March 2011 - Haskell-Cafe

Need help to solve this question
by Chatura Roche 12 Mar '11

12 Mar '11

Both sections relate to the case study: Index for a document of text. SECTION A: Given the attached Haskell code which produces an index of words, make the following alterations by modifying existing functions and including new functions where necessary : 3) Treat a capitalised word (one or more capitals) as being different from the word in all lower case (but they should still be sorted alphabetically) – unless it is at the start of a sentence with only the initial letter capitalised. A sentence is terminated by a ‘.’, ‘?’ or ‘!’. import Prelude type Doc = String type Line = String type Word = String makeIndex :: Doc -> [ ([Int], Word) ] makeIndex = shorten . -- [([Int], Word)] -> [([Int], Word)] amalgamate .-- [([Int], Word)] -> [([Int], Word)] makeLists . -- [(Int, Word)] -> [([Int], Word)] sortLs . -- [(Int, Word)] -> [(Int, Word)] allNumWords .-- [(Int, Line)] -> [(Int, Word)] numLines . -- [Line] -> [(Int, Line)] splitUp -- Doc -> [Line] splitUp :: Doc -> [Line] splitUp [] = [] splitUp text = takeWhile (/='\n') text : -- first line (splitUp . -- splitup other lines dropWhile (==’\n’) . -- delete 1st newline(s) dropWhile (/='\n')) text -- other lines numLines :: [Line] -> [(Int, Line)] numLines lines -- list of pairs of = zip [1 .. length lines] lines -- line no. & line -- for each line -- a) split into words -- b) attach line no. to each word splitWords :: Line -> [Word] -- a) splitWords [] = [] splitWords line = takeWhile isLetter line : -- first word in line (splitWords . -- split other words dropWhile (not.isLetter) . -- delete separators dropWhile isLetter) line -- other words where isLetter ch = (‘a’<=ch) && (ch<=’z’) (‘A’<=ch) && (ch<=’Z’) numWords :: (Int, Line) -> [(Int, Word)] -- b) numWords (number, line) = map addLineNum ( splitWords line) -- all line pairs where addLineNum word = (number, word) -- a pair allNumWords :: [(Int, Line)] -> [(Int, Word)] allNumWords = concat . map numWords -- doc pairs sortLs :: [(Int, Word)] -> [(Int, Word)] sortLs [ ] = [ ] sortLs (a:x) = sortLs [b | b <- x, compare b a] -- sort 1st half ++ [a] ++ -- 1st in middle sortLs [b | b <- x, compare a b] -- sort 2nd half where compare (n1, w1) (n2, w2) = (w1 < w2) -- 1st word less (w1 == w2 && n1 < n2) -- check no. makeLists :: [(Int, Word)] -> [([Int], Word)] makeLists = map mk -- all pairs where mk (num, word) = ([num], word) -- list of single no. amalgamate :: [([Int], Word)] -> [([Int], Word)] amalgamate [ ] = [ ] amalgamate [a] = [a] amalgamate ((n1, w1) : (n2, w2) : rest)-- pairs of pairs | w1 /= w2 = (n1, w1) : amalgamate ((n2, w2) : rest) | otherwise = amalgamate ((n1 ++ n2, w1) : rest) -- if words are same grow list of numbers shorten :: [([Int], Word)] -> [([Int], Word)] shorten = filter long -- keep pairs >4 where long (num, word) = length word > 4 -- check word >4

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Uncatchable error
by Daniel Díaz 12 Mar '11

12 Mar '11

Hi, cafe, I'm working in a program where I use many connections with Network.HTTP. Sometimes, connections are closed while my program is reading them, and an error appears: <socket: XXX>: Data.ByteString.hGetLine: invalid argument (Bad file descriptor) All I need is to handle this error. The function 'catch' doesn't work. I guess this error comes from a call to 'error' function, or something similar. What I can do? Thanks in advance, Daniel Díaz

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Help with how to concatenate with own datatypes
by eldavido 12 Mar '11

