- Beginners - Haskell.org

TCP Server
by Raphael Päbst 16 May '11

16 May '11

Hi! I need a TCP server for my current project and am now wondering if I have to write it from scratch or if there is a package out there that lets me avoid this. I have seen examples of TCP servers for example in the Real World Haskell book, but they are a bit confusing to me at the moment, so I'd like to avoid tackling this one. Thanks Raf

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Genetic algorithm for knapsack problem
by Karol Samborski 16 May '11

16 May '11

Hi all, I'm a student and I got recently a task to do (and I want to do it in haskell). I have to implement "genetic algorithm for knapsack problem" and it would be nice if it will be fast. So my question is: what haskell datatypes, modules etc. should I use for this algorithm to achieve this goal? I mean fast as compared to implementation in imperative languages like C. Any advices ? Best regards, Karol Samborski

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Multiple versions
by Tom Murphy 16 May '11

16 May '11

Hi all, Despite my best efforts, there are multiple versions of the Haskell Platform on my system. I'm running Mac OSX, 10.6. I've installed the newest Haskell Platform, from its installer package. In /Library/Frameworks/GHC.framework/Versions, there is only one folder: the latest. Previously, though, I had to delete by hand an older version of GHC in the Versions folder. There are no Haskell or GHC folders in ~/Library. When I run the ghci that's in the /Library/Frameworks/GHC.framework/Versions folder, it's the right version. Yet when I type "ghci" in the prompt, it gives me "GHCi, version 6.10.4" Any help would be very appreciated! Tom

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Generic exception available?
by Christopher Howard 16 May '11

16 May '11

I have one spot in some code where I just want to raise an exception that contains the message "Database is formatted incorrectly." Is there some kind of generic way to throw that (like, e.g., in Ruby you just call 'raise "exception message"'. In this case, I don't particularly want to take the time to think about creating an Exception type or finding a suitable one. (BTW: Is there a hierarchical list of standard Haskell Exception instances somewhere?) -- frigidcode.com theologia.indicium.us

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Printing the entries of a Data.Map m
by Manfred Lotz 16 May '11

16 May '11

Hi all, I have a map with some two thousand entries. Now I want to print that map. I could do: print m but I like to use my own print function: mapM_ myprint (Map.toList m) I may be wrong but having to use Map.toList looks pretty inefficient. Question: I'd like to know if there is a more efficient way to do it? -- Manfred

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More direct binding of IO result
by Christopher Howard 15 May '11

15 May '11

I understand that one can bind the unwrapped results of IO functions to variables, and pass them to functions, like so: main = do filecontents <- readFile "data.txt" putStrLn filecontents But does the syntax allow you to cut out the middle man, so to speak, and bind the results directly to the parameter? Like -- Tried this and it didn't work. main = do putStrLn (<- readFile "data.txt") -- frigidcode.com theologia.indicium.us

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hidden package error when using Snap
by Todd Porter 15 May '11

15 May '11

Hi, I am using the parseTime function from Data.Time.Format. The first parameter is a TimeLocale defined in System.Locale (old-locale-1.0.0.2). I first wrote a trivial little module to test my understanding in ghci: module Tbasic where import Data.Time import System.Locale parseDate :: String -> Maybe UTCTime parseDate s = parseTime defaultTimeLocale "%d/%m/%Y" s Works fine. I then tried using this same function in my Snap web application. The application builds (via cabal install) fine. However when I start up the application and make my first request I get the following error: A web handler threw an exception. Details: Compile errors: Could not find module `System.Locale': It is a member of the hidden package `old-locale-1.0.0.2'. Use -v to see a list of the files searched for. I have added old-locale to Build-depends in the applications .cabal file. old-locale-1.0.0.2 appears in the global list of packages when I run ghc-pkg list Unsure what to try next. To be honest I don't understand a compile error at runtime given I didn't activate dynamic recompilation (i.e. I last built using cabal install, not cabal install -fdevelopment). appreciate any advice, Todd

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do-notation question
by blackcat＠pro-ns.net 15 May '11

15 May '11

In The Yesod Book, I see the following code: defaultLayout contents = do PageContent title headTags bodyTags <- widgetToPageContent $ do addCassius ... How does this work? Is bodyTags drawn from widgetToPageContent and then used as the third argument to PageContent? tia, Lee

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Example Connection String to MySQL via ODBC?
by Christopher Howard 14 May '11

14 May '11

Hi. I'm trying to connect for the first time to a 5.1 MySQL database with HDBC via the ODBC driver. Would someone be kind enough to show me an example connect string to pass to connectODBC? I've tried to put together what I can from the "example" at Hackage and old posts I found on the Web, but all I ever get is *** Exception: <<Database.HDBC.SqlError>> If someone could give me a boilerplate example with the standard elements (host, database, user, password) that would probably help. Thank you. -- frigidcode.com theologia.indicium.us

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cabal question, asked in a foul mood
by Philippe Sismondi 14 May '11

14 May '11

What exactly *is* a cabal package? I an not asking about the purpose of a package, but very literally: is it a single file? a set of files? a hierarchy of directories? I apologize if this seems like a silly question. However, I have just read the first five pages of the cabal user's guide, and am none the wiser for it. That particular piece of documentation is in serious need of review by a good editor, to put it mildly. I cannot read any further in it without losing my patience completely. Thanks; sorry for the bit of ranting. - Phil -

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