMonadReaderTest ------------------------------------------------ when not commenting the line above I'm getting this error: HListTest/MonadReader.hs|35| 26: Couldn't match expected type `Int' against inferred type `[Char]' Expected type: HCons Int (HCons [Char] HNil) Inferred type: HCons [Char] (HCons Int HNil) In the second argument of `printBoth', namely `l2' In the expression: printBoth printAndRerun l2 Thanks Marc

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functional database queries
by Henning Thielemann 23 Feb '07

23 Feb '07

At http://www.haskell.org/hawiki/HaskellDbTutorial it is described, how database queries can be modelled with a monad. However, I wonder if this is also possible without monads. Say, writing "DB.map col1 $ DB.filter (\row -> col2 row == 10+2) myTable" for "SELECT col1 FROM MyTable where col2 = 10+2"

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TFP 2007: Registration and Program
by TFP 2007 23 Feb '07

23 Feb '07

Dear Colleagues, You may now resgister for TFP 2007! TFP 2007 will be held April 2-4, 2007 in New York City, USA. Our invited speaker is John McCarthy, Stanford University. Further details can be found at our homepage: http://cs.shu.edu/tfp2007/ . You may register at: http://cs.shu.edu/tfp2007/registration.html . The registration deadline is March 2, 2007 (11:59 p.m. EST). Accomodations information may be found at: http://cs.shu.edu/tfp2007/accomodations.html . We kindly remind you that the deadline to make a hotel reservation at the guaranteed rates offered to TFP 2007 participants is also quickly approaching. We are proud to announce our program of accepted talks: Unifying Hybrid Types and Contracts Jessica Gronski and Cormac Flanagan A Dual Semantics for the Data Description Calculus Yitzhak Mandelbaum, Kathleen Fisher, and David Walker A Metalanguage for Structural Operational Semantics Matthew Lakin and Andrew Pitts An Arrow Based Semantics for Interactive Applications Peter Achten, Marko van Eekelen, Maarten de Mol, and Rinus Plasmeijer Dependent Types: Easy as Pie Dimitrios Vytiniotis and Stephanie Weirich Constructing Correct Circuits -- Hardware Modelling with Dependent Types Edwin Brady, James McKinna, and Kevin Hammond Why Would Extensible Dependent Types Matter Pablo Nogueira and Bruno Oliveira Bytecode Verification for Haskell Robert Dockins and Samuel Z. Guyer UnreadTVar: Extending Haskell Software Transactional Memory for Performance Nehir Sonmez, Cristian Perfumo, Srdjan Stipic, Adrian Cristal, Osman S. Unsal, and Mateo Valero A New Functional Implementation of Grover's Fast Search Algorithm Justin Stallard and Murray Gross An Inference Algorithm for Guaranteeing Safe Destruction Manuel Montenegro, Ricardo Peña, and Clara Segura Hierarchical Master/Worker Skeletons Jost Berthold, Mischa Dieterle, Rita Loogen, and Steffen Priebe Property Directed Generation of First-Order Test Data Fredrik Lindblad Refactoring for Comprehension Gustavo Villavicencio Towards a Box Calculus for Hume Gudmund Grov and Greg Michaelson Scaled Regression: A Refinement of Primitive Recursion Daniel Leivant Equality-Based Uniqueness Typing Edsko de Vries, Rinus Plasmeijer, and David Abrahamson Lightweight Static Resources: Sexy Types for Embedded and Systems Programming Oleg Kiselyov and Chung-chieh Shan Space-Efficient Gradual Typing David Herman, Aaron Tomb, and Cormac Flanagan Use-Based Reference of Polymorphism Dave King and John Hannan Designing a Generic Graph Library Using ML Functors Sylvain Conchon, Jean-Christophe Filliatre, and Julien Signoles The SCIence Joint Research Activity Kevin Hammond, Dana Petcu, Phil Trinder, Abdallah Al Zain, Steve Linton, and Greg Michaelson Generic and Index Programming Jeremy Gibbons, Meng Wang, and Bruno C d. S. Oliveira The AHA Project Marko van Eekelen, Olha Shkaravska, Ron van Kesteren, Bart Jacobs, Sjaak Smetsers, and Erik Poll Studying Helium Program Bahaviour with the Neon Library Jurriaan Hage and Peter van Keeken Design and Implementation of JFP Hao Xu Hop Client-Side Compilation Florian Loitsch Adaptive High-Level Scheduling in a Generic Parallel Runtime Environment Jost Berthold, Abyd Al-Zain, and Hans-Wolfgang Loidl Bundles Pack Tighter than Lists Francisco Lopez-Fraguas, Juan Rodriguez-Hortala, and Jaime Sanchez-Hernandez Model-Based Testing of Thin-Client Web Applications and Navigation Input Pieter Koopman, Peter Achten, and Rinus Plasmeijer We look forward to seeing you at TFP 2007! Cheers, Marco ************************************************************************************ Dr. Marco T. Morazan TFP 2007 Program Committee Chair http://cs.shu.edu/tfp2007/

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Haskell vs Ruby as a scripting language
by Joel Reymont 23 Feb '07

23 Feb '07

Is anyone using Haskell as a scripting language in their app? I'm thinking of viable it would be to embed ghc in a Mac (Cocoa) app. TextMate [1] uses Ruby as the extension language and quite successfully at that. Everybody loves Ruby since it's simple. I need a trading systems language and I'm afraid the combination of ... everything that makes Haskell otherwise great will prove a drawback in my case. What do you think? Thanks, Joel [1] http://macromates.com/ -- http://wagerlabs.com/

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Debugging concurrent program - no threads apparently running, but RTS still doing something.
by Alistair Bayley 22 Feb '07