12 Mar '11

Hi, I´m doing a project in haskell and I need to define an operator that concatenate some own defined data types, just like the operator ++ does for lists. I don´t see how to define the operator recursively since this adding function (:) doesn´t work on my own data types. This is my code: data Allele = Aone | Atwo deriving Show type Lifespan = Int data Population = Pop [(Allele,Allele,Lifespan)] deriving Show genepool :: Population genepool = Pop [] --can only concatenate 2 elements (+-) :: Population -> Population -> Population Pop [(a,b,c)] +- Pop [] = Pop [(a,b,c)] Pop [(a,b,c)] +- Pop [(d,e,f)] = Pop [(a,b,c),(d,e,f)] --and thats why this function goes non-exhaustive. gpinsertaoneaone :: Int -> Int -> Population gpinsertaoneaone 0 age = genepool gpinsertaoneaone n age = Pop [(Aone,Aone,70-age)] +- gpinsertaoneaone (n-1) age Thx for help and advice! -- View this message in context: http://haskell.1045720.n5.nabble.com/Help-with-how-to-concatenate-with-own-… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ContT and ST stack
by Anakim Border 11 Mar '11

11 Mar '11

Dear list, I have the following (simplified) piece of code: find :: Int -> [Int] find i = runST . (`runContT` return) $ callCC $ \escape -> do return [] which used to compile correctly under GHC 6.12.3. Now that I've switched to 7.0.2 it gets rejected with the following error: Couldn't match expected type `forall s. ST s c0' with actual type `m0 r0' Expected type: ContT r0 m0 a0 -> forall s. ST s c0 Actual type: ContT r0 m0 a0 -> m0 r0 In the second argument of `(.)', namely `(`runContT` return)' In the expression: runST . (`runContT` return) I'm a little bit lost at what exactly is the problem. Anyone can suggest a solution? Thanks! AB

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Convert a function to a string of operators?
by Evgeny Grablyk 11 Mar '11

11 Mar '11

Hello! I was wondering if it was possible to "convert" a function (which may also call functions) to a plain list of operators on parameters. Example: foo a b = a + b bar a b = a * b baw a b c = bar (foo a b) c baw' a b c = (a + b) * c Any way to get `baw'' from `baw'? Preferrably as a String. -- Evgeny

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Re: [Haskell-cafe] possible bug for ghc 7 + xcode 4 on snow leopard?
by Carter Schonwald 11 Mar '11

11 Mar '11

to answer the question of the other poster, ghc 7.0.2 packaged installer, 64 bit build Jurrien, I wound up doing something similar, namely just wholesale copying the 10.5 sdk into the sdks folder. i'm still curious why this problem even exists! 2011/3/10 Jurriën Stutterheim <j.stutterheim(a)students.uu.nl> > I solved it by making a symlink from /Developer/SDKs/MacOSX10.6.sdk to > /Developer/SDKs/MacOSX10.5.sdk (which is missing with XCode 4). So far it > seems to work :) > > > Jurriën > > > On 10 Mar, 2011, at 06:53 , Carter Schonwald wrote: > > > after experimenting by just naively moving the old Developer directory > back to its original location preceding the installation of xcode 4, > everything builds properly now. > > > > still, this is not a reasonable long term solution. Any ideas about how > this might be fixed? > > > > On Thu, Mar 10, 2011 at 12:33 AM, Carter Schonwald < > carter.schonwald(a)gmail.com> wrote: > > Hey All, > > As of installing xcode 4 earlier today, > > > > i'm getting errors of the form > > ld: library not found for -lcrt1.10.5.o > > collect2: ld returned 1 exit status > > > > for any haskell code that invokes ld in its cabal install process. > > 1) is this related to some problem in xcode / what the haskell code > defaults to linking to? > > 2) is this something completely unrelated? > > > > thanks! > > -Carter > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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hsc2hs not figuring out #include paths?
by Edward Amsden 11 Mar '11

11 Mar '11

I'm trying to write some bindings using hsc2hs. At the top of my .hsc file, I have #include <jack/types.h> #include <jack/jack.h> When I compile a .c file with just these two lines in gcc, it works fine (the jack directory is in /usr/local/include). When I try to run hsc2hs, it complains that it cannot find the header files. Any ideas? How do I set the include path (via cabal or just in hsc2hs) for hsc2hs? -- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com

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Parallel Haskell stories
by Simon Peyton-Jones 11 Mar '11

