February 2010 - Haskell-Cafe

sendfile leaking descriptors on Linux?
by Bardur Arantsson 25 Mar '10

25 Mar '10

Hi all, I've been using the sendfile package off Hackage, but it seems to be leaking file descriptors when using the Linux-native build. What's happening in my specific case is the following: 1) client requests a range of a file 2) server starts sending the range 3) client disconnects before receiving the whole file This happens over and over with the client requesting different ranges of the file (so the client does make progress). If I use the portable build of the sendfile package, everything works fine for hours and hours of this happening. If I use the Linux-native build of the sendfile package, the server will eventually run out of file descriptors. According to "lsof" the files that are being kept open are the data files being sent in 2) above. This is on GHC 6.10.x (Ubuntu). Is anyone else seeing this? Anyone got any idea what's going on? Cheers,

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really difficult for a beginner like me...
by Ivan Amarquaye 24 Mar '10

24 Mar '10

Hi everyone, this is my first posting on here and this was what drove me here and the quest to know more as i anticipate a lot of help and direction in this quite new and different environment haskell.I have this paper that i'm working on and need to solve these scenarios/ cases based on some sample codes: Scenarios/cases: 1) Allow words to be hyphenated and treat the hyphenated word as a single word (including the hyphen). 2) As for no. 2 but if the hyphen is the last character on a line treat the hyphenated word as a single word without the hyphen. 3) Treat a capitalised word (one or more capital letters) the same as lower case, i.e. only the lower case word appears in the index. 4) Treat a word ending in an ‘s’ as a plural and thus the same as the singular, i.e. only the singular appears in the index. 5) As for no. 5 but (a) treat suffix ‘ss’ as not a plural; and (b) treat the plural suffixes ‘sses’, ‘zzes’, ‘oes’, ‘xes’, ‘shes’, ‘ches’ the same as the singular, i.e. without the ‘es’, e.g. “branches” (except for 4- and 5-letter plurals with suffices ‘oes’ and ‘ches’, e.g. “floes”, and 4-letter plural suffix ‘xes’); and (c) treat the plural suffix ‘ies’ (except for 4-letter plurals, e.g. “pies”) as the singular suffix ‘y’. 6) Make the output more readable in the form of an index table. 7) Use an output file 8) Include a user-friendly menu by which the user can choose input and output file names. This is the code i'm supposed tomodify and in some cases create new functions to support.I also need some explanation as the various approaches in solving them.............................................................................................. The function makeIndex given a document produces a list of entries. Each entry is a word and a list of line numbers (for words > 4 letters) Type definitions: import Prelude -- hiding (Word) -- predefined Word hidden, so we can define ours -- type String = [Char] defined in Prelude type Doc = String type Line = String type Word = String -- our version makeIndex :: Doc -> [ ([Int], Word) ] A data-directed design considers a sequence of functions (i.e using composition operator ‘.’) to transform the document of type, Doc, into an index of type, [ ([Int], Word) ]. splitUp the document, doc, into a list of lines, [Line]. numLines pairs each line with a line number, [(Int, Line)]. allNumWords splits lines into words and line no., [(Int, Word)]. sortLs sorts words into alphabetical order, [(Int, Word)]. makeLists makes a list for each line number, [([Int], Word)]. amalgamate nos. into a list of nos. for each word, [([Int], Word)]. shorten into a list for words > 4 letters, [([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] Last -- [a] -> [a ] splitUp function 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 Example: splitUp “hello world\n\nnext world” => [“hello world”, “next world”] numLines function: numLines :: [Line] -> [(Int, Line)] numLines lines -- list of pairs of = zip [1 .. length lines] lines -- line no. & line Example: numLines [“hello world”, “next world”] => [(1, “hello world”), (2, “next world”)] splitWords function: -- 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 && Last ==’-’) . -- delete separators dropWhile isLetter) line -- other words where isLetter ch = (‘a’<=ch) && (ch<=’z’) || (‘A’<=ch) && (ch<=’Z’) Example: splitWords “hello world” => [“hello”, “world”] allNumWords function: 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 Examples: addLineNum “hello” => (1, “hello”) numWords (1, “hello world”) => [(1, “hello”), (1, “world”)] allNumWords [(1, “hello world”), (2, “next world”)] => [(1, “hello”), (1, “world”), (2, “next”), (2, “world”)] SortLs function: 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. Example: sortLs [(1, “hello”), (1, “world”), (2, “next”), (2, “world”)] => [(1, “hello”), (2, “next”), (1, “world”), (2, “world”)] makeLists function: makeLists :: [(Int, Word)] -> [([Int], Word)] makeLists = map mk -- all pairs where mk (num, word) = ([num], word) -- list of single no. Examples: mk (1, “hello”) => ([1], “hello”) makeLists [(1, “hello”), (2, “next”), (1, “world”), (2, “world”)] => [([1], “hello”), ([2], “next”), ([1], “world”), ([2], “world”)] Amalgamate function: 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 Example: amalgamate [([1], “hello”), ([2], “next”), ([1], “world”), ([2], “world”)] => [([1], “hello”), ([2], “next”), ([1, 2], “world”)] Shorten function: shorten :: [([Int], Word)] -> [([Int], Word)] shorten = filter long -- keep pairs >4 where long (num, word) = length word > 4 -- check word >4 Example: shorten [([1], “hello”), ([2], “next”), ([1, 2], “world”)] => [([1], “hello”), ([1, 2], “world”)] _________________________________________________________________ Connect to the next generation of MSN Messenger http://imagine-msn.com/messenger/launch80/default.aspx?locale=en-us&source=…