22 Feb '07

Below is a test case for a threading problem I can't figure out. It models a socket server (here I've replaced the socket with an MVar, to keep it simple). The idea is to have a listener which accepts incoming requests on the socket. When one arrives, it forks a handler thread to deal with the request, and returns to listening on the socket. The handler thread is run in parallel with a timeout thread. In the test case below, the handler takes too long, so the timeout thread completes and kills the handler. The problem is that when the main thread ends, the RTS doesn't stop for another 6 or so seconds. The only thread that runs this long is the handler (waitFor (secs 8.0)) but it has already been killed. So I'm scratching my head a bit. Also, any pointers to better techniques for designing and debugging concurrent code are appreciated (is there a better way than putStrLn?) Platform: ghc-6.6, Windows XP. Thanks, Alistair --------------------------------------------------------------------- module Main where import Prelude hiding (catch) import Control.Concurrent import Control.Exception import System.IO secs :: Float -> Int secs n = round (n * 1000000) -- e.g. waitFor (secs 25) waitFor n = debug ("waitFor start " ++ show ((fromIntegral n) / 1000000.0)) >> threadDelay n >> debug ("waitFor end " ++ show ((fromIntegral n) / 1000000.0)) debug msg = myThreadId >>= ( \t -> putStrLn $ (show t) ++ ": " ++ msg ) -- Thread 1 - start server then send message main = do hSetBuffering stdout LineBuffering blockingInput <- newEmptyMVar -- like a socket; accept on a socket blocks listnr <- forkIO (listener blockingInput) putMVar blockingInput "hello" waitFor (secs 2.0) debug "main: done" -- Thread 2 - listener. -- Blocks on input; when input arrives, starts handler thread. listener blockingInput = do msg <- takeMVar blockingInput serveRequest msg -- in a real server we'd loop: --listener blockingInput debug "listener: done" -- Start threads 3 and 4 - handler and timeout. serveRequest msg = do let acquire = return () let release _ = debug "handler release action" forkIO (runTimeout 0.1 acquire release (handler msg)) return () handler msg _ = do debug ("handler start: " ++ msg) catch (waitFor (secs 8.0) >> debug "handler end") (\e -> debug ("Exception: " ++ (show e)) >> throwIO e) runTimeout timeout acquire release action = do handlerTid <- myThreadId -- Use this to ensure the timeout thread doesn't start before the handler -- i.e. give the handler a chance to start. startTimeout <- newEmptyMVar timeoutTid <- forkIO (runTimeoutThread startTimeout timeout handlerTid) bracket acquire ( \a -> release a >> killThread timeoutTid ) ( \a -> do debug "runTimeout: start handler action" putMVar startTimeout True action a debug "runTimeout: end handler action" ) runTimeoutThread startTimeout timeout handlerTid = do takeMVar startTimeout debug "runTimeoutThread: start" waitFor (secs timeout) killThread handlerTid debug "runTimeoutThread: end"

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Re: Leaves of a Tree
by Chad Scherrer 22 Feb '07

22 Feb '07

Neil, I think this idea is better than what I had suggested, but as it stands it doesn't typecheck. Did you mean something like this? leaves :: Tree -> [Int] leaves = f [] where f rest (Leaf n) = n : rest f rest (Branch l r) = f (f rest r) l -Chad ------------------------------- (from Neil Mitchell) Hi Tom > data Tree = Branch Tree Tree | Leaf Int deriving (Eq, Ord) > leaves :: Tree -> Set Int > leaves (Leaf n) = singleton n > leaves (Branch left right) = union (leaves left) (leaves right) The standard method for a traversal over leaves with accumulation is: leaves :: Tree -> Set Int leaves x = f [] where f (Leaf n) rest = n : rest f (Branch l r) rest = f l (f r rest) This makes the construction of the list quite cheap. Then you can do the fromList trick, and that might give you a speed up. Thanks Neil

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Leaves of a Tree
by Tom Hawkins 22 Feb '07

22 Feb '07

Hello, Any recommendations for speeding up extracting the set of leaves from a tree? data Tree = Branch Tree Tree | Leaf Int deriving (Eq, Ord) My slow, naive function: leaves :: Tree -> Set Int leaves (Leaf n) = singleton n leaves (Branch left right) = union (leaves left) (leaves right) In my case, many of the branches in the tree are the same. I suspect the fixed point operation mentioned in the thread "speeding up fibonacci with memoizing" is applicable, but I'm having a tough time making the connection. -Tom

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Selecting overlapping instances
by Emil Axelsson 22 Feb '07

22 Feb '07

Hello! I have a problem with overlapping instances for which I already know a workaround. However, I'm still curious to know if there isn't a simpler solution available. I have a feeling that -fallow-incoherent-instances might be the answer, but I can't get it to work. In the code at the end of the message, (Block w n s e) is a representation of a geometrical block whose west-/north-/south-/eastern edge has type w/n/s/e respectively. (*||*) composes two blocks horizontally and (*=*) composes vertically. In this simplified example, there is only one primitive block -- unitBlock. Now, I want to be able to make grids of unitBlock:s where the edges can have any type that can be obtained using (,) and (). This is what the Grid class attempts. The program compiles fine until I add the test program > test :: Block ((),()) ((),()) ((),()) ((),()) > test = grid which raises an "Overlapping instances" error. What I would like is for the compiler to pick *either* of the two matching instances first, and then the other one, because in this particular case, it doesn't matter which one it picks first. The workaround is to explicitly pick an order by giving the additional instance (requires -fallow-overlapping-instances): > instance ( Grid x1 y1 > , Grid x1 y2 > , Grid x2 y1 > , Grid x2 y2 > ) => Grid (x1,x2) (y1,y2) > where > grid = grid *||* grid Is there any way to make this work without adding this last instance? Thanks! / Emil -------------------------------------------------------- > data Block w n s e = Block > > (*||*) :: Block w1 n1 s1 x -> Block x n2 s2 e2 -> Block w1 (n1,n2) (s1,s2) e2 > (*||*) = undefined > > (*=*) :: Block w1 x s1 e1 -> Block w2 n2 x e2 -> Block (w1,w2) n2 s1 (e1,e2) > (*=*) = undefined > > unitBlock :: Block () () () () > unitBlock = undefined > > class Grid x y > where > grid :: Block x y y x > > instance Grid () () > where > grid = unitBlock > > instance (Grid x1 y, Grid x2 y) => Grid (x1,x2) y > where > grid = grid *=* grid > > instance (Grid x y1, Grid x y2) => Grid x (y1,y2) > where > grid = grid *||* grid

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Re: Leaves of a Tree
by Chad Scherrer 22 Feb '07

22 Feb '07

Hi Tom, Tom Hawkins wrote: > Folding was my first approach: > > leaves :: Tree -> Set Int > leaves tree = accumLeaves Set.empty tree > > accumLeaves :: Set Int -> Tree -> Set Int > accumLeaves set (Leaf n) = insert n set > accumLeaves set (Branch l r) = foldl accumLeaves set [l,r] > > However, with this approach I quickly ran out of stack space. I found > this odd, since I thought this program was tail recursive and > shouldn't be using the stack. This is a problem of tail recursion and lazy evaluation not playing nicely. See this page: http://www.haskell.org/haskellwiki/Stack_overflow > My next attempt was to use some form of memorization. Since many of > the branches in my trees are equal, memorization should prevent > following branches that have already been calculated. My question is, > what is the best way to transform the original function to incorporate > memorization? I think something like this could be done, if there's some invariants maintained by the data structure. Is there any additional structure you're imposing beyond that required for the "data" line? -Chad