11 Mar '11

Friends I'm giving a talk at a developer conference in London on Friday 18th, about parallel programming in Haskell. http://skillsmatter.com/event/scala/functionalpx-2011/ad-1382 I know that some of you have been using Haskell for parallel or concurrent programming quite a bit, so this email is just to ask could you contribute a little vignette or story about using Haskell in a *parallel/concurrent* application that I could use to illustrate my talk? I can rant all I like about the glories of Haskell, but I'm a biased witness. It's much more convincing if I can illustrate with true tales from the trenches. I'm interested in both parallelism and concurrency, so for example the success of Warp in using Concurrent Haskell is in scope. I can't say a lot about any one example, obviously, but what would be great would be - an idea of how Haskell helped (esp if you have a head to head comparison) - code snippets that illustrate how lovely it all is - brief performance indicators *Insight* is the key word. I don't just want to say "Company X used Haskell to do Y" because that doesn't convey any re-usable insights or ideas. What is the essence? I'll start writing in earnest on Monday 14th; the talk is on Friday. I have plenty to say already, so treat this as an opportunity not an obligation. But still, examples would be good! Do let me know if you have one. Simon

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Why don't my OS threads terminate?
by Bas van Dijk 11 Mar '11

11 Mar '11

Dear all, At work, I'm developing a webapplication with happstack. When I let it run for some time (< 5min.) it crashes with the following message: "failed to create OS thread: Resource temporarily unavailable" This happens because the server continuously creates OS threads but never terminates them. This keeps going on until I hit the OS thread limit. I would like to know why the OS threads are created and more importantly why they aren't terminated. Unfortunately I can't publish the source code because it's proprietary software. Hopefully the following description may give an idea what's going on: The webapplication is basically a simple measurement application: The client side (HTML, Javascript, JQuery) contains a plot which shows the measurements that the server generates. The client contains a loop which first longpolls[2] the server to wait for a new measurement, the server will then block (using takeMVar) until (using putMVar) a measurement is available. The client then retrieves the measurement from the server including all other relevant data. These requests all happen asynchronously using ajax calls[1]. The server starts by forking a thread which runs the measure loop. At each iteration the measure loop reads data from a file then fits that data to a model. For fitting I use my levmar fitting library[3]. The coefficients of the fitted model are then used to calculate a measurement. I believe the OS threads are created by my levmar library. This library uses bindings-levmar[4] which is a binding to a C library. bindings-levmar uses safe FFI calls because the levmar C procedures are reentrant (they need to call back into Haskell to execute the model function). I believe the RTS creates an OS thread for each safe FFI call. When I replace my levmar fit function with a function which just returns a random number the problem disappears. The problem also doesn't appear in a standalone application I have which just uses my levmar fit function to fit all the datafiles, without the webserver. It seems that the combination of happstack and levmar causes the OS threads to not terminate. I tried space profiling the application but that did not reveal anything interesting. I even used threadscope to look at the application eventlog (see: [5]) but that also looked normal. Hopefully someone can point me where to look next. Thanks, Bas [1] http://api.jquery.com/jQuery.ajax/ [2] http://en.wikipedia.org/wiki/Push_technology#Long_polling [3] http://hackage.haskell.org/package/levmar [4] http://hackage.haskell.org/package/bindings-levmar [5] http://bifunctor.homelinux.net/~bas/crash.eventlog

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ANNOUNCE: cinvoke 0.1 released
by Remi Turk 11 Mar '11

11 Mar '11

I am happy to finally announce cinvoke 0.1, a binding to the C library cinvoke[1], allowing functions to be loaded and called whose names and types are not known before run-time. Why? Sometimes you can't use the Haskell foreign function interface because you parse the type of the function from somewhere else, i.e. you're writing an interpreter for a language that has an FFI itself. What? The main function it exports is: cinvoke :: Symbol -> RetType b -> [Arg] -> IO b And because code is worth a thousand words, here's a small program that uses libc to write a 1Gb buffer of random garbage to a file: > module Main where > > import Foreign.CInvoke > > main = do > cxt <- newContext > libc <- loadLibrary cxt "libc.so.6" > malloc <- loadSymbol libc "malloc" > creat <- loadSymbol libc "creat" > write <- loadSymbol libc "write" > free <- loadSymbol libc "free" > let sz = 2^30 > buf <- cinvoke malloc (retPtr retVoid) > [argCSize sz] > fd <- cinvoke creat retCInt [argString "/tmp/test", argCUInt 0o644] > n <- cinvoke write retCSize [argCInt fd, argPtr buf, argCSize sz] > cinvoke free (retPtr retVoid) [argPtr buf] It hopefully works on any machine on which cinvoke works, but has only been tested on linux x86_64. As the current version of cinvoke only installs a static library, it does not work from GHCi at the moment (without hacking cinvoke to build a shared library). More interesting examples are included in examples/ in the package. Where? Hackage: http://hackage.haskell.org/package/cinvoke Cheers, Remi [1] http://www.nongnu.org/cinvoke/

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