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trees on Haskell : Do I understand it right ?
by Roelof Wobben 01 Mar '15

01 Mar '15

Hello, Suppose we have this definition of a tree : data MessageTree = Leaf | Node MessageTree LogMessage MessageTree deriving (Show, Eq) let Message = LogMessage "E 1 this is a test error" let Message = LogMessage "e 2 this is the second test error " As I understand it right I can make the first entry like this : first_entry = Node Messagetree Message Messagetree And the second one like this second_entry = Node Message Messagetree Message2 Messagetree ?? Roelof

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Using findIndex and then splitAt in Data.List
by Geoffrey Bays 28 Feb '15

28 Feb '15

Hi. An elementary question here about two functions in Data.List: how to use a value from findIndex which returns a Maybe Int, and then use that result in splitAt which takes a regular Int? This is in an IO() do Like so: (This is in an IO() do block) let gradeItemIndex = findIndex (\g -> (itemName g) == (itemName gradeItemP)) gradeItemList let twoListsTuple = splitAt gradeItemIndex gradeItemList // does not compile obviously Many Thanks, Geoffrey

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Bounded STM Queues with load shedding
by Alex Hammel 28 Feb '15

28 Feb '15

Hi list, It would be handy for me to have a bounded STM Queue along the lines of TBQueue <https://hackage.haskell.org/package/stm-2.4/docs/Control-Concurrent-STM-TBQ…> with the difference that when the queue is full, a call to writeTBQueue is a non-blocking no-op. This is in the context of an asynchronous logging system. If the load (somehow) gets heavy enough that the log message queue is taking up a lot of memory, It's better to lose a few log messages than to have processes block until the logger catches up. Are there any off-the-shelf solutions? Cheers, Allex <https://hackage.haskell.org/package/stm-2.4/docs/Control-Concurrent-STM-TBQ…>

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Testing Yesod applications.
by Njagi Mwaniki 27 Feb '15

27 Feb '15

Could I get help with testing yesod applications when it comes to situations requiring authentication and authorization? Here is my full question on stack overflow. http://stackoverflow.com/questions/28764807/performing-authentication-durin…

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Random variable holding a function
by Ondrej Nekola 27 Feb '15

27 Feb '15

Hi I try to simulate some processes, that contain randomness and I would like do it idiomatic way and keep as much code not to know about randomness as possible. I have a following code (simplified to make email more readable): import Data.List import Data.Functor import Genes import Data.Random.RVar import Data.Random.Extras data Individual = Individual (Int, Int) deriving (Eq, Show) -- this part is a bit more complicated... data Population = Population { individuals :: [Individual] } deriving (Eq, Show) type Selection = Population -> Population so far, it's easy: allSurvive :: Selection allSurvive = id extinction :: Selection extinction _ = Population [] The issue comes, when I want to have some randomness in the process. I am able to write a "pick some individuals into next generation" sort of "Selection" fairChance :: Int -> Population -> RVar Population fairChance newSize p = Population <$> (Data.Random.Extras.sample newSize $ individuals p) But it obviously doesn't fit into the "Selection" type. I would like to have something like fairChance :: Int -> RVar Selection (e.g. fairChance :: Int -> RVar (Population -> Population)) Is there a way to do this? Or should I give up this way and try to give up some purity and go withtype Selection :: RVar Population -> RVar population? Thanks Ondrej

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Re: [Haskell-beginners] Relatively simple undirected graph library?
by Stefan Höck 27 Feb '15

27 Feb '15

Dear list Sorry that it took so long. The usual excuses apply: Two little children, other priorities at work, had to write two more monad tutorials. For better or worse, here is a first draft of a labeled graph library. https://github.com/stefan-hoeck/labeled-graph This is all highly experimental. No unit tests so far and lots of stuff is still missing, so feel free to contribute. In the next couple of days, I'll add some more type class instances to LGraphs (Foldable, Traversable, Bifunctor, Bifoldable, Bitraversable). The implementation might well still change. One implementation detail that is different compared to other graph libraries: Edges are represented as Ints internally, so the maximal vertex is limited by the square of the largest Int, which is rather low on 32 bit systems. This is a typical case of bad preliminary optimisation, since I hope that using this implementation we can use efficient IntMaps to store edge labels. It's well possible, that this will change in the future. Please note also that I will use the library mainly in a cheminformatics project, where graphs typically are small and sparse (vertices are almost always of degree four or lower). Grüsse aus Zürich Stefan On Wed, Feb 25, 2015 at 11:30:54PM +0100, Torsten Otto wrote: > I second that it would be terrific to see this posted. > > Regards from Hamburg, > Torsten >

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Re: [Haskell-beginners] how to do this ?
by Roelof Wobben 24 Feb '15