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Type-level lambdas in Haskell? ( was Multiparameter class error)
by oleg＠pobox.com 22 Feb '07

22 Feb '07

Alfonso Acosta wrote: > class Synchronous s f1 f2 | s -> f1, s -> f2 where > mapSY :: f1 a b -> s a -> s b > delaySY :: a -> s a -> s a > zipWithSY :: f2 a b c-> s a -> s b -> s c > > The goal of this class is to extend the name of the following > functions (which BTW are already present in a working library and for > that reason _it is a must_ that their types remain untouched) ... > > mapSY :: (a->b) -> Signal a -> Signal b > delaySY :: a -> Signal a -> Signal b -> Signal c > zipWithSY :: (a->b->c) -> Signal a -> Signal b -> Signal c > > .. accepting these definitions as well > > mapSY :: (HDPrimType a, HDPrimType b) => > HDFun (a->b) -> HDSignal a -> HDSignal b > delaySY :: HDPrimType a => a -> HDSignal a -> HDSignal a > zipWithSY :: (HDPrimType a, HDPrimType b, HDPrimType c) => HDFun > (a->b->c) -> HDSignal a -> HDSignal b -> HDSignal c First of all, the design already exhibits the problem: delaySY :: HDPrimType a => a -> HDSignal a -> HDSignal a cannot be made _at all_ the member of an instantiated Synchronous class. The reason is that in the class definition > class Synchronous s f1 f2 | s -> f1, s -> f2 where > delaySY :: a -> s a -> s a the function delaySY is declared *fully* polymorphic over 'a' -- there are no constraints on a and no restrictions. However, > delaySY :: HDPrimType a => a -> HDSignal a -> HDSignal a shows that 'a' is constrained to satisfy the HDPrimType a. That's the problem: the latter function is not generic enough. The problem is described (and solved) in a message `Restricted Datatypes Now' http://www.haskell.org/pipermail/haskell-prime/2006-February/000498.html I'm not certain if there is a compelling reason to place mapSY, delaySY and zipWithSY in the same class. If not, the following is the solution to the problem. Both sets of mapSY, delaySY and zipWithSY are unified in overloaded functions: {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module SY where class SynchronousM arg1 s a b | s a b -> arg1 where mapSY :: arg1 -> s a -> s b class SynchronousD s a where delaySY :: a -> s a -> s a class SynchronousZ arg1 s a b c | s a b c -> arg1 where zipWithSY :: arg1 -> s a -> s b -> s c -- stubs newtype Signal a = Signal a newtype HDSignal a = HDSignal a newtype HDFun a = HDFun a class HDPrimType a where cnv :: a -> a; cnv = id instance HDPrimType Int instance HDPrimType Bool -- (not so) Grand Unification instance SynchronousM (a->b) Signal a b where mapSY f (Signal x) = Signal (f x) instance (HDPrimType a, HDPrimType b) => SynchronousM (HDFun (a->b)) HDSignal a b where mapSY (HDFun f) (HDSignal x) = HDSignal (cnv . f . cnv $ x) instance SynchronousD Signal a where delaySY _ = id instance HDPrimType a => SynchronousD HDSignal a where delaySY _ (HDSignal x) = HDSignal (cnv x) instance SynchronousZ (a->b->c) Signal a b c where zipWithSY f (Signal x) (Signal y) = Signal (f x y) instance (HDPrimType a, HDPrimType b, HDPrimType c) => SynchronousZ (HDFun (a->b->c)) HDSignal a b c where zipWithSY (HDFun f) (HDSignal x) (HDSignal y) = HDSignal (cnv (f (cnv x) (cnv y)))

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Is anyone using CUDA with haskell yet?
by Jefferson Heard 22 Feb '07

22 Feb '07

I don't want to duplicate anyone's work, and I'm not sure that NDA would allow me to release the code in any case (have to check on it carefully), but is anyone currently using the CUDA framework from nVidia inside of Haskell for highly parallel programming? -- Jeff

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Re: Leaves of a Tree
by Chad Scherrer 22 Feb '07

22 Feb '07

Tom, I think inserting elements would be a lot faster than multiple unions. I would try: leafList :: Tree -> [Int] leafList (Leaf n) = [n] leafList (Branch left right) = leafList left ++ leafList right leaves = fromList . leafList If you're writing many functions on Trees (or maybe even if you're not), you might consider writing a fold function and putting leafList in terms of this. Do you have experience with folds? -Chad > Hello, > > Any recommendations for speeding up extracting the set of leaves from a tree? > > data Tree = Branch Tree Tree | Leaf Int deriving (Eq, Ord) > > My slow, naive function: > > leaves :: Tree -> Set Int > leaves (Leaf n) = singleton n > leaves (Branch left right) = union (leaves left) (leaves right) > > In my case, many of the branches in the tree are the same. I suspect > the fixed point operation mentioned in the thread "speeding up > fibonacci with memoizing" is applicable, but I'm having a tough time > making the connection. > > -Tom

2 1

Type-level lambdas in Haskell?
by oleg＠pobox.com 22 Feb '07

22 Feb '07

On 2/21/07, Alfonso Acosta <alfonso.acosta at gmail.com> wrote: > In my opinion adding Type-level lambdas would be the way to go, but > they unfortunately are not part of Haskell. Type-level lambdas are already present in Haskell. Please see the messages On computable types. I. Typed lambda and type closures http://www.haskell.org/pipermail/haskell/2006-September/018486.html On computable types. II. Flipping the arrow http://www.haskell.org/pipermail/haskell/2006-September/018487.html

1 0

Type-level lambdas in Haskell? ( was Multiparameter class error)
by Alfonso Acosta 22 Feb '07