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Anyone up for Google SoC 2010?
by Malcolm Wallace 10 Mar '10

10 Mar '10

Google has announced that the Summer of Code programme will be running again this year. If haskell.org people would like to take part again this year, then we need volunteers: First, * suggestions for suitable projects (in the past this was organised using a reddit) * an administrator to co-ordinate the application to Google (I have done it for the last three years but am very willing to hand on to someone else) Google will accept applications from organisations in the period 8th - 12th March 2010, approx 1900UTC. If haskell.org is accepted again, students can apply between 29th March - 9th April. More volunteers will be required: * to review student applications and choose which to accept * to supervise the accepted students Both of these roles are called "mentor" in the Google system. Putting together a good team of mentors before applying as an organisation is helpful towards us being accepted into the programme. Regards, Malcolm

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Real-time garbage collection for Haskell
by Luke Palmer 09 Mar '10

09 Mar '10

I have seen some proposals around here for SoC projects and other things to try to improve the latency of GHC's garbage collector. I'm currently developing a game in Haskell, and even 100ms pauses are unacceptable for a real-time game. I'm calling out to people who have seen or made such proposals, because I would be willing to contribute funding and/or mentor a project that would contribute to this goal. Also any ideas for reducing this latency in other ways would be very appreciated. Thanks, Luke

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New OpenGL package: efficient way to convert datatypes?
by Peter Verswyvelen 05 Mar '10

05 Mar '10

The newest package seems to require using GLdouble/GLfloat. What is the most efficient way to convert Double/Float to GLdouble/GLfloat? I'm currently using realToFrac. But essentially the operation should be a nop on my machine. I haven't looked at the core code yet (on Windows, last time I checked, the ghc-core util did not work, and without it, reading core is even harder) Thanks, Peter

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ANN: uuid-0.1.2
by Mark Lentczner 03 Mar '10

03 Mar '10

Antione and I are please to announce the release of uuid-0.1.2. CHANGES: - added functions toByteString and fromByteString - added 'nil' UUID - added unit tests and benchmarks, built when configured -ftest - major speed up of to/from functions (as well as in general) - added version-3 generation (deterministic based on MD5) - major changes to internal representation - now uses four strict Word32 values - internal ByteSource classes for easy construction (see Builder.hs) - Storable instance now stores in memory as system libraries in C do: 16 bytes derived from the network order of the fields, no matter what the host native endianess is. - fixed bugs in V1 time and clock stepping, and V1 generated values - builds cleanly under GHC's -Wall - added CHANGES file String conversions were sped up 4x to 7x! - Mark Mark Lentczner (MtnViewMark) http://www.ozonehouse.com/mark/

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using haskell to serve with apache
by brad clawsie 03 Mar '10

03 Mar '10

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 hi i'm currently doing some work on a personal site and was considering giving a haskell web tool a spin. i have a fairly complex apache configuration, so i don't want to try to replace it with a native haskell server, but instead use haskell via an interface to apache (i.e. something like wsgi, plack, etc). some searching shows hyena and yesod in various states of development, but i can't figure out if they are appropriate or ready for use. anyone have experiences using haskell behind apache? should i just try out something based on fastcgi? any anecdotes welcome thanks brad -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (FreeBSD) iEYEARECAAYFAkuKoJEACgkQxRg3RkRK91PVJwCdFD4dS47KjJuMcNVZmPkYXKyU ItsAnRoYBE39XtXyXDluh+pHSxzP+Qcd =91Ql -----END PGP SIGNATURE-----

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Multiple Interpretations for a monad?
by Günther Schmidt 02 Mar '10

02 Mar '10

Hi everyone, in my attempts to remove boilerplate and thus to do more "abstraction" I come across a number of interesting things and suggestions. Especially blog posts from Dan Piponi and also Heinrich Apfelmus. I think what they both are saying is that you can construct / implement some sort of 2 layered monad which can then have more than one interpretation. In the responses to one of my posts on DSLs Dan Piponi also points out that he considers monads also to be DSLs. It didn't click with me when he said it, but reading more of his blog posts made me remember that. Now I know this is probably something obvious to most haskellers, but to me it's not. Did I understand this correctly so far? Günther

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What are "free" Monads?
by Günther Schmidt 02 Mar '10

02 Mar '10

Hello, I see the term "free monad" quite a lot, but don't really see an explanation what a free monad is. What sets a monad free and why in this day and age are there unfree monads? Günther

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Linear programming in Haskell
by Louis Wasserman 01 Mar '10

01 Mar '10

Is there a nice package out there somewhere with a linear programming implementation? Preferably with a nicely functional interface? Kthxbai, Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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