24 Feb '15

Yes, that is one of the books I have to read. For this exercises chapter 2 and 3 . Roelof Francesco Ariis schreef op 24-2-2015 om 10:37: > off list: try to give a more meaningful title to your posts > (i.e. "cis194 help exercise 2" instead of "how to do this"). > It might lead to more answers! > > Also, have you tried "Learn you a haskell for great good"? > > > On Tue, Feb 24, 2015 at 10:28:28AM +0100, Roelof Wobben wrote: >> Hello, >> >> Im trying to solve exercise 2 of this course : >> http://www.seas.upenn.edu/~cis194/spring13/hw/02-ADTs.pdf >> >> I have this till so far : >> >> -- | Main entry point to the application. >> {-# OPTIONS_GHC -Wall #-} >> >> module LogAnalysis where >> >> import Data.Char (isDigit, isLetter) >> import Log >> >> -- | checks if the String is of the format char int >> isValid :: String -> Bool >> isValid s = >> case words s of >> [a]:b:_ -> isLetter a && all isDigit b >> _ -> False >> >> -- | Parse the String to make the right logmessage >> parse_line :: String -> LogMessage >> parse_line s = >> case words s of >> ("I":time:text) -> LogMessage Info (read time) >> (unwords text) >> ("W":time:text) -> LogMessage Warning (read time) >> (unwords text) >> ("E":errorcode:time:text) -> LogMessage (Error (read >> errorcode)) (read time) (unwords text) >> _ -> Unknown "This is not in the >> right format" >> >> >> -- | here the actual function which uses isValid and parse to make >> the right log messages >> parseMessage :: String -> LogMessage >> parseMessage s = >> if isValid(s) then parse_line(s) else Unknown "This is not the >> right format" >> >> >> parse :: String -> [logMessage] >> parse s = parse_line s >> >> >> -- | The main entry point. >> main :: IO () >> main = do >> print $ show (parseMessage "I 4681 ehci 0xf43d000:15: regista14: >> [0xbffff 0xfed nosabled 00-02] Zonseres: brips byted nored)") >> print $ show (parseMessage "W 3654 e8] PGTT ASF! 00f00000003.2: >> 0x000 - 0000: 00009dbfffec00000: Pround/f1743colled") >> print $ show (parseMessage "E 47 1034 'What a pity it wouldn't >> stay!' sighed the Lory, as soon as it was quite") >> >> But I do not see how I can get the output of the parse_line into a List. >> >> Can anyone give me a tip ? >> >> Roelof >> >> >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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How can I print the outcome with using main.
by Roelof Wobben 24 Feb '15

24 Feb '15

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Re: [Haskell-beginners] How can I print the outcome with using main.
by Joel Williamson 24 Feb '15

24 Feb '15

testParse returns an IO action. show expects an argument that is in Show. You will need to bind the result of testParse to a name and ensure that LogMessage is in Show.

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how to do this ?
by Roelof Wobben 24 Feb '15

24 Feb '15

Hello, Im trying to solve exercise 2 of this course : http://www.seas.upenn.edu/~cis194/spring13/hw/02-ADTs.pdf I have this till so far : -- | Main entry point to the application. {-# OPTIONS_GHC -Wall #-} module LogAnalysis where import Data.Char (isDigit, isLetter) import Log -- | checks if the String is of the format char int isValid :: String -> Bool isValid s = case words s of [a]:b:_ -> isLetter a && all isDigit b _ -> False -- | Parse the String to make the right logmessage parse_line :: String -> LogMessage parse_line s = case words s of ("I":time:text) -> LogMessage Info (read time) (unwords text) ("W":time:text) -> LogMessage Warning (read time) (unwords text) ("E":errorcode:time:text) -> LogMessage (Error (read errorcode)) (read time) (unwords text) _ -> Unknown "This is not in the right format" -- | here the actual function which uses isValid and parse to make the right log messages parseMessage :: String -> LogMessage parseMessage s = if isValid(s) then parse_line(s) else Unknown "This is not the right format" parse :: String -> [logMessage] parse s = parse_line s -- | The main entry point. main :: IO () main = do print $ show (parseMessage "I 4681 ehci 0xf43d000:15: regista14: [0xbffff 0xfed nosabled 00-02] Zonseres: brips byted nored)") print $ show (parseMessage "W 3654 e8] PGTT ASF! 00f00000003.2: 0x000 - 0000: 00009dbfffec00000: Pround/f1743colled") print $ show (parseMessage "E 47 1034 'What a pity it wouldn't stay!' sighed the Lory, as soon as it was quite") But I do not see how I can get the output of the parse_line into a List. Can anyone give me a tip ? Roelof

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