22 Feb '07

Now I'm facing another problem, sorry if it takes too long to reach the Type level lambdas issue ... The full definition of my class is class Synchronous s f1 f2 | s -> f1, s -> f2 where mapSY :: f1 a b -> s a -> s b delaySY :: a -> s a -> s a zipWithSY :: f2 a b c-> s a -> s b -> s c The goal of this class is to extend the name of the following functions (which BTW are already present in a working library and for that reason _it is a must_ that their types remain untouched) ... mapSY :: (a->b) -> Signal a -> Signal b delaySY :: a -> Signal a -> Signal b -> Signal c zipWithSY :: (a->b->c) -> Signal a -> Signal b -> Signal c .. accepting these definitions as well mapSY :: (HDPrimType a, HDPrimType b) => HDFun (a->b) -> HDSignal a -> Signal b delaySY :: HDPrimType a => a -> HDSignal a -> HDSignal a zipWithSY :: (HDPrimType a, HDPrimType b, HDPrimType c) => HDFun (a->b->c) -> HDSignal a -> HDSignal b -> HDSignal c The problem is: How to choose the f1 and f2 parameters when defining the instances? instance Synchronous Signal (->) ?? where instance Synchronous HDSignal ?? ?? where I'm facing one of the problems discussed at Type Classes with Functional Dependencies (http://web.cecs.pdx.edu/~mpj/pubs/fundeps-esop2000.pdf )section 3.1: "Some of the remaining instances can be reworked to ﬁt the constructor class framework by introducing dummy type and value constructors" The following is a solution following the one proposed by MJones in his paper, but is not acceptable as it requires changing the types of the original functions. newtype F2 a b c = F2 (a->b->c) newtype HDF1 a b = HDF1 (HDFun (a->b)) newtype HDF2 a b c = HDF2 (HDFun (a->b->c)) instance Synchronous Signal (->) F2 where .... instance Synchronous HDSignal HDF1 HDF2 where ... Following MJones advice, redefining the class to something such us ... class Synchronous2 a sa sb sc f1ab f2abc | {- dependencies ommited -} where mapSY :: f1ab -> sa -> sb delaySY :: a -> sa -> sa zipWithSY :: f2abc-> sa -> sb -> sc ... is feasible but not elegant at all because the number of class parameters and dependencies are dramatically increased. In my opinion adding Type-level lambdas would be the way to go, but they unfortunately are not part of Haskell. Something like this would be much more expressive and useful. Using the first definition of the class we could do something as instance Synchronous Signal (->) (\a b c -> (a->b->c)) where .... instance Synchronous HDSignal (\a b -> HDFun (a->b)) (\a b c -> HDFun (a->b->c)) where ... Is there any extension to the language covering type-level lambdas or even a plan to include them in next revision? Thanks, Fons On 2/21/07, Alfonso Acosta <alfonso.acosta(a)gmail.com> wrote: > Thanks, the functional dependency solved the problem > > On 2/21/07, Yitzchak Gale <gale(a)sefer.org> wrote: > > Hi Alfonso, > > > > You wrote: > > > Could not deduce (Synchronous s f11) > > > from the context (Synchronous s f1) > > > \begin{code} > > > class Synchronous s f1 where > > > mapSY :: f1 a b -> s a -> s b > > > delaySY :: a -> s a -> s a > > > sourceSY :: f1 a a -> a -> s a > > > sourceSY f s0 = o > > > where > > > o = delaySY s0 s > > > s = mapSY f o > > > \end{code} > > > > > > Can anyone explain a bit further than GHC what am I doing wrong? > > > > Every method of a multiparameter class must refer > > to every parameter in its signature. Otherwise, there > > is no way for the compiler to know which instance > > of the class you want when you use the method. > > > > There are two ways to get around this restriction. > > One is to use a functional dependency: > > > > class Synchronous s f1 | s -> f1 where > > > > That promises that for each type s, you will > > only define an instance Synchronous s f1 > > for at most a single type f1. Now, whenever > > you mention s in a type signature, it is as if > > you also mentioned f1. > > > > If you can't keep that promise, then you will > > have to use a phantom parameter. Change > > the type of delaySY to > > > > delaySY :: f1 -> a -> s a -> s a > > > > and ignore the f1 parameter when you implement > > delaySY: > > > > delaySY _ x y = ... > > > > Then, when you use delaySY, you specify the > > type f1 by writing: > > > > delaySY (undefined :: T) ... > > > > Regards, > > Yitz > > >

2 1

Saving the AST generated by Template Haskell
by Alfonso Acosta 22 Feb '07

22 Feb '07

Hi all, I'm using Template Haskell to design a small subset of a Hardware Description DSEL (Domain Specific Embedded Language). My language supports higher order as the user can supply small functions as arguments. I chose to parse them with TH because it allows me to use plain Haskell for the function syntax (instead of reinventing the wheel) but mostly because gives parsing for free. The AST of the functions must be kept by the embedded compiler so that it can be later translated to a target language by one of the potential backends of the embedded compiler. The problem is ... how to store that AST? Let me show an example ---- import Language.Haskell.TH.Syntax -- sample function from the DSLE library hdMapSY :: (HDPrimType a, HDPrimType b) => HDFun (a -> b) -> HDSignal a -> HDSignal b -- We keep the function's AST (to make program transformations in the backends) newtype HDPrimFun = HDPrimFun [Dec] deriving Show -- Type safety layer, we keep the function to make sure TH checks the type (and possible further simulations) data HDFun a = HDFun [Dec] a deriving Show -- Helper constructor function, which suffers from the Saving-the-AST problem -- mkMetaAST currently returns a phony value mkHDFun :: Q [Dec] -> Q Exp mkHDFun qd = do dx <- qd metaASTnm <- newName "metaAST" let funnm = getFunName dx metaAST = mkMetaAST dx metaASTdec = ValD (VarP metaASTnm) (NormalB metaAST) [] return $ LetE (metaASTdec:dx) (AppE (AppE (ConE $ mkName "HDFun") (AppE (ConE $ mkName "HDPrimFun") (VarE metaASTnm))) (VarE funnm)) where getFunName :: [Dec] -> Name getFunName [FunD nm _] = nm getFunName _ = error "mkHDFun: toy example, just and exactly one dec!" -- This function should create an AST expression from the AST -- but it would be a big pain to code mkMetaAST :: [Dec] -> Exp mkMetaAST _ = AppE (ConE (mkName "LitE")) (LitE (StringL "big pain to code!")) --- An example program coded in the DSLE could could be something like .. myCircuit :: HDSignal Int -> HDSignal Int myCircuit = hdMapSY ($mkHDFun [d| f input = input+1 |]) So the question is .. How can mkHDfun save the AST of "f input = input+1" (for which it needs to create and return a metaAST) without the effort of having to create boiler plate code for the whole TH library types? Did anyone do something similar before? I workaround would be saving the String of the AST with show, but Dec nor Exp belong to the Read class, :S Thanks in advance, Alfonso Acosta

2 3

Fwd: [Haskell-cafe] Newbie: generating a truth table
by Peter Berry 22 Feb '07

22 Feb '07

Sigh, I seem to have done a reply to sender. Reposting to the list. On 06/02/07, phiroc(a)free.fr <phiroc(a)free.fr> wrote: > Hello, > > I would like to create a Haskell function that generates a truth table, for > all Boolean values, say, using the following "and" function : > > and :: Bool -> Bool -> Bool > and a b = a && b A fairly old thread, but I had an interesting idea: > combos :: (Enum a, Enum b) => a -> b -> (a -> b -> c) -> [(a, b, c)] > combos min1 min2 op = [(x, y, x `op` y) | x <- [min1..], > y <- [min2..]] Then: *Main> combos False (&&) [(False,False,False),(False,True,False),(True,False,False),(True,True,True)] In the case of Bool and a few others, you can use a slightly nicer one: > bCombos :: (Enum a, Bounded a, Enum b, Bounded b) => > (a -> b -> c) -> [(a, b, c)] > bCombos op = [(x, y, x `op` y) | x <- [minBound..maxBound], > y <- [minBound..maxBound]] And: *Main> bCombos (&&) [(False,False,False),(False,True,False),(True,False,False),(True,True,True)] The secret of these is of course in the Enum and Bounded type classes, which define, respectively, * enumFrom and enumFromTo (which have syntactic sugar in [foo..] and [foo..bar] respectively), and * minBound and maxBound. You can do the same with any instance of these classes. -- Peter Berry <pwberry(a)gmail.com> Please avoid sending me Word or PowerPoint attachments. See http://www.gnu.org/philosophy/no-word-attachments.html

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Multiparameter class error
by Alfonso Acosta 21 Feb '07

21 Feb '07

Hi, I'm a newbie to multiparameter classes and I'm getting this error from GHC when compiling the following class definition: Could not deduce (Synchronous s f11) from the context (Synchronous s f1) arising from use of `delaySY' Possible fix: add (Synchronous s f11) to the class or instance method `sourceSY' In the expression: delaySY s0 s In the definition of `o': o = delaySY s0 s In the definition of `sourceSY': sourceSY f s0 = o where o = delaySY s0 s s = mapSY f \begin{code} class Synchronous s f1 where mapSY :: f1 a b -> s a -> s b delaySY :: a -> s a -> s a sourceSY :: f1 a a -> a -> s a sourceSY f s0 = o where o = delaySY s0 s s = mapSY f o \end{code} Can anyone explain a bit further than GHC what am I doing wrong? Thanks, Alfonso Acosta

2 2

Implementation of scaled integers
by Stefan Heinzmann 21 Feb '07

21 Feb '07

Hi all, is there a library for Haskell that implements scaled integers, i.e. integers with a fixed scale factor so that the scale factor does not need to be stored, but is part of the type? In particular it would be useful (i.e. for signal processing) to have numbers based on Int scaled such that they fall into the range [-1.0 .. 1.0). Or other scale factors which are powers of 2. Addition and subtraction would then map to the ordinary operations for Int, while Multiplication and Division would have to apply the scale factor to correct the result of normal Int operations (which would be a shift operation). If it doesn't exist yet, have you got ideas how to implement that for maximum efficiency? Cheers Stefan

6 7

nobench: now with hbc support (!)
by dons＠cse.unsw.edu.au 21 Feb '07

21 Feb '07

Just a quick note. I've tweaked the benchmarks some more, adding support for Lennart's hbc compiler. (Go hbc!). Also, we have nice html output (thanks to Text.XHtml!). http://www.cse.unsw.edu.au/~dons/nobench/results.html Pretty :-) Remaining tasks left are to port the rest of the 'real' category from nofib, and implement output diffing (note the nhc winning entries, due to erroneous output), and add nhc's bytestring support. The particular errors are logged here, http://www.cse.unsw.edu.au/~dons/nobench/bench.results -- Don

1 0

Call for testers: using Cabal to build RPMs
by Bryan O'Sullivan 21 Feb '07

21 Feb '07

I've got a branch of Cabal that adds a new command, "rpm", that lets you build an RPM package with a single invocation: runhaskell Setup.*hs rpm I've tested this pretty extensively with GHC 6.6, and I'm quite happy with it, but if there are other users of RPM-based distros out there, I'd appreciate it if you could kick the tyres a bit. Once it's in solid shape, I hope to plug this capability into Hackage so that there will automatically be fresh RPM packages available for Linux distributions such as Fedora. See the brief writeup here: http://www.serpentine.com/blog/2007/02/20/haskell-cabal-now-with-extra-crun… Cheers, <b

1 0

Haskell School of Expression
by Andrew Wagner 21 Feb '07

21 Feb '07

Has anyone worked through HSOE lately? I'm wondering if I'm going to be able to work through the examples that use their graphic library, under gtk2hs on fedora. Is there a workaround for this?

2 1

Re: Genuine Eratosthenes sieve [Was: Optimization fun]
by Melissa O'Neill 20 Feb '07

20 Feb '07

Sorry, I'm a little late to this thread, but the topic of > sieve [] = [] > sieve (x:xs) = x : sieve [y | y <- xs, y `mod` x /= 0] (as posted by Rafael Almeida) is somewhat dear to my heart, as I wrote a paper about it last summer and sent it in to JFP. Still waiting for a reply though. Let's go back to the beginning with the classic complaint and the excuses... Creighton Hogg wrote: > So a friend and I were thinking about making code faster in > Haskell, and I was wondering if there was a way to improve the > following method of generating the list of all prime numbers. It > takes about 13 seconds to run, meanwhile my friend's C version took > 0.1. Here come the excuses, like this one from apfelmus, > While Haskell makes it possible to express very complicated > algorithms in simple and elegant ways, you have to expect to pay a > constant factor (roughly 2x-10x) when competing against the same > algorithm in low-level C. and this one from Nicolas Frisby, > I have yet to need my Haskell to perform well Matthew Brecknell came up with something much faster, namely > primes :: [Int] > primes = 2 : filter isPrime [3,5..] where > f x p r = x < p*p || mod x p /= 0 && r > isPrime x = foldr (f x) True primes FWIW, another way to write this code (without needing to think about how "fold bails early") is primes = 2: [x | x <−[3,5..], all (\p −> x `mod` p > 0) (factorsToTry x)] where factorsToTry x = takeWhile (\p −> p*p <= x) primes Both of these algorithms best the "sieve" we began with and run quickly, but as you can see (possibly more clearly from my rephrasing), this algorithm is not actually the Sieve of Eratosthenes -- it's actually a classic naive primes algorithm which checks a number for primality by trying to divide it by every prime up to its square root. But that's okay, because our initial algorithm ISN'T THE REAL SIEVE EITHER. Markus Fischmann hits the nail on the head when he says > The characteristics of a sieve is, that it uses the already found > primes to generate a list of non-primes that is then removed from a > list of candiates for primeness. But then we get distracted by a discussion about avoiding division. It's true that the real sieve avoids division, but it is NOT true that every algorithm that avoids division is the sieve. The thread ends with this algorithm from Yitzchak Gale: > -- Delete the elements of the first list from the second list, > -- where both lists are assumed sorted and without repetition. > deleteOrd :: Ord a => [a] -> [a] -> [a] > deleteOrd xs@(x:xs') ys@(y:ys') > | x > y = y : deleteOrd xs ys' > | x < y = deleteOrd xs' ys > | otherwise = deleteOrd xs' ys' > deleteOrd _ ys = ys > > sieve (x:xs) = x : sieve (deleteOrd [x+x,x+x+x..] xs) > sieve _ = [] > > primes = sieve [2..] Which seems reasonable, until you realize that every prime p we come up with is still "considered" by k deleteOrd filters, where k is the number of primes that preceeded it. So, let's recap: the original algorithm is beautiful and simple, but it is NOT the actual Sieve of Eratosthenes, NOT because it uses modulus, but because fundamentally, at the highest level, it is a different algorithm. At the risk of beating a dead horse, let's see why it's not the real sieve. What makes the sieve an efficient algorithm are the details of *what* gets "crossed off", *when*, and *how many times*. Suppose that we are finding the first 100 primes (i.e., 2 through 541), and have just discovered that 17 is prime. We will begin crossing off at 289 (i.e., 17 * 17) and cross off the multiples 289, 306, 323, ... , 510, 527, making fifteen crossings off in total. Notice that we cross off 306 (17 * 18), even though it is a multiple of both 2 and 3 and has thus already been crossed off twice. (The starting point of 17 * 17 is a pleasing, but actually *minor*, optimization for the *genuine* sieve.) The algorithm is efficient because each composite number, c, gets crossed off f times, where f is the number of unique factors of c less than sqrt(c). The average value for f increases slowly, being less than 3 for the first 10^12 composites, and less than 4 for the first 10^34. None of the algorithms we've discussed correspond to the above algorithm. It is not merely that they don't perform "optimizations", such as starting at the square of the prime, or that some of then use a divisibility check rather than using a simple increment. Rather, at a fundamental level, they all work differently than the real sieve. Following our earlier example, after finding that 17 is prime, the "phony" algorithm will check to see if 19 is divisible by 17 (in the case of Yitzchak's algorithm, divisibility is checked by comparing against 17*2), followed by 23, 29, 31, ... , 523, 527, checking a total of ninety-nine numbers for divisibility by 17. In fact, even if it did (somehow) begin at 289, it would examine all forty-five numbers that are not multiples of the primes prior to 17 (289, 293, 307, ... , 523, 527). It is also worth realizing that the set of numbers examined by the phony algorithm is not a superset of the ones examined by the real sieve (as we saw, the real algorithm crossed off 306, which the phony algorithm skips). In general, the speed of the phony algorithm depends on the number of primes that are *not* factors of each composite, whereas the speed of the real algorithm depends on the number of (unique) primes that *are*. Hopefully at this point I've piqued your interest. Maybe you're wondering what our original sieve algorithm is, if it isn't the real sieve and the mod issue is just a red herring...? Maybe you're wondering what the the real sieve looks like, coded in Haskell. Or maybe you're still not convinced about any of it. If so, I invite you to read a draft of my JFP paper, available at: http://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf Best Regards, Melissa.

5 7

return?
by Vikrant 20 Feb '07

20 Feb '07

Hi, I am writing a recursive function which does some IO operation. I encounter situation in which my function has to end recursion by doing "nothing" and otherwise keep calling same function with some different parameters. I did not find anything equivalent to "pass" or "return" statement in Haskell. Presently I have got a workaround. I am doing it by putting a dummy print as described below. can anybody help me in this? e.g f some_params = if some_condition then do putStr "Hacky way to do nothing!" else do perform_some_IO f some_other_params == Vikrant

3 2

trouble installing ghc 6.6: xargs: /usr/bin/ar: terminated by signal 11
by tphyahoo 20 Feb '07

20 Feb '07

I'm having trouble installing ghc 6.6. On ubuntu, virtual server (user mode linux). Something seems to be killing the process, no idea why. Anyone seen this? I looked around with ps to see if anything looked suspicious. I do have a lot of screens open, but I don't see why that should matter. There is no instance of haskell or gnci running. after running configure with no problem..... ~$ cd ghc-6.6/ ~/ghc-6.6$ sudo make &>makeOut Password: ~/ghc-6.6$ tail -n8 makeOut (echo Control/Concurrent_stub.o System/CPUTime_hsc.o System/Time_hsc.o ; /usr/bin/find Control/Applicative_split Control/Arrow_split Control/Concurrent_split Control/Concurrent/Chan_split ...long mess... Text/PrettyPrint/HughesPJ_split Text/Printf_split Text/Read_split Text/Read/Lex_split Text/Show_split Text/Show/Functions_split -name '*.o' -print) | xargs /usr/bin/ar q libHSbase.a /usr/bin/ar: creating libHSbase.a xargs: /usr/bin/ar: terminated by signal 11 make[2]: *** [libHSbase.a] Error 125 make[2]: *** Deleting file `libHSbase.a' make[1]: *** [all] Error 1 make[1]: Leaving directory `/home/thartman/ghc-6.6/libraries' make: *** [stage1] Error 2 ~/ghc-6.6$ -- View this message in context: http://www.nabble.com/trouble-installing-ghc-6.6%3A-xargs%3A--usr-bin-ar%3A… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANNOUNCE: nobench: Haskell implementaion benchmarks. GHC v Hugs v Yhc v NHC v ...
by dons＠cse.unsw.edu.au 20 Feb '07

20 Feb '07

Following recent discussion about a cross-implementation performance benchmark suite, based on nofib, I've gone and combined nofib with the great language shootout programs, and rewritten the build system to support cross implementation measurements. The result is: nobench http://www.cse.unsw.edu.au/~dons/nobench.html The benchmark suite runs regularly, and currently reports the speed of each program in the suite, running under each system. The results are quite interesting. The most recent run is available: http://www.cse.unsw.edu.au/~dons/nobench/bench.results http://www.cse.unsw.edu.au/~dons/nobench/bench.log The programs are a mixture of traditional nofib style Haskell, with more performance-tuned code from the shootout. More tweaking is required to help better support nhc and yhc (and jhc, and ...). The entire benchmark set and framework is available via darcs: darcs get http://www.cse.unsw.edu.au/~dons/code/nobench Currently todo are porting the rest of nofib, pretty graphs of the results (and html), and memory use measurements. Patches welcome! Cheers, Don

9 12

our worst unsafePerformIO nightmares are upon us!
by Nicolas Frisby 20 Feb '07

20 Feb '07

http://www.thinkgeek.com/geektoys/cubegoodies/86b8/ Now you can really show your coders why unsafePerformIO is to be avoided! -Nick ps - Please don't consider this post a product advertisement or endorsement of any kind.

2 1

Is lazyness make big difference?
by Nick 20 Feb '07

20 Feb '07

Hi all, (Another topic stolen from a Russian forum discussion). As everyone know, there are lot of strict languages, that have possibilities to "switch on" lazy evaluation when needed. But the examples that I saw on Haskell, there was not much use of lazy evaluation, often there were just several lazy points, and the rest could be done strictly without loss of generality. For example, in finding primes: main = print primes primes = 2:filter is_prime [3,5..] is_prime n = all (\p-> n `mod` p /= 0) (takeWhile (\p-> p*p<=n) primes) We can rewrite this in strict languages with lazy constructs. For example, in Scala (of course stream is not only lazily evaluated thing there) def main(args: Array[String]): Unit = { val n = Integer.parseInt(args(0)) System.out.println(primes(ints(2)) take n toList) } def primes(nums: Stream[Int]): Stream[Int] = Stream.cons(nums.head, primes ((nums tail) filter (x => x % nums.head != 0)) ) def ints(n: Int): Stream[Int] = Stream.cons(n, ints(n+1)) I think the Haskell solution is more compact due to syntactic sugar, curring and "parentheses-free-ness", *not* lazy evaluation. According to one guy's analogy: the Real World is strict - in order to drink tea, you have to put the cattle on the fire, wait until water boils, brew tea and then drink. Not the cattle is put on the fire, water boils and the tea is brewed when you take the empty cup to start drinking. :-) The question is the following: how big the gap between strict languages with lazy constructs and Haskell? Does the default lazyness have irrefutable advantage over default strictness? Thank you for the attention. With best regards, Nick.

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ANNOUNCE: Derangement version 0.1.0
by dgriffi3＠uiuc.edu 20 Feb '07

20 Feb '07

This is the inital version of a very small library, mostly using the functional graph library, to find a derangement of a set, or rather a list. A derangement of a set is a permutation with no fixed points, like many constrained matching problems it is susceptible to solution via a Max-flow algorithm. The source and its darcs repository is available at http://www.acm.uiuc.edu/~dgriffi3/projects/derangement/ Of course questions, comments and patches are welcome. -- Dennis Griffith, ACM@UIUC Treasurer

1 0

Recursion in Haskell
by P. R. Stanley 20 Feb '07

20 Feb '07

Hi I understand the basic principle of recursion but have difficulty with the following: -- a recursive function -- for calculating the length of lists myLen [] = 0 myLen (x:xs) = 1 + myLen xs What's happening here? Top marks for a comprehensive jargon-free explanation. Thanks in advance Paul

9 15

Why do I have to specify (Monad m) here again?
by Marc Weber 20 Feb '07

20 Feb '07

Why do I have to specify (Monad m) here again? class (Monad m) => GetMV m a where ... instance GetMV m c where ... No instance for (Monad m) arising from the superclasses of an instance declaration possible fix: add (Monad m) to the instance declaration superclass context Marc

7 21

Lambada and connecting Haskell to a Weblogic server
by Joel Reymont 20 Feb '07

20 Feb '07

Folks, Where can I find Lambada these days and would it be of any use to me in trying to connect to a Weblogic server? To make the long story short, my broker's Java software connects to a remote Weblogic server and I would like to do the same. I suppose this would require me to implement Java RMI in Haskell which is non- trivial. I wonder if it would be easier for me to implement a Java server that talks to Weblogic and then connect to that server. What do you think? Thanks, Joel -- http://wagerlabs.com/

6 7

stateful walk through a tree?
by David Tolpin 20 Feb '07

20 Feb '07

Hi, I am looking for help in design of a stateful tree walker. I have a tree-like data structure (an AST for a small language) for which there is a general stateful walk procedure. That is, each node type is an instance of a walker class, and the walk function is monadic: class Walker a where walk :: a -> State Context b The context is used to store names in the scope, for example. Now, I'd like to use Walker as a general class for implementing several different transformations on the tree (like cross-referencing, code emission, tree visualisation). Those transformations will need expanded state. What is the proper design for that? In Common Lisp, for example, I would Context to contain an opaque slot (visitor), and would assign additional state information to it; a would also define a generic function that would dispatch on the visitor (and probably on walker if I end up having more than one walker). How would I do that in Haskell? I'd like to avoid using mutable variables for now (mostly for didactic puproses). David

2 2

A new code search engine
by Stephane Bortzmeyer 20 Feb '07

20 Feb '07

http://www.krugle.com/ Unlike Google, you can specify Haskell as a language. But the very useful (and, again, unlike Google) parsing service, which allows you to specify that you search "foldl" only in function calls, not function definitions, seems broken for Haskell ("map" is never found in function calls...)

4 3

GHC throws IOError on Win32 when there is no console
by John Ky 20 Feb '07

20 Feb '07

Hi, I noticed on Windows that when I use IO functions that write to stdout when the process is lacking a console, those functions throw an IOError. I'm not sure if this also occurs for stderr because I haven't tried it. Some classes of processes are created without a console because they never interact with the user and include System services. Crashing with IOError in this case is difficult to diagnose because because the only symptom is the process crashes with no visible output. I believe the most sensible behaviour should be for those functions to not throw, but instead do nothing. -John

7 13

questions about core
by Matt Roberts 19 Feb '07

19 Feb '07

Hi all, I am trying to get a deeper understanding of core's role in GHC and the compilation of functional languages in general. So far I have been through - The hackathon videos, - "A transformation-based optimiser for Haskell", - "An External Representation for the GHC Core Language (DRAFT for GHC5.02)", and - "Secrets of the Glasgow Haskell Compiler inliner". and I am still a bit hazy on a few points: - What role do *the semantics* of core play (i.e. how and where are they taken advantage of)? - Exactly what are the operational and denotational semantics of core? - The headline reasons (and any other arguments that emerge) for having core *and* stg as separate definitions. I have an intuition on a few of these points, but I would love something concrete to latch on to. Thanks Matt R

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GHCi and multi-line support ?
by Dunric 19 Feb '07

19 Feb '07

Is it possible to write multi-line definitions in GHC interactive interpreter ? (like in Python interp. ?) When I try to write f.E. if-then-else in more then 1 line I get the following error: <interactive>:1:30: parse error (possibly incorrect indentation) ------------------------------------------ http://icqsms.cz/

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Summarize of Why do I have to specify (Monad m) here again?
by Marc Weber 19 Feb '07

19 Feb '07

Thanks for all the feedback. It did help me a lot. Now I know that if there is something left to discuss the topic should be: Would it make sense to specify partial type declarations ? I don't need an answer right now. Marc

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Re: Re[2]: [Haskell-cafe] Why do I have to specify (Monad m) here again?
by Sebastian Sylvan 19 Feb '07

19 Feb '07

On 2/18/07, Dean Herington <heringtonlacey(a)mindspring.com> wrote: > At 12:42 AM +0400 2/19/07, David Tolpin wrote: > >On Mon, 19 Feb 2007 00:30:47 +0400, Sebastian Sylvan > ><sebastian.sylvan(a)gmail.com> wrote: > >> Well, I guess the H98 report would be a good start. But there are > >> multiple tutorials on type classes that will cover this, most of which > >> are available from haskell.org > > > >Sebastian, > > > >I did read H98 and would like an exact reference. > > See section 4.3.2, the third bullet item in the bulleted list. (Note > that the last sentence of that bullet item says that context > inference--though often possible--is deliberately eschewed.) Yes, but the key point still stands: when the user supplies a type, is should get taken seriously. Just because there may be a way to infer a context which may work in some/most cases, doesn't mean it's a good idea to do so because in all other parts of Haskell the user supplied type is taken seriously -- if you don't include a context, that means there *is* no context. Period. In the original example you really are telling Haskell that you *don't* want any context for "m". So if you want some inference to happen there, you need to find a way to specify that you want "any" contexts rather than no contexts. E.g. instance ( _ m) => GetMV m c where I don't think there's another way to do what you want without seriously messing Haskell up. -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862

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Re: Re[2]: [Haskell-cafe] Why do I have to specify (Monad m) here again?
by Sebastian Sylvan 19 Feb '07

19 Feb '07

On 2/18/07, David Tolpin <dvd(a)davidashen.net> wrote: > >> Why the compiler cannot infer class constraint on m from class > >> definition in instance definition while it can in function type > >> definition? > > > > But it can't! If you give a type to a function, it will assume zero > > class constraints unless you specify them (just like it will when you > > give a type to an instance declaration). If you do something like: > > > > Hi Sebastian, > > it is not the example I brought. In the example I brought I showed how in > function type declaration assertion that an instance of a class is also an > instance of the other class is used. Take a look at my example. According > to what part of the type system logic type inference in instances is not > implemented That's completely different. The class in that case guarantees that the type has an "Eq" class, so it's okay to use the functions in the "Eq" class. You're using the guarantees supplied by the class. When you write instances, it's the other way around, the class has *requirements* that you must fulfill -- and there are multiple ways of doing it (Haskell won't guess, it will obey what you tell it -- if you don't give any class constraints it won't assume that they are there). The point I'm trying to make is that if you say that something is of a given type, Haskell will assume that you know what you're talking about. I.e. if you give a type which has no class constraints, Haskell will assume that you don't want any class constraints -- if this conflicts with something else, you will get an error. I think maybe this is where you're missing the point. *Leaving out* class constraints says something about the type. In other words, if you leave out a class constraint that does *not* mean that you don't care what class constraints the type variable has, it explicitly means that there are *no* class constraints on it (and using it in a context where a class constraint is required, is thus an error). In this case you *require* that any instances of a class is also an instance of the Monad class. But then you try to instantiate a type which is *not* in the monad class in this class. This is a conflict, and you should get an error. If you want to construct a type which is in the Monad class by constraining the type variable "m" to the Monad class, then that's fine, but you have to tell Haskell that this is what you want to do -- if you say that your type has *no* class constraints, it will take your word for it an report the conflict. Another alternative is that you want to produce the type by using a concrete type called, say, "M" which is in the Monad class, then that's also fine. Haskell won't guess what you meant (it can either be a spelling error, "m" for "M", or that you forgot to add "Monad m =>" in the instance declaration, or anynumber of other errors) and change it for you. Whenever you write something out, Haskell will take your word for it, if it fails, it's your fault, and Haskell will tell you what the problem is. -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862

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Very fast loops. Now!
by dons＠cse.unsw.edu.au 19 Feb '07

19 Feb '07

The following C program was described on #haskell #include <stdio.h> int main() { double x = 1.0/3.0; double y = 3.0; int i = 1; for (; i<=1000000000; i++) { x = x*y/3.0; y = x*9.0; } printf("%f\n", x+y); } Which was translated to the following Haskell: {-# OPTIONS -fexcess-precision #-} import Text.Printf main = go (1/3) 3 1 go :: Double -> Double -> Int -> IO () go !x !y !i | i == 1000000000 = printf "%f\n" (x+y) | otherwise = go (x*y/3) (x*9) (i+1) To everyone's surprise, GHC 6.6 beats GCC (3.3.5) here, at least the two test machines: $ ghc -O -fexcess-precision -fbang-patterns -optc-O3 -optc-ffast-math -optc-mfpmath=sse -optc-msse2 A.hs -o a $ time ./a 3.333333 ./a 0.96s user 0.01s system 99% cpu 0.969 total ^^^^^ Versus gcc 3.3.5: $ gcc -O3 -ffast-math -mfpmath=sse -msse2 -std=c99 t.c -o c_loop $ time ./c_loop 3.333333 ./c_loop 1.01s user 0.01s system 97% cpu 1.046 total ^^^^^ Note that newer gcc's will statically compute that loop. Note also that -fexcess-precision must currently be provided as a pragma only. I declare GHC Haskell numerics (with -fexcess-precision) not so shabby! -- Don

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Fwd: Re: Re[2]: [Haskell-cafe] Why do I have to specify (Monad m) here again?
by David Tolpin 19 Feb '07

19 Feb '07

------- Forwarded message ------- From: "David Tolpin" <dvd(a)davidashen.net> To: "Sebastian Sylvan" <sebastian.sylvan(a)gmail.com> Cc: "David House" <dmhouse(a)gmail.com>, haskell-cafe(a)haskell.org Subject: Re: Re[2]: [Haskell-cafe] Why do I have to specify (Monad m) here again? Date: Sun, 18 Feb 2007 23:38:59 +0400 >> Why the compiler cannot infer class constraint on m from class definition in instance definition while it can in function type definition? > > But it can't! If you give a type to a function, it will assume zero > class constraints unless you specify them (just like it will when you > give a type to an instance declaration). If you do something like: > Hi Sebastian, it is not the example I brought. In the example I brought I showed how in function type declaration assertion that an instance of a class is also an instance of the other class is used. Take a look at my example. According to what part of the type system logic type inference in instances is not implemented? David